Fouling on ion-exchange membranes: Classification, characterization and strategies of prevention and control Sergey Mikhaylin, Laurent Bazinet PII: DOI: Reference:
S0001-8686(15)00226-2 doi: 10.1016/j.cis.2015.12.006 CIS 1609
To appear in:
Advances in Colloid and Interface Science
Please cite this article as: Mikhaylin Sergey, Bazinet Laurent, Fouling on ion-exchange membranes: Classification, characterization and strategies of prevention and control, Advances in Colloid and Interface Science (2015), doi: 10.1016/j.cis.2015.12.006
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ACCEPTED MANUSCRIPT Fouling on ion-exchange membranes: classification, characterization and strategies of prevention and control
Institute of Nutrition and Functional Foods (INAF) and Dairy Research Center (STELA), Department of Food
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Sergey Mikhaylina and Laurent Bazineta*
*Corresponding
author:
Laurent
Bazinet,
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Sciences, Pavillon Comtois, Université Laval, Québec (Qc), Canada G1V 0A6
[email protected],
website:
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www.laurentbazinet.fsaa.ulaval.ca, (+1) 418 656-2131 poste 7445
Abstract
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The environmentally friendly ion-exchange membrane (IEM) processes find more and more applications in the modern industries in order to demineralize, concentrate and modify
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products. Moreover, these processes may be applied for the energy conversion and storage.
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However, the main drawback of the IEM processes is a formation of fouling, which significantly decreases the process efficiency and increases the process cost. The present
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review is dedicated to the problematic of IEM fouling phenomena. Firstly, the major types of IEM fouling such as colloidal fouling, organic fouling, scaling and biofouling are discussed along with consideration of the main factors affecting fouling formation and development. Secondly, the review of the possible methods of IEM fouling characterization is provided.
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This section includes the methods of fouling visualization and characterization as well as methods allowing investigations of characteristics of the fouled IEMs. Eventually, reader will find the conventional and modern strategies of prevention and control of different fouling types.
Keywords
Ion-exchange membrane, fouling, fouling characterization, fouling prevention and control
Contents of paper
I. Introduction
ACCEPTED MANUSCRIPT II. Classification of IEMs fouling II.1 Colloidal fouling II.2 Organic fouling
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II.3 Scaling
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II.4 Biofouling
III.1 Visualization of IEMs fouling III.2 Membrane characteristics
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III. Characterization of IEMs fouling
III.2.1 Membrane electrical resistance and conductivity
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III.2.2 Voltammetry and chronopotentiometry III.2.3 Electrical impedance spectroscopy
III.2.5 Zeta potential III.2.6 Contact angles
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III.3 Fouling composition
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III.2.4 Transport numbers
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III.3.1 Nitrogen content analysis by combustion III.3.2 Spectroscopy
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III.3.2.1 Molecular spectroscopy III.3.2.1.1 Ultraviolet-visible (UV/vis) spectroscopy III.3.2.1.2 Attenuated Total Reflectance Fourier Transform
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Infrared spectroscopy (ATR–FTIR)
III.3.2.2 Atomic spectroscopy III.3.2.2.1 Atomic absorption spectrometry (AAS) III.3.2.2.2 Optical emission spectrometry (OES) III.3.2.2.3 Energy dispersive X-ray spectroscopy (EDS) III.3.2.3 X-ray diffraction analysis (XRD)
III.3.3 Chromatography III.3.4 Biofouling characterization IV. Strategies of prevention and control of IEMs fouling IV.1 Modification of IEMs IV.2 Cleaning agents IV.3 Pretreatment IV.3.1 Pressure-driven membrane processes IV.3.2 Other pretreatment techniques
ACCEPTED MANUSCRIPT IV.4 Mechanical action IV.5 Regimes of ED treatment IV.5.1 Control of hydrodynamic conditions
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IV.5.2 Electrodialysis with reversal polarity
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IV.5.3 Pulsed electric field IV.5.4 Overlimiting current regimes
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Conclusion
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I Introduction
Fouling is the phenomenon of undesirable attachment of certain species (living
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organisms or non-living substances) to the surface or inside the material. This phenomenon is one of the key problems for the modern chemical, agricultural, food, pharmaceutical processing and water treatment. The cost of cleaning procedures and membrane replacement
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may vary from 20-30 % [1] for the pressure-driven membrane processes to 40-50 % for the
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electromembrane processes [2]. In the scope of the present review, we will consider only the fouling on ion-exchange membranes (IEMs), which are applied in electrodialysis (ED),
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electrolysis, diffusion dialysis, Donnan dialysis, fuel cells etc. IEMs are a special membrane type providing ionic selectivity since they carry electrical charges. This particular type of selectivity leads to separation of ionic species from the neutral media allowing demineralization, concentration and modification of products. Moreover, IEMs are involved
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in the energy conversion and storage processes. However, the formation of deposit may occur on the IEM surface and/or inside the membrane causing an increase in electrical resistance, a decrease in permselectivity and membrane alteration [3, 4]. Thus, the industrial application of IEM based processes is hampered due to fouling problems. Therefore, the aim of the present paper is to describe the fouling types occurring during processes using IEMs, the methods for fouling detection and characterization, and the ways of fouling prevention and control. The main emphasis will be put on the ED processes since they are the most widespread processes using IEMs.
II Classification of IEMs fouling
II.1
Colloidal fouling
ACCEPTED MANUSCRIPT Colloids are non-dissolved suspended solids, which are presented in natural and processed waters and many effluent streams in forms of clay minerals, colloidal silica, iron oxide, aluminum oxide, manganese oxide, organic colloids etc. [5, 6]. The size of colloid
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particles may vary from 10 Å to 2 μm in diameter. For instance, the majority of colloidal
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particles found in natural waters are small aluminum silicate clays. These clays are in the 0.3 to 1.0 micron diameter size range. The main feature of colloids is an excessive charge on the
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surface (net charge), which leads to adsorption of ions from the surrounding solution. Fig.1 represents a general model of the colloid structure which was developed by Gouy [7] and Chapman [8] and later by Stern [9]. The solid in this model has an excessive positive surface
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charge, which attracts negatively charged molecules from the solution. The Stern layer is tightly adjusted to the solid due to electrostatic forces compensating most part of excessive
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positive charge. The ions from diffusion layer neutralize the rest of excessive charge. The diffusion layer acts as a caution preventing colloids coming in contact with one another and
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coagulating [6].
Fig.1: Model of positively charged colloidal particle.
Thereafter, colloid particles have a net charge, which plays an important role in terms of colloid stability (according to the theory of Derjaguin, Landau, Verwey and Overbeek (DLVO) [10, 11]) and also may lead to the attachment of colloid to the membrane surface. Number of works is dedicated to the investigation of nature, structure and stability of colloidal fouling as well as to the mechanisms of its formation on the membrane surface. Most of these investigations are related to membrane filtration processes [6, 12-20]. In electrodialytic processes, the attention usually focuses on the anion-exchange membranes (AEMs) colloidal deposits since most part of colloids treated by ED is negatively charged, which leads to interactions with positively charged ion-exchange groups of AEMs [5, 21-24].
ACCEPTED MANUSCRIPT For instance, Lee et al. [25] studied the formation of a special colloidal fouling such as fouling by a surfactant agent (sodium dodecylbenzenesulfonate (SDBS)). These authors developed a mechanism of fouling formation where SDBS micelles formed the fouling layer
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on the membrane surface due to increased molecular size and highly negative charges.
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Ghalloussi et al. reported the sorption of organic colloids inside the IEMs during their use in food industrial processes [26, 27]. The factors affecting colloidal fouling are concentration of
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fouling particles as well as dissolved salt concentration, pH, temperature, membrane properties, mode of operation and hydrodynamic conditions.
Organic fouling
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II.2
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Organic fouling is similar to organic colloidal fouling except for the fact that organic foulants are initially dissolved in the solution treated by ED. Additionally, the colloidal state of organic molecules is maintained by weak, long-range van der Waals forces of attraction
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and electrostatic forces of repulsion, in contrast to ordinary organic molecules, which
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predominantly have covalent bonds [28]. Organic fouling occurs when treated solution contains organic substances such as oil, carbohydrates, proteins, aromatic substances, humic
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acid and anti-foaming agents [4, 29-45]. These organic substances stick to the surface of the membrane and/or lodge themselves inside the membrane. This type of membrane fouling has a real importance due to the large number of treated products with different matrices
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containing organic matter (Fig.2).
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Fig.2: Examples of organic fouling (adapted with permissions): A) Chitosan fouling [37], B)
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Antocyanin fouling [35], C) Protein fouling [36], D) Polyacrylamide fouling [30].
Several investigations have been reported providing information about mechanisms of
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organic fouling formation [30, 33, 42, 43, 46-48]. Organic fouling on the IEM surface and inside the IEM may be due to electrostatic and hydrophobic interactions, which is different from colloidal fouling where the majority of interactions have electrostatic nature only. One
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can distinguish several parameters affecting organic fouling formation such as concentration, size and structure of organic molecules as well as membrane composition and structure, and regimes of electromembrane treatment. For instance, Tanaka et al. [49]
reported that
aromatic organic substances cause more severe organic fouling on AEMs with divinylbenzene and chloromethylstyrene base matrix than aliphatic organic substances due to affinity interaction between the aromatic substances and the membrane matrix (Fig.3). Bukhovets et al. [50] reported importance of current regimes applying during ED. It has been found that at current densities close to the limiting values, where water splitting takes place (see section III.2.2), IEM becomes fouled by phenylalanine. Langevin et al. [46] studied effect of acidification and basification of the IEM surface on the fouling by peptides. These conditions are close to those occurring when current reaches limiting value and water splitting takes place. It was found that the crucial role in linking between peptides and IEMs was due to electrostatic interactions.
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Fig.3: Schematic diagram of organic fouling mechanism due to affinity interaction between membrane matrix and organic substance ( a) AMX/sodium octylbenzene-sulfonate (OBS)
Scaling
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and b) AMX/dodecylsulfonate (DS)) (adapted with permission from [49]).
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Scaling on IEM occurs when salts present in the water precipitate out and settle on the membrane surface and/or within membrane channels. The major scaling ions present in
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solutions treated by ED include magnesium, calcium, barium, bicarbonate and sulfate [3, 4, 38, 51-57]. The precipitation occurs when the equilibrium of solution is changed in a way that decreases the solubility below the concentration of salts causing them to precipitate out of the solution. Conventionally, two main factors affect precipitation in solutions such as
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concentration of ions and solution temperature [51, 58, 59]. When speaking about the precipitation, it is worth to emphasize the effect of the membrane structure and regimes of electromembrane treatment. Bazinet et al. [60] reported that scaling may be reversible or irreversible depending on the membrane permselectivity. Van Geluwe et al. [55] compared how different types of scaling may be prevented on homogeneous and heterogeneous membranes. A very important factor affecting scaling formation is pH of treated solution. Basic pH values mean excessive presence of OH- ions favoring the scaling build-up. On the one hand, it leads to the precipitation of hydroxides due to the interaction of OH- with Ca2+ and Mg2+ as follow [61]:
(1)
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On the other hand, OH- can shift the balance of weak-acid anions. For instance, in the
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(Fig.4) which leads to precipitation of carbonates (Eq.3, 4).
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solution with basic pH the balance between carbonic ions is shifted towards carbonate ions,
Fig.4: Speciation of major carbon species depending on pH (total concentration 0.003 mol/l,
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T = 20°C, closed system, and ionic strength I = 0) (reprinted with permission from [62]).
(3)
(4)
Detailed studies of pH influence on scaling on IEMs was carried out by Casademont et al. [63]. These authors maintained a certain pH value in the concentrate compartment during ED treatment while pH of the diluate compartment was not kept constant changing from its initial value (6.5). It was reported that the membrane scaling formed by minerals of Ca2+ and Mg2+ on AEM surface mostly takes place at neutral pH values, however CEM surface was scaled at basic pH. Additionally to pH factor, the type of IEM (anion-exchange or cation-exchange) can play an important role in the scaling formation. Furthermore, as for the organic fouling,
ACCEPTED MANUSCRIPT application of current higher than the limiting current affects scaling quantity and composition. Generation of OH- ions by CEM provides conditions for the scaling formation, however H+ ions generated by AEM may create a “proton barrier” preventing precipitation of
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minerals [63-65]. An additional factor influencing the scaling formation is the ratio of the
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scaling ions due to the competition during their migration from the diluate compartment towards the concentrate and due to the cross-effects of different scaling ions on nucleation
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and crystal growth [63, 64, 66-72]. Moreover, consecutive ED treatments lead to the formation of scaling multilayers having different composition [73]. Recent studies of Cifuentes-Araya et al. revealed mechanisms of the membrane scaling formation and graphical
Biofouling
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II.4
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models including possible interactions between scaling ions and IEMs were created [64, 74].
Biofouling seems to be a less studied fouling type occurring in the processes
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involving the IEMs. Most of the biofouling studies are dedicated to the micro-, ultra-,
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nanofiltration and reverse osmosis [75-83]. However, rapid development of biotechnologies with application of IEMs makes biofouling an actual problem. For instance, biofouling is a
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major problem in microbial desalination cells (Fig.5). The main feature of these microbial cells is implication of special bacteria on the anode surface. These bacteria may oxidize biodegradable substrates and produce electrons and protons. Electrons move towards the cathode completing the electric loop. Separating anode and cathode by CEM and AEM, one
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can obtain a classical desalting channel [84-91]. There are number of different microbial electrochemical systems aiming production of electricity and chemicals, desalination, waste treatment etc. Most part of these systems was recently been described by Wang et al. [92].
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Fig.5: Schematic of a three-chamber microbial desalination cell for simultaneous substrate removal (anode), desalination (middle chamber), and energy production (reprinted with
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permission from [93]).
The lifecycle of the biofilm is represented on Fig.6 with microphotograph of each stage of
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biofilm development [94]. Initial interaction of bacteria with a surface is reversible (1) whereas subsequent adhesion is irreversible (2). After attaching to the membrane surface, bacteria start production and excretion of extracellular polymer substances (2), which allow
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cells to become cemented to the surface. Continuous growth results in the development of microcolonies (3). As the microcolonies continue to increase in size (4), cells in the interior of the microcolonies will experience overcrowding, decreased availability of nutrients, increased concentrations of waste-products, toxins and excreted metabolites including cell-tocell signaling molecules, along with changes in their physicochemical environment. At last, matrix within a microcolony is digested, cells become free to move by active motility or browning motion (5). Eventually, a breach is made in the matrix at a margin of the microcolony through which the bacteria are able to escape into the surrounding bulk liquid.
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Fig.6: Biofilm lifecycle. Stages in the development and dispersion of biofilm are shown proceeding from right to left. Lower panel shows photomicrographs of bacteria at each of the
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five stages shown in the schematic above (reprinted with permission from [94]).
Fleming et al. [76] distinguished factors affecting formation and development of biofilms on
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the membrane surfaces (Tab.I)
Tab.1: Factors affecting biofilm formation on membrane surface. Microorganisms factor
Membrane surface
Solution factor
factor Species
Chemical composition
Temperature
Composition of mixed
Surface charge
pH
Surface tension
Substances forming a
population Cell number
conditioning film Growth phase
Surface
Dissolved
hydrophobicity
organic/inorganic
ACCEPTED MANUSCRIPT substances Nutrient status
Roughness
Suspended matter and colloids
Porosity
Viscosity
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Hydrophobicity Surface charge
Surface tension
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Physiological response
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on adhesion
Flow rate
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III. Characterization of IEMs fouling
This section will provide an information about the main methods for the characterization of IEMs fouling. A more exhaustive list of IEMs fouling characterization
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III.1 Visualization of IEMs fouling
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methods (Table 2) is presented at the end of the section with the respective list of references.
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Conventional methods allowing visualization of membrane fouling are the following ones (Fig.7):
Photo imaging
Optical microscopy
Scanning electron microscopy (SEM)
Confocal laser scanning microscopy (CLSM)
Atomic force microscopy (AFM)
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Fig.7: Visualization of membrane fouling by photo imaging (adapted with permissions):
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(A,B) [36, 95], optical microscopy (C) [96], SEM (D) [32], CLSM (E) [97], AFM (F) [98].
These methods may be applied to identify the presence of membrane fouling, to reveal the
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fouling structure and distribution along the membrane surface or inside the membrane, to study the effects of different experimental conditions on fouling development and effectiveness of cleaning procedures. Thereafter, the fouling type and the goal of research
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should determine the choice of one or another method. For instance, the majority of membrane fouling investigations are performed with the use of SEM, however CLSM is used mostly for biofouling investigations.
III.2
Membrane characteristics
III.2.1 Membrane electrical resistance and conductivity
Membrane electrical resistance and conductivity are typical parameters for characterization of IEMs. A wide range of methods was developed to measure the membrane electrical resistance and conductivity [99-101]. The widely used methodology was proposed by Belaid et al. [102] and Lteif et al. [103]. According to their approaches, membrane electrical conductivity can be obtained via measuring the membrane conductance using the
ACCEPTED MANUSCRIPT clip-cell containing two platinum electrodes (Fig.8). After measuring conductance of the solution and conductance of the solution with membrane, it is possible to calculate the
(5)
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electrical resistance of the membrane as
where Rm is the transverse electric resistance of the membrane, Rm+s the resistance of the
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membrane and the reference solution measured together, Rs the resistance of the reference solution, Gm the conductance of the membrane, Gm+s the conductance of the membrane and of the reference solution measured together, and Gs the conductance of the reference solution.
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Finally, the membrane electrical conductivity κ can be calculated as [103]:
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where L is the membrane thickness and A the electrode area. Since membrane conductance depends on the temperature, all measurements should be carried out at the same temperature what can be provided by using a thermostat. Usually fouling phenomena lead to the decrease
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in membrane conductivity due to the 1) increase in membrane resistance by formation of surface fouling layer or internal membrane fouling and 2) deposition of fouling agents on membrane ion-exchange groups [24, 33, 36, 50, 56, 92, 104-107]. The disadvantage of this method is that during immersion of the fouled membranes into the working solution (usually NaCl), the fouling could be detached from the membrane leading to the underestimation of results. Thus, the analysis of fouling, which is weakly attached to the IEM (e.g. colloidal, organic) may give inappropriate data.
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Fig.8: Schematic representation of the system for measuring membrane conductance (adapted
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with permission from [103]).
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III.2.2 Voltammetry and chronopotentiometry
Voltammetry and chronopotentiometry are privilege methods to study electrochemical behavior of IEMs with the presence of fouling [108-114]. The dependence of current from
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potential difference applied to the electrodes on membranes in the ED system is called current-voltage curve (CVC). CVC method provides information about limiting current density (LCD) values and coupled effects of concentration polarization such as water splitting phenomenon (mentioned in sections II.2 and II.3) and current-induced convection (gravitational convection and electroconvection). Conventional approach considers three regions of CVC (Fig.9), however more precise description of CVC curves includes interfacial Donnan potential drop and effective resistance of the membrane system at low current densities [115]. The first region represents the “ohmic” region where current increases linearly with the voltage increasing according to the Ohms law. Second region starts, when the electrolyte concentration near the diluate side of the membrane surface becomes close to zero (ED system reaches LCD). “Limiting” current region is reflected on the CVC in the form of a plateau. However, on the “plateau” region increase of current still takes place due to the development of water splitting phenomenon which provides new current carriers (H+
ACCEPTED MANUSCRIPT and OH-) and the development of gravitational convection and/or electroconvection, which improve the delivery of current carriers to the membrane surface [116-119]. When the current-induced convection is strongly developed, ED system passes to the third
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“overlimiting” current region. The formation of fouling on the membrane surface or inside
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the membrane leads to the changes in shape of CVC. For instance, Fig.9 demonstrates the significant changes in the slope of the ohmic region (Region 1) of fouled membrane in
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comparison with fresh membrane. Moreover, there are slight changes in the limiting current density values and plateau length. Concerning CVC investigations, it is possible to obtain important information on operations of ED processes to minimize fouling effects. However,
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this method is time consuming and usually requires additional knowledge to interpret the results. Moreover, during the sample preparation and CVC registration, the weakly bounded
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fouling can be modified and/or detached.
Fig.9: Current-voltage curves of fresh and fouled by sodium dodecylbenzenesulfonate (SDBS) AMX membranes (adapted with permission from [25]).
Chronopotentiogram (ChP) represents the dependence of voltage from time under application of fixed current (Fig.10). Conventional ChP includes the initial vertical region (point 1 on Fig.10) corresponding to the “ohmic” drop of potential. Then voltage increases slowly due to the decrease of electrolyte concentration near the membrane surface (point 2 on Fig.10). The next part of the curve corresponds to a drastic increase of potential when the electrolyte concentration becomes zero (point 3 on Fig.10). Finally, the system reaches steady
ACCEPTED MANUSCRIPT state without any substantial changes in voltage (point 4 on Fig.10). Duration of the potential drop increasing corresponds to the transition time (τ), which can be calculated according to
where D is the diffusion coefficient of electrolyte,
(7)
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the Sand eq. [108, 120] as
and the charge of the counter-ion, respectively, and
and zi the concentration in the bulk
the transport numbers of the counter-
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ion in the membrane and solution respectively, i the current density, F the Faraday constant. From Eq.7 it is possible to see that transition time depends on the transport number.
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Thereafter, this parameter allows making a conclusion about the membrane permselectivity: transition time is minimal for an ideally permselective membrane with ti=1 [121]. Krol et al. [112] and Choi et al. [114] proposed using the ChP for determination of membrane reduced
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permeable area. This is possible if one considers the current density (i) as a current density in
(8)
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the membrane conducting region (i*) as
where I is the current intensity, A the membrane area and ε the fraction of conducting region.
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Hence, substituting Eq. (8) into Eq. (7) yields
(9)
Indeed, Park et al. [122] reported that bovine serum albumin fouling reduces the conducting phase from 0.98 to 0.84 for CMX and from 0.99 to 0.02 for AMX membranes. The same negative tendency was demonstrated by Kang et al. [123] during the separation of organic molecules having the molecular weight larger than 70 g mol-1. The deposition of inorganic salts on the surface of IEMs as well reduces the membrane conducting abilities. For instance, Kang et al. [124] reported the decrease of the fraction of conducting region with deposited silica compounds from 10 to 25 % for different AEMs and with embedded hydroxide and oxide of Fe3+ from 7 to 27 % respectively for the Nafion 117 membrane. This method is quite
ACCEPTED MANUSCRIPT rapid however, as in the case of CVC, the membrane fouling could be modified and/or
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detached affecting the final data.
Fig.10 Example of chronopotentiometric curve of 0.1 M NaCl at current density 15 mA/cm2
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(reprinted with permission from [112]).
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III.2.3 Electrical impedance spectroscopy
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Electrical impedance spectroscopy (EIS) is a noninvasive method providing an information about conduction and capacitive properties of membrane systems [125]. It involves the application of an alternating sinusoidal voltage V(t) (or current) with small
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amplitude Va and measurements of the output current I(t) (or voltage) response shifted in phase (θ) with amplitude Ia (Fig.11 a):
(10) (11)
Here ω is a radial frequency of the applied voltage, which relates to the applied voltage frequency f as (12)
The ratio V(t)/I(t) called the complex impedance of the system (Eq.13). As one can see, this voltage/current ratio reminds the Ohm’s Law for the pure electrical resistance. However,
ACCEPTED MANUSCRIPT impedance is a more complex parameter than the electrical resistance since it takes into
(13)
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account phase differences:
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The impedance (Z) is therefore expressed in terms of an amplitude (absolute value), Za=|Z|, and a phase shift, θ. The representation of the current-voltage dependence yields a
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“Lissajous figure” (Fig.11b).
Fig.11: a) Impedance experiment: sinusoidal voltage input V at a single frequency f and
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current response I; b) Lissajous figure (adapted with permission from [126]).
Nowadays, the results of the EIS are no more presented in the form of a Lissajous figure. The involvement of modern amplifiers and frequency analyzers allowing application of a voltage input with variable frequencies (μHz-GHz) and timescales (hours-nanoseconds) [126] led to the different data expressions. The two most common graphical representations of impedance data are Niquist and Bode plots (Fig.12). These plots are obtained using the Euler’s formula:
(14)
The impedance is then represented as a complex number (Z*) and can be expressed as a combination of a “real” (Re(Z) or Z') and “imaginary” (Im(Z) or Z") parts:
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The calculations in the EIS are carried out using the equivalent circuit models, which contain
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common ideal electrical elements such as resistors, capacitors and inductors and nonideal circuit elements such as constant phase element [5, 122, 126-129]. For instance, Lee at al. [5]
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used two equivalent circuits to describe the deposition of silica sol on the surface of AEM (Fig.12). It is possible to see that the circuits contain resistors, capacitor and constant phase element presented as a Warburg impedance. The impedance of a resistor is independent of
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frequency and has only a real component (Z=R). The current through a capacitor is phase-
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shifted (π/2) with respect to the voltage and the impedance could be calculated as
(16)
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where C is the capacitance (in F). The Warburg impedance (Zw) is the particular case of a
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constant phase element and it represents the diffusion processes [126]. The impedance of a
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constant phase element (ZCPE) is defined as:
(17)
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If α=1, then equation (17) describes an impedance of a pure capacitor (Q=C). For the case of α=0, the above equation describes an impedance of a pure resistor (Q=1/R). If α=0.5, the impedance of a constant phase element describes a special (Warburg) case for homogeneous semi-infinite diffusion (Q=1/Rw), where Rw is diffusion resistance [126]. Thus, a Warburg impedance (Zw) could be calculated as:
(18)
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Fig.12: Equivalent circuits of an electrochemical cell and its impedance Niquist plot (RΩ, the
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solution resistance; Rct, the charge transfer resistance; Cd, the double layer capacitance; Zw,
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the Warbug impedance) (adapted with permission from Lee at al. [5]).
Lee at al. [5, 25] demonstrated the changes in the impedance spectra (Niquist plots) when the membranes were fouled with silica sol and SDBS (Fig.13 a and b). These authors reported
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that silica sol deposition leads to the decrease in the charge transfer resistance and increase in the double layer capacitance (Fig.13a). This is possible due to the increase in the ionexchange capacity of the fouled AEM. The silica sol fouling did not affect significantly the ED efficiency. However, the deposition of SDBS led to the double increase in the charge transfer resistance and significant decrease in the double layer capacitance, which considerably decreased the current efficiency and increased the power consumption. Park et al. [34] used the Bode’s plots representing the dependency of conductance and capacitance with the frequency. These authors studied the fouling of IEMs by bovine serum albumin. One can see that the conductance of the fouled membrane is lesser than fresh membrane indicating the negative effect of this fouling type. The results of capacitance however present a little differences between the fresh and fouled samples. Overall, the impedance spectroscopy is very sensitive to fouling effects in the systems with IEMs. However, this method is time consuming and demands profound knowledge of studying membrane system and fouling
ACCEPTED MANUSCRIPT composition in order to find a reasonable explanation of obtained impedance spectra. Moreover, the measurements of zeta potential usually take a fairly long time (5-8 hours) and need two measurements per sample, one measured from neutral pH to acidic pH and another
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one from neutral pH to basic pH to not affect the membrane structure or nature following
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reaching extreme conditions of pH.
Fig.13: a) Niquist plot of fresh and fouled by silica sol CEMs, b) Niquist plot of fresh and fouled by sodium dodecylbenzenesulfonate (SDBS) AEMs, c) Bode plot of conductance from the frequency of fresh and fouled with bovine serum albumin (BSA) AEMs, and d) Bode plot of capacitance from the frequency of fresh and fouled with BSA AEMs (adapted from Lee at al. [5, 25] and Park et al. [34]) (all figures are reprinted with permissions).
III.2.4 Transport number
Transport numbers measurements allow to indicate the presence of fouling since fouling phenomena usually affects the migration of counter-ions through the IEMs [85, 92, 130, 131].
ACCEPTED MANUSCRIPT This method is quite simple and rapid. However, the problem of fouling modification and/or detachment during the experiment may lead to the underestimation of the final results. The procedure of transport numbers determination consists of monitoring the potential difference
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near the membrane-solution interfaces via the Luggin capillary and Ag/AgCl electrodes.
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Usually, a specific ionic specie is chosen to verify its transport number trough the fresh and fouled IEM. For instance, Choi et al. [85] estimated the influence of biofouling on the
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transport of Na+ ions through the CEM. To determine transport number of Na+ (t+), these authors used two NaCl solutions with different concentration to create the concentration gradient and measured potential difference. The Na+ transport (1:1 electrolyte) can be
(19)
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obtained from [132]:
Here E is potential difference, R the molar gas constant, T the temperature, F the Faraday
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constant and C1 and C2 the NaCl concentrations (C1>C2). The Na+ transport number through
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the CEM decreases in the presence of biofouling from 0.96 to 0.94 [85]. However, more substantial decrease of transport number due to the biofouling was observed by Pointié et al.
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[130]. These authors reported the decrease of Li+ transport through the Nafion 117 fouled by Pseudomonas putida DSM 50026 from 0.95 to 0.89. The presence of humate, SDBS and bovine serum albumin fouling on the surface of AMX decreases the Cl - transport number
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from 0.96 to 0.94, 0.93 and 0.92 respectively [25]. Interesting experiments were conducted by Sata et al. [133] to estimate the influence of cyclodextrins fouling. These authors estimated the transport numbers of different divalent cations in relation to the transport number of sodium cations. The adsorption of cyclodextrins, having relatively high molecular weight, significantly decreased the transport of alkaline earth metals trough the CEM.
III.2.5 Zeta potential
Zeta potential measurements can provide an information about magnitude of the electrostatic or charge repulsion/attraction forces of fouling agents and IEMs surfaces allowing control of their possible interactions [82, 125]. Zeta potential is an electrokinetic potential appearing on a slip (shear) plane between the Stern and Gouy layers (Fig.14). It is worth to note that the exact location of the slip plane is hard to define. Discussions
ACCEPTED MANUSCRIPT concerning the slip plane locations were recently published by Lyklema [134, 135]. Zeta potential depends on several factors such as surface properties (i.e. charge, roughness, chemical heterogeneity etc.) [136], composition and concentration of solution, pH and
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temperature [137, 138]. There are several methods allowing zeta potential determination such
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as electrophoresis, electro-osmosis, streaming potential, sedimentation potential, vibrational
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characterization of IEMs and fouling phenomena.
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potential etc. Electrophoresis and streaming potential are the most abundant methods for the
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Fig.14: Schematic representation of zeta potential between solid surface and liquid (adapted with permission from [136]).
Electrophoresis is the movement of charged particles or polyelectrolytes in the liquid under the influence of an external electric field. The electrophoretic mobility of a charged particle is determined by the balance of electrical and viscous forces and could be measured. Hence, zeta potential could be expressed as
(20)
Here u is the electrophoretic mobility equal to particle velocity per unit of electric field strength, η the solution viscosity, ε the dielectric constant,
the dielectric permittivity of
vacuum, κa the particle radius normalized to the Debye length κ-1. The present formula is
ACCEPTED MANUSCRIPT valid for the small ζ values (< 50 mV) [138]. Most part of works implying the electrophoresis for the investigation of fouling phenomena is devoted to the measurements of the zeta potential of the fouling agents in order to predict the possible interactions with IEMs. For
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instance, Lee et al. [139] analyzed the charge of the humate and studied its influence on the
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ED performance. These authors reported that the humate have a highly negative values of zeta potential (from -18 to -25 mV) over the entire pH range. However, in the case of silica
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sol, the zeta potential changes form the highly negative values at high pH to positive values below pH 3 [5]. These results may be useful for the prediction of fouling either on negatively charged CEM or positively charged AEM when pH of solution is known. The interesting
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approach of the humate fouling mitigation was proposed by Park et al. [140]. These authors used the addition of different water-soluble polymers capable to interact with humate and
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decrease its charge and ability to foul the AEM. The control of the zeta potential in this work allowed finding the optimal ratio between humate solution and polymer for the formation of the interpolymer complex, which does not foul the AEM. The investigation of the IEM zeta
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potential by electrophoresis method is possible by using a standard reference particle. Kim et
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al. [141] studied the influence of the natural organic matter on the zeta potential at different pH values. These authors used a polylatex (in 10-3 M NaCl solution) as a standard reference
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particle and found a good relationship between the zeta potential values and amount of adsorbed natural organic matter. The influence of different factors on determination of zeta potential by electrophoresis (e.g. Joule heating, pH changes, roughness and heterogeneity of the surfaces etc.) is given elsewhere [136-138].
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The streaming potential is possible to measure when electrolyte solution is forced through a plug, membrane, capillary or diaphragm [138]. In the frames of the present work, only flow across the top surface of the membrane caused tangential streaming potential will be discussed. Tangential streaming potential appears when an external pressure gradient is applied. This pressure gradient leads to the hydrodynamic flow and displacement of counterions from slip plane (Fig.14). Accumulation of ions downstream results in formation of electrical field causing a back current. Streaming potential is measured in a steady state of the system when the difference between streaming and back currents becomes zero [142]. Zeta potential is directly proportional to the streaming potential and can be calculated as [136]
(21)
ACCEPTED MANUSCRIPT Here ΔEstr is the streaming potential, ΔP the applied pressure, K the conductance determining a back current. Reynard et al. [143] described the possible device and procedure for the streaming potential measurements concerning the surface charge of IEM. Lee et al. [139]
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measured zeta potentials in order to explore the fouling phenomena by humate on different
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types of AEMs. These experiments allowed estimations of the fouling tendency as well as development of the fouling formation mechanisms. Several studies used the streaming
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potential in order to evaluate the antifouling potential of the modified AEMs. Mulyati et al. [144, 145] and Vaselbehagh et al. [146] applied different modification agents leading to increase in the surface negative charge, which allowed increasing of the antifouling potential
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by the model organic fouling agent such as sodium dodecylbenzene sulfonate. It is necessary to note that highly charged and heterogeneous surfaces as well as low solution concentrations
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cause substantial deviations from the Eq.21. Several approaches taking into account the above-mentioned deviations could be found elsewhere [136-138, 142, 143]. One has to take into account that the highly charged surfaces also demand the use of very stable electrodes,
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which are usually not included in the commercially available equipment. Moreover, the
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measurements of zeta potential usually take a fairly long time (5-8 hours) and need two measurements per sample, one measured from neutral pH to acidic pH and another one from
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neutral pH to basic pH to not affect the membrane structure or nature following reaching extreme conditions of pH.
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III.2.6 Contact angles
Contact angles measurements allow to investigate one of the most important parameters affecting the IEM performances and fouling phenomena such as surface hydrophobicity [116, 147, 148]. Usually, contact angles of different membranes are measured by goniometer with a measurement range of contact angle 0–180o. The measurement procedure includes the registration of the contact angles between a drop of a liquid (distilled water or organic liquid) and a membrane surface [149-153]. A drop of liquid may be applied onto a dry membrane or onto a swollen membrane (sessile drop method) mopped with a filter paper to remove the excessive water from the surface (Fig.15a) [151, 154-156]. Furthermore, membrane can be immersed into a liquid and a drop of another liquid (immersed method) [152, 157] or air bubble (captive bubble method) [158, 159] could be applied onto a membrane surface
ACCEPTED MANUSCRIPT (Fig.15b). Contact angle could be calculated using the Young equations 22 and 23 for the cases of sessile drop and immersed method respectively:
Here θ is the contact angle,
,
and
liquid and liquid-vapor interfaces and
(23)
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(22)
the interfacial energies of the solid-vapor, solid,
and
the interfacial energies of the solid-
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water, solid-octane and water-octane interfaces , respectively. Nowadays, it is well known that the Young’s equation should be modified for the contact angles calculations taking into
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account several phenomena such as surface roughness and heterogeneity. These modifications could be found elsewhere [152, 159, 160]. The studies of Bukhovets et al.[50] revealed the increase of the AEM hydrophobicity
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when membrane was fouled by phenylalanine. Similar increases in the AEM surface
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hydrophobicity were reported by Guo et al. [30] who conducted studies on polyacrylamide fouling and Lee et al. [92] who conducted studies on humate, SDBS and bovine serum
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albumin fouling. Hence, the development of the anti-fouling membrane properties is usually aims to create more hydrophilic surfaces, which are concerned to be disadvantageous for the organic fouling and biofouling [95, 144-146, 161]. The simplicity and rapidity of the contact
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angles method is very attractive to analyze the fouling phenomena. However, one should take into account that the fouling can be partially removed during the mopping procedure aiming removal of excessive water at the surface. Furthermore, IEMs lose their inner water on the air and become dry quite quickly, which demands certain skills from researcher to accomplish an analysis in appropriate quick way. When the problem of quick drying appears, the contact angles measurements in liquid can be an interesting solution (Fig.15b). Additionally, the results of contact angles could be strongly affected by the surface heterogeneity (e.g. IEM modified with a mesh).
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ACCEPTED MANUSCRIPT
Fig.15: Examples of contact angles measurement methods a) in air and b) in liquid (adapted
Fouling composition
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III.3
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with permissions from [152, 162]).
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III.3.1 Nitrogen content analysis by combustion
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The most widespread method for the protein, peptide and amino acid fouling characterization by combustion is Dumas method. This method consists of combusting a
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sample of known mass with oxygen at a temperature about 1000 oC. The combustion process results in release of water, carbon dioxide and oxides of nitrogen. A gas mixture is swept through a reduction reagent tube where nitrogen oxides are converted to the molecular
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nitrogen and then gas mixture passes through the absorber to remove water and carbon dioxide. The molecular nitrogen with helium, which is used as a carrier gas, pass through a thermal conductivity cell for nitrogen detection. However, the lower detection limit of the Dumas method is 0.05 % (w/w). Langevin et al. [46] studied the fouling phenomena by peptides in a soy protein hydrolysate solution. Analysis of nitrogen content revealed the sensitivity of different IEMs to the peptides fouling and the effect of the IEM pretreatment procedures on the fouling formation. It was demonstrated that CEMs were almost two times more sensitive to the peptide fouling than AEMs. Furthermore, HCl pretreatment resulted in a 12 % increase in the fouling content in comparison with distilled water and NaOH pretreatments. The studies of protein fouling conducted by Ayala-Bribieska et al. [44] revealed the formation of protein film only on the surface of AEM during ED of whey protein solutions. Moreover, these authors demonstrated that whey protein fouling on AEM takes place only at acidic and
ACCEPTED MANUSCRIPT neutral pH. Subsequent studies of AEMs nitrogen content after the ED of whey protein solutions conducted by Casademont et al. [163] allowed to reveal the positive effect of pulsed electric field on the decrease of the protein fouling. The increase in CEM nitrogen content
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after ED with bipolar membranes of industrial glutamate production wastewater indicated the
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amino acid and/or protein fouling which was completely removed after the acid cleaning with 180s of ultrasound [54]. The advantage of this method is an ability of direct analysis of the
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fouled membrane, which should be dried prior to the analysis. Although the nitrogen content gives an important information about the fouling nature and quantity, it cannot help to reveal the fouling composition and structural features. Thus, this method is recommended as an
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additional to other methods such as chromatography with mass spectrometry, infrared
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spectroscopy etc.
III.3.2 Spectroscopy
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Spectroscopy is a broadly used analytical technique for studying the interactions
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between matter and electromagnetic radiation. Two spectroscopic classes can be distinguished:
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1) Transfer of energy between electromagnetic radiation and sample. When sample absorbs electromagnetic radiation, it undergoes a change in energy. This energy change could be associated with the transitions of electrons from the lower energy orbitals (molecular or atomic) to the higher energy orbitals or with inducing the vibrational excitation of
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covalently bonded atoms and groups. The electromagnetic radiation can be represented in a form of a beam of particles (photons) having its frequency, wavelength, wavenumber and energy. Energy of photons determines their level of penetration inside the atom or molecule. For example, energy of infrared (IR) radiation (1.24 meV-1.24eV) is sufficient just to change the vibrational energy of a molecule while X-ray radiation (124 eV-124 keV) carries enough energy to ionize atoms. There are two phenomena used to acquire the spectroscopic analytical information such as absorption (photons are absorbed by the sample) and emission (photons are liberated from the sample when it returns from the higher-energy state to a lower-energy state). The methods based on this principle are ultraviolet-visible spectroscopy, atomic absorption spectroscopy, infrared spectroscopy, luminescence spectroscopy, X-ray spectroscopy etc. 2) The electromagnetic radiation is refracted, reflected, scattered, diffracted or dispersed by the sample, which results in a change in energy amplitude, polarization, phase angle or
ACCEPTED MANUSCRIPT direction of propagation. These methods include X-ray diffraction, low energy electron diffraction, photoelectron diffraction etc.
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III.3.2.1.1 Ultraviolet-visible (UV/vis) spectroscopy
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III.3.2.1 Molecular spectroscopy
The ultraviolet-visible spectroscopy (UV/vis) is based on the interactions of the UV (190-380 nm) or visible (380-750 nm) electromagnetic radiations with molecules and ions,
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which cause transitions of valence electrons (most important are transition from n→π and π→π* molecular orbitals). These interactions lead to the attenuation of radiation intensity.
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The measurement of the energy attenuation, called absorbance, serves as an analytical signal.
(24)
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According to the Beer-Lambert’s law, absorbance is defined as
Here I0 and It are the intensities of incident and transmitted electromagnetic radiation, ε the
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molar absorptivity, l the sample path length and C the solution concentration. The limitations of a Beer-Lambert’s law having chemical and instrumental nature are discussed elsewhere [164].
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Most part of studies involving a UV/vis technique is dedicated to fouling by organic matter. For instance, Lee et al. [48, 92, 105] and Park et al. [125, 140] determined the concentrations of abundant organic fouling agents such as humate, SDBS and bovine serum albumin using the respective wavelengths of 254 nm, 224 nm and 280 nm. Banasiak et al. applied the UV/vis technique to study the effects of humic acid on sorption of steroid hormones and pesticide endosulfan [42, 43] and ED removal of boron, fluoride and nitrate by electrodialysis in the presence of organic matter [29]. The UV/vis technique was applied by Kusumoto et al. [131, 165] to explore an antifouling potential of modified AEMs. Furthermore, investigations conducted by Vijayakumar et al. [166] of the UV/vis spectra of the fouling on Nafion membranes in vanadium redox flow batteries allowed to reveal the types of the fouling agents. These authors reported that among different ions of vanadium only V4+ ions participated to the Nafion membrane fouling. The UV/vis spectroscopy is a
ACCEPTED MANUSCRIPT relatively rapid method. However, there are several important points, which should be considered to perform the appropriate analysis such as: impurities in the sample which can scatter radiation and react with analytical reagents;
-
low concentration of the analyzed sample which can lead to the difficulties in the
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-
-
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distinction between the analyzed sample and the instrumental “noises”; high concentration of the analyzed sample which can lead to the deviations from the
-
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Beer-Lambert’s law due to the saturation of the absorption bands; method cannot be applied directly to the membrane, the fouling must be removed
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from the membrane and solubilized.
III.3.2.1.2 Attenuated Total Reflectance Fourier Transform Infrared spectroscopy
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(ATR–FTIR)
Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR–FTIR)
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is one of the most used and powerful methods of the modern analytical chemistry. As in the
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case of UV/vis, during ATR-FTIR sample absorbs the electromagnetic radiation. However, the energy of IR radiation is much weaker than energy of UV radiation. Hence, no electronic
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transitions occur and the energy of IR radiation is sufficient only to produce a change in the vibrational energy of a molecule or polyatomic ion. IR spectrum is a fingerprint of the studied sample because each sample has its unique distribution of the different vibration modes. Thus, when studying fouling, IR spectra of fresh and fouled IEMs could be very useful for
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the determination of the fouling structure and mechanisms of formation especially when treated solution contains different fouling agents. The qualitative analysis in ATR-FTIR could be performed as for the UV/vis spectroscopy using Beer-Lambert’s law. However, it is worth to note that in the case of IR spectroscopy, where absorption bands are relatively narrow, deviations from the Beer-Lambert’s law are more pronounced in comparison with UV/vis [164]. The advantage of ATR-FTIR spectroscopy as compared to UV/vis spectroscopy is that the fouled membrane can be analyzed directly after the drying procedure. The studies of IR spectra of the IEMs immersed in a solution of cyclodextrins detected the formation of membrane fouling due to the appearance of new characteristic peaks of ether groups (1155 cm-1) and primary alcohol groups (1032 cm-1) on a fouled IEM [133]. The adsorption of cyclodextrins, having relatively high molecular weight, occurs around cation-exchange groups and leads to the decrease in ionic permeation during ED treatment. The protein and peptide fouling could be as well detected by ATR-FTIR [46]. The
ACCEPTED MANUSCRIPT peaks indicating the presence of protein fouling lie in the region of wavenumbers 1700-1600 cm-1 (amide I) and 1580-1510 cm-1 (amide II). The AEM fouling by polyacrylamide causes the appearance of already mentioned amide I peak, two peaks corresponding to the
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carboxylate groups (C=O at 1662 cm-1 and C-O at 1322 cm-1) and two peaks corresponding
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to the primary amides (NH2 at 3349 cm-1 and 3196 cm-1) [30]. Moreover, the characteristic peaks of polyacrylamide and fouled AEM response at a lower wavenumbers indicating the
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increase in the bonds length, which might be a result of electrostatic interactions between polyacrylamide and ion-exchange groups of AEM. The above-mentioned peaks of amides and carboxylate groups may be attributed to the proteins originated from microorganisms,
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which could confirm presence of membrane biofouling [167]. ATR-FTIR can be also used to explore the colloidal fouling. For instance, Lee et al. [5] studied the IR spectra and found that
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during ED operations silica sol becomes chemically bonded to the ion-exchange groups of AEM.
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III.3.2.2 Atomic spectroscopy
Atomic spectroscopy deals with the transitions of electrons in atoms. Hence, the
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sample should be atomized prior to detection. The sample atomization can be carried out using flames, furnaces and electrical discharges.
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III.3.2.2.1 Atomic absorption spectrometry (AAS)
Atomic absorption spectrometry (AAS) determines the concentration of a particular element in a sample. In AAS, two types of atomizers are usually applied to convert a sample into free atoms such as flames (e.g. air-acetylene, oxygen-acetylene) and electrothermal atomizers (graphite furnaces). The flames are advantageous from the point of the reproducibility with which the sample is introduced in the spectrometer though they have relatively poor atomization capacity. The atomization in furnaces is more sensitive with larger detection limits compare to flames though this way of atomization is less reproducible. Usually, the flame atomization is applied for the higher concentrations of the analyte as compared to electrothermal atomization in the furnace (e.g. detection limits for Ca are 0.5 ppb and 0.01 ppb for the flame and furnace atomization respectively) [164]. The atoms absorb radiation of a specific wavelength and by measuring the amount of radiation absorbed, quantitative analysis is possible according to the Beer-Lambert’s law (eq.23). The different
ACCEPTED MANUSCRIPT drawbacks of AAS such as nonlinearity of calibration curves, spectral and chemical interferences are broadly discussed elsewhere [164, 168]. Moreover, when the AAS uses a line source of radiation, just one element per analysis could be determined. The use of
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continuum radiation source, allowing multielement analysis, requires a high-resolution
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monochromator.
The AAS technique could be applied to monitor the concentration of fouling agent in
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the treated solution [169] or of fouling (particularly scaling) deposited on the IEM after ED treatment [60, 170, 171]. Prior to determination of the IEMs fouling content, membranes should be pretreated. The pretreatment procedure consists of membrane soaking into the
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solution of acid (HCl or H2SO4) in order to dissolve the precipitates and to convert them into ionic form. Another possible way of the sample pretreatment includes membrane ashing in
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the furnace at 550 oC for 16 hours and then dissolution in acid solution (HCl or HNO3). Then solutions containing fouling ions can be analyzed by AAS. This method is mostly suitable for the analysis of scaling and colloidal fouling having inorganic nature. Moreover, AAS gives a
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quantative information and do not reveal the fouling forms and structures. Halogens and H,
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C, S, O, N and P elements cannot be analyzed by AAS since the resonance lines of these
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elements are under the detection limit.
III.3.2.2.2 Optical emission spectrometry (OES)
The optical emission spectrometry includes two steps. Firstly, the sample is atomized
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and electrons become excited. Secondly, transition of the valence electrons from the higherenergy orbitals to the lower-energy orbitals occurs and the emission specter can be obtained. The most common sources for the atomization and excitation are flames and plasmas (the macroscopically neutral gas containing ions and electrons). The detection limits are 1 – 1000 ppb and 0.1 – 50 ppb for the flame and plasma atomizers respectively. Halogens and H, C, S, O, N and P elements cannot be analyzed by OES. The plasmas are the most abundant sources for the OES due to their higher temperatures in comparison to flames, which allows better atomization and excitation. Plasma can be generated by several ways such as 1) application of a direct electric field across the electrodes (direct current plasma (DSP)); 2) application of high radiofrequency field (inductively coupled plasma (ICP)) and 3) application of microwave field (microwave-induced plasma (MIP)) [172]. The majority of works dedicated to the fouling phenomena applied ICP-OES technique. As for the AAS, ICP-OES could be applied for the analysis of fouling agents in
ACCEPTED MANUSCRIPT the treated solution [29, 93, 173] (e.g. heavy metals, silicon, boron etc.) or in the fouled membrane [44, 54, 174]. The determination of fouling composition by ICP-OES demands a sample pretreatment, which can be performed as in the case of AAS (section III.3.2.2.1). As
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for the AAS technique, the ICP-OES gives just a quantitative information concerning the
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scaling and inorganic colloidal fouling, and demands the use of additional analytical methods
multielement determination in the same sample.
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to reveal the fouling structure. The advantage of this method is a possibility of the
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III.3.2.2.3 Energy dispersive X-ray spectroscopy (EDS)
This technique is based on electronic transitions between inner atomic shells. EDS
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comprises two steps (Fig.16):
1) Excitation of an electron situated on the orbital of shell with low energy (E1) by the certain source of energy such as electrons, protons or X-ray beams. As a result of this excitation,
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electron leaves its orbital with the formation of free vacancy (electron hole);
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2) An electron from an outer shell having higher energy (E2) fills the free vacancy and the difference in energy (E2-E1) appears in the form of an X-ray detected by energy dispersive
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detector. The final data are displayed as a histogram of intensity by voltage (energy).
Fig.16: Scheme of energy dispersive X-ray spectroscopy.
EDS method allows determination of the elemental composition directly on a fouled membrane sample, which should be dried prior to the analysis. The most part of studies using EDS method are dedicated to membrane scaling since EDS is able to detect only elements heavier than Berillium and there are difficulties to detect nitrogen. Hence, the analysis of fouling having organic nature seems to be inappropriate without the information concerning nitrogen content. The one of the main scaling agents are the compounds of Ca2+ and Mg2+ ions. The identification of scaling by EDS during the electroacidification of skim milk was
ACCEPTED MANUSCRIPT studied by Bazinet et al. [56, 60]. These authors reported that the scaling could precipitate in a form of calcium carbonate, and hydroxides of calcium and magnesium. The same types of IEMs scaling was explored by Wang et al. during ED of industrial glutamate production
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wastewater [54]. Ling Teng Shee et al. revealed by EDS the precipitation of magnesium
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hydroxide on cation-exchange membrane during cheddar cheese whey electroacidification [53]. The precipitation of calcium compounds was detected by Ren et al. [32] during the
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recovery of sulfuric acid and ammonia from simulated monosodium glutamate fermentation wastewater. Recently, Asraf-Snir et al. used EDS method to study the formation of another abundant scaling type such as gypsum [175]. Numbers of studies provide a very useful
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information about the nature of the IEMs scaling by Ca and Mg compounds in different conditions, such as ionic concentration [106], Mg/Ca ratio and pH [66, 67, 73], ED stack
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configurations and current modes [64, 74, 163, 176]. The EDS spectra of fouled AEM during the recovery and concentration of ammonia from swine manure revealed the presence of two fouling types such as scaling (calcium carbonate) and colloidal fouling (colloidal silica) [24].
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Finally, the EDS studies were most helpful in revealing the fouling nature in microbial cells
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[85]. It is worth to note, that EDS method has a special advantage such as opportunity to explore the elemental distribution on the membrane surface, which could be very useful, for
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example, in the case of non-uniform fouling distribution. Moreover, Mikhaylin et al. demonstrated the multilayer scaling organization on the CEM [177]. The underlying layer of Mg(OH)2 was explored by focusing the EDS detector in the free space taking place in the upper layer of CaCO3. Despite the attractiveness of EDS method, there are several factors
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affecting the accuracy of EDS spectrum such as overlapping peaks of different elements, nature, inhomogeneity and roughness of the sample.
III.3.2.3 X-ray diffraction analysis (XRD)
X-ray diffraction is a method for the identification of atomic and molecular structure of a crystal. XRD diffractometer consists of a source of monochromatic X-rays, which are focused on a sample at some angle θ (Fig.17). The intensity of diffracted X-rays is analyzed by a detector, which is placed at 2θ from the source path. The diffraction happens when the X-rays strike the crystalline surface resulting in their partially scattering by atoms from different layers of crystalline lattice having a certain interlayer distance d. If X-ray beams diffracted by two different layers are in phase, constructive interference occurs and diffraction results in a peak on a diffractogram.
However, if beams are out of phase,
ACCEPTED MANUSCRIPT destructive interference occurs and there is no peak on a diffractogram. Hence, only crystalline solids will be detected by XRD while the limit is that amorphous materials will remain undetected [178].
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The studies of XRD spectra allow revealing of the crystalline structures of precipitates
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formed on the IEMs treated by ED. The only pretreatment of a fouled membrane prior to XRD is drying. Casademont et al. [73] reported the transformation of calcium carbonate and
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hydroxides of Ca and Mg during the consecutive ED treatments. For example, no crystalline scaling was detected on the surface of AEM after the first ED run and scaling was composed of amorphous Ca and Mg hydroxides. However, after the third ED run the calcite (calcium
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carbonate) formation was detected. These data allowed authors to develop mechanism of scaling growth and development on CEM and AEM. The more detailed studies using XRD of
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scaling formation mechanisms and effects of non-stationary electric fields were conducted by Cifuentes-Araya et al. [64, 74, 176]. These authors reported the formation of multilayer scaling composed of calcite, brucite and aragonite crystals during consecutive ED treatments.
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Moreover, they emphasized the importance of water splitting phenomenon on the
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development and growth of crystalline scaling. The XRD studies of the scaling nature were also conducted by Wang et al. [54] for the ED of industrial glutamate production wastewater
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and Ping et al. [107] for microbial cells.
Fig.17: Schematic representation of X-ray diffraction analysis.
III.3.3 Chromatography
ACCEPTED MANUSCRIPT
Chromatography is based on a separation of components moving through a system containing mobile and stationary phases. For this purpose, the sample is dissolved in a mobile
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phase, carrying it through a stationary phase, which could be placed in a column (column
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chromatography) or on the plane surface (planar chromatography). The separation of mixture occurs due to the different rate of migration of each component trough the stationary phase.
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For instance, the mixture of components A and B is moved by the mobile phase through the column with a steady phase (Fig.18). Component B has a stronger affinity towards the steady phase and moves slower in comparison with component A. Eventually, component A leaves
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the column in the first place at time tA, is detected and transformed into a chromatographic peak and then a peak of component B appears at time tB. The retention time is a qualitative
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characteristic and the peak height and area are quantitative characteristics. There are different classification of chromatographic methods [164]. The most important classification in the scope of the present review is based according to the physical state of mobile phase, which
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comprises liquid and gas chromatography. Moreover, there are number of chromatographic
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methods including the use of different steady phases, detectors, sample pretreatments etc. Hence, one should take into account the nature of the analyzed sample in order to choose an
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appropriate chromatographic method.
The most used chromatographic method for the IEM fouling investigations is a highperformance liquid chromatography (HPLC). Kim et al. [141] used a size-exclusion (SEC) HPLC for the characterization of fouling by natural organic matter. The advantage of size-
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exclusion HPLC is the ability to separate according to the molecular weight, which is convenient for the case of natural organic matter with a large relative molecular mass. The same HPLC-SEC method with UV detector was applied by Lee et al. [105] to investigate the fouling phenomena by humate. The distribution of humate molecular weight before, after ED treatments, and after the cleaning procedures allowed estimating the quantity of reversible and irreversible fouling. Furthermore, Park et al. used HPLC-SEC to investigate the possible mitigation of fouling by humate by addition of different water-soluble polymers and use of optimal pulsed electric field conditions [125, 140]. The molecular weight distribution of peptides forming IEM fouling was studied by Langevin et al. [46] using HPLC with detection by mass spectrometry (HPLC-MS). Mass spectrometer allows detection of chemicals by measuring the mass-to-charge ratio. The HPLC-MS data provided an essential information about the fouling composition on the CEM and AEM and were very useful for the development of the fouling formation mechanisms. Another method used in IEM fouling
ACCEPTED MANUSCRIPT investigations is ion-exchange or ion chromatography (IC) with electrical conductivity detection. This method is mostly used for the determination of concentration of different inorganic ions, which could cause membrane scaling [48, 87, 125]. Recently, the gas
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chromatography with electron capture detection (GC-ECD) was applied by Banasiak et al. to
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study the fouling phenomena by pesticides and hormones [43]. Despite many advantages of chromatographic methods, the main disadvantage is the inability of direct analysis of fouling
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the liquids prior to the chromatographic analysis.
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composition of fouled membranes. Indeed, the fouled membranes should be converted into
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Fig.18: Scheme of a typical chromatographic separation in a column.
III.3.4 Biofouling characterization
The problem of IEM biofouling appeared quite recently due to the development of microbial fuel, electrolysis and desalination cells. Thereby, there is relatively small amount of studies provided an information about the structure and properties of biofilms formed on IEMs. However, there are studies and reviews concerning the biofouling on filtration membranes [75, 80, 89, 98, 179]. These studies could be very useful for the researchers willing to explore different aspects of IEMs biofouling. The techniques of biofouling characterization are very complex and time consuming. Moreover, there are difficulties in microorganism’s quantification and elicitation of the role of each microorganism in formation of complex biofilms [94]. Currently, the IEMs biofouling is widely characterized by
ACCEPTED MANUSCRIPT visualization using SEM, CSLM and AFM microscopies [85, 93, 95, 107, 130, 167, 180, 181]. Luo et al. characterized the composition of IEMs biofouling by denaturing gradient gel electrophoresis (DGGE) [182]. DGGE is a fingerprint technique enabling separation of DNA
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fragments [183]. During DGGE, a sample of DNA migrates trough a gel that contains a
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denaturing agent. Different sequences of DNA will denature at different denaturant concentrations resulting in appearance of a bands pattern. Each band represents a different
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bacterial population from the community. There are several steps prior to DGGE analysis. First of all, is a careful extraction of the bacterial genomic DNA from a biofilm. Second of all, is a polymerase chain reaction (PCR) including amplification of a single or few copies of
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a piece of DNA. Recently, Zhi et al. [184] published a comprehensive review concerning methods for understanding microbial community structures and functions in microbial fuel
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cells, which could be useful for the prospective investigations of IEMs biofouling.
Tab.2: Methods for the fouling analysis
Photo imaging
M
Optical microscopy
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Visualization
Scanning Electron Microscopy (SEM)
M
Reference [35-37, 63]
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M*
M
Fouling type
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Method
[3, 96, 165] O*, C, S, B
[24, 30, 32, 53, 54, 56, 57, 63, 66, 73, 74, 85, 96, 107, 130, 146, 161, 163, 167, 175, 177, 180, 182, 185]
Confocal Laser Scanning Microscopy
[167, 180, 186]
(CLSM)
characteristics
Membrane
M
Atomic Force Microscopy (AFM)
[85, 98, 130]
Electrical Resistance and Conductivity
[24, 29, 33, 35, 42, 43, 46, 50, 53, 54, 56, 57, 66, 67,
-
85, 92, 96, 133, 139, 141, 145, 170, 171, 187-191]
ACCEPTED MANUSCRIPT Voltammetry and chronopotentiometry
[2, 25, 30, 50, 131, 161,
-
167, 169, 170, 182, 185, 187, 192-194] [25, 34, 85, 93, 107, 122,
(EIS)
182, 195]
-
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O, C, S, B
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Transport Number -
-
141, 144-146]
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139, 141, 167, 189, 196]
[35, 36, 46, 53, 56, 57, 67,
Water Uptake
[47, 56, 60, 92, 189, 196]
Current(Voltage, Global Resistance)-
[2, 23, 29, 30, 32, 33, 35,
Time Curves
36, 45, 48-50, 57, 85, 96,
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-
[21, 25, 30, 92, 95, 139-
[24, 30, 54, 92, 105, 130,
Membrane Thickness QT
144, 189, 191]
144-146, 161]
Ion-Exchange Capacity -
[85, 92, 122, 130, 133,
[30, 50, 92, 95, 105, 139,
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Contact Angles
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Zeta potential -
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Electrical Impedance Spectroscopy
96, 177, 196]
107, 122, 125, 131, 144, -
146, 163, 165, 167, 169, 170, 173, 177, 187, 196-
QT
Nitrogen Content by Combustion
O, C
[36, 46, 54, 60, 163]
QT
Carbon Content by Combustion
O, B
[29, 31, 48, 88]
fouling
Quantification of
Qualification and
199]
QT
Ash Content
S
[56, 175, 200, 201]
QT
Ultraviolet-Visible Spectroscopy
O, S
[29, 43, 48, 92, 105, 131,
QL
(UV/vis)
ST
Attenuated Total Reflectance Fourier
140, 141, 165, 166] O, C, B
[30, 31, 46, 50, 92, 96,
ACCEPTED MANUSCRIPT Transform Infrared Spectroscopy
133, 141, 144, 167, 185]
(ATR-FTIR) QT, QL
Atomic Absorption Spectrometry
S
[60, 169-171]
O, C
[5, 29, 36, 93, 163, 173,
QT, QL
Optical (or Atomic) Emission
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Spectrometry (with inductively coupled
QT, QL
Energy Dispersive X-ray Spectroscopy (EDS) X-ray Diffraction (XRD)
QT, QL
Chromatography
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QT, QL
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ST
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plasma atomization) (ICP-OES (or ICP-AES)
X-ray Photoelectron Spectroscopy
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(XPS)
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(AAS)
O, C, S, B
S
175, 201]
[5, 24, 32, 53, 54, 85, 93, 167, 175] [54, 64, 66, 67, 74, 169, 202]
O, C
[43, 46, 48, 105, 125, 139141]
O, S
[133, 161, 166]
Radiation Analysis
O
[42, 43, 47, 188]
QT, QL
Denaturating Gradient Gel
B
[89, 98, 182]
DNA, RNA sequence
B
[98, 182, 184]
Chemical Oxygen Demand (COD)
O, B
[89, 185]
Dubois Method
B
[89]
Transparent Exopolymer Particles
B
[89]
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QT
ST QT QT
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QT
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Electrophoresis (DGGE)
Staining Method
ST
Terminal Restriction Fragment Length Polymorphism (T-RFLP)
B
[184]
ST
Clone library
B
[184]
QT, ST
Fluorescence
B
[184]
in
situ
hybridization
(FISH) ST
Pyrosequencing
B
[184]
QT
Metatranscriptomics
B
[184]
QT
Quantitative polymerase chain reaction
B
[184]
(qPCR) QT
Reverse transcription quantitative polymerase chain reaction (RT-qPCR)
B
[184]
ST
Phylogenetic analysis
B
[98, 184]
ACCEPTED MANUSCRIPT * The letters O, C, S and B refer to organic, colloidal, scaling and biological fouling types respectively and the letters M, QT, QL and ST refer to morphological, quantitative, qualitative and structural analysis respectively, symbol “-“ means non-applicability of
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IV. Strategies of prevention and control of IEMs fouling
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method for determination of fouling characteristics.
There are numbers of different strategies allowing fouling prevention and control in ED processes. However, one should always take into account the fouling nature because
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certain strategy can be appropriate against one fouling type and ineffective against another. Thereafter, in ED systems containing different fouling types a complex procedure comprising
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application of number of procedures may be involved. This section will provide a general information about the strategies of prevention and control of IEMs fouling.
Modification of IEMs
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IV.1
Generally, IEM modification aims to change the membrane surface properties such as Surface charge
Hydrophobic/hydrophilic balance
Roughness
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Grebenyuk et al. [2] demonstrated that the modification of AEM by high molecular mass surfactants was successful against organic fouling what led consequently to 1.7 times reduction in consumption of electric power. Kusumoto et al. [131] found that the oxidizing treatment of the AEM may generate thin and neutral layer on the membrane surface making AEM resistant to organic fouling. Modification of the AEM surface by sulfonating agents considerably improves antifouling potential [145, 165]. This improvement happens due to the decrease in membrane surface hydrophobicity and increase in its negative surface charge density (Fig.19). Moreover, using layer-by-layer modification, simultaneous improvement of antifouling potential and monovalent anion selectivity may be performed [144]. Recently, Vaselbehagh et al. [146] also reported IEM modification by polydopamine against fouling. Authors also emphasized that the increase in surface negative charge and the decrease in surface hydrophobicity has a positive influence on antifouling potential, while the increase in surface roughness decreased the antifouling potential.
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Fig.19: Example of fouling prevention by modification of CEM surface (reprinted with
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permission from [145]).
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The situation with biofouling seems to be more complex. Surface hydrophobicity and charge
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density are although important factors [81] but not effective criteria in membrane modification for biofouling control. The nature of surface charges plays the more important
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role. Membrane modification for biofouling control aims two goals such as anti-adhesive approach that prevents the initial attachment of bacteria on a membrane and anti-microbial approach that aims to kill bacteria already attached to the membrane. An example of antibiofouling approach was demonstrated in studies involving membrane surface modification
IV.2
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with biocides such as nanosilver particles [95, 130, 203, 204].
Cleaning agents
Taking into account the fouling nature, one can chose cleaning agents to remove the fouling form the surface of IEMs or to prevent fouling formation during ED treatment (Tab.3). However, rinsing by chemical addition may have a negative influence on the membrane performance [23]. For instance, IEMs cleaning with alkali solutions can lead to the degradation of ion-exchange groups [205] and cleaning with oxidants provokes the degradation of ion-exchange groups and polymeric matrix [206]. Furthermore, addition of some chemicals may have consecutive negative influence on the quality of ED products what becomes really important when speaking about food industry.
ACCEPTED MANUSCRIPT Tab.3: Cleaning agents for different types of membrane fouling Examples
Cleaning agents
References
Colloidal
SiO2, Fe(OH)3,
Chlorination, alkali
[22, 23, 207]
Al(OH)3, etc.
rinsing, anticoagulants
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Fouling type
Polysaccharides,
Alkali rinsing salt
proteins, peptides,
solutions, isopropanol
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Organic
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and dispersants
fatty acids, humate, surfactans CaCO3, Ca(OH)2,
Citric acid, EDTA,
Mg(OH)2, CaSO4, etc.
hydrochloric acid,
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Scaling
[46, 51, 198]
[54, 198, 208]
antiscalants
Enzymes, surface active
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Bacteria, biofilms, transparent exopolymer
substances, chaotropic
particles, etc.
agents, biocides, nitric
[94, 209]
oxide, etc.
Pretreatment
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IV.3
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Biofouling
IV.3.1 Pressure-driven membrane processes
Rejection of particles causing formation of a fouling layer on the ion-exchange
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membranes or clogging of the ED stack is possible by application of pressure-driven membrane processes as a pretreatment technique prior to ED. These filtration techniques use the pressure gradient as a driving force and allow separation of particles according to their size or molecular weight. General classification distinguishes pressure-driven membrane processes according to the pore size and applied pressure (Fig.20), and includes microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO).
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Fig.20: Classification of pressure-driven membrane processes (adapted with permission from
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Bazinet, Firdaous and Pouliot, Chap.17 in [210]).
Filtration processes can operate in two modes: dead-end and cross-flow. In dead-end mode,
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the stream of the feed solution flows perpendicularly to the membrane surface and only one
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flow leaves the membrane module. In cross-flow mode, the feed solution flows tangentially to the membrane surface, and there are two streams leaving the membrane module. For the
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majority of the modern pressure-driven membrane processes dead-end mode is inappropriate due to the severe accumulation of the rejected particles on the membrane surface leading to an abrupt decrease of the permeate flux. The tangential flow can help to shear away the accumulated rejected particles and attain relatively high flux of permeate. Assuredly,
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pressure-driven membrane processes as electromembrane processes also suffer from the fouling phenomena, however this subject is out of consideration in the present review. The more recent information concerning fouling in pressure-driven membrane processes is described in the following papers [51, 211-214]. Successful applications of pressure-driven membrane processes as a pretreatment step prior ED were performed during production and separation of proteins [215-218], production and fermentation of organic acids and its derivatives [219-224], juice deacidification [225227], waste water treatment [228-230], green production of polyaluminium salts [231] and caseins [201].
IV.3.2 Other pretreatment techniques
ACCEPTED MANUSCRIPT Avoiding some types of membrane fouling may be successfully carried out by application of following techniques:
Activated carbon allows removal of dissolved organic matter prior to ED. The
Filtration is used to reject high molecular weight residues which can cause fouling of
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IEMs and clogging of ED stack [221-223, 233];
Centrifugation can be used to separate suspensions from solutions treated by ED [218]
Pellet reactor is a column filled with a garnet sand, which acts as absorber of mineral
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activated carbon with a large surface area [223, 232, 233];
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mechanism of action is based on absorption of organic particles by the porous
ions such as Ca2+ and Mg2+ preventing the formation of scaling on IEMs [234, 235]; UV irradiation inactivates bacteria and can be used for the control of biofouling [94,
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236];
Phytoremediation is a cost-effective technique, which uses various plants to remove
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the organic and inorganic compounds present in a ground water and solid. Willow
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trees were reported being suitable for the wastewater treatment prior to ED [237]. Willow trees act as biological filters using their root system to remove the nutrients
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(needing for the growth) from the treated media. Furthermore, willow trees are salt tolerant and allow the treatment of concentrates from RO. The phytoremediation by willow field leads to 20% removal of total organic carbon and 32 % removal of total
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nitrogen and phosphorous preventing the formation of organic fouling and biofouling on IEMs during the subsequent ED treatment [237].
IV.4
Mechanical action
Prevention and destruction of fouling by mechanical procedures such as ultrasound, vibration, air sparge etc. are involved in filtration processes [51, 238-240]. However, in ED processes mechanical cleaning is applied rarely due to the changes in membrane properties and even membrane deterioration [51, 241]. Nevertheless, recently Wang et al. [54] reported that ultrasound treatment in combination with acid treatment can be effective against CEM fouling. In addition, Parvizian et al. [242] showed that impose of ultrasound could successfully improve the membrane potential, transport number and selectivity and decrease its electrical resistance. These investigations of ultrasound application may be attractive in
ACCEPTED MANUSCRIPT terms of fouling prevention and ED performance improvement, however additional studies in this field are necessary.
Changing regimes of ED treatment
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IV.5
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IV.5.1 Control of hydrodynamic conditions
In some cases, control of hydrodynamic conditions of ED treatment may be sufficient to avoid fouling formation. Grossman et al. [193, 243] reported, that an increase in flow rate
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and introduction of spacers promoting turbulence are advantageous for fouling mitigation. The design of the spacers is important in terms of increasing current efficiency and number of
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works are dedicated to this issue [90, 91, 244, 245]. The tendency of spacers design moves towards the creation of ion-exchange spacers. On the one hand, ion-exchange spacers have positive effect on ED performance, on the other hand, they tend to be more fouled, due to
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their surface charge, in comparison with uncharged spacers. Balster et al. [246] proposed an
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approach alternative to spacers such as air sparging. This approach allows increasing solution
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turbulence, which may be also advantageous in the reduction of a membrane deposit.
IV.5.2 Electrodialysis with reversal polarity
Other method industrially applied for the fouling control is ED with reversal polarity
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[247-254]. This ED mode consists of inverting the polarities of the electrodes periodically (i.e., at time intervals varying from a few minutes to several hours), as well as the hydraulic flow streams. The reverse of polarity immediately converts diluate compartments into concentrate compartments and vice versa (Fig.21), which requires a special equipment to interchange the diluate and concentrate streams [247]. When the polarity of electrodes is reversed, foulants attaching to the membrane surface (usually charged oppositely to ionexchange groups) become detached moving in the opposite direction (Fig.21). There are evident advantages of ED with reversal polarity such as prevention of membrane fouling, dissolution of scale seeds and absence of chemical additives. However, there is inconvenience related to the time (usually 1-2 minutes) after the reverse of polarity when both diluate and concentrate streams become “off specification” and diverted automatically to waste or back to feed tank [248, 251].
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ACCEPTED MANUSCRIPT
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IV.5.3 Pulsed electric field
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Fig.21: Scheme of ED with reversal polarity and presence of foulants.
The use of pulsed electric field (PEF) demonstrated very good results in terms of
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fouling prevention. PEF procedure consists in application of consecutive pulse and pause
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lapses (Ton/Toff) of certain duration (Fig.22).
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ACCEPTED MANUSCRIPT
Fig.22: Scheme of continuous current and pulsed electric field (PEF) modes with representation of PEF influence on casein fouling mitigation (adapted with permission from
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[36]).
PEF has number of advantages such as Increase of current efficiency due to the suppression of concentration polarization phenomenon; Effectiveness against fouling; Simplicity of equipment, which makes integration of such approach into industrial scale easy and inexpensive. First of all, the positive influence of PEF was found in terms of decrease of concentration polarization phenomenon and consequently decrease in water dissociation and increase in ED power efficiency [116, 255-257]. Secondly, Lee et all. [198] and Park et al. [125] reported mitigation of fouling containing humates and Ruiz et al. [36] successfully mitigated protein deposit. As could be seen from the Fig.22, the duration of pulse/pause is very important.
ACCEPTED MANUSCRIPT Indeed, the pause lapse of 40 s is advantageous in comparison with 10s in terms of casein fouling prevention. This positive effect of a longer pause duration could be explained by the better detachment of caseins from the membrane surface by the solution flux. Different
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situation was explored during the studies of IEMs scaling. Though scaling mitigation was
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demonstrated in several studies [57, 64, 65, 74, 163, 176, 201], there was no entire scaling prevention due to the strong interactions of scaling ions with membrane ion-exchange groups.
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Deep investigations has been made in order to reveal the mechanisms of scaling formation under the action of PEF [64, 65, 74, 163]. The results of these works show also the importance of pulse/pause duration in terms of scaling inhibition. Moreover, authors of above
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mentioned works tested relatively long PEF modes (20s – 5s) and created a 3D model of influence of pulse/pause duration on demineralization rate. Subsequent studies conducted by
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Mikhaylin et al. revealed that short PEF modes (1s-3s) allow better mitigation of concentration polarization and scaling phenomena [177]. Indeed, the application of the optimal pulse/pause mode of 2s/0.5s allows a 40 % decrease of the CEM scaling and
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complete inhibition of AEM scaling (just traces of AEM scaling were detected) in
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comparison with a continuous current mode [177].
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IV.5.4 Overlimiting current regime
Contemporary investigations revealed reasonableness of overlimiting currents for improving ED performances [118]. The overlimiting currents lead to the formation of
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current-induced convection (particularly electroconvection), which has already been discussed in section III.2.2. Electroconvective vortices facilitate the transport of ions towards the IEM surface allowing control of water splitting phenomenon and decrease in the area of IEMs used, which subsequently leads to decrease in the ED cost [118]. Furthermore, electroconvective vortices may affect the formation of fouling. Bukhovets et al. [187] proposed the “washing out” effect of electroconvection on the organic fouling. The positive effect of electroconvection on CEM scaling mitigation was recently reported by Mikhaylin et al. [202].
Tab.4 Methods for the prevention and control of fouling on ion-exchange membranes Method
Scale
Advantages
Disadvantages and operation limitations
ACCEPTED MANUSCRIPT Membrane
Laboratory
modification
Less power consumption,
Higher membrane costs; may be
decrease of pretreatment
ineffective for the treatment of
costs, additional
complex solutions, which contain
improvement of
fouling having different nature
Cleaning-in-place of
Generation of additional effluents,
membrane fouling having
additional expenses including the
different nature
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Industrial
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Cleaning
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selectivity
costs of chemicals and pumping
energy; may affect the membrane
Industrial
Avoidance of the passage
Additional costs concerning
of fouling agents in the
installation of new modules
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Pretreatment
performance
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system with ion-
exchange membranes Mechanical action
Laboratory
Cleaning-in-place of
Possible membrane deterioration
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membrane fouling having different nature
Industrial
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with reversal polarity
Pulsed electric
Cleaning-in-place of
Requires a special equipment to
membrane fouling having
interchange the diluate and
different nature
concentrate streams; not suitable in
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Electrodialysis
Laboratory
field
the systems with bipolar membranes; generation of “offspecification” effluents Cleaning-in-place of
Unknown influence on the
membrane fouling having
membrane integrity
different nature; decrease of concentration polarization; simplicity in installation
Overlimiting current regime
Laboratory
Cleaning-in-place of membrane fouling having different nature by electroconvection; decrease of concentration polarization; lower membrane area is
May affect membrane integrity
ACCEPTED MANUSCRIPT required in comparison to underlimiting current
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regime
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Conclusion
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The present review demonstrated the wide range of IEMs processes facing the problem of membrane fouling. Fouling is a major drawback hampering the industrial application of these processes. However, if fouling phenomena are well studied and
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understood, it is possible to find a right solution in order to minimize or completely avoid the fouling. To do so, it is necessary to carry out thorough exploration of the fouling structure and composition as well as fouling influence on the performances of IEMs process. This
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paper presents a wide range of techniques allowing fouling investigations and approaches for the following fouling control or/and prevention. The modern tendencies are directed to the
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creation of membrane materials with antifouling properties. However, the modified
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membranes usually have anti-fouling properties against certain fouling type, which could be ineffective for complex solutions containing different fouling types. From this point, the perspective direction seems to be the investigation of effectiveness of already existed
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modified membranes against different fouling agents as well as the IEM modification to create the membrane resisted to the fouling having different nature. Also, the complete characterization of the membrane physico-chemical properties as well as those of the
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different fouling types and their mixtures is of great importance in order to well understand the overlaying mechanisms. Additionally, pretreatment techniques, such as pressure-driven processes, are very perspective since they avoid contact of fouling agents with IEMs. The main disadvantage of these processes is the involvement of additional investments, which do not always suit to the financial expectations of the industries. In the light of above, two current modes such as PEF and overlimiting current seem to be a feasible solution in the nearest future since they do not demand additional investments. These modes were reported to be appropriate against different fouling types. Moreover, PEF and overlimiting current allow the decrease of concentration polarization phenomenon, which is the one of the major problems of ED leading to the increase of the process costs. Thus, anti-fouling action of these current modes will be accompanied by the economic benefits. However, the approach of using overlimiting current mode is relatively new and demands more detailed investigations
ACCEPTED MANUSCRIPT to reveal underlying mechanisms of action, possible disadvantages and the reasonableness of their use on all types of IEMs (cationic, anionic and bipolar membranes).
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Abbreviations
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AAS - Atomic Absorption Spectrometry AEM - Anion-Exchange Membranes
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AFM - Atomic Force Microscopy
ATR–FTIR - Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy CEM - Cation-Exchange Membranes
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ChP – Chronopotentiogram
CLSM - Confocal Laser Scanning Microscopy
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CVC - Current-Voltage Curve
DGGE - Denaturing Gradient Gel Electrophoresis EDS - Energy Dispersive X-ray Spectroscopy
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EIS - Electrical Impedance Spectroscopy GC – Gas Chromatography
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ED – Electrodialysis
IR – Infrared
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HPLC - High-Performance Liquid Chromatography IEM – Ion-Exchange Membrane LCD - Limiting Current Density
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MS – Mass Spectrometry MF – Microfiltration NF – Nanofiltration
OES - Optical Emission Spectrometry PCR - Polymerase Chain Reaction PEF – Pulsed Electric Field RO – Reverse Osmosis SDBS - Sodium Dodecylbenzenesulfonate SEM - Scanning Electron Microscopy SEC – Size Exclusion UF – Ultrafiltration UV/vis - Ultraviolet-Visible Spectroscopy
ACCEPTED MANUSCRIPT XRD - X-ray Diffraction Analysis
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Acknowledgements
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The financial support of the Natural Sciences and Engineering Research Council of Canada
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(NSERC) is acknowledged.
References
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Graphical abstract
ACCEPTED MANUSCRIPT Highlights
There are for principle types of ion-exchange membrane fouling such as colloidal,
The thorough exploration of the fouling phenomena may include: 1) fouling
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organic, scaling and biofouling;
visualization, 2) characterization of fouling composition and 3) characterization of
The strategies of fouling control and preventions are following: 1) membrane
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changes of the process regimes.
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modification, 2) pretreatment procedures, 3) cleaning, 4) mechanical action and 5)
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fouled ion-exchange membranes;
