Bismuth recovery from acidic solutions using Cyphos IL-101 immobilized in a composite biopolymer matrix

water research 42 (2008) 4019–4031

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Bismuth recovery from acidic solutions using Cyphos IL-101 immobilized in a composite biopolymer matrix K. Camposa, R. Domingoa,b, T. Vincenta, M. Ruizb, A.M. Sastrec, E. Guibala,* a

Ecole des Mines d’Ale`s, Laboratoire Ge´nie de l’Environnement Industriel, 6, avenue de Clavie`res, F-30319 Ale`s cedex, France Universitat Politecnica de Catalunya, Department of Chemical Engineering, E.U.P.V.G., av. Victor Balaguer, s/n, E-08800 Vilanova i la Geltru, Spain c Universitat Politecnica de Catalunya, Department of Chemical Engineering, E.T.S.E.I.B., Diagonal 647, E-08028 Barcelona, Spain b

article info

abstract

Article history:

Impregnated resins prepared by the immobilization of an ionic liquid (IL, Cyphos IL-101,

Received 9 May 2008

tetradecyl(trihexyl)phosphonium chloride) into a composite biopolymer matrix (made of

Received in revised form

gelatin and alginate) have been tested for recovery of Bi(III) from acidic solutions. The

15 July 2008

concentration of HCl slightly influenced Bi(III) sorption capacity. Bismuth(III) sorption

Accepted 16 July 2008

capacity increased with IL content in the resin but non-linearly. Maximum sorption

Available online 25 July 2008

capacity reached 110–130 mg Bi g1 in 1 M HCl solutions. The mechanism involved in Bi recovery was probably an ion exchange mechanism, though it was not possible to establish

Keywords:

the stoichiometric exchange ratio between BiCl 4 and IL. Sorption kinetics were investi-

Bismuth

gated through the evaluation of a series of parameters: metal concentration, sorbent

Ionic liquid

dosage, type and size of sorbent particles and agitation speed. In order to reinforce the

Cyphos IL-101

stability of the resin particles, the IL-encapsulated gels were dried; this may cause

Impregnated resin

a reduction in the porosity of the resin particle and then diffusion limitations. The intra-

Biopolymer encapsulation

particle diffusion coefficients were evaluated using the Crank’s equation. Additionally, the

Bismuth

pseudo-first-order and pseudo-second-order equations were systematically tested on

Sorption isotherms

sorption kinetics. Metal can be desorbed from loaded resins using either citric acid or KI/

Uptake kinetics

HCl solutions. The sorbent could be recycled for at least three sorption/desorption cycles. ª 2008 Elsevier Ltd. All rights reserved.

Desorption

1.

Introduction

The strong demand on nonrenewable resources such as strategic and precious metals has drawn in the past decade the attention of research community toward the development of alternatives processes for the recovery of these metals from low-level ores and from industrial wastes, including the recycling of end-of-life products (Chmielewski et al., 1997; Saleh et al., 2001; Sheng and Etsell, 2007). The increases of the cost of base metals and the stringent regulations concerning the recycling of materials have extended this interest for the

recovery of metals such as copper. Waste materials (from electronic materials for example) have become an interesting resource for multi-metal recovery (Hall and Williams, 2007; Sheng and Etsell, 2007). In the past only platinum group metals were considered for valorization of industrial wastes, nowadays a considerable attention is also paid to some base metals (Yokoyama and Iji, 1998; Kinoshita et al., 2003). In this case the challenge is the separation of the base metals in order to make efficient and competitive the valorization of these wastes (Moawed et al., 2007). Bismuth is frequently found in the materials issued from the dismantling of printed circuit

* Corresponding author. Tel.: þ33 (0)466782734; fax: þ33 (0)466782701. E-mail address: [email protected] (E. Guibal). 0043-1354/$ – see front matter ª 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2008.07.024

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water research 42 (2008) 4019–4031

boards (Hall and Williams, 2007), the purification of copper refinery electrolytes (Navarro and Alguacil, 2002; Giannopoulou and Panias, 2007), and more generally in polymetallic wastes (Giannopoulou and Panias, 2007), where it is associated with copper among other metals. It is thus interesting to consider the possibility to develop resins that could separate Bi from Cu in acidic solutions, proceeding from the acidic leaching of waste materials or the copper refinery electrolyte. Due to the low toxicity of bismuth, the literature focusing on Bi recovery is not abundant compared to other base metals such as Hg, Cd and so on. Solvent extraction has been tested for Bi recovery from various types of aqueous solutions, using essentially Cyanex liquid extractants (Argekar and Shetty, 1995; Sarkar and Dhadke, 1999; Iyer and Dhadke, 2003). Some resins have been developed for Bi recovery, essentially for analytical purpose, more rarely for metal recovery. For example, Arpadjan et al. (1997) immobilized dithiocarbamate on polyurethane foam as a solid phase extractant for a series of metals (including Bi and As, Hg, Se, Sn and Sb), prior to analysis by ICP-AES analysis (inductively coupled atomic emission spectrometry). Tokman et al. (2003) proposed silica gel grafted with aminopropyltriethoxysilane as another solid phase extractant. Suvardhan et al. (2006) used similar systems with resins prepared by immobilization of 2-propylpiperidine1-carbodithioate on a resin prior to desorption and flame atomic absorption spectrometry. Focusing on the recovery/ concentration of Bi traces (in groundwater and seawater) prior to analysis, these resins have been essentially tested for Bi sorption at near neutral pH or in slightly acidic solutions. This means that the comparison of experimental data with literature data is difficult. Another resin was synthesized by grafting of salen chelate onto Amberlite XAD-4 resin for Bi(III) sorption in slightly acidic solutions (Kim et al., 2005). Biosorbents or sorbents derived from materials of biological origin were also tested for pre-concentration of bismuth (Bakircioglu et al., 2003; Wang et al., 2006; Oshita et al., 2007a,b). Sorbents can be produced by grafting specific reactive groups onto conventional resin backbone or by impregnation of porous support with a suitable liquid extractant or chelating dye (Sutton et al., 1997; Reyes-Aguilera et al., 2008). A new generation of impregnated resins have been recently developed consisting in the immobilization of liquid extractants into polymer capsules (Kamio et al., 2002), or biopolymer capsules (Fournel et al., 2001; Mimura et al., 2001a; Mimura et al., 2001b; Mimura et al., 2002; Mimura et al., 2004; Ngomsik et al., 2006; Outokesh et al., 2006; Guibal et al., in press; Vincent et al., 2008a,b). Liquid extractants such as Cyanex organophosphinic products have been frequently used (Fournel et al., 2001; Mimura et al., 2001a,b, 2002; Ngomsik et al., 2006; Outokesh et al., 2006); however, some ionic liquids (IL) bearing phosphonium groups have attracted attention for their capability to extract a variety of organic compounds (Martak and Schlosser, 2006, 2007), and metal ions (Regel-Rosocka et al., 2006; Cieszyn´ska et al., 2007). It was also used for the biodegradation of phenol (Baumann et al., 2005) and for homogeneous catalysis (Wong et al., 2006). Cyphos IL-101 (tetradecyl(trihexyl)phosphonium chloride) is one of the IL recently synthesized at industrial scale by Cytec (Bradaric et al., 2003; Del Sesto et al., 2005). Cyphos IL-101 was immobilized in composite biopolymer capsules for the binding of

Pd(II) (Vincent et al., 2008a), Pt(IV) (Vincent et al., 2008b), Hg(II) (Guibal et al., in press), and Au(III) (Campos et al., in press). This IL was immobilized by a procedure involving (a) the stabilization of an emulsion (prepared by mixing the IL with gelatin); (b) the mixing of the emulsion with alginate solution, followed by (c) the ionotropic gelation of the mixture in a calcium chloride solution (extruded as small drops). The spherical resins were tested for Bi(III) sorption from HCl solutions. The influence of a series of parameters on Bi(III) sorption was investigated at equilibrium, for example: HCl concentration, presence of competitor ions (mineral anions and metal ions), IL content in the resins. The sorption isotherms have been established at 1 M HCl concentrations and the sorption kinetics were investigated changing experimental conditions (sorbent dosage, IL content, metal concentration, agitation speed). Finally the desorption of the resin was carried out with the double objective of Bi(III) recovery and resin recycling.

2.

Materials and methods

2.1.

Materials

The Cyphos IL 101 was kindly supplied by Cytec (Canada). This ionic liquid is a phosphonium salt (tetradecyl(trihexyl) phosphonium chloride, C.A.S. Number: [258864-54-9]). Alginate and gelatin were supplied by Acros (Switzerland/ France). Other reagents (BiCl3, competitor metals (under the form of chloride salts), NaCl, NaNO3, mineral acids) were supplied as reagent grade products by Fluka AG (Switzerland).

2.2.

Synthesis of Cyphos-impregnated capsules

The synthesis of the Cyphos-impregnated capsules was performed in 2 steps: (a) Cyphos IL 101 (6.25X g, previously mixed with 1.25X g of a 10 M solution of NaOH) was mixed with 25 g of a 20% w/w aqueous solution of gelatin. An amount of 475–7.5X g of alginate sodium solution (2% w/w) was then added to the gelatin–ionic liquid solution and mixed under ultrasonic treatment until a slightly viscous white solution was obtained, with X ¼ 1, 2 or 3, respectively, for C1, C2 and C3 series. (b) The composite solution was then extruded through a nozzle (internal diameter 0.6 mm) into an ionotropic gelling solution (CaCl2, 6% in water, w/w). The beads were maintained in the coagulation bath overnight before being rinsed with 0.1 M HCl solution. They were stored in 0.1 M HCl, in order to prevent possible degradation of the composite biopolymer matrix. The proportion of ionic liquid was modified to prepare resins with different IL content, changing the amount of gelatin and alginate according to the appropriate percentage required. Table 1 gives the main characteristics of the resins prepared by this procedure. The uncontrolled drying of biopolymers causes the irreversible collapse of the structure. Several experiments have shown that this modification of the biopolymer gel (chitosan, alginate and so on) induces strong reduction in

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water research 42 (2008) 4019–4031

Table 1 – Characteristics of resins Resin C1S C1L C2S C2L C3L

Diameter (wet, mm)

Diameter (dry, mm)

Cyphos content (%, d.w.)

Cyphos content (mmol/g d.w.)

Water contenta

Water regaina

1.00  0.04 2.33  0.02 0.85  0.06 2.67  0.03 2.58  0.02

435  80 988  74 472  75 1313  48 1600  51

35.9  1.3 31.4  0.6 50.3  0.9 44.6  0.9 57.3  1.8

0.692 0.604 0.968 0.860 1.103

84/81 90/81 76/ND 88/82 86/80

11/43 15/51 15/40 12/29 ND/31

ND: not determined. a In percentage (weight), first number shows data for oven drying; second number shows data for air drying.

kinetic properties. The size of the pores and the specific surface is considerably reduced after uncontrolled drying. Controlling the drying step (using for example solvent exchange – alcohol instead of water – or drying under supercritical CO2 conditions) is not appropriate for the present system since it would cause loss of the IL. Freeze-drying is an alternative but previous investigation has shown that the resulting gels were not mechanically stable. Fig. 1 shows the SEM photographs of a resin sample, irregularly circular shapes are observed due to the drying step involving a little deformation of the beads. The characterization of the resins was performed according to the following procedures. The water content of the resin was determined by weight loss at 60  C for 24 h in an oven. Water regain was obtained by weight measurement of dry beads that were wetted in 0.1 M HCl solution overnight. The size of particles was determined by measurement of the beads on photographs and measurement from retro-projection of the beads. Ionic liquid content was determined by phosphorus analysis using ICP-AES (JY 2000, Jobin-Yvon, Longjumeau, France) after chemical degradation of the polymer matrix. A known amount (50–100 mg depending on IL content) was mixed with 2 mL of sulfuric acid and heated till complete mineralization occurred (destruction of polymer capsule), after cooling, 1 mL of hydrogen peroxide was added drop by

Fig. 1 – SEM photograph of C2S resin (dry state).

drop. The mixture was heated until bubbles disappeared and discoloration was complete. After cooling and volumetric adjustment, the P content was measured to evaluate the molar IL concentration in the resin.

2.3.

Sorption experiments

For the evaluation of equilibrium performance a volume of acid solution (80 mL in most cases) containing the appropriate concentration of Bi was mixed with a known amount of resin (i.e., 20 mg). The slurry was agitated for 96 h to be sure to reach equilibrium. The residual concentration of Bi was measured by ICP-AES analysis after filtration. The sorption capacity (q, mg Bi g1 or mmol Bi g1) was determined by the mass balance equation: q ¼ V(Co  Ceq)/m, where C0 and Ceq are the initial and equilibrium concentrations (mg Bi L1 or mmol Bi L1) of Bi, respectively; V is the volume of solution (L) and m the mass of resin (g). The distribution coefficient, D, is defined as the ratio of sorption capacity to residual metal concentration D ¼ q/Ceq, L g1. To investigate the influence of competitor ions, sodium sulfate and sodium nitrate were added to the initial solution as salts, and CuCl2, NiCl2 and ZnCl2 salts were added to the solution to examine the effect of competitor metals. Sorption isotherms were obtained by mixing fixed amounts of resin (i.e., 20 mg) with fixed volumes (i.e., 80 mL) of HCl solutions of increasing concentrations (0.1 M, 1 M and 2 M), containing Bi concentrations in the range 10–200 mg Bi L1. At equilibrium (i.e., after 96 h of contact), the residual concentration was used to calculate the sorption capacity. Uptake kinetics were investigated by adding, under agitation, a known amount of resin (i.e., 250 mg, dry weight, in most cases except when investigating the impact of sorbent dosage) to 1 L of Bi (C0: 10–70 mg Hg L1) solutions prepared in 1 M HCl solutions. Low sorbent dosage has been selected in order to detect the contribution of intraparticle diffusion resistance, which has been proved to be a limiting step in these materials (Guibal and Vincent, 2006). Samples were collected, filtered and analyzed at fixed times. Desorption was performed by mixing loaded sorbent (about 20 mg, with known concentration of bismuth) with a known volume of eluent (citric acid, potassium iodide in acidic media, sodium sulfate, etc.) for 2 h. The objective of this step was only to demonstrate the reversibility of the sorption process. The volume of the eluent was the same as the volume used for the sorption step. For practical applications

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a concentration effect is expected and thus it would be necessary to substantially decrease the volume of eluent. The amount of bismuth released was used for calculating Bi desorption yield. For the study of resin recycling, a rinsing step was carried out, using a 0.1 M HCl solution, before using the resin for the next adsorption step. The amount of Bi bound to the resin and subsequently desorbed was obtained by the mass balance equation for the sorption and elution steps, respectively. Comparison of these values enabled the desorption yield (%) to be calculated.

2.4.

Characterization of resins by SEM-EDAX

Element distribution (especially bismuth and phosphorus, as the tracer of Cyphos IL-101) in the beads was investigated by Environmental Scanning Electron Microscopy (ESEM) Quanta FEG 200, equipped with an OXFORD Inca 350 Energy Dispersive X-ray microanalysis (EDX) system. The system can be used to acquire qualitative or quantitative spot analyses and qualitative or quantitative X-ray elemental maps and line scans. This ESEM allows samples to be analyzed at pressures and humidity which approach normal laboratory conditions and avoids experimental artifact. More specifically, this is possible to analyze the samples at much higher pressure than with conventional SEM. Dry samples of free or saturated extractant-encapsulated beads were embedded in a synthetic resin made of Araldite D glue and HY 956 lubricant. After a drying step of 24 h at 60  C, the embedded beads were cut and sections were polished with a series of abrasive GEOPOL disks

of decreasing grain size (2400 mm, 1000 mm, 600 mm and 320 mm). The sections were rinsed with water between each pair of abrasive disks after the polishing step. Finally the sections were polished with a fine tissue using a DP emulsion lubricant (DP lubricant) and diamond spray HQ (successive sizes 6 mm, 3 mm and 1 mm). The polishing step, though operated under water rinsing, may involve dispersion of the elements; for this reason complementary analysis was performed using another procedure consisting in the freezing of the resin in liquid nitrogen followed by the mechanical breaking of the spherical particles. Though the cross-section is less regular than with thin-slice cutter this procedure prevents the section to be contaminated by element dispersion. Actually the same trends were observed for each procedure.

3.

Results and discussion

3.1.

Characterization of IL-immobilized resins

SEM-EDAX was used for the characterization of both textural and chemical properties of the IL-immobilized resins before and after metal binding. Fig. 2 shows the cross-section of C3L and C2S resins. These particles are characterized by the presence of large vesicles homogeneously distributed in the whole mass of the resin. The size of the vesicles varies in the range 5–12 mm; this is analogous to macroporosity. However, these vesicles are not interconnected; this means that

Fig. 2 – SEM photograph of cross-sections of C3L and C2S resins (dry state).

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diffusion properties will not be favorable, despite these large and numerous vesicles. After Bi(III) binding the vesicles appear filled (See Supplementary data, AM1): metal binding caused a decrease in the apparent porosity of the resin. This appears clearly with C3L resin, while for C2S material the difference was less marked. SEM-EDAX analysis showed that the distribution of P element (representative of IL distribution) was homogeneous throughout the resin particles, regardless of the type of resin (not shown). Fig. 3 shows the distribution of P and Bi along a section of C3L and C2S resins after metal sorption. The IL was homogeneously distributed in the whole mass of the resin as shown by the distribution of P element. The distribution of Bi was significantly different for C3L and C2S resins: while for the smallest particles (with intermediary IL content) the Bi profile was almost identical along the cross-section, in the case of the largest particles, Bi was not present in the central part of resin particle. The resin was not saturated under selected experimental conditions; complementary experiment performed under saturation conditions demonstrated that this center part can be also loaded (not shown). However, this figure shows that the diffusion front is very limited (moving boundary rather than concentration gradient) and that the sorption can be described by a shrinking core mechanism. This result can be also linked to the observations made by SEM analysis: the sorption of Bi is followed by the

filling of the vesicles (See Supplementary data, AM1). This may contribute to controlling the diffusion rate of metal species in the non-connected ‘‘porosity’’.

3.2.

Influence of HCl concentration on Bi(III) sorption

The impact of HCl concentration on Bi(III) sorption was investigated using C3L resin (Fig. 4). Sorption capacity slightly decreased with increasing HCl concentration: in the range 0.3– 1 M the sorption capacity remained around 120–140 mg Bi g1 (i.e., 0.57–0.67 mmol Bi g1), while at 5 M the value tended to decrease to 107 mg Bi g1 (i.e., 0.51 mmol Bi g1). These values are significantly higher than those found by Wang et al. (2006) using bayberry tannin immobilized on collagen fibers (i.e., 0.35 mmol Bi g1 in acidic solutions) or by Kim et al. (2005) who used XAD-4-salen resin and found a sorption capacity close to 0.19 mmol Bi g1 at pH 5. In HCl solutions, Bi(III) is present under the form of chloro species. The distribution of Bi species was calculated using the Hydra software developed by Puigdomenech (2002), based on the constants proposed by Martell and Smith (1976): 3þ

Bi



3i

þ iCl 4 BiCli

with log Ki ¼ 3:6; 5:5; 7:1 and 8:1;

for i ¼ 1  4; respectively Under selected experimental conditions, BiCl 4 is systematically the predominating species. However, the percentage of

Fig. 3 – SEM-EDAX analysis of cross-sections of C3L and C2S resins after Bi(III) sorption – distribution of P and Bi element.

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water research 42 (2008) 4019–4031

a

log (HCl) (M) -1

-0.5

0

0.5

0.3

120

0.2 90 60 0.1

log Kd (L/g)

C1L 60

C1S

30

q

C2S C3L

0

log Kd

0

50

100

0

1

2

3

4

5

6

(HCl) (M) Fig. 4 – Influence of HCl concentration on Bi(III) sorption capacity and distribution ratio (Kd) using C3L resin (C0: 100 mg Bi LL1; sorbent dosage (SD): 250 mg LL1).

this anionic species increases with HCl concentration from 73% in 0.3 M HCl solutions to above 97% in solutions of concentration higher than 2 M. The mechanism involved in Bi(III) binding is probably based on ion exchange properties: BiCl 4 anions are bound to R3R0 Pþ cations by exchange of chloride anions. Cieszyn´ska et al. (2007) investigated the extraction of Pd(II) from HCl solutions using Cyphos IL-101 diluted in toluene. The extraction mechanism varied with HCl concentration. At low HCl concentration (i.e., 0.1 M HCl) the extracted complex was [R3R0 Pþ][PdCl 3 ], while at high HCl concentration (i.e., 3 M HCl) palladium was extracted under the form [R3R0 Pþ]2[PdCl2 4 ]. The change in extracted species was oriented by the variation in metal speciation (species predominance and co-existence of anionic species). Though Bi(III) may be present under the form of cationic, neutral and anionic species, BiCl 4 is the only anionic species and the sorption mechanism is thus less sensitive to metal speciation. This sorbent was also tested for the recovery of Pt(IV) (Vincent et al., 2008b), Pd(II) (Vincent et al., 2008a), Hg(II) (Guibal et al., in press), and Au(III) (Campos et al., in press). All these metal ions readily form chloroanionic species that were efficiently bound to IL-encapsulated resins according similar reaction mechanisms. Fig. 4 also shows the logarithmic plot of the distribution ratio Kd (q/Ceq, L g1) as a function of HCl concentration. The distribution ratio was hardly affected by HCl concentration; indeed Kd was close to 1.62  0.16 L g1 in the concentration range 0.3–5 M. This result confirms that the resin was not sensitive to HCl concentration in acidic region. Salen XAD-7 resin, chitosan derivatives and bayberry tannin immobilized on collagen fibers showed poor effect of pH on Bi(III) sorption (for lightly acidic solutions) (Kim et al., 2005; Wang et al., 2006; Oshita et al., 2007a,b).

Sorption isotherms

The influence of IL content in the resin was tested in 1 M HCl solutions by comparison of the sorption isotherms for the different lots of resins (Fig. 5a). The shape of the sorption

150

200

1

1.2

Ceq (mg Bi/L)

0

0

3.3.

90

C2L

b

1 0.8

qm (mmol Bi/g)

q (mg Bi/g)

120

q (mg Bi/g)

150

30

150

1

qm = 0.82 IL - 0.08 R2 = 0.963

0.6 0.4 0.2 0 0.4

0.6

0.8

IL content (mmol IL/g) Fig. 5 – Influence of IL content on Bi(III) sorption (a) sorption isotherms; (b) correlation between IL content and maximum sorption capacity (1 M HCl solutions).

isotherms was systematically characterized by a steep initial slope followed by a saturation plateau reached for low residual Bi concentration. The shape of the curves suggests that the isotherm could be described by the Langmuir equation. The parameters used for plotting the modeled curves are summarized in Table 2, the modeled curves are shown on Fig. 5a (continuous lines). Two distinct trends can be observed comparing the five isotherms. For small particles (C1S and C2S) the initial slope was steeper than those of large resin particles (C1L, C2L and C3L); this is confirmed by the distribution ratio (qm  b), which was almost doubled compared to the values obtained for large resin particles. The small size resins were characterized by a greater affinity for Bi(III). The maximum sorption capacity significantly varied with IL content. Increasing IL content generally increased the maximum sorption capacity except for C3L resins. Indeed, for the highest IL content (and for large particles, i.e., C3L) the maximum sorption capacity tended to level off (Fig. 5b). The maximum sorption capacity linearly increased with IL content (for IL content lower than 1 mmol g1) and the slope of this linear curve was close to 0.82. This means that the stoichiometric ratio between IL and Bi(III) was close to 1.2. The molar ratio IL/Bi(III) appearing in Table 2 was in the range 1.3–1.6. This is consistent with the preceding conclusion. Actually, the molar ratio IL/Bi(III) was higher than the 1:1 expected value, based on the suggested reaction occurring between R3R0 Pþ and BiCl 4 . This probably means that some of the reactive groups

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Table 2 – Sorption isotherms – model parameters for the Langmuir equation (influence of resin type) Resin

IL (mmol g1)

qm (mg Bi g1)

qm (mmol Bi g1)

IL/qm (mol/mol)

b (L mg1)

qmb (L g1)

R2

0.604 0.692 0.860 0.968 1.103

84.7 106.3 123.2 152.7 141.3

0.405 0.509 0.589 0.731 0.676

1.49 1.36 1.46 1.32 1.63

0.46 0.61 0.20 0.38 0.46

39.0 64.8 24.6 58.0 65.0

0.987 0.997 0.995 0.996 0.995

C1L C1S C2L C2S C3L

may be not accessible or available, perhaps due to steric hindrance. This could explain the decrease in sorption capacity for C3L resin (higher IL content and larger size). This is also consistent with the SEM observations that showed that vesicles were filled after metal binding. In the case of Pt(IV) binding using similar resin the molar ratio at saturation of the resin (in the range 1.5–1.8) was less than the 2:1 expected value; in this case the IL was expected to react with PtCl2 6 . By analogy with results presented by Cieszyn´ska et al. (2007) on Pd(II) liquid–liquid extraction using Cyphos IL-101 it is possible to suggest that another mechanisms involves a 1:1 interaction of that the resin bound to ion pairs (associateing hexachloroplatinate species and one proton). However, in the case of Bi(III) BiCl 4 is the unique species to be present in the solution (additionally to the IL excess at resin saturation, compared to stoichiometric ratio) confirms that all the reactive groups are not accessible.

3.4.

Uptake kinetics

The sorption kinetics may be controlled by various diffusion mechanisms: (a) bulk diffusion, (b) film diffusion, and (c) intraparticle diffusion. Three models were used for the description of kinetic profiles based on the pseudo-firstorder equation (the so-called Lagergren equation), on the pseudo-second-order equation described by Ho (2006), and on the intraparticle diffusion equation defined by Crank (1975). Pseudo-first-order equation:   dqt ¼ k1 qe  qt dt

(2)

(3)

After integration of Eq. (3): qt ¼

q2e k2 t 1 þ qe k2 t

tan qn ¼

with

Pseudo-second-order equation:  2 dqt ¼ k2 qe  qt dt

q(t) and qeq are the concentrations of the metal in the resin at time and equilibrium, respectively, r being the radius of resin particles. The fractional approach to equilibrium represents the ratio between the sorption capacity at time t and the sorption capacity at equilibrium. This is a measure of the velocity of the system for reaching the equilibrium. And qn are the non-zero roots of the equation: 3qn 3 þ aq2n

(6)

(1)

After integration of Eq. (1): qt ¼ ð1  exp½k1 tÞ qe

The modeling of sorption kinetics requires taking simultaneously into account film diffusion, intraparticle diffusion and equilibrium distribution (boundary condition at the interface) for an accurate simulation of experimental data. This approach requires a complete knowledge of sorbent characteristics (including homogeneity of the material) and requires extensive and complex numerical analysis. Simplifying hypotheses on the characteristics of the sorbent obviously induce discrepancy in the modeling and in a first approach we decided to separate film diffusion from intraparticle diffusion. The poor effect of agitation speed on the kinetic profiles indicated that this mechanism hardly contributed to mast transfer resistance. For this reason, film diffusion was neglected and the intraparticle diffusion coefficient (De, effective diffusivity, m2 s1) was determined using Crank’s equation, assuming the solid to be initially free of metal (Crank, 1975): De q2n t N 6aða þ 1Þexp X qðtÞ r2 ¼1 ¼ FATE (5) 2 a2 qeq 9 þ 9a þ q n n¼1

(4)

where qe is the amount of metal ion sorbed at equilibrium (mg Bi g1), qt is the amount of metal sorbed (mg Bi g1) at any time, t; k1 (min1) and k2 (g mg1 min1) are the pseudo-firstorder and pseudo-second-order rate constants. The parameters qe, k1 and k2 are pseudo-constants, depending on experimental conditions; they were obtained by non-linear regression analysis using the Math Package Mathematica.

qeq 1 ¼ VCo 1 þ a

(7)

The three kinetic models were compared for a given experimental series (but similar trends were systematically observed for other series); the results are presented in AM2 (Supplementary data). The intraparticle diffusion model fitted well with experimental and better than pseudo-second-order rate equation (and even better than the pseudo-first-order rate equation). Table 3 shows that the estimated error variance was systematically lower for pseudo-second-order rate equation than for pseudo-first-order rate equation. Additionally, it is noteworthy that the experimental sorption capacity at equilibrium was systematically overestimated by the pseudo-second-order equation model while it was underestimated by the pseudo-first-order equation model. The agitation speed is suspected to control the formation (and the thickness) of the film surrounding the particles (i.e., resistance to film diffusion). In order to verify the contribution

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Table 3 – Modeling of Bi sorption kinetics (pseudo-first-order equation and pseudo-second-order equation) – influence of agitation speed (v, rpm), metal concentration (Co, mg Bi LL1), and sorbent dosage (SD,g LL1), IL content (IL: mmol IL gL1) Model Pseudo-first-order Resin C3L C3L C3L C3L C3L C1L C1S C2L C2S

3

Pseudo-second-order

v

Co

SD

IL

qeq,exp

qe,model

k1  10

EV

qe,model

k2  105

EV

500 500 500 500 500 500 500 500 500

10 40 70 40 40 40 40 40 40

0.25 0.25 0.25 0.125 0.50 0.25 0.25 0.25 0.25

1.1 1.1 1.1 1.1 1.1 0.60 0.69 0.86 0.97

42.7 114.8 150.2 71.7 75.5 69.6 91.4 83.8 153.4

37.9 107.3 145.7 70.5 76.5 60.6 86.7 72.4 143.4

2.2 0.82 0.95 1.5 1.5 2.6 3.2 1.8 2.4

13.6 39.0 62.5 39.2 5.6 32.8 23.5 37.8 125.3

43.2 132.2 179.4 85.6 92.9 68.8 96.2 84.6 162.4

6.1 0.66 0.56 1.86 1.65 4.3 4.3 2.4 1.8

9.0 21.6 46.3 33.6 7.8 20.2 13.0 21.9 75.7

EV: estimated error variance; q: mg Bi g1; k1: min1; k2: g mg1 min1.

of film diffusion on the kinetic control, several kinetics were tested using identical experimental conditions (C2S resin type; metal concentration: 40 mg Bi L1; sorbent dosage: 250 mg L1; 1 M HCl concentration), except the agitation speed, which was varied from 180 rpm to 400 rpm. The kinetic profiles were superimposed (not shown, see in Supplementary data, AM3). This preliminary result showed that film diffusion was probably not the controlling step. This conclusion is confirmed by the modeling of sorption kinetics using the Homogeneous Diffusion Models (applied to Film diffusion and Intraparticle Diffusion) and the Shrinking Core Models (applied to Film Diffusion, Intraparticle Diffusion and Reaction Rate). In Supplementary data (AM4) an example of application of these models shows that the model involving intraparticle diffusion resistance as the main controlling parameter was the most appropriate for simulating the kinetic profile. The time required for reaching equilibrium strongly depends on experimental conditions as it will appear in the next sections. Two or three days of contact were necessary; this is long compared to other systems. However, the strict comparison of the results is made difficult by the differences in the experimental conditions. Herein the sorbent dosage was quite low compared to other systems in order to identify the controlling step: for example, Wang et al. (2006) using bayberry tannin immobilized on collagen fibers commented that 10–12 h were sufficient for reaching the equilibrium but they used a high sorbent dosage (i.e., 1 g L1, 2–5 times higher than the conditions selected in the present work).

1 Co: 10 mg/L

The impact of Bi(III) concentration on sorption kinetics was tested between 10 mg L1 and 70 mg L1 for C3L resin (Fig. 6). The rate parameter (k2, for pseudo-second-order equation) decreased when increasing metal concentration from 10 mg Bi L1 to 40 mg Bi L1, but it tended to level off above 40 mg Bi L1 (Table 3). Similar trends were observed with the rate parameter k1 (for the pseudo-first-order equation). In the case of Pt(IV) and Pd(II) binding using a similar resin the rate parameter k2 also decreased with increasing metal concentration and their values were about one order of magnitude greater than those obtained with Bi(III) (Vincent et al., 2008a,b). A reciprocal trend was observed when considering

Co: 40 mg/L

0.8

Influence of Bi(III) concentration on sorption kinetics

Co: 70 mg/L

C(t)/Co

3.4.1.

the intraparticle diffusion coefficient: De increased with initial metal concentration; it is doubled when the concentration increased from 10 mg Bi L1 to 70 mg Bi L1 (Table 4). For Pt(IV) and Pd(II) sorption on the same resins, the variations of the intraparticle diffusion coefficients were also limited: in the ranges 2–6  1012 m2 min1 for Pt(IV) and 7–9  1012 m2 min1 for Pd(II) in the concentration range 10–30 mg metal L1 (Vincent et al., 2008a,b); this means 4 orders of magnitude 2 lower than the free diffusivity of PdCl2 4 and PtCl6 in water 8 2 1 (i.e., 8.3–8.4  10 m min ) (Marcus, 1997). Marcus reported the values of the self-diffusion coefficient for a broad range of inorganic ions: the diffusivity varied between 1.1  108 m2 min1 and 5.5  107 m2 min1; though the free diffusivity of Bi(III) and BiCl 4 in water are not known, the probable values are also several orders of magnitude greater than the values found for Bi(III) diffusion in the resins. This is another evidence for the strong contribution of diffusion restrictions in the control of sorption kinetics. It is noteworthy that for both Pd(II) and Pt(IV) sorption on similar resins the comparison of the diffusion properties of these metals in wet resins were systematically increased by a factor 5–10. The drying step caused an irreversible collapse of the porous structure of the polymer matrix, which in turn limits the diffusion of solute through the resin.

0.6 0.4

0.2 0

0

24

48

72

96

Time (h) Fig. 6 – Influence of initial concentration (C0) on sorption kinetics (C3L resin; SD: 250 mg LL1; [HCl]: 1 M; v: 500 rpm).

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water research 42 (2008) 4019–4031

Table 4 – Modeling of Bi sorption kinetics (intraparticle diffusion, i.e., Crank’s equation) – influence of agitation speed (v, rpm), metal concentration (Co, mg Bi LL1), and IL content (IL: mmol IL gL1) Resin C3L C3L C3L C3L C3L C1L C1S C2L C2S

v

Co

SD

IL

De  1011 m2 min1

MSR

500 500 500 500 500 500 500 500 500

10 40 70 40 40 40 40 40 40

0.25 0.25 0.25 0.125 0.50 0.25 0.25 0.25 0.25

1.1 1.1 1.1 1.1 1.1 0.60 0.69 0.86 0.97

3.4 4.0 6.9 14.3 1.2 8.6 1.5 6.0 0.43

0.151 0.044 0.057 0.155 0.132 0.100 0.028 0.006 0.079

MSR: minimum square of residuals: fractional approach to equilibrium FATE (see Eq. 5). P MSR ¼ ni¼1 ½FATEexp ðiÞ  FATEcalc ðiÞ2 ; SD: sorbent dosage (g L1).

3.4.2.

Influence of sorbent dosage on sorption kinetics

Fig. 7 shows the impact of sorbent dosage on Bi(III) sorption kinetics for C3L resin. Three scenarios can be observed: (a) Low sorbent dosage: the excess of metal contributed to a fast saturation of sorption sites and the equilibrium was reached after about 2 days of contact. (b) Intermediate sorbent dosage: a long contact time is required since the intraparticle diffusion controls the transfer of the metal. The decreasing trend for the relative concentration extended up to 3 days (and probably more). (c) High sorbent dosage: bismuth was almost completely removed from the solution (residual concentration was less than 5%). The excess of sorbent contributes to limiting the sorption at the external surface of the sorbent and the impact of the resistance to intraparticle diffusion is limited. This may explain that in this case the intraparticle diffusion model (solid line) fitted worst experimental data compared to other sorbent dosages. The pseudo-first-order rate parameter hardly varied in the range 0.8–1.5  103 min1, while the pseudo-second-order rate parameter was in the range 0.6–1.9  105 g mg1 min1. Changing the sorbent dosage had a limited impact on the

reaction rates. The intraparticle diffusion coefficient increased by an order of magnitude (from 1.2  1011 m2 min1 to 14.3  1011 m2 min1) when the sorbent dosage decreased from 0.5 g L1 to 0.125 g L1. At low sorbent dosage the intraparticle diffusion is favored by a higher concentration gradient between the liquid phase and the center of resin particles. At high sorbent dosage, sorption at the surface of resin particles is enhanced in a first step of the process; in the second step the residual concentration in the liquid phase decreasing diminishes the concentration gradient between the liquid and the core of the resin.

3.4.3.

Influence of resin type on sorption kinetics

Fig. 8 compares the kinetic profiles for the different lots of resins. Substantial differences are observed. The interpretation of the differences is made difficult by the simultaneous variation of the size of the particles and the IL content of the resin (see Table 1). Increasing the size of the particles is expected to slow down the kinetics since the solute has a greater distance to run before reaching the internal reactive groups. The size is a critical parameter in the case of sorption systems controlled by intraparticle diffusion (Helfferich, 1995). This effect is reinforced in the case of poorly porous systems. SEM and SEM-EDAX analyses have shown that large particles were characterized by a change in the morphology/ texture of the vesicles after metal sorption and that several zones were identified in the cut particles (a saturation zone and a free zone, Fig. 3 and AM1 in Supplementary data); this is typical of a shrinking core mechanism that may contribute to a certain extent to the limitation of the diffusion of the solute. This is also confirmed by the rupture in the slope of the sorption capacity plotted versus IL content (Fig. 5b) at the highest IL content. C3L resins was significantly slower to reach equilibrium than the other resins (Fig. 8), while the other curves showed a homogeneous behavior: 1–2 days of contact were necessary for reaching equilibrium, depending on the excess of IL compared to metal concentration. As expected for C2L and C2S resins, the equilibrium was reached earlier for the smallest resins. In the case of C1L and C1S the impact of the size of the beads was less significant; this is probably due to the lower IL content (compared to C2S/C2L resins), which resulted in faster saturation of reactive groups.

1

1 C1L

0.8

C2L

0.8

C3L

0.6

m/V: 0.25 g/L

C(t)/Co

C(t)/Co

m/V: 0.125 g/L m/V: 0.5 g/L 0.4 0.2

C2S C1S

0.6 0.4

0.2

0

0

0

24

48

Time (h) Fig. 7 – Influence of sorbent dosage (SD) on sorption kinetics (C3L resin; C0: 40 mg LL1; [HCl]: 1 M; v: 500 rpm).

72

0

24

48

72

Time (h) Fig. 8 – Influence of resin type on sorption kinetics (C0: 40 mg LL1; SD: 250 mg LL1; [HCl]: 1 M; v: 500 rpm).

96

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water research 42 (2008) 4019–4031

3.5.

Influence of competitor ions – sorption selectivity

Since various acids may be used for the leaching of waste materials, sulfate and nitrate anions may be present in these industrial leachates. It is thus important to evaluate the impact of counter anions on the sorption of Bi(III), increasing concentrations of sodium nitrate and sodium sulfate were added to Bi(III) solutions prepared in 1 M HCl solutions. Table 5 shows that the sorption capacity progressively decreased with increasing nitrate and sulfate concentrations. However, even in the presence of anion concentrations as high as 5 g L1, the decrease did not exceed 25% of the reference value (Bi(III) in 1 M HCl solutions, without addition of anions). Sulfate ions had slightly higher depreciating effect than nitrate. The weak effect of anion is a positive criterion for the application of the sorption process to industrial leachates. Though Bi(III) is suspected to be bound by an anion exchange mechanism, the resin appears to be selective against these anions. The acidic leaching of solid wastes may also induce the presence of base metals such as copper, zinc or nickel. These metal ions can be classified in function of their ability to form chloroanionic complexes. Indeed, copper and nickel can form complexes with chloride ions in HCl media; however, these complexes are exclusively neutral (CuCl2, NiCl2) or cationic (CuClþ, NiClþ). On the opposite hand, Zn(II) can form anionic 2 complexes with chloride ions (ZnCl 3 and ZnCl4 ) additionally þ to neutral (ZnCl2) and cationic forms (ZnCl ). Hence, up to 30–40% of Zn(II) may appear under the form of anionic chlorocomplexes. Based on the sorption mechanism involved in Bi(III) binding, it is expected that base metals that do not form anionic complexes will be less competitive than those forming such chloroanionic species. Fig. 9 confirms this

160

Zn Ni Cu

120

q (mg Bi/g)

The pseudo-first-order rate constant varied in the range 0.8– 3.2  103 min1 without a clear trend relatively to IL content (Table 3); on the opposite hand the pseudo-second-order rate constant linearly decreased (from 4.3  105 g mg1 min1 to 0.7  105 g mg1 min1) when increasing IL content according to: k2  105 (g mg1 min1) ¼ 9.2–7.7 IL (mmol g1) (R2: 0.972). The intraparticle diffusion coefficient was not correlated to IL content. The intraparticle diffusivity varied between 0.4  1011 m2 min1 and 8.6  1011 m2 min1. The system was more sensitive to the size of resin particles. Hence for the same range of IL content (C1S/C1L and C2S/C2L) De decreased (by a factor of 6–14) when decreasing the size of the particles, while for the same range of size (C1L/C2L/C3L and C1S/C2S) the variation in the same class of resin was less marked. For large particles, De only varied by a factor 2 for C3L–C1L and by a factor 3–4 for C1S and C2S.

80

40

0 0

250

500

750

1000

5000

Competitor metal (mg/L) Fig. 9 – Influence of competitor metal ions (Zn(II), Ni(II) and Cu(II)) on Bi(III) sorption capacity (C3L resin; C0: 100 mg LL1; SD: 250 mg LL1; [HCl]: 1 M).

assumption. For Ni(II) and Cu(II) sorption the sorption capacity remained, respectively, close to 137  8 mg Bi g1 and 132  7 mg Bi g1, regardless of the concentration of the competitor metal. Wang et al. (2006) also observed that the presence of Cu(II) did not interfere on Bi(III) removal using bayberry tannin immobilized on collagen fibers. With Zn(II) the sorption capacity decreased below 30 mg Bi g1 for a 5 g L1 concentration of Zn(II). The resin is thus selective against metal ions (providing these metal ions do not form anionic complexes) and against ‘‘simple’’ anions; competition of anions occurred only with anionic complexes.

3.6.

Bi desorption and sorbent recycling

The recycling of the sorbent is a critical parameter for the competitiveness of the process. Several eluents were tested for the recovery of Bi from loaded resin (Table 6); other eluents such as EDTA–Na salt 0.05 M, KBr 0.5 M, KSCN 1 M and KSCN 1 M in 0.5 M HCl solution were tested but the desorption efficiency ranged between 10% and 15%. Since alginate is not stable in alkaline media (biopolymer dissolving) the resin should be desorbed in acidic solutions. The most efficient systems are 0.5 M KI (in 0.1 M HCl) and 0.05 M citric acid. In the case of bayberry tannin immobilized on collagen fibers, Wang et al. obtained high desorption efficiency with EDTA–Na salt, compared to citric acid and nitric acid. In the case of citric acid the desorption efficiency in the first single step was almost

Table 6 – Bi desorption from loaded resins Table 5 – Influence of increasing concentrations of sulfate and nitrate anions on Bi(III) sorption capacity (mg Bi gL1) using C2L resins (SD: 250 mg LL1; HCl: 1 M; Co: 100 mg Bi LL1) Concentration (g L1) Anion Sulfate Nitrate

0 131.7 131.7

0.25 97.5 110.4

1 102.8 106

3 97.8 99.9

5 93.2 98.1

20 109.2 101.5

Eluent (M)

Citric acid, 0.05 M

KI, 0.25 M; HCl, 0.1 M

Na2SO4, 0.5 M

HNO3, 2M

Desorption efficiency (%)

>99

74.9

36.4

33.1

m (loaded resin): 20 mg; V (eluent): 80 mL; adsorption from 1 M HCl solution of 40 mg Bi L1.

water research 42 (2008) 4019–4031

complete. The efficiency of citric acid for Bi(III) desorption can be explained by their ability to form a series of stable complexes (Sadler et al., 1999). Kim et al. (2005) tested Bi(III) desorption from Amberlite XAD-4-Salen resin using nitric, sulfuric and hydrochloric acids and they concluded that nitric acid was the most efficient. For analytical purpose, Oshita et al. (2007a) used nitric acid for the desorption of Bi(III) loaded on glycine-modified chitosan at pH 5: the desorption was complete with 0.1 M HNO3 solutions. The interaction of chitosan-based sorbent with Bi(III) was certainly weaker than that established between Bi(III) and Cyphos IL-101 in the biopolymer particles. In the case of KI, the complexation effect is less strong than with citric acid. These two eluents were selected for carrying out three sorption/desorption cycles using C3L resin. Fig. 10 shows the amounts of Bi that were adsorbed and desorbed at each cycle for both citric acid and KI/HCl eluents. In the case of citric acid, the sorption remained almost constant at each step: the amount of Bi adsorbed on the resin slightly decreased from 3.2 mg to 2.8 mg; the desorption considerably varied at each step with efficiencies that did not exceed 73%. The cumulative desorption yield for citric acid eluent was around 63%; this means far from the expected value based on preliminary results (Table 6). Similar trends were observed by Wang et al. (2006) with bayberry tannin: the sorption capacity progressively decreased and the desorption efficiency dropped much faster with increasing the number of cycles. This decreasing trend in the efficiency (for both sorption and desorption) was more marked when using citric acid and nitric acid compared to EDTA–Na salt. For KI/HCl eluent, large variations were obtained at each cycles: the amount of Bi adsorbed on the resin varied between 3.2 mg and 1.6 mg and the desorption at the first cycle did not exceed 45%; however, at the second and the third cycles the desorption efficiency was restored (desorption yield exceeding 93%). It is noteworthy that the sorption increased at the third cycle with a sorption capacity close to the initial sorption level. The cumulative desorption yield at the end of the third cycle was close to 75%, consistently with data presented in Table 6. It seems that the mixed eluent KI/HCl allowed after stabilization of the sorption/desorption process to reach resins with more

4 Ads.

mAds. # mDes. (mg Bi)

Citric Acid

KI + HCl

Des.

3

2

1

0

1

2

3

1

2

4029

stable performance than the system involving citric acid as the eluent. In other experiments based on gold recovery using the same sorbent the loss of IL was determined after three sorption/desorption cycles (Campos et al., in press): the decrease in sorption capacity was partly correlated to the loss of about 15% of the IL initially present in the IL-immobilized particles. In the following sorption/desorption cycles the decrease in sorption capacity and desorption could be explained by several reasons: (a) loss of IL resulting for the successive sorption/desorption treatments, (b) release of citric acid that may contribute to complex Bi(III) at the following sorption step, and (c) saturation of IL with Bi(III) that was not desorbed in the previous cycles. Complementary experiments would be required, using for example liquid/liquid extraction for anticipating the impact of metal-Cyphos IL-101-Eluent interactions. Regardless of the eluent, despite an incomplete desorption, sorption performance remained relatively stable. These results show that the resin can be used for a minimum of three sorption/desorption cycles maintaining appreciable Bi(III) recovery.

4.

Conclusions

The encapsulation of Cyphos IL-101 into a biopolymer composite matrix allowed synthesizing a resin that revealed to be very efficient for the recovery of Bi(III) from HCl solutions. Bismuth chloroanionic complexes can be bound through ion exchange 0 þ mechanism by reaction of BiCl 4 with R3R P . The sorption properties were hardly affected by HCl concentration or by the presence as nitrate and sulfate anions at concentrations as high as 20 g L1. The resin was also poorly affected by the presence of base metals such as Cu(II) or Ni(II) at concentrations as high as 1 g Cu L1 and 5 g Ni L1, respectively. On the opposite hand, Zn(II) being able to form chloroanionic complexes interacts with the resin and contributed to reducing Bi(III) sorption. The resin can thus separate Bi(III) from other metals, providing these metals do not form chloroanionic species. The sorption kinetics were efficiently modeled using simple models such as the pseudo-second-order equation and taking into account the effect of the resistance to intraparticle diffusion. The Crank’s equation was used for modeling the kinetic profiles. While agitation speed poorly affected sorption kinetics (indicating that film diffusion is probably not controlling sorption rate), parameters such as sorbent dosage, IL content and particle size have a more marked effect on kinetic rates. Bismuth can be recovered from loaded resins using either citric acid (0.05 M) or KI (0.25 M)/HCl (0.1 M) solutions. Three sorption/desorption cycles have been performed maintaining good sorption efficiency (the decrease in the amount of Bi removed by the resin was less than 15%) and a cumulative desorption yield close to 75%.

3

Cycle number Fig. 10 – Bi(III) sorption/desorption cycles (C3L resin; [HCl]: 1 M; SD: 625 mg LL1; adsorption: C0: 40 mg Bi LL1; desorption: 0.05 M citric acid solution – 0.25 M KI D 0.1 M HCl solution).

Acknowledgements Authors thank Cytec (Dr. Al Robertson) for the gift of Cyphos IL-101 sample. Authors also thank The European Commission

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water research 42 (2008) 4019–4031

for R.D. Socrates fellowship during his stay at EMA. E.G. and T.V. acknowledge Carnot Label for supporting the project ‘‘Proce´de´s d’encapsulation d’extractants’’. K.C. and E.G. thank the Franco-Peruvian Collaboration Program ‘‘Raul Porras Barrenechea’’ for the fellowship support. Authors also acknowledge Mr. Jean-Marie Taulemesse from the Centre des Mate´riaux de Grande Diffusion at Ecole des Mines d0 Ale`s for the SEM analysis of resin samples.

Supplementary data Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.watres.2008.07.024.

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