Copper removal from aqueous solution using mixed ...

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The Journal of Energy and Environmental Science. Photon 128 (2014) 457-466 https://sites.google.com/site/photonfoundationorganization/home/the-journal-of-energy-and-environmental-science Original Research Article. ISJN: 1784-6372: Impact Index: 4.18

The Journal of Energy and Environmental Science

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Copper removal from aqueous solution using mixed mineral systems injected with zinc sulfide in sulfidic- anoxic conditions II. The role of solution composition and ageing D.E. Egirania*, J.E. Andrewsb, A.R. Bakerb a b

Faculty of Science, Niger Delta University, Wilberforce Island, Nigeria School of Environmental Sciences, University of East Anglia, Norwich, UK

Article history: Received: 24 March, 2014 Accepted: 05 April, 2014 Available online: 23 April, 2014 Abbreviations: Cu: Copper Keywords: Ageing, copper, particle concentration, mixed minerals, zinc sulfide Corresponding Author: Egirani D.E.* Lecturer (Professor Assistant) Geochemistry Email: [email protected] Phone: +23408104826459

of

Environmental

Andrews J.E. Professor of Geochemistry Email: [email protected] Phone: +441603592536 Baker A.R. Associate Professor of Geochemistry Email: [email protected] Phone: +441603592536

Abstract Pollutants, especially heavy metals Cu inclusive discharged by acid mine drainage and wastewater treatment are the major causes of degraded stream water chemistry (Ridge and Sedlak, 2004). Limited studies on Cu removal exist but the use of mixed mineral systems injected with zinc sulfide under

sulfidic-anoxic condition is a new dimension of research (Egirani et al., 2013). This study investigates Cu removal onto binary mixed mineral sorbents from simulated wastewater relevant to effluents. Batch mode study conducted at room temperature involves injecting zinc sulfide into mineral systems of kaolinite, montmorillonite, goethite and their mixtures. Variables investigated include pH, initial metal concentration, solid concentration and residence time. The results indicate that all mineral systems but goethitemontmorillonite exhibits a linear increase in Cu sorption as pH increases. Sorption capacity increases with metal concentration. Some mineral systems neither demonstrated promotive nor nonpromotive sorption of Cu over the range of particle concentration investigated. Cu sorption exhibit a complex behavior over the range of residence time (ageing) investigated. Sorption behavior is due to the presence of different reactive sites. Studies on solution composition and longer residence time under complete oxic condition are required to ascertain the effects of different aqueous environments on Cu removal. Citation: Egirani, D.E., Andrews, J.E., Baker A.R., 2014. Copper removal from aqueous solution using mixed mineral systems injected with zinc sulfide in sulfidic- anoxic conditions II. The role of solution composition and ageing. The Journal of Energy and Environmental Science. Photon 128, 457-466. All Rights Reserved with Photon. Photon Ignitor: ISJN17846372D679823042014

1. Introduction Pollutants, especially heavy metals discharged by acid mine drainage, weathering of mineralized bedrock, and wastewater treatment are the major causes of degraded stream water chemistry (Ridge and Sedlak, 2004). The mere presence of a variety of metals such as copper, cadmium, lead, nickel and chromium in an aquatic environment is of great concern because they are toxic and do not biodegrade in nature. These metal ions Ph ton

may show acute toxicity to aquatic organisms and terrestrial life and cause remarkable adverse physiological effects in humans and animals. From this perspective, the need to reduce or eliminate these pollutants and minimize their adverse effects has led to a significant increase in recent decades in the number of investigations aimed at controlling these metals in the environment (Santana et al., 2010).

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The need to reduce metal concentrations discharged into water bodies remains a priority in both developed and most developing countries. Currently, research has become more focused on developing locally sourced materials that have the ability to remedy this problem. Many studies use natural or synthetic materials whose roles follow the requirements of green chemistry, which aims to reduce waste and optimize the wastewater treatment processes (Airoldi, 2008; Staudinger, 1920). The removal of dissolved metal species can be hampered by the absence of reliable sorbents and solution chemistry adequate to understanding metal sorption (Pant et al., 2011). The removal of Cu ions from wastewater is controlled by the solution composition, namely, pH, initial metal concentration, and solid or particle concentration Cp and the residence time (ageing) of the solid phase in solution (Altin et al., 1999; Kitano et al., 1980; Banks et al., 1997; Schlegel et al., 2001). Since pH is considered a master variable in heavy metal removal in aqueous environments, its effects on Cu removal by mixed suspensions of clays and (hydr)oxides under sulfidic-anoxic condition is an area of research interest (Younger et al., 2002). Solution pH controls (a) the solubilities of metal hydroxides; (b) hydrolysis behavior of metals; and (c) surface charge of the sorbent (Appel and Mao, 2002). Initial metal concentration effects on heavy metal adsorption depend on the predominant sorption mechanism. Adsorption may increase as initial metal concentration increases (outer sphere complexation) or not be significantly affected as initial metal concentration increases (inner sphere complexation) (Petruzzelli and Pezzarossa, 2003; Di Toro et al., 1986). . Increase in adsorption as initial metal concentration increases (promotive initial metal concentration effects) for organic and inorganic substances sorbed on colloidal clay and oxide particles still remains an area of research interest in conventional surface complexation theory (Lutzenkirchen, 2001). The solid concentration effect is an anomalous adsorption phenomenon (i.e., the adsorption isotherm declines as particle concentration increases). Although the cause of this phenomenon remains unclear, the nature of metal species formed in solution is affected by changes in the mineral/ solution ratio (Fotovat et al., 1997). Prolonging the aging of a solid mineral phase in the absence of a sorbate results in much mineral surface reorganization

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as high and new reactive sites are formed (Philips, 1999). Details of the Freundlich isotherm and the associated empirical model used in validating Cu behavior in this study are provided elsewhere in the companion paper I. In that paper the behavior of Cu in single and mixed mineral systems in terms of reactivity and reaction kinetics was investigated. In this paper the sorption relationship between simulated wastewater containing Cu ions using single and mixed mineral systems containing zinc sulfide i.e. under sulfidicanoxic condition, kaolinite/montmorillonite, kaolinite/ goethite and montmorillonite/goethite in sulfidic-anoxic conditions based on different solution composition and ageing was investigated. The choice to investigate Cu ions is based on the reasoning that this metal is one of the most commonly occurring base metals in wastewaters associated with effluents and mine discharge. 1.1. Objective of Research The main objective of this work is to determine the effects of solution composition (i.e. pH, initial metal concentration, particle size) and ageing (residence time) Cu removal from simulated mine wastewater using mixed mineral systems of kaolinite, montmorillonite and goethite. . These minerals were chosen to simulate natural minerals found in acid mine impacted areas subjected to sulfidic-anoxic condition. To achieve this objective, an empirical model derived from existing models is required and these models and isotherms are used to determine sorption characteristics of these single and mixed mineral systems 2. Materials and Methods In an attempt to understand materials involved in Cu removal, sorbents used were characterized for particle size, % colloid, pH and surface area as provided in companion paper 1. 2.1 Preparation of sulfidic-anoxic iron sulfide suspension Sulfidic-anoxic conditions are characterized by depletion of dissolved oxygen. These conditions will occur if the rate of oxidation is greater than the supply of dissolved oxygen (Cullen and Reimer, 1989). In sulfidic-anoxic environment, hydrogen sulfide occurs as a product of sulfate and sulfide reduction (Velasco et al., 2008). In this study, 1% acidified zinc sulfide sulfidic-anoxic

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suspension was prepared using deoxygenated deionized water. Purified nitrogen gas was bubbled through the zinc sulfide suspension continuously for 24 hours. The content, securely sealed was stored in airtight containers in the anaerobic chamber in dark environment before use. The formation of hydrogen sulfide was prototypically characterized by a “rotten egg” odor (Wilkin, and Barnes, 1996; Dunnette, et al., 1985; Wu et al., 2011). 2.2. System characterization Detailed characterization is provided in companion paper I. All solutions were prepared using de-aerated and deionized water. This water was prepared by bubbling purified nitrogen gas through deionized water for at least 24 hours. Deionized water was obtained from a Millipore Milli-Q system. Then the water was purged overnight in an anaerobic chamber containing a mixture of 5% hydrogen and 95% nitrogen gases. Clay minerals and zinc sulfide used in this study provided by the Richard Baker Harrison Company and Acros Organics Ltd and goethite provided by Iconofile Company Inc. were nitrogen flushed and stored in airtight containers in the anaerobic chamber before use to avoid oxidation. For sorbent characterization, the (a) Coulter laser method was used to determine the particle sizes; (b) % colloid was estimated from the particle size distribution curves, (c) equilibrium pH of the untreated mineral suspensions was determined using the Model 3340 Jenway ion meter; (d) the standard volumetric Brunauer, Emmett, and Teller (BET) method was used to determine the surface areas (Brunauer et al., 1938; Anirudhan et al., 2009; Banks et al., 1997), (f) spectral analysis was performed using scanning electron microscopy, energy dispersive spectroscopy and x-ray diffraction to identify the mineral sorbent, (g) Cu concentrations in the mineral samples were determined using ICP-OES (Ridge and Sedlak, 2004; Silva et al., 2011; Egirani et al., 2013; 2005). 2.3 Experimental methods Batch mode experiments in this study 1% sulfidic-anoxic suspension of zinc sulfide was added to 1:1 single mineral suspensions of kaolinite, montmorillonite and goethite. Also, 1% sulfidic-anoxic suspension of zinc sulfide was added to 1:1 mixed mineral suspensions of kaolinite/montmorillonite, kaolinite/goethite and montmorillonite/ goethite were used to elucidate the difference in sorption between the single and mixed mineral phases. Ph ton

Characterization of sorbents used in this study included (a) particle size; (b) pH; (c) specific surface area (SSA); and (d) point of zero salt effect (PZSE). Details of characterization are provided elsewhere in a companion paper, I. In all batch mode experiments two components of the batch mode reactions Cu was added at the same time to the mineral systems (t = 0) and equilibrated for 24 h at the desired pH. For batch mode pH investigation, 1% sulfidic-anoxic suspension of zinc sulfide was added to single and 1:1 mixed mineral suspensions made up to 50 ml containing 1% (by mass) mineral suspension and 5 mg/l initial Cu concentration at zero electrolyte background were adjusted to the required pH (ranging from pH 4 to 8) using 0.1 M HNO3 and 0.1 M NaOH. The treated mineral systems were equilibrated for 24 h and pH measured using a Model 3340 Jenway ion meter.For batch mode solid or particle concentration investigation, 1% sulfidic-anoxic suspension of zinc sulfide was added to 1% single and 1:1 mixed mineral systems were made up to 50 ml containing solid concentrations (kg/l) of 0.002 to 0.01 and 10, 15, 20, and 40 mg/l of Cu ions. The treated suspensions adjusted to pH 4 and zero ionic strength was equilibrated for 24 h. Batch mode ageing investigations were carried out from 24 to 720 h using 1% sulfidic-anoxic suspension of zinc sulfide added to 1% single and 1:1 aged mixed mineral systems containing 1% (by mass). These mineral systems made up to 50 ml contained 10, 15, 20, and 40 mg/l initial concentrations of Cu as single sorbates. The treated mineral systems adjusted to required pH with no added electrolyte, were equilibrated for 24 h. Twentyfour hours was sufficient to establish equilibrium between the solid and liquid phases because metal ion sorption reactions occur in milliseconds or minutes (Dursun, 2006). Equilibrium experiments conducted at shorter equilibration times (i.e., beginning at 18 h) showed the same sorption behavior as at 24 h. Details of mineral suspension treatment including acidification and storage are provided in paper I. Three replicates were used for each treatment, and metal concentration sorbed (S) in mgkg−1 was calculated from difference between the initial metal concentration C0 (metal concentration before sorption) and the equilibrium concentration C (the metal concentration in solution after sorptionequilibrium), using Equation 1:

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(1) where Vtotal is the suspension volume and W is the mass of mineral solid. The effect of polycarbonate tubes on the sorption of metal ions is so small that it can be neglected (Rao and Khan, 2009). The amount of metal remaining in solution was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES). Detailed system characterization and an empirical model involving the distribution coefficient (Kd) as used in this paper are provided in companion paper I. Due to the low range of metal concentration used in this study, adsorption isotherms were of the C type, resulting in linear isotherms. Therefore, Kd was calculated from the Freundlich model Equation 2:

(2) where S, C, and N are as described in companion paper I. 2.4 Justification of Research The results of these experiments and models employed showed complex behavior of the role of pH, initial metal concentration, particle size and ageing of these single and mixed mineral systems in removing Cu under sulfidicanoxic conditions. They are different from previous experiments conducted in the absence of zinc sulfide. 3. Results and Discussions 3.1. Mineral systems and pH effects on Cu removal In previous studies, Cu sorption increased with increasing pH for both single mineral systems of kaolinite, montmorillonite and goethite and mixed mineral systems of kaolinite/montmorillonite, kaolinite/goethite, and montmorillonite/goethite (Egirani et al., 2005; 2013). In this study under sulfidicanoxic condition, all mineral systems but goethite-montmorillonite exhibits a linear increase in Cu sorption as pH increases. Goethite-montmorillonite exhibit a step-wise increase in Cu sorption over the range of pH investigated as shown in Fig. 1 Mineral systems behavior in Cu sorption is attributed to deprotonation of reactive sites and the presence of thiol (≡S-H) and hydroxyl (≡Me-OH) functional groups and reactive sites on surface of zinc sulfide (Appel and Ma, Ph ton

2002). Kaolinite exhibited highest sorption behavior relative to the rest of the mineral systems. On the contrary, goethite exhibits the lowest sorption behavior. Differences in sorption behavior of these mineral systems may also be attributed to differences in particle size which affects the specific surface area. Mineral mixing results in greater aggregation of flocs impeding Cu diffusion and can account for the low efficiency of the mixed mineral systems in sorbing metals by ion exchange (Helios-Rybicka et al., 2012). In addition, reactive sites of the mineral surfaces are masked by mineral mixing thus impeding sorption. These findings do not completely agree with the view of Puppa et al., 2013, that mineral mixing does not significantly affect heavy metal sorption on the birnessite/montmorillonite mixed mineral systems as shown in Table 1. The behavior of goethite-montmorillonite tends to demonstrate a more complex sorption behavior involving outer and inner sphere complexation. This may be due to the higher efficiency of –Al– +2 OH type sites to sequester other ions in solution (Idris et al., 2011). 3.2. Mineral systems and effects of initial metal concentration on Cu removal Previous studies except montmorillonite and kaolinite/goethite, Cu sorption on mineral systems decreased as metal concentration increases on single and mixed mineral systems (Egirani et al 2005; 2013]. Also, Cu sorption exhibited a crossover of Kd for kaolinite mineral system. In this study in sulfidic-anoxic condition, all mineral systems exhibit linear increase in sorption capacity as metal concentration increases and suggests a progressive change in the mode of particle association such as (a) edge-to-edge (EE); (b) edge-to-face (EF); and (c) face-to-face (FF) to favor Cu sorption as the metal concentration increases as shown in Fig. 2 (Helios-Rybicka et al., 2012). 3.3. Mineral systems and Cp effects on Cu removal In previous studies in the absence of sulfidicanoxic condition, Cu sorption on single mineral systems decreased as solid concentration (Cp) increased and differences in sorption between the single and mixed mineral systems revealed (a) enhanced sorption which decreases with increasing Cp for Cu sorbed on kaolinite/montmorillonite and kaolinite/goethite mineral systems (Egirani et al., 2005; 2013). In this report, Cu Kd is highest for the zinc sulfide, decreasing with increase in Cp as shown in Fig. 3. Except for

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kaolinite-goethite and goethite-montmorillonite, the remaining mineral systems neither demonstrated promotive or non-promotive sorption of Cu, exhibiting a near flat sorption characteristics. Differences between the actual and predicted Kd are positive for kaolinitegoethite and kaolinite-montmorillonite and near flat characteristics within the zero range for goethite-montmorillonite. This means that mineral mixing based on the isotherm model enhances Cu sorption for the former and not making any significant difference for the latter as shown in Fig. 4. Decrease in metal sorption as Cp increases may be attributed to increase in particle size and aggregation of the mineral systems. In addition, the presence of thiol (≡S-H) and hydroxyl (≡Me-OH) functional groups and reactive sites on surface of zinc sulfide may account for differences in sorption in the absence of sulfidic-anoxic condition and the presence of zinc sulfide in the mineral systems.

The Cp effect is also related to effective surface area, pressure, and force at the mineral/water interface (Liu et al., 2011). Increase in Cp results in low pressure at the interface and a subsequent decrease in sorbing ion diffusion to reactive sites. 3.4. Mineral systems and ageing effects on Cu removal In previous studies in the absence of sulfidicanoxic condition (Egirani et al., 2005; 2013), Cu removal from simulated wastewater by aged mineral systems increased with increase in residence time. In this report under sulfidicanoxic condition, Cu sorption by all mineral systems increase with increase in ageing up to 288 contact hour, flattening out over the rest of the residence time investigated as shown in Figure 5. Differences

Figure 1: Plots of Cu sorbed versus pH (a) Zinc sulfide, (b) kaolinite, (c) goethite-kaolinite, (d) kaolinite/montmorillonite., (e) montmorillonite, (f) goethite-montmorillonite, (g) goethite, sulfidic-anoxic mineral systems ZnS K GK KM M GM G

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Figure 2: Plots of sorption capacity versus Initial ion concentration for Cu sorbed on (a) Zinc sulfide, (b) goethite/montmorillonite; (c) goethite; (d) kaolinite/montmorillonite., (e) kaolinite/goethite., (f) kaolinite., (g) montmorillonite, sulfidic-anoxic mineral systems a. b. c. d. e. f. g.

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Figure 3: Plots of actual Kd versus particle concentration for Cu sorbed on (a) Zinc sulfide., (b) goethite/montmorillonite., (c) goethite., (d) kaolinite/montmorillonite., (e) kaolinite/goethite.,(f) kaolinite; (g) montmorillonite; sulfidic-anoxic mineral systems a. b. c. d. e. f. g.

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Figure 4: Plots of actual and theoretical Kd differences versus particle concentration-Cp for Cu sorbed on: (a) goethite/montmorillonite; (b) kaolinite/montmorillonite;and (c) kaolinite/goethite a. b. c.

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Figure 5: Plots of Cu sorbed versus Ageing (residence time) for (a) Zinc sulfide., (b) montmorillonite., (c) kaolinite., (d) goethite., (e) kaolinite/montmorillonite., (f) goethite/kaolinite., (g) goethite/montmorillonite., sulfidicanoxic mineral systems

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Figure 6: Plots of actual and theoretical Cu sorbed differences versus residence time for (a) kaolinite/montmorillonite; (b) goethite/kaolinite; and (c) goethite/montmorillonite., sulfidic-anoxic mineral systems a. b. c.

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between the actual and theoretical sorption capacity for goethite-montmorillonite remains on the positive territory indicating that mineral mixing enhances Cu sorption as shown in Fig. 6. Differences between the actual and theoretical sorption capacity for goethite-montmorillonite remains on the positive territory indicating that mineral mixing enhances Cu sorption as shown in Fig. 6, Cu sorption using this isotherm model decreases with increase in ageing. Differences in actual and predicted Cu sorption for goethite-kaolinite and kaolinitemontmorillonite are in the negative territory, indicating that mineral mixing for these mineral systems attenuated Cu sorption over the range of residence time investigated. Differences decrease with increase in ageing. This may be attributed to increased hydroxylation of the mineral surfaces, resulting in the formation of new reactive sites (Schlegel Ph ton

et al., 2001). Complex behavior of Cu sorption under sulfidic anoxic-condition may be attributed to differences in the hydrolysis behavior of the sorbing ions and the presence of thiol (≡S-H) and hydroxyl (≡Me-OH) functional groups and reactive sites on surface of zinc sulfide (Iskander and Selim, 1999). Research Highlights • Anthropogenic sources remain the major input of toxic elements, • Such as Cu into the aqueous environment, • Sorption of Cu by single and mixed mineral systems of kaolinite, montmorillonite and goethite in the presence of zinc sulfide is influenced by solution composition and ageing, • Mineral systems demonstrate complex behaviour in Cu removal. Limitations

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Due to lack of resources and time, authors could not measure the actual content of hydrogen sulide generated under sulfidic anoxic-condition. However, phreeqc model has been to calculate the content of hydrogen sulfide. Recommendations • Conduct similar experiments using other geological materials. • Replicate these experiments in pilot field studies.

aggregation of the mineral systems. In addition, the presence of thiol (≡S-H) and hydroxyl (≡Me-OH) functional groups and reactive sites on surface of zinc sulfide may account for differences in sorption in the absence of sulfidic-anoxic condition and the presence of zinc sulfide in the mineral systems. Cu sorption by all mineral systems increase with increase in ageing up to 288 contact hour, flattening out over the rest of the residence time investigated. This may be attributed to increased hydroxylation of the mineral surfaces, resulting in the formation of new reactive sites.

Justification of Research The role of solution composition and ageing of mixed mineral systems in Cu removal, under sulfidic-anoxic conditions have not been carried out previously. This work has opened up a new dimension of research in this regard using geological materials of interest. The results have shown the possibility of using empirical models derived from existing models to validate the effects of solution composition and ageing on Cu removal under sulfidicanoxic condition. Funding and Policy Aspect This work has been funded by Dr. Davidson Egirani from research payments made by the Niger Delta University, Nigeria. Conclusion The possibilities of using mixed sorbents of kaolinite, montmorillonite, and goethite to remove Cu from simulated wastewater have been studied in sulfidic-anoxic condition as a function of solution composition and ageing. All mineral systems but goethitemontmorillonite exhibits a linear increase in Cu sorption as pH increases. Goethitemontmorillonite exhibit a step-wise increase in Cu sorption over the range of pH investigated. Mineral systems behavior in Cu sorption is attributed to deprotonation of reactive sites and the presence of thiol (≡S-H) and hydroxyl (≡Me-OH) functional groups and reactive sites on surface of zinc sulfide.Also, all mineral systems exhibit linear increase in sorption capacity as metal concentration increases. Except for kaolinite-goethite and goethitemontmorillonite, the remaining mineral systems neither demonstrated promotive or non-promotive sorption of Cu, exhibiting a near flat sorption characteristics. Decrease in metal sorption as Cp increases may be attributed to increase in particle size and Ph ton

Author’s Interests

Contribution

and

Competing

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