Removal of heavy metals by surfactant-enhanced ultrafiltration from ...

Desalination 207 (2007) 125–133

Removal of heavy metals by surfactant-enhanced ultrafiltration from wastewaters Francesco Ferella, Marina Prisciandaro*, Ida De Michelis, Francesco Veglio’ Department of Chemistry, Chemical Engineering and Materials, University of L’Aquila, Monteluco di Roio, 67040 L’Aquila (AQ), Italy Tel. +39 (0862) 434255; Fax: +39 (0862) 434203; email: [email protected] Received 11 November 2005; Accepted 12 July 2006

Abstract This study examines the effects of cationic and anionic surfactants on the performance of surfactant-enhanced ultrafiltration process. In particular, the separated removal of lead and arsenic from wastewater is investigated by using dodecylbenzenesulfonic acid (DSA) as anionic surfactant and dodecylamine as cationic surfactant. The ultrafiltration process is performed by means of a monotubular ceramic membrane of nominal pore size 20 nm (molecular weight cut-off: 210 kDa). Pb and As ions are removed from the water flow one at a time using both DSA and dodecylamine. Metals and surfactants are added in a biologically treated wastewater from a municipal wastewater treatment plant and then ultrafiltered. Concentrations of lead and arsenic introduced in the water vary from 4.4 to 7.6 mg/l, while DSA and dodecylamine concentrations are equal to 10!5 M and 10!6 M, respectively, both of them below their critical micelle concentration. The results show a quantitative removal of lead ions >99% in the Pb/DSA and Pb/dodecylamine systems, while it was observed a decrease of arsenate ions in both of systems of about 19%. The almost complete retention of lead cannot be due only to surfactants, but also to its bioabsorption by the abundant organic matter present in solution, while AsO4- ions, being very toxic, are not adsorbed by microorganisms, therefore the removal is very low. The rejection of other main constituents found in wastewaters is already presented. Keywords: Ultrafiltration; Surfactant; Wastewaters; Heavy metals; Lead; Arsenic

1. Introduction Heavy metal pollution of civil and industrial wastewaters represents a major problem for the environment since metal ions are nonbiodegrad*Corresponding author.

able, have a very high toxicity and some of them have proved to be carcinogenic. If directly discharged in sewers, metal ions may seriously damage the subsequent biological treatments in depuration plants and render treatment sludge not reusable for agriculture. Adsorption and chemical

0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.07.007

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precipitation are the most widely used techniques to remove heavy metals from wastewaters, but they work in reducing metal concentrations below the law imposed limits only in few cases; anyway, they are very expensive and too low selective processes. Membrane processes represent a viable and almost cheap solution in the treatment of metal containing wastewaters since membranes can be added as a retrofit of existing plants, moreover efficiency removal of 99% can be almost easily gained [1,2]; these are some of the reasons why membrane processes are becoming more and more widespread. In addition, membrane filtration process is carried out at room temperature, without any yield decrease. As for the membrane cut-off to be used to remove metal ions, nanofiltration or reverse osmosis membranes should be employed, but a very high transmembrane pressure is required, rendering the process very expensive. The problem could be overcome by using an additive, able to fix metal ions rendering their dimensions higher than those of ultrafiltration membrane pores. This is the principle of micellarenhanced ultrafiltration (MEUF), a process in which a surface active agent is added to the liquid stream to be treated in a concentration higher than critical micellar concentration (CMC); therefore some micelles are formed in solution that trap metal ions on their surfaces forming micellemetal complexes. The stream is then fed to an ultrafiltration membrane, which retains complexes on its surface. To obtain the highest retentions, surface active agents of electric charge opposite to that of the ions to be removed have to be used. At the moment, MEUF is mostly used to remove single pollutants through the addition of anionic surfactants, cationic surfactants or polyelectrolytes. More interesting is the simultaneous removal of two or more chemical species in the same stream, which is made complex by the different ions affinity towards micelles, with the

subsequent inhibition of the component with the lowest binding power. Baek and coworkers [1] experimented with the simultaneous removal of chromate and ferriccyanide from the wastewater or groundwater with MEUF using octadecylamine acetate (ODA) as a cationic surfactant: the removal of ferriccyanide and chromate in the ferriccyanide/ODA and chromate/ODA systems were 98% and >99.9%, respectively. As for the interactions between different species if added together in solution, authors found that the removal of chromate was inhibited by the presence of ferriccyanide, which has a higher binding power towards micelle surface. In a subsequent work [3] the binding characteristics of chromate and ferriccyanide with cetylpyridinium chloride (CPC) micelles were investigated. The removal of chromate and ferriccyanide in the chromate/CPC and ferriccyanide/ CPC systems were all >99% with the 5 molar ratio of CPC. In the co-existence of chromate and ferriccyanide, the removal of ferriccyanide was similar to that in the ferriccyanide/CPC system, while chromate removal was significantly inhibited by ferriccyanide. Authors concluded that even though the bind of chromate to CPC micelles was significantly inhibited by the existence of ferriccyanide, MEUF could be a promising technology to remove or separate chromate and ferriccyanide simultaneously. Other studies [2,4–7] focused on the influence that some parameters (e.g., transmembrane pressure, membrane material, pH) have on metal removal efficiency. As expected, surfactant retention decreases while increasing membrane cut-off [8], while not so evident is the circumstance that also while surfactant concentration is lower than CMC; therefore, no micelles should be present in solution and some retention is obtained. The explanation stands in the fact that during filtration some surfactant amasses on membrane surface on a gel layer: on it, concentration becomes higher than CMC, thus some micelles probably form [5].


Obviously, concentration polarization, together with biofouling phenomena, causes flux to decrease. As for the influence of TMP, retention coefficient is in practice constant in the interval 3×105÷5×105 Pa [4]. Moreover, studies on the influence of membrane material have shown that micelles adsorb preferably on the hydrophilic surface of polyamide membranes, rather than on the hydrophobic surfaces of ceramic or polysulfone membranes [8]. Thus, hydrophobic interactions between membrane material and surfactant play an important role in determining the performance of MEUF process. In this work, the removal of both Pb2+ and As! ions from synthetic industrial wastewaters by surfactant-enhanced ultrafiltration is investigated. The obtained results confirm that this is a promising and reliable method for the removal of heavy metals from industrial effluents.

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Fig. 1. Experimental apparatus used for ultrafiltration. 1: jacketed feed tank; 2: pump; 3: membrane module; 4: back-flush device; 5, 6: manometers (0÷4×105 Pa); 7: temperature gauge; 8: muffler; 9, 10, 11: valves; 12: air purge valve.

2. Materials and methods 2.1. Apparatus description

2.2. Feed water characteristics

Experimental studies have been carried out in a tangential flow Membralox® XLAB 3 (Exekia, Bazet, France) laboratory pilot plant with a single tube Membralox® Tl-70 ceramic ultrafiltration membrane (Fig. 1). The recirculation pump gives a fixed tangential velocity of about 7 m/s. All experiments were performed at room temperature; for the cleaning procedure, in which water was at 40°C, temperature was controlled by the tank jacket connected to a Crioterm 10–80 thermostat. The plant is equipped with a backflush system BF3, controlled by an electrovalve (pressure 7×105 Pa, re-injected volume 3 ml). Backflush was utilized only during membrane cleaning, with intervals and lengths regulated manually (frequency 2 min, length 1 s, approximately). The pore size of membrane used in experimental work was 20 nm (MWCO .210 kDa).

Water utilized in this study was withdrawn from the municipal wastewater treatment plant (MWTP) of Ponte Rosarolo in L’Aquila (Italy) after the chlorination section. Forty litres of wastewater were microfiltered with a press-filter with pore size of 0.45 µm and then stored at 5°C. This pretreatment was necessary to remove suspended material preventing excessive membrane fouling due to pore plugging. Several wastewater parameters were measured before and after the microfiltration step. Results of these analyses are reported in Table 1. Alkalinity, hardness, calcium and magnesium were measured by titration, total suspended solids (TSS) and total dissolved solids (TDS) by evaporation at 105°C according to the Standard Methods [9]. pH was determined using a Backman Φ72 pH-meter, and electrical conductivity using a microprocessor LF196 conductivity-

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Table 1 Wastewater analysis in MWTP effluent and after MF step Parameter

Units

MWTP effluent

After MF

Rejection [%]

Conductivity at 25°C pH TSS TDS Hardness Mg2+ Ca2+ BOD5 COD Total P Total N N-NH3 N-NO2 N-NO3 Alkalinity Cationic surfactants Anionic surfactants Non-ionic surfactants Total Coliforms

[µS/cm]

475 7.47 5 275 17.8 12.2 51.2 12 15 2.60 / 0.85 7.44 4.88 0.22 / 0.71 2.27 / 10.1 195 0.33 0.39 99 18

abundant organic matter present in solution, according to the rejection coefficient of total coliforms (>94%) [11–13]. Lead is then accumulated on the biofilm that covers membrane surface where nutrients amass. As for arsenic, the rejection percentage is of about 19%. AsO4! ions, being very toxic, are not adsorbed by microorganisms; therefore, the removal is very low. The plausible mechanism of removal is therefore the adsorption on the few micelles present on membrane surface.

As for metals, lead has showed rejection higher than 99% in the presence of surfactant, while arsenic a less good 19%: this difference has been explained by considering that Pb has been adsorbed by microorganisms, in turn retained by membrane. The next step will be the use of membrane with lower cut-off, and/or the increase in surfactant concentrations above CMC. It is worth noting that surfactant concentration used in solution is to be optimized, in order to reduce operating costs and to limit the presence of such monomers in the permeate that are pollutants and have to be removed. Moreover, it could be interesting to study processes for surfactant recycling, and, where convenient, also for the recovery of metal ions entrapped in micelles. The operating costs control of these two processes is the basic condition to realize a large-scale plant. Acknowledgements

4. Conclusions In this paper, the removal of Pb2+ and As! ions from industrial wastewaters by surfactantenhanced ultrafiltration has been investigated. The surface active agents used are either anionic (dodecylbenzenesulfonic acid) and cationic (dodecylamine) surfactants, which have been added to a synthetic industrial liquid waste, obtained by the addition of lead and arsenic ions to a municipal wastewater treatment plant effluent. The mixture is then forced through an ultrafiltration membrane with molecular cut-off of 210 kDa. Surfactants have been added with a concentration lower than CMC; notwithstanding, average rejections are about 21% as for anionic surfactant and about 68% as for cationic surfactant in the investigated conditions: it represents a good result, taking into account for the used membrane cut-off.

The authors are very grateful to Ms. Lia Mosca and Mr. Marcello Centofanti for the helpful collaboration in the experimental work. This work was carried out with the financial support of the Italian Ministry of Education, University and Research (MIUR – Project PRIN 2003).

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