Remediation of Solutions Containing Oxyanions of Selenium by

Article pubs.acs.org/IECR

Remediation of Solutions Containing Oxyanions of Selenium by Ultrafiltration: Study of Rejection Performances with and without Chitosan Addition Sébastien Déon,*,† Julien Deher,†,‡ Boukary Lam,† Nadia Crini,‡ Gregorio Crini,‡ and Patrick Fievet† †

Institut UTINAM (UMR CNRS 6213), Université de Bourgogne Franche-Comté, 16 route de Gray, Besançon, 25030 CEDEX, France ‡ Laboratoire Chrono-Environnement (UMR CNRS 6249), Université de Bourgogne Franche-Comté, 16 route de Gray, Besançon, 25030 CEDEX, France ABSTRACT: Among the various technical options for removing ionic contaminants from wastewaters, membrane processes and especially their coupling with polymer addition have been proven to provide worthwhile prospects for the removal of metal cations. Nevertheless, their use for the removal of anionic pollutants such as oxyanions has been little studied in the literature. In the present work, the rejection of oxyanions forms of Se(IV) and Se(VI) by tight ultrafiltration membranes was deeply investigated under various experimental conditions. This paper aims at understanding the mechanisms governing oxyanion rejection and determining the potential ways to improve performances. It is first shown that selenium concentration and salt content play a leading role on performances, with high selenium rejection even if pores are noticeably larger than ions. Additionally, results reveal that the pH value also has a tremendous impact on performances due to its influence on both membrane and selenium charges. Finally, chitosan addition as a preliminary step was found to have a positive influence on selenium rejection for ceramic ultrafiltration (UF) membranes with a large pore size whereas it had no influence on the performances of organic UF membranes with a lower molecular weight cutoff.

1. INTRODUCTION Far from highly industrialized areas, more than 700 million people lack access to safe drinking water and some 2.5 billion people do not use basic systems to treat their sewage.1 For this reason, wastewater treatment has become a major challenge all around the world, especially for countries facing water scarcity. Water pollution is obviously mainly generated by human activity, and most of the ionic contaminants are released from industrial and urban streams.2 This pollution can be induced by many types of contaminants.3 Among them, toxic ions represent a substantial part due to their disrupting impact on the functioning of the human body and their noxious consequences on health.4 Removal of heavy metal ions is often investigated in the literature.5,6 However, some nonmetal elements such as selenium can also be strongly hazardous, and their recovery from wastewaters is an issue of the utmost importance.7−9 Selenium is a mineral element required by the body as an essential nutrient but only in extremely low quantities. Unfortunately, it becomes noxious in its oxyanion forms such as selenate SeO42− (Se(VI)) and even more in the form of selenite ions SeO32− (Se(IV)), which is more commonly found in the environment. For instance, Hamilton10 has reported that © 2017 American Chemical Society

most toxicity thresholds mentioned in the literature are below the current national water quality criterion for the protection of aquatic life, which is 5 mg L−1.11 For this reason, selenium has recently emerged as a contaminant of concern for various industries such as agriculture, mining, oil refining, or power generation.12 Many processes are available to remove ions from wastewaters. However, they all exhibit benefits and weaknesses.13,14 Among them, membrane processes can succeed in the removal of ionic pollutants such as metal ions15,16 due to the electrostatic repulsions between ions and membrane material, which allow selective rejection of ions depending on their electrical charge.17,18 Unfortunately, pressure-driven membrane processes such as nanofiltration or ultrafiltration also exhibit some drawbacks.19 In this context, it was mentioned many times that, even if nanofiltration has shown outstanding rejection performances, unfortunately, it produces relatively low permeation fluxes and requires high applied pressure to Received: Revised: Accepted: Published: 10461

June 26, 2017 August 24, 2017 August 25, 2017 August 25, 2017 DOI: 10.1021/acs.iecr.7b02615 Ind. Eng. Chem. Res. 2017, 56, 10461−10471

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Industrial & Engineering Chemistry Research Scheme 1. Scheme of the Experimental Setup Used for Filtration Experiments

obtain significant flux. Oppositely, ultrafiltration allows filtration of larger volumes but the removal performances are often lower. For this reason, potential ways to improve membrane performances were increasingly explored in the past years.20,21 The first option consists of chemically modifying the membrane surface or material to adapt its performances to the requirements of a given application. This can be done by several methods such as surface coating,22,23 chemical grafting,24,25 or inclusion of additives such as polymers,26,27 nanotubes, or nanoparticles28−30 in membrane material. This way appears pertinent but requires strong skills in chemistry and may be difficult and expensive to implement for industrial or urban issues. The second possible way is based on the coupling of filtration experiments with a preliminary step aiming at increasing the effective size of pollutants for improving their rejection. The step is usually implemented by complexation of metal ions with either synthetic polymers, such as polyethylenimine (PEI),31 poly(acrylic acid) (PAA),32 and macrocycle and macromolecular compounds,33 or natural polymers, such as alginate34 and chitosan35 as chelating agent. Due to their many potential donor sites, these polymers have shown very interesting trends for complexation of various metal ions such as copper, lead, nickel, and cobalt by coordinate bounds with lone pairs of ligand. This mechanism of metal complexation is well-known36 and has been investigated many times. However, the possibility of using polymers to improve removal of oxyanions has not yet been accurately investigated to the best of our knowledge. In this case, physicochemical mechanisms leading to links between ions and polymers are different, and electrostatic interactions should probably play a key role. In this context, the present study aims at understanding physicochemical mechanisms that govern selenium rejection. The possibility of enhancing removal performances by addition of a dissolved natural polymer to the feed solution is also

investigated. In this study, chitosan was chosen for its ecofriendly properties, even if some other chelating or macrocylic ligands have more potential donor sites. Chitosan is usually used in its solid form for metal removal, but its dissolved form can also be interesting within the framework of polymerenhanced ultrafiltration (PEUF). Under acid conditions, it behaves like a positive polyelectrolyte and its electrostatic interaction with negative ions can lead to rejection enhancement. In this paper, rejection of Se(IV) and Se(VI) by ultrafiltration is first investigated under various conditions. The influence of concentration or pH value is particularly discussed to understand the mechanisms governing selenium rejection. Finally, the impact of a preliminary step of chitosan addition is investigated to assess if chitosan can be used to improve selenium rejection by organic and ceramic membranes, even when complexation does not occur.

2. EXPERIMENTAL DESCRIPTION 2.1. Filtration Experiments. The experimental setup used for filtration is a cross-flow unit, which is schematically described in Scheme 1. The feed solution is injected tangentially in the suitable cell containing either a tubular or flat-sheet membrane. In this study, two composite organic flat-sheet membranes Desal GH and GK (polyamide active layer) and a ceramic 3channels tubular membrane Ceram 60 (TiO2 active layer), respectively supplied by GE Water & Process Technologies (Trevose, USA) and TAMI Industries (Nyons, France), were studied. The data for these three membranes are summarized in Table 1. The temperature of solution in the tank is maintained constant at 25 °C by circulating cooled water in a double shell exchanger. Applied pressure was varied from 5 to 28 bar, which corresponds to a range of permeation flux between 0.5 and 5 × 10462

DOI: 10.1021/acs.iecr.7b02615 Ind. Eng. Chem. Res. 2017, 56, 10461−10471

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Industrial & Engineering Chemistry Research

2.2. Ions and Polymer Solutions. All experiments were performed using demineralized water with a residual conductivity lower than 0.1 μS/cm. Selenate and selenite sodium salts provided by Acros Organics were used as pollutants. Selenium concentrations of 1.7 × 10−3 and 1.7 × 10−5 mol L−1 were chosen since they correspond to concentrations that were found in an industrial wastewater before and after treatment. The amount of chitosan was chosen so that 12 mol of repeat unit are present in the solution for each mole of Se, which corresponds to 2 × 10−2 and 2 × 10−4 mol L−1 of chitosan/ chitin monomer unit. 2.2.1. Various Forms of Selenium. Before investigating the rejection of selenium(IV) and selenium(VI), it is primordial to understand that each element can be present in different forms depending on pH value compared to acid dissociation constant (pKa) of the various acid/base equilibriums. Figure 1 shows the distribution diagrams depicting the various forms taken by Se(IV) and Se(VI) according to the pH value of the solution. Distribution diagrams clearly show that, for acidic pH, Se(IV) can be in the form of selenous acid and/or hydrogen selenite ion whereas it can be in the form of hydrogen selenite and/or selenite ions at basic pH values. Se(VI) is mainly in selenate form for pH above 3. These diagrams will be essential in the following paragraphs for discussing experimental rejections depending on the pH value considered. 2.2.2. Chitosan. Chitosan is a polysaccharide obtained by the deacetylation of chitin, which is a seafood waste product. Therefore, biodegradable and biocompatible properties of chitosan make it a relevant ecological substitute to treat effluents compared to synthetic polymers from petrochemical industries.38 The chitosan structure contains amino (−NH2) and acetamide (−NHCOCH3) groups as well as hydroxyl groups, which confer hydrophilic properties. Chitosan is characterized by its degree of acetylation DA39 (see Scheme 2). For DA < 50%, the molecule is considered as chitosan whereas it remains chitin if DA > 50%. In this study, the DA of chitosan was 15% since it represents a sound compromise between price and quality. Under acid conditions (pH < pKa), amine groups are protonated and chitosan behaves like a positively charged polyelectrolyte. Oppositely, in basic conditions, the lone pair of the nitrogen atom can lead to complexation of ions. The pKa value of chitosan is around 6.3, and its overall charge is positive

Table 1. Properties of the Three Membranes Used in This Study membranes

GH

supplier type configuration material of the active layer MWCO from provider (kDa) filtrating surface area (m2)

GK

Ceram 60

GE Power & Water organic flat-sheet

GE Power & Water organic flat-sheet

polyamide

polyamide

TAMI Industries ceramic tubular (3 channels) TiO2

2.5

3.5

8

14 × 10−3

14 × 10−3

22 × 10−3

10−5 m3 m−2 s−1. It should be noted that feed concentrations were kept constant by recycling permeate stream into the feed tank except during flow-rate measurements and sampling for concentration analyses. Filtration experiments were carried out at the maximum feed flow-rate that can be supplied for each cell to ensure that the concentration polarization phenomenon at the membrane vicinity is minimized. Indeed, it was shown in a previous study that polarization layer thickness is strongly decreased by increasing feed flow-rate37 and its contribution can be assumed to be almost negligible at high velocities. Before the experiments, membranes were first washed by an acido-basic cycle before being equilibrated by water filtration until permeability becomes constant. For each applied pressure, permeation flux (Jv) and observed rejections (Ri,obs) are calculated from the mass of permeate (mp) collected during a time Δt and the concentrations of retentate (Ci,r) and permeate (Ci,p) streams: mp Jv = ΔtρSm (1) R i ,obs = 1 −

Ci ,p Ci ,r

(2)

where ρ and Sm are the density and the membrane surface, respectively. Polymer-assisted ultrafiltration was investigated by adding a given amount of chitosan (provided by France Chitine, Orange, France) to the ionic solution before filtration.

Figure 1. Distribution diagrams of the various forms of Se(IV) (a) and Se(VI) (b). 10463


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Industrial & Engineering Chemistry Research Scheme 2. Synthesis of Chitosan by Deacetylation of Chitin

below this value and neutral if pH is beyond pKa.40 For this reason, chitosan is first dissolved in water at pH = 3 (adjusted by acetic acid) during several hours under stirring to ensure that macromolecular chains are uncoiled and that active sites are available for interactions with ions. Afterward, pH of the feed chitosan solution is adjusted to the desired value by adding NaOH or HCl. Finally, selenium salts are added to the chitosan solution, and the solution is stirred for 2 h before being filtered in the pilot plant. 2.3. Analyses. Concentrations of polymers (chitosan, PEG) were determined by total organic carbon measurements after an adequate dilution to fit the range of calibration. Concentrations of ions were assessed by ICP-AES iCAP 6500 model (ThermoFisher, Courtaboeuf, France) or ionic chromatography 883 Basic IC Plus (Metrohm, Courtaboeuf, France) depending on the form of selenium. Zeta potential (ζ) values were calculated from streaming current (Is) measurements and the coupling between streaming potential (Δφs) and system conductance (Gt) measurements. Potential and current were measured at various pressures ΔP with a zeta-meter ZETACAD (CAD Inst., France), and electric conductance was estimated from impedance measurements carried out with a SI 1260 electrochemical impedance analyzer (Solartron, Farborough). ζ-potential was calculated from both streaming potential and current using eq 3: Δφs ΔP

Gt = −

Is ε ε ⎛h L ⎞ = 0 r ⎜ c c ⎟ς ΔP η ⎝ l ⎠

Figure 2. Evolution of Se(VI) rejection with permeation flux with or without salt addition for the two organic membranes investigated {Desal GH and GK}.

membrane leads to lower rejection when salt is added to filtered solution. The decrease in rejection in the presence of salt can be attributed to lower electrostatic interactions between the negatively charged membrane and the Se(VI), which is mainly in the form of selenate ion SeO42− at this pH value. Throughout the rest of the paper, performances of organic membranes are all investigated from rejections obtained with the GK membrane. Indeed, it is better that rejection is as low as possible to analyze rejection evolution with experimental conditions. 3.1.1.2. Influence of Selenium Concentration and Salt Presence. The influence of Se(VI) concentration on rejection was investigated at 1.7 × 10−5 and 1.7 × 10−3 mol L−1, corresponding to usual concentrations that can be found in discharge and untreated effluents, respectively (Figure 3). Figure 3a shows that rejection is close to 95% irrespective of the concentration considered, which means that treating effluent by ultrafiltration as main treatment or purification step is similar. Rejections of selenium(IV) and selenium(VI) were also investigated in the presence of salt at 0.2 M. Curves are shown in Figure 3b. This figure highlights that selenium rejection is notably lower in the presence of NaCl. It can also be noticed that Se(VI) is more rejected than Se(IV) in the same conditions. It may sound surprising since these species are relatively similar in terms of both size and charge. However, it should be considered that Se(IV) is mainly in the form of hydrogen selenite ion form at natural pH whereas Se(VI) is in the form of selenate ion. Hydrogen selenite being a monovalent oxyanion, repulsive interactions with the negative membrane charge are therefore weaker compared to those generated by divalent selenate. These behaviors prove that electrostatic

(3)

where hc, Lc, and l denote the channel height, width, and length, ε0 is the vacuum permittivity, εr is the relative dielectric constant of the solution, and η is its dynamic viscosity. It should be stressed that measurements were performed for a single channel height. Zeta-potential values provided here are probably apparent ones because they do not take the possible contribution of the porous support to the streaming current into account.41−43 In any case, this impacts neither the isoelectric point nor the charge sign of the membrane.

3. RESULTS AND DISCUSSION 3.1. Study of Organic Membranes. 3.1.1. Ultrafiltration of Selenium Solutions. 3.1.1.1. Influence of Membrane Porosity. First, the influence of the membrane molecular weight cutoff (MWCO) on selenium(VI) rejection was investigated. These filtration experiments were conducted with two organic UF membranes (Desal GH and GK) at pH = 5.5 with (concentration of 0.2 M) and without the presence of NaCl (Figure 2). From this figure, it can be seen that fluxes provided by the GK membrane are noticeably higher than those provided by the GH membrane, probably due to its larger pore size. Concerning the rejection performances, it seems that the use of the GK 10464


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Figure 3. Evolution of selenium rejection with permeation flux at two Se(VI) concentrations (a) and for Se(IV) and Se(VI) in the presence of 0.2 mol L−1 of NaCl (b) {Membrane Desal GK}.

Figure 4. Evolution of (a) Se(VI) and (b) Se(IV) rejection with permeation flux for various NaCl concentrations {Membrane Desal GK}.

interactions play a key role in separation performances. The impact of ionic strength should thus be examined more specifically. 3.1.1.3. Influence of Ionic Strength on Selenium Rejection. The influence of NaCl on both Se(IV) and Se(VI) rejections was investigated at various salt concentrations in Figure 4. Figure 4a,b clearly shows that, the larger the quantity of added NaCl, the lower is the rejection, irrespective of the kind of selenium considered. It can be concluded that the increase of the ionic strength of the solution by addition of salt leads to a collapse of the electrical double layer (EDL). Indeed, the increase in ionic strength compresses the double layer on the pore surface, which leads to a decrease of the ratio of Debye length to membrane pore radius and thus to a lower impact of electrostatic interactions on rejection at the pore entry.44 In order to assess this phenomenon, Figure 5 depicts the rejection evolution of Se(IV) and Se(VI) with NaCl concentration at a given flux (4.5 × 10−5 m3 m−2 s−1). Due to the progressive collapse of the EDL, rejection decreases with adding salt down to a minimum value that corresponds to negligible repulsions. From experiments conducted with usual

Figure 5. Evolution of Se(IV) and Se(VI) rejections with NaCl concentration at a given permeation flux of 4.5 × 10−5 m3 m−2 s−1 {Membrane Desal GK}.

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size considered: ionic, hydrated, or Stokes radius). It is therefore questionable that the selenium rejection does not decrease toward zero at high salinity. This means that the pore size may have an influence on electrostatic interactions even if steric exclusion is negligible. Indeed, physical models usually consider that Donnan exclusion is governed by the charge inside pores and not by that at the membrane surface.48 In this case, the pore size can act on electrostatic interactions if its value is in the same order of magnitude as the length of electrostatic interactions. Consequently, a smaller pore size leads to stronger electric exclusion at the pore entry. This physical mechanism can also explain why two membranes that only differ in terms of MWCO (Desal GH and GK) can lead to different rejection performances in the same conditions (see Section 3.1.1.1). In any case, the link between steric and electrostatic interactions is probably complicated and this concern could perhaps be explored further from mixture filtration. Indeed, it is widely agreed that the filtration of ionic mixtures provides more information than the filtration of single salt solutions since the various exclusion phenomena have a different influence on separation selectivity.49 For this reason, a mixture containing 0.85 × 10−3 mol L−1 of both Se (IV) and Se(VI) was filtered at pH = 1.5 for which Se(VI) is mainly in HSeO4− form whereas Se(IV) is mainly in the neutral form of H2SeO3 (Figure 7).

salts, it is usually considered that interactions become negligible for concentrations close to 0.2 or 0.4 mol L−1.45 However, in the case of selenate and hydrogen selenite, it can be noted that electrostatic interactions still act even for concentrations higher than 1 mol L−1. The decrease of rejection with increasing NaCl concentration seems slightly more pronounced for Se(IV) than for Se(VI). This trend may be assigned to the different valence of oxyanions. Indeed, divalent anions such as selenate ion exhibit stronger interaction with membrane charge compared with monovalent anions such as hydrogen selenite. Hence, the amount of NaCl required to screen electrostatic interactions is higher with divalent ions. 3.1.1.4. Study of Steric Hindrance. To understand the mechanisms governing selenium rejection, it is crucial that the membrane pore radius is known. For this purpose, filtration of a neutral solute (namely, polyethylene glycol with a MW of 1000 g mol−1) was implemented, and the rejection curve was fitted with a hydrodynamic model (eq 4) by adjusting the value of the membrane mean pore radius as it is usually done.46,47 Ri = 1 −

ϕiK i , c

⎡ ⎛ ⎞⎤ K (ΔP − Δπ ) 1 − ⎢(1 − ϕiK i ,c)⎜exp − i8,cK D η rp2 ⎟⎥ ⎝ ⎠⎦ ∞ i ,d i , ⎣

(

)

(4)

where ϕi is the steric coefficient and Ki,c and Ki,d are the steric hindrance coefficients to convection and diffusion, respectively. By considering cylindrical pores, they can be calculated with eqs 5−7: 2 ⎛ ri ⎞ ϕi = (1 − λi) = ⎜⎜1 − ⎟⎟ rp ⎠ ⎝

(5)

K i ,d = 1 − 2.30λi + 1.154λi 2 + 0.224λi 3

(6)

K i ,c = (2 − ϕi)(1 + 0.054λi − 0.988λi 2 − 0.441λi 3)

(7)

2

The experimental PEG rejections and curve simulated with the best-fitted mean pore radius is given in Figure 6. The estimated pore radius is 1.15 nm, which means that the pore size is considerably larger than the ion size (regardless of Figure 7. Evolution of Se(IV) and Se(VI) rejections with permeation flux for a mixture containing 0.85 × 10−3 mol L−1 of each species at pH = 1.5 {Membrane Desal GK}.

Figure 7 shows that Se(VI) is more strongly retained than Se(IV) at pH = 1.5 due to its different form, the former being charged whereas the latter is uncharged. It is worth mentioning that the curve of Se(VI) corresponds to all selenate ions since it was not possible to dose the two forms (HSeO4− and SeO42−) separately. The curves provided in Figure 7 seem to confirm that steric exclusion is negligible and that rejection performances are mainly governed by electrostatic interactions. This is especially corroborated by the very low rejection of the neutral form of Se(IV) (H2SeO3) for which rejection can only be induced by steric exclusion. It should be mentioned that charged species are hydrated by a certain number of water molecules, but this hydration differs from that of uncharged species. Hence, hydrated HSeO4− or SeO42− ions are probably larger in size than uncharged H2SeO3. In theory, it is possible that size difference between H2SeO3 and

Figure 6. PEG rejection evolution with permeation flux obtained experimentally and simulated by a hydrodynamic model with a mean pore radius of 1.15 nm {Membrane Desal GK}. 10466


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Industrial & Engineering Chemistry Research hydrated HSeO4− ions may also play a role in rejection. Nevertheless, this contribution is probably negligible in this case since the size of membrane pores is sufficiently large to neglect steric exclusion, even for hydrated ions. Finally, it is worth noting that, even if the interactions with membrane charge are screened at the membrane surface, they may occur within the confined pores in which concentration is lower. This work should enrich discussion within the scientific community about the role played by pore size and ion concentration inside pores in ion rejection even if a firm conclusion cannot be drawn at this time. 3.1.1.5. Influence of the Solution pH. It is well-known that pH is a key parameter for membrane separation. For this reason, filtration of Se(VI) was carried out at various pH values. Rejection curves are provided in Figure 8.

Evolution of membrane charge with pH was investigated by streaming current Is and streaming potential Δφs. The values of apparent ζ-potential calculated from eq 3 and the HelmholtzSmoluchowski relation are provided in Figure 9.

Figure 9. Evolution of ζ-potential of the membrane Desal GK calculated from streaming potential and current with pH.

This figure clearly proves that membrane charge is strongly affected by the pH of the solution, and the sign is even switched within the pH range studied. Indeed, the isoelectric point of the membrane was found at 3.5, which means that, below this pH value, the membrane is positively charged and electrostatic interactions as well as rejection are therefore strongly impacted. It should also be stressed that both streaming current and streaming potential coupled with conductance measurements lead to the same ζ-potential value, which validates the reliability of the measurements. Additionally, the fact that values estimated by the classical Helmholtz-Smoluchowski (H-S) relation are much lower than those calculated with eq 3 shows that this relation is not applicable in the case of conducting materials such as porous membranes. Indeed, electrical conductance of the membrane porous body provides a kind of partial “short-cut” of streaming current, and calculations with H-S relation lead to underestimated values.42,52 Although the three previously mentioned mechanisms to explain rejection fall with pH occur simultaneously, the membrane positive charge at pH below 3.5 is probably the prime phenomenon that leads to the sharp decrease of selenium rejection at pH = 2.5. Indeed, negatively charged oxyanions are therefore no longer repelled by the membrane charge and rejection decreases. It is worth mentioning that change in pH value can potentially lead to a slight change in membrane mean pore size due to electrostatic repulsion between the pore walls. This mechanism was proposed in a previous paper53 to explain the decrease of polyethylene glycol rejection in the presence of mineral salts. However, even if the impact of such a phenomenon is noticeable for nanofiltration membranes (which exhibit tighter pores), it probably becomes negligible in the case of UF membranes for which steric exclusion is negligible. 3.1.2. Ultrafiltration Assisted by Chitosan Addition. Before investigating the benefit of chitosan addition on selenium rejection, solutions of pure chitosan containing 20 mol of monomer units (which corresponds to 12 mol of units per mole of Se) were filtered to ensure that the polymer is totally

Figure 8. Se(VI) rejection evolution with permeation flux at various pH values {Membrane Desal GK}.

Variation of pH does not appear to have any influence on selenium rejection for values above 2.5, whereas rejection starts decreasing at pH = 2.5 and drastically falls at pH = 1.5. This trend can be explained by three different mechanisms: • Addition of hydrochloric acid for pH adjustment necessarily increases the ionic strength. For instance, concentration of hydronium and chloride ions is 3 × 10−2 mol L−1 at pH = 1.5, but such a concentration should cause a weaker rejection decrease in the light of what has been observed with an equivalent NaCl addition (see Figure 5). • As it is highlighted in Section 2.2.1, the form of Se(VI) varies depending on the pH value. At pH = 1.5, 70% of Se(VI) is in the monovalent form of hydrogen selenate (HSeO4−) and 30% is in the divalent form of selenate (SeO42−). Conversely, at pH ≥ 2.5, the divalent selenate form is largely predominant. Electrostatic interactions are lessened with monovalent ions, and the rejection is therefore lowered at very low pH values. • Finally, it should be regarded that membrane charge also varies with pH value, which thus leads to modification of electrostatic interactions.50 Indeed, it is acknowledged that polyamide membranes are differently charged depending on the solution pH.51 Hence, discussion requires an in-depth study of membrane charge properties in this pH range. 10467


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Additionally, flux was found to lessen by 30−50% due to the osmotic pressure increase induced by chitosan. This observation does not allow a high degree of optimizm about the use of chitosan to enhance oxyanions removal performances of UF membranes. Nonetheless, the low MWCO of the organic membrane used in this study is perhaps not relevant to conclude about the reliability of such a procedure. For this reason, the same study with a ceramic membrane having larger MWCO and pore size has been implemented. 3.2. Study of the Ceramic Membrane. First, rejections of single selenium and single chitosan are examined in Figure 12

rejected by the membrane. The corresponding curves are depicted in Figure 10.

Figure 10. Evolution of chitosan rejection with permeation flux for three pH values {Membrane Desal GK}.

The extremely high rejections of chitosan for pH = 1.5 and 3.0 show that selenium rejection could be highly enhanced by the potential interaction with chitosan. For pH = 4.5, it is assumed that all the amine groups are still protonated and the slight decrease of rejection can probably be attributed to the change in the sign of membrane charge, as it is highlighted in Figure 9. Selenium rejections by UF membrane are compared with those obtained after a preliminary step of chitosan addition in Figure 11. Unfortunately, Figure 11 shows that preliminary

Figure 12. Evolution of chitosan and selenium rejection with permeation flux for single solute solutions at pH = 4 {Membrane Ceram 60}.

for comparison with organic membrane before filtrating mixed solutions. Selenium rejections were similar irrespective of their concentration, but they were substantially reduced compared with those obtained with the organic membrane at similar pH (pH = 4.0). Oppositely, chitosan remains hugely retained by the membrane despite its larger pore size. The impact of chitosan addition on selenium is finally analyzed in Figure 13a,b, which shows rejection of 1.7 × 10−3 mol L−1 of Se(IV) and Se(VI) with and without chitosan. From these figures, it is obvious that chitosan addition before filtration contributes substantially to raising rejection of selenium, the form of which is monovalent (HSeO3−) or divalent (SeO42−). The addition of 120 mol of monomer units per mole of Se even allows one to obtain rejection close to 95%. It is worth mentioning that, even if chitosan addition sharply reduces permeation flux, flux values obtained with the ceramic membrane are still higher than those of organic membranes, although the applied pressures are at least three times lower (i.e., 0−9 bar instead of 0−25 bar). The strong enhancement of selenium rejection after chitosan addition proves that chitosan can interact with ions to increase their effective size and therefore their rejection. Besides, this enhancement occurs even with negative ions for which complexation with nitrogen atoms does not occur. Hence, chitosan addition before filtration appears to be a relevant option to increase selenium removal. However, this solution is only of interest if membrane pores are sufficiently large to maintain substantial flux. Considering the pKa of chitosan (almost 6.3), it is expected that this way should be efficient at least up to pH = 5.3 for which more than 90% of the amino groups are in their positive −NH3+ form. Finally, it is

Figure 11. Evolution of Se(VI) rejection with permeation flux with (12 units/Se(VI)) or without adding chitosan for three pH values {Membrane Desal GK}.

addition of chitosan does not perceptibly enhance rejection of selenium. Indeed, for pH ≥ 3.0, rejections are close to 90% and the enhancement due to polymer addition is not observable in these conditions. In the case of pH = 1.5, rejections without chitosan are lower but the increase in ionic strength, induced by HCl addition, probably screens the electrostatic interactions between oxyanions and chitosan. Consequently, rejection enhancement does not occur at such a pH value. 10468


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Figure 13. Evolution of (a) Se(IV) and (b) Se(VI) rejections with and without chitosan addition at pH = 4 {Membrane Ceram 60}.

(Nirhofex program, Innovative techniques for wastewater treatment) for their financial support.

worth noting that real effluents often contain sulfate or phosphate at high concentration, and it is probable that such ions have a negative impact on performances due to competitive interactions with polymers.

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4. CONCLUSION This study has experimentally demonstrated that organic membranes allow removal of selenium irrespective of its form due to electrostatic interactions even if pore size is considerably larger than oxyanions. It was especially highlighted that ionic strength has a very strong influence on performances since it screens electrostatic interactions between ions and membrane charge. The pH value was also proven to have an overriding impact on rejection performances since it governs the form of the selenium species and the membrane charge. It was found that chitosan addition has a negligible impact on selenium removal by organic ultrafiltration membranes with low MWCO. On the other hand, this addition has a noticeable positive impact during filtration of selenium by ceramic membranes with higher MWCO. This improvement of selenium rejection has met with great success although mechanisms governing interactions are different from those leading to complexation of metal cations. Consequently, polymer-enhanced ultrafiltration appears as a potential option to remove anionic pollutants from wastewaters, provided that ion content allows electrostatic interactions to occur.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +33 3 63 08 25 81. ORCID

Sébastien Déon: 0000-0003-4775-5964 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the region of Bourgogne Franche-Comté (Grant number: 2016Y-04563), the French embassy in Mauritania, and the Agence de l’Eau and FEDER 10469


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