Al

Ra

b

a

ARRA

KPCAPESU

1

mcTtosVtiuwm(mpat

1d

Chemical Engineering Journal 185– 186 (2012) 226– 235

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

jo u r n al hom epage: www.elsev ier .com/ locate /ce j

mberlite XAD-7 impregnated with Cyphos IL-101 (tetraalkylphosphonium ioniciquid) for Pd(II) recovery from HCl solutions

icardo Navarroa,∗, Imelda Saucedoa, Carmen Gonzaleza, Eric Guibalb

Universidad de Guanajuato, División de Ciencias Naturales y Exactas, Departamento de Química, Cerro de le Venada s/n, Pueblito de Rocha, C.P. 36040 Guanajuato, Gto, MexicoEcole des Mines d’Alès, Laboratoire Génie de l’Environnement Industriel, Equipe BioPhysicoChimie des Interfaces, 6, avenue de Clavières, F-30319 Alès cedex, France

r t i c l e i n f o

rticle history:eceived 11 December 2011eceived in revised form 18 January 2012ccepted 19 January 2012

eywords:hosphonium ionic liquidyphos IL 101mberlite XAD-7alladium

a b s t r a c t

Cyphos IL-101 ionic liquid (IL; tetradecyl(trihexyl)phosphonium chloride) was immobilized in AmberliteXAD-7 for the preparation of an extractant impregnated resin (EIR) that was used for Pd(II) sorptionfrom HCl solutions. Chloro-palladate anionic species are bound to the EIR by electrostatic/anion exchangemechanism between anionic species and phosphonium cation (IL). Maximum sorption capacity increaseswith extractant loading and depends on HCl concentration reaching values up to 71 mg Pd g−1 EIR foran EIR with 401 mg IL g−1 EIR, in 0.5 M HCl solutions. Uptake kinetics are controlled by the resistanceto intraparticle diffusion (effective diffusivity varying in the range 1 × 10−11 to 20 × 10−11 m2 min−1) asconfirmed by the limited impact of agitation speed (negligible resistance to film diffusion). IL loading isa key parameter since it controls the filling of resin porous network and consequently the mass transfer

xtractant impregnated resinsorption isothermsptake kinetics

properties: diffusivity in the extractant phase is reduced compared to diffusivity in water. IncreasingIL loading increases sorption capacity but induces supplementary limitations to intraparticle diffusion.Increasing temperature decreases IL viscosity, which, in turn, enhances diffusion in this extractant phase.Nitric acid and thiourea in hydrochloric acid solutions allows complete desorption of Pd(II) from loadedEIR. The resin can be recycled for at least five cycles maintaining high sorption and desorption efficiencies.

. Introduction

The demand for precious metals, and especially platinum groupetals (PGMs) is increasing with the development of catalytic pro-

esses for industry, automotive applications, fuel cells and so on.his growing demand and the cost of these metals have focusedhe interest of many research groups for recovering metals notnly from primary sources (minerals) but also from secondaryources (waste materials such as used catalysts, electronic devices).alorization of wastes is also part of the current politics on sus-

ainable growth: in many countries a waste can only be disposedn landfill when it is proved that no other way to valorize it existsnder economical constraints. Metals can be recovered from solidastes by a series of unitary processes including grinding, gravi-etric separation, magnetic separation, bio- or chemical leaching

solid to liquid transfer). When transferred in the liquid phase theetals can be recovered by several processes such as electrolytic

rocesses, or solvent extraction [1–4]. However, these processesre generally designed for metal removal from concentrated solu-ions and alternative processes should be used for the treatment

∗ Corresponding author. Tel.: +52 473 732 7555; fax: +52 473 732 7555.E-mail address: navarrm@ugto.mx (R. Navarro).

385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2012.01.090

© 2012 Elsevier B.V. All rights reserved.

of dilute solutions (as a polishing treatment, for example). Sorp-tion is frequently used for metal recovery from low-concentrationsolutions. A number of different sorbents have been tested includ-ing biosorbents (biomass, biopolymers) [5–10], and resins [11–19].Biopolymers bearing amine groups such as chitosan have showninteresting properties for PGMs; however, the sorption efficiencyis generally limited to medium acidity range (i.e., pH 2). Acidicleachates generated during the extraction of PGMs are generallymuch more acidic. This is necessary to modify the polymers tobring more specific reactive groups with broader range of activepH: phosphinic [20], sulfur [21], or imidazolium groups [15,16], forexample.

Extractant impregnated resins (EIR) are alternative systems thatcombine the advantages of both resins and solvent extractants[22–26]. Indeed, solvent extraction is very efficient (fast and effec-tive); however, its major drawback is the possible release (loss) ofhazardous and expensive extractants (partially soluble in water).Resins offer possibilities for confining the extractant in a matrixpreventing extractant loss, reducing economical constraints andenvironmental issues. A new class of extractants has been recently

used for the preparation of EIRs: ionic liquids (ILs) offer interestingproperties such as higher thermal stability, lower solubility in waterand lower vapor pressure that make these extractants competitiveagainst conventional materials for metal extraction [27]. Cyphos

R. Navarro et al. / Chemical Engineering

Table 1Physical characteristics of Amberlite XAD-7 resin [24].

Parameter Value

Particle size 20/60 mesh – 250–850 �mSpecific surface area 450 m2 g−1

Resin porosity 0.55

Itcbtpfi[s

trT(ibpimp(som

im(seba

glkIm

2

2

L(saiatasof

Pore size (mean value) 85–90 APore volume 0.97–1.14 cm3 g−1

Skeletal density 1.24 g cm−3

L-101 (tetradecyl(trihexyl)phosphonium chloride) is a member ofhe Cyphos ionic liquid family based on phosphonium cation (asso-iated to different anions: mineral or organic compounds). It haseen used for metal recovery in conventional liquid/liquid extrac-ion processes [28,29]. Recently, these ILs have been also used forreparing new EIRs [30–36]. Two techniques have been developedor manufacturing these EIRs: (a) the conventional method consist-ng in the impregnation of the resin followed by solvent evaporation30,32,34] and (b) the immobilization of the IL in biopolymer cap-ules [31,35,36].

The present study focuses on the sorption of Pd(II) from HCl solu-ions using an EIR prepared by the conventional method. Amberliteesins are supports commonly used for the preparation of EIR.hey have been designed with different characteristics of porositypore volume and pore size) and with different surface character-stics (the most important being hydrophobicity that is controlledy the type of polymer (acrylic-ester or styrene-divinyl benzeneolymers). In this study, Amberlite XAD-7 was used. This resin

s a macroporous acrylic ester polymer, moderately polar (dipoleoment: 1.8), with interesting textural properties such as high

ore volume (1.14 mL g−1), pore diameter (90 A) and surface area450 m2 g−1). This resin impregnated with different extractants hashowed greater efficiencies and faster extraction of metal ions thatther hydrophobic macroporous styrene-divinyl benzene poly-ers such as Amberlite XAD-2 (dipole moment 0.3) [37].This is part of a more extended research program consisting

n the testing and comparison of the extraction of a series ofetal ions using Cyphos IL-101 immobilized in different systems

i.e., Amberlite XAD-7 resin, but also alginate biopolymer cap-ules) [30–36,38–41]. The different systems allow identifying andxplaining the impact of the mass transfer properties of the immo-ilization matrices and the effect of metal speciation on equilibriumnd kinetic performance.

The effect of HCl concentration on sorption capacity is investi-ated, before determining the sorption isotherms for different ILoadings, and at different temperatures. In a second step uptakeinetics are carried out considering the influence of agitation speed,L loading, temperature and metal concentration. In a third step,

etal desorption and resin recycling are investigated.

. Materials and methods

.1. Materials

Amberlite XAD-7 was supplied by Sigma–Aldrich (Saint-ouis, U.S.A.). This is a polyacrylic acid ester type resin[CH2 CH(COOR)]n). The physical characteristics of the resin areummarized in Table 1. Amberlite XAD-7 can be considered as

nonionic, moderately hydrophilic polymer. It is commercial-zed as a macroporous polymer, although it must be considereds a mesoporous material (pore diameter: 20–500 A) accordingo IUPAC. The resin was conditioned by the supplier with NaCl

nd Na2CO3 to inhibit or delay bacterial growth. It was neces-ary to clean it to remove salts and monomeric material presentn the resin. The resin was therefore put into contact with ketoneor 24 h at 25 ◦C. After filtration under vacuum to remove excess

Journal 185– 186 (2012) 226– 235 227

ketone, the resin was rinsed with de-mineralized water, beforebeing washed with nitric acid (0.1 M) for 24 h. The resin was fil-tered under vacuum and then rinsed with de-mineralized water toconstant pH. Finally, the resin was put into contact with ketone for12 h before being filtered under vacuum and dried in a roto-vaporat 50 ◦C. Cyphos IL-101 was kindly supplied by Cytec (Canada). Thisis a phosphonium salt (tetradecyl(trihexyl)phosphonium chloride,C.A.S. number: 258864-54-9, formula weight: 519.42 g mol−1). Itis a slightly viscous room temperature ionic liquid. It is less densethan water and colorless to pale yellow. It is immiscible with wateralthough it is sparingly soluble in water and can dissolve up to8% water. The chemical structure is [R3R′P]+ Cl−, where R = hexyland R′ = tetradecyl. Other reagents (salts, acids, etc.) were analyti-cal grade and supplied by KEM (Mexico). Standard metal solutionswere supplied by Perkin Elmer (U.S.A.).

2.2. Resin impregnation

In the present work the extractant was immobilized on the resinby a physical technique. Different processes may be used for thephysical impregnation of the resin including (i) the wet method,(ii) the dry method, (iii) the impregnation in the presence of amodifying agent, or (iv) the dynamic method [42]. Previous stud-ies have shown that the dry method increases the stability of theextractant on the resin. The dry impregnation of the resin was actu-ally performed by contact of 5 g of conditioned Amberlite XAD-7with 25 mL of ketone for 24 h. Varying amounts of Cyphos IL-101diluted in ketone (0.5 M) were added to resin slurry for 24 h, underagitation. The solvent was then slowly removed by evaporation ina roto-vapor. The amount of extractant immobilized on the resin(qCyphos IL-101) was quantified by the following procedure. A knownamount of impregnated resin (250 mg) was mixed with methanol(3 mL) for 24 h to remove the extractant by dissolving. The washingtreatment was carried out four times. The solvent was finally sepa-rated from the resin, which was dried at 50 ◦C for 24 h for completeevaporation of solvent. The mass difference (MCyphos IL-101) betweenimpregnated (MXAD-7/Cyphos IL-101) and washed resin (MXAD-7) wasused to calculate the amount of extractant immobilized on the EIR:

qCyphos IL-101 = MXAD-7/Cyphos IL-101 − MXAD−7

MXAD-7/Cyphos IL-101(1)

The experimental procedure allowed the preparation of EIR con-taining 59, 106, 152, 207, 291, 401, 498, and 586 mg IL g−1 EIR.Previous studies have shown that an excessive loading of IL in theEIR (around and above 600 mg IL g−1 EIR) resulted in a partial leak-age of the IL: exudates were observed by optical microscope at thesurface of the resin during metal sorption due to partial water andHCl co-extraction [30]. In addition, iridescence (typical of hydro-carbon release) was also observed at the surface of water. In thepresent study, the IL loading was maintained below 500 mg IL g−1

EIR for systematic sorption experiments (i.e., sorption isothermsand uptake kinetics) to prevent IL leakage. In a matter of fact, theobservation of resin surface and water surface did not show any ofthese phenomena, indicating the stability of the IL in the porousnetwork.

The drying of the resin at the end of the impregnation pro-cess may increase the hydrophobicity of the resin, which, in turn,hinders the intraparticle diffusion of metal ions inside the EIR. Tominimize this effect the EIR particles were pre-hydrated at thetarget pH (or appropriate HCl concentration), prior to use in thekinetics studies.

2.3. Characterization

Element distribution (especially Pd and P, as the tracers ofmetal sorption and IL distribution, respectively) in the beads was

2 eering

i(DuiEttrb

2

c3bw(as(stwwcc

a

Ff

28 R. Navarro et al. / Chemical Engin

nvestigated by Environmental Scanning Electron MicroscopyESEM) Quanta FEG 200, equipped with an OXFORD Inca 350 Energyispersive X-ray microanalysis (EDX) system. The system can besed to acquire qualitative or quantitative spot analyses and qual-

tative or quantitative X-ray elemental maps and line scans. ThisSEM allows analyzing samples at pressures and humidity levelshat approach normal laboratory conditions and avoids experimen-al artifact. The preparation of the cross-section of impregnatedesin was performed by particle freezing in liquid nitrogen followedy knife cutting of the spherical particles.

.4. Sorption and desorption studies

Pd(II) solutions were prepared in HCl solutions of different con-entrations (0.01–8 M) with metal concentrations ranging between0 and 400 mg Pd L−1. The sorption experiments were performedy mixing the resin (0.02 g) with Pd(II) solutions (10 mL) for 48 hith a solid/liquid ratio (sorbent dosage, SD) fixed to m/V = 2 g L−1

m: mass of sorbent, V: volume of solution). The contact was oper-ted on a reciprocal shaker (Cole Parmer 51502) with an agitationpeed of 150 movements per minute at constant temperature10, 20 and 40 ◦C). After filtration the samples were analyzed bypectrophotometry UV–vis at 280 nm (Varian Cary 50 UV-Vis Spec-rophotometer). The amount of metal adsorbed (q, mg Pd g−1 EIR)as calculated by the mass balance equation: q = V(C0 − Ceq)/m,here C0 and Ceq (mg Pd L−1) are the initial and equilibrium Pd(II)

oncentrations, respectively. The distribution coefficient was cal-ulated by the equation: D = q/Ceq.

Sorption kinetic experiments were performed by contact undergitation of a fixed amount of EIR (0.02 g; loading in the range

ig. 1. SEM–EDX analysis (Element cartography) of raw Amberlite XAD-7 resin (a), Cyphosrom 0.01 M HCl solution (c), and 0.5 M HCl solution (d) (qCyphos IL-101: 401 mg IL g−1 EIR).

Journal 185– 186 (2012) 226– 235

106–401 mg IL g−1 EIR) with a fixed volume (50 mL; SD:0.4 g L−1) of0.5 M HCl solution containing varying Pd(II) concentrations (in therange 15–35 mg Pd L−1). In order to compare the different lots of EIRon the same basis of equilibrium concentration, a series of kineticswas performed adjusting the amount of EIR to reach a similar equi-librium metal concentration (i.e., Ceq = 6.5 mg Pd L−1). Temperaturewas varied in the range 20–40 ◦C. Samples were collected at fixedtimes and analyzed for determination of residual metal concen-trations. The modeling of uptake kinetics is described (along withthe equations of the different models that were used) in AdditionalMaterial Section.

For the study of Pd(II) desorption, an amount of 0.02 g of EIR(extractant loading: 401 mg IL g−1 EIR) was mixed with 10 mLof Pd(II) solution (0.5 M HCl solution (SD: 2 g L−1), initial metalconcentration: 25 mg Pd(II) L−1) for 48 h at 20 ◦C. The residualconcentration measured by atomic absorption spectrometry (AASPerkin Elmer AAnalyst 200) after filtration served to determinethe amount of metal bound to the resin. The metal-loaded resinwas mixed for 24 h (150 rpm at 20 ◦C; SD: 2 g L−1) with two kindsof eluents: 5 M HNO3, and 1 M thiourea (in 0.5 M HCl solution).This desorption step was carried out twice. After filtration theconcentration in the eluent was determined by AAS in order toobtain the amount of Pd desorbed from the resin and to calcu-late the desorption yield. For the evaluation of sorption/desorptioncycles, the same procedure was used for five cycles. The presenceof thiourea in the porous or extractant phase (after desorption)

may limit the efficiency of the sorbent for next sorption cycles.For this reason, three washing runs (with 10 mL of water by 8 hat 150 rpm and 20 ◦C) were operated before the next sorptionstep.

IL-101 impregnated resin (b), Cyphos IL-101 impregnated resin after Pd(II) sorption

R. Navarro et al. / Chemical Engineering Journal 185– 186 (2012) 226– 235 229

F from

(

3

3S

sdgiAenPsttcoh

trasatoipssPfi(afEtoH

ig. 2. SEM–EDX analysis of Cyphos IL-101 impregnated resin after Pd(II) sorptionqCyphos IL-101: 401 mg IL g−1 EIR).

. Results and discussion

.1. Characterization of extractant impregnated resin byEM–EDX analysis

The distribution of elements in the EIR (before and after metalorption) is a helpful parameter for evaluating (a) the homogeneousistribution of the IL in the EIR, (b) the accessibility of reactiveroups in the EIR. SEM–EDX analysis is a very useful tool for access-ng this information. Fig. 1 shows the cartography of elements formberlite XAD-7 (raw material, C and O elements as characteristiclements of polymer, Fig. 1a), for EIR (Amberlite XAD-7 impreg-ated with Cyphos IL-101 at the IL loading of 401 mg IL g−1 EIR,

and Cl tracers of Cyphos IL-101) before (Fig. 1b) and after metalorption (Fig. 1c and d, from 0.01 M HCl solution and 0.5 M HCl solu-ion, respectively, P tracer of Cyphos IL-101, Cl element for bothhe metal chloride complexes and the IL, and Pd). On Fig. 1b, theartography of P and Cl elements is homogeneous: no gradient isbserved between the outer and inner zones of the EIR. The IL isomogeneously distributed in the EIR.

Further experiments (effect of HCl concentration) will show thathe sorption mechanism differs with HCl concentration. For thiseason, the cartography of P, Cl and Pd elements was determinedt both 0.01 M (Fig. 1c) and 0.5 M HCl solutions (Fig. 1d). Fig. 1chows that there is a correlation between P, Cl and Pd elementsnd a slight gradient between the outer and the center of the par-icle. This gradient is probably due to a heterogeneous distributionf the IL (resulting from the impregnation procedure). The majornterest of this figure is to confirm the interaction between phos-honium cation and tetrachloropalladate anions confirmed by theimilar spatial distribution of these elements (P, Cl and Pd). Fig. 1dhows that for this experiment performed in 0.5 M HCl solutions, Cl and Pd elements are homogeneously distributed. This is con-rmed by Fig. 2: the cross-section analysis shows that Pd elementidentified on the X-ray spectrum) is at the same concentrationlong the cross-section. Similar experiments (not shown) were per-ormed with EIR loaded with lower IL amount (i.e., 106 mg IL g−1

IR). They confirmed that the IL is homogeneously distributed, thathe metal accessed internal groups (with a constant concentrationver the cross-section) for Pd(II) sorption in both 0.01 and 0.5 MCl solutions. This study shows that the metal can access the center

0.5 M HCl solution – Pd distribution along the cross-section of the particle (arrow)

of the particle and can react with IL reactive groups present at thecenter of the EIR.

3.2. Evaluation of the effect of HCl concentration and IL loadingon sorption capacity

The sorption capacity of the EIR (at different IL loadings) wascompared with those of raw Amberlite XAD-7 for different HClconcentrations. Varying the concentration of HCl allows simulatingdifferent types of media and also evaluating the impact of metalspeciation on extraction efficiency. Fig. 3 shows that increasingHCl concentration results in the drastic decrease of sorption capac-ity: this effect is particularly observed for HCl concentration above4 M (sorption capacity does not exceed 10 mg Pd g−1 for 8 M HClsolutions even with the highest IL loading, close to 600 mg IL g−1

EIR). It is noteworthy that at low HCl concentration (i.e., 0.01 M),Pd(II) could be sorbed on raw Amberlite XAD-7 (sorption capac-ity close to 20 mg Pd g−1). The resin completely loses its sorptionproperties for Pd(II) when HCl concentration reaches 0.1 M. Thereactivity of the Amberlite XAD-7 (without extractant) was previ-ously observed in the case of Au(III) extraction from HCl solution[34,43]. Under very acidic conditions, the resin is chemically mod-ified by partial hydrolysis of acrylic ester group ( O· · ·H+Cl−), andAu(III) is extracted as an anionic chloro-complex ( O· · ·H+AuCl4−).Hydrophobic interactions of resin matrix with tetrachloroauric acid(a neutral species: HAuCl4) have been also proposed to explainthe high sorption levels of Au(III) in mild acid conditions. In thesame way, the sorption of a series of trivalent metals, includ-ing Fe(III), on Amberlite XAD-7 resin in acidic solutions has beenreported [43–45]. In the present case, metal binding occurred atpH close to 2; this is not strong enough to induce a degradationof Amberlite XAD-7 support. Several mechanisms could be sug-gested for justifying Pd(II) binding: (a) the binding of specific metalspecies (such as palladium hydroxide or palladium hydroxo-chlorocomplexes) different to those involved in metal binding on IL-impregnated resins; (b) a partial reduction of metal particles onthe surface due to oxidative effect of Pd(II) (though a higher HCl

concentration would be probably more efficient for the oxidationprocess). These mechanisms are supported by some color changesthat occurred during metal sorption at 0.01 M HCl concentrationand at low IL loading: Fig. AM2 (Additional Material Section) shows

230 R. Navarro et al. / Chemical Engineering

Fc

srXavaEtrPrpbstbtiwcCtae

P

P

ig. 3. Impact of variations of HCl concentration and IL loading on Pd(II) sorptionapacity (T: 20 ◦C; SD: 2 g EIR L−1; C0: 150 mg Pd L−1).

ome pictures of the flasks (containing both metal solution andesin particles). At low HCl concentration (pH close to 2) AmberliteAD-7 changes color (from white to black): palladium hydroxide,s well as reduced palladium, is black colored; this supports pre-ious hypotheses on the binding mechanism. The black color islso observed for low IL loadings (qCyphos IL-101 below 200 mg IL g−1

IR). However, this color no more appears when HCl concentra-ion increases (above 0.1 M). Starting with 0.1 M HCl concentration,aw Amberlite XAD-7 (without IL impregnation) no more bindsd(II): on the 0.1–8 M concentration range the sorption capacityemained below 1.7 mg Pd g−1 EIR. This mechanism does not takelace for HCl concentration equal or higher to 0.1 M and metalinding proceeds through interactions between IL and palladiumpecies. Increasing the amount of IL in the EIR increases the sorp-ion capacity: this increase is almost linear up to 401 mg IL g−1 EIRut tends to less increase above (especially at low HCl concentra-ion). At high HCl concentration (i.e., 8 M) the increase of IL loadings not sufficient to maintain appreciable sorption capacity (which

as below 6 mg Pd g−1 EIR). Cieszynska and Wisniewski [28] dis-ussed the liquid/liquid extraction of Pd(II) from HCl solutions usingyphos IL-101 diluted in toluene. More specifically, they comparedhe stoichiometric ratio between Pd(II) and phosphonium cationt 0.1 M and 3 M HCl concentrations. They suggested the followingquilibriums:

At 0.1 M HCl concentration:

dCl42−(w) + [R3R′P+][Cl−](o) ↔ [R3R′P+][PdCl3−](o) + 2Cl−(w) (2)

At 3 M HCl concentration:

dCl42−(w)+2[R3R′P+][Cl−](o) ↔ [R3R′P+]2[PdCl42−](o)+2Cl−(w) (3)

Journal 185– 186 (2012) 226– 235

where w and o subindex represent the species in aqueous andorganic phase, respectively.

According these equations an excess of chloride ions (occur-ring at high HCl concentration) contributes to reversing the bindingreaction. These two mechanisms may explain the strong decreaseof sorption capacity at increasing HCl concentration. The drasticdecrease in sorption capacity is more specifically identified whenHCl concentration exceeds 4 M. The concavity of the curve q = f(CHCl)decreased with increasing qCyphos IL-101 (Fig. 3b).

The log–log plot of the distribution coefficient D versus qCyphos

IL-101 and aHCl can be used to determine the stoichiometric ratiobetween the metal and the extractant, and between the metal andchloride ions, respectively. The slope of the plot allows identify-ing the number of phosphonium cations and chloride ions thatwere exchanged. Here, aHCl is the mean activity of H+ and Cl−

ions and was used for estimating the chloride ion activity in orderto determine the number of chloride ions that are involved inmetal sorption (taking into account the impact of ionic strength).The mean activity coefficients used in this section were obtainedfrom Robinson and Stokes [46]. The slope of log D versus log aHClapproaches −2 and indicates that two moles of chloride wereexchanged for the binding of one mole of metal (not shown, seeAdditional Material Section, Fig. AM3). The plot of log D versuslog qCyphos IL-101 (see Additional Material Section, Fig. AM3) showsthat the slope significantly changed with HCl concentration: thisslope (i.e., the number of moles of IL per mole of Pd) decreaseswith increasing HCl. Above 4 M HCl concentration, the slope tendsto decrease (below 2) while at lower concentration the slopeexceeds 2. These values are higher than the values suggested byEqs. (10) and (11) (i.e., 1 and 2, respectively). At low concentrationthe extraction mechanism may include a reaction between R3R′P+

and PdCl3−(Eq. 10); while at high HCl concentration the reactioninvolves 2 R3R′P+ and 1 PdCl42− (Eq. 11). Previous studies on Au(III)and Cd(II) binding using the same EIR have shown that a fractionof the IL remained inactive, probably bound to the support [30,34].Moreover, Avila-Rodriguez et al. [47] observed that Cyphos IL-101can extract significant amounts of HCl. This makes the discussion ofthe exchange molar ratio debatable; moreover variations in metalspeciation with increasing HCl concentration contribute to makingdifficult slope analysis.

3.3. Sorption isotherms

Previous section showed the strong impact of HCl concentrationon metal efficiency and also on the sorption mechanism. For thesereasons, sorption isotherms at different IL loadings were performedat two HCl concentrations: weakly acidic conditions (i.e., 0.01 MHCl, with possible contribution of the support to metal binding,Fig. 4a), and acidic conditions (i.e., 0.5 M HCl, sorption only occursthrough reaction with the IL, Fig. 4b). The sorption isotherms arecharacterized, in most cases, by a Langmuir-type shape (sharp ini-tial slope and appearance of a saturation plateau; i.e., asymptotictrend):

q = qmbCeq

1 + bCeq(4)

where qm (mg Pd g−1 EIR or mmol Pd g−1 EIR) is the maximumsorption capacity reached at saturation of the monolayer and b isthe affinity coefficient (L mg−1 Pd or L mmol−1 Pd), q is the sorptioncapacity (mg Pd g−1 EIR or mmol Pd g−1 EIR) in equilibrium with theresidual concentration Ceq (mg Pd L−1 or mmol Pd L−1). On Fig. 4, thelines show the plots of the Langmuir equation using the parameters

summarized in Table 2.

In 0.01 M HCl solutions, Amberlite XAD-7 (resin without impreg-nation) binds limited amounts of Pd(II) with a linear trend thatcorresponds to the typical Henry’s law. More conventional profiles

R. Navarro et al. / Chemical Engineering Journal 185– 186 (2012) 226– 235 231

Table 2Coefficients of the Langmuir equation for the modeling of Pd(II) sorption isotherms using Cyphos IL-101 impregnated resins.

T (◦C) qCyphos IL-101 (mg IL g−1 EIR) CHCl (M) qm (mg Pd g−1 EIR) b (L mg−1) R2

20 0 0.01 KH = 0.196a – 0.97720 106 0.01 45.6 0.016 0.94520 207 0.01 39.4 1.16 0.97120 401 0.01 80.7 0.805 0.99920 0 0.5 NS NS NS20 106 0.5 13.5 0.030 0.92120 207 0.5 33.7 0.073 0.96120 401 0.5 71.1 0.207 0.99810 401 0.5 73.0 0.290 0.99940 401 0.5 69.7 0.280 0.999

Na

wLitarPCubw

FnHt

S: no sorption.Linear isotherm (Henry equation) for Pd(II) above 24 mg L−1.

ere obtained after IL impregnation with the typical shape ofangmuir equations. The maximum sorption capacities obviouslyncrease with IL loading up to 80 mg Pd g−1 EIR. In 0.5 M HCl solu-ions, the raw Amberlite XAD-7 (without IL impregnation) does notdsorb at all Pd(II), sorption capacity remains below 2 mg Pd g−1

esin. Sorption capacities increase with IL loading, up to 71 mgd g−1 EIR (i.e., 0.67 mmol Pd g−1 EIR, or 0.87 mmol Pd mmol−1 IL).omparable sorption capacities were obtained by Parajuli et al.

sing lignophenol derivatives (obtained by dimethylamine immo-ilization) [48]. This is about 2 times lower than the levels reachedith Cyphos IL-101 immobilized in biopolymer capsules under

ig. 4. Pd(II) sorption isotherms using Amberlite XAD-7 and Cyphos IL-101 impreg-ated resin (at different IL loadings) from 0.01 M HCl solution (a), and from 0.5 MCl solution (b) (T: 20 ◦C; lines represent the modeling of experimental data using

he parameters of the Langmuir equation reported in Table 2).

comparable experimental conditions (i.e., 0.1–2 M HCl solutions)[36]. Wołowicz and Hubicki [17] used a series of synthetic resinsfor extraction of Pd(II) complexes: sorption capacities reached val-ues close to 20 mg Pd g−1 at weakly acidic pH (i.e., pH 1) but sorptioncapacity significantly decreased with increasing HCl concentra-tion. Higher sorption capacities were obtained by Parodi et al. [15]using an imidazol-based resin (i.e., 180, 130 and 60 mg Pd g−1 for0.1, 1 and 2 M HCl concentrations, respectively). Fujiwara et al. [7]reached sorption capacities close to 100 mg Pd g−1 using l-lysine-modified cross-linked chitosan at pH 2, while Ramesh et al. [9]immobilized glycine on chitosan for Pd(II) binding: they reacheda sorption capacity close to 120 mg Pd g−1 at pH 2. At the same pH,Ruiz et al. [49] reached sorption capacities as high as 200 mg Pd g−1

for glutaraldehyde cross-linked chitosan. The modification of chi-tosan (obtained by grafting supplementary amine groups, such aspolyethyleneimine, or sulfur groups) allowed both increasing sorp-tion capacity and decreasing the impact of pH [5,10].

The maximum sorption capacity (qm value in Table 2) was plot-ted versus IL loading (Fig. 5). This figure is supposed to show thecorrelation between IL loading and maximum sorption capacity. Athigh HCl concentration (0.5 M), the sorption only proceeds throughinteraction of the metal anions with the IL and the sorption capacitycan be directly correlated to IL loading. At low HCl concentration(0.01 M), as pointed out above, sorption may proceed through thecombined effect of binding on the resin (free form, interaction withhydroxo and chlorohydroxo complexes) and interaction with theIL (anionic chlorocomplexes with phosphonium cation). At low ILloading, Amberlite XAD-7 is able to bind small (but non negligi-

ble) amounts of Pd(II) and the sorption capacity is over-evaluatedwhen compared to the curve representing the sorption capacityas a function of IL loading. As the IL loading increases the inter-nal surface is progressively covered by the IL, reducing the surface

Fig. 5. Correlation between Pd(II) maximum sorption capacity and IL loading for0.01 M and 0.5 M HCl solutions (T: 20 ◦C).

2 eering Journal 185– 186 (2012) 226– 235

aXtaactw

lc1iin(mPtuo0iatAt

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3

litd

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Fig. 6. Pd(II) uptake kinetics. Influence of IL loading at constant sorbent dosage (SD:0.4 g EIR L−1; T: 20 ◦C; CHCl: 0.5 M; v: 150 rpm; C0: 25 mg Pd L−1; lines represent themodeling of experimental data using the intraparticle diffusion coefficients of theCrank equation reported in Table 3).

Fig. 7. Pd(II) uptake kinetics. Influence of IL loading varying sorbent dosage to obtainsimilar equilibrium metal concentrations. qCyphos IL-101: 106 mg IL g−1 (SD: 6.45 gEIR L−1); qCyphos IL-101: 207 mg IL g−1 (SD: 1.07 g EIR L−1); qCyphos IL-101: 401 mg IL g−1

comparable and this may explain similar initial slopes. However,Table 3 shows that the intraparticle diffusion coefficient signif-icantly decreases with increasing IL loading (from 1.05 × 10−10

to 1.6 × 10−11 m2 min−1). Similar trends were observed for Pd(II)

32 R. Navarro et al. / Chemical Engin

vailable for binding Pd(II) (through the interaction of AmberliteAD-7 with hydroxo and chlorohydroxo complexes) and the sorp-

ion only proceeds through ion exchange (electrostatic attraction ofnionic chlorocomplexes with phosphonium cations). The discrep-ncy observed at the first point of the curve was thus expectable andonfirms previous hypotheses. Removing this point makes sense forhe determination of the stoichiometric ratio between Pd(II) and IL,hich is determined by the slope of the curves qm = f(qCyphos IL101).

Since the slope was determined using a limited number of ILoadings this should be taken as indicative values. However in bothases (i.e., 0.01 M and 0.5 M HCl solutions), the slope was close to; this is consistent with the results showed by Vincent et al. [36]

n the case of Cyphos IL-101 immobilized in alginate capsules. Thiss also consistent with the results obtained by Cieszynska and Wis-iniewski [28] for Pd(II) solvent extraction using Cyphos IL-101dissolved in toluene): in 0.1 M HCl solutions the stoichiometric

olar ratio was close to 1 (indicating the interaction of R3R′P+ anddCl3−). It is noteworthy to observe on Fig. 5 that the projection ofhe curves does not pass by 0 (see equations included on the fig-re). The curves tend to indicate that a fraction of IL immobilizedn the EIR does not serve to bind metal. This fraction is evaluated to.04 mmol IL g−1 EIR in 0.01 M HCl solutions and to 0.07 mmol IL g−1

n 0.5 M HCl solutions. Similar trends were observed for cadmiumnd gold sorption using the same EIR [30,34]. A fraction of the IL isightly bound to the support losing its reactivity for metal binding.t low HCl concentration, the reactivity of the support contributes

o minimize this inactive fraction.The effect of temperature on Pd(II) sorption isotherms (in 0.5 M

Cl solutions) was tested (Fig. AM4, Additional Material Sec-ion). The maximum sorption capacity does not significantly varybetween 70 and 73 mg Pd g−1 EIR), while the variation in theffinity coefficient (b coefficient) hardly changed (between 0.21nd 0.29 L mg−1). These variations are not sufficient for estimat-ng the thermodynamic parameters of the reactions (enthalpyhange, free energy and entropy) and to conclude on thendothermic/exothermic characteristic of the sorption process. Theemperature hardly influences the sorption process (at least in theemperature range investigated in the study). This means that therocess involves low-energy mechanisms that are consistent withhe ion exchange reaction proposed for describing Pd(II) recovery.

.4. Uptake kinetics

Uptake kinetics have been carried out varying agitation speed, ILoading, metal concentration and temperature. Table 3 reports thentraparticle diffusion coefficients obtained from the Crank’s equa-ion. These coefficients have been used for simulating experimentalata (lines on Figs. 6–9).

Effect of agitation speed: Increasing agitation speed hardly affectsd(II) sorption kinetics (Fig. AM5, Additional Material Section). Thenitial section of the kinetics can be controlled by the resistanceo film diffusion: when increasing the agitation speed the initiallope increases. The variation of the intraparticle diffusion coeffi-ient with agitation is negligible (1.6 × 10−11 m2 min−1 at 150 rpmo 1.7 × 10−11 m2 min−1 at 250 rpm). These results are consistentith previous comments on the predominance of the resistance to

ntraparticle diffusion (Fig. AM1): the SCM-PD model fitted exper-mental data demonstrating that the kinetics are controlled byntraparticle diffusion and that the resistance to film diffusion cane neglected.

Effect of IL loading: IL loading influences the density of reactiveroups and then the amount of metal that can be adsorbed. Obvi-

usly, increasing IL loading, for a given sorbent dosage, decreaseshe equilibrium concentration. Fig. 6 shows that the initial slopef the curve is weakly influenced by IL loading: the initial sec-ions of the kinetic curves are very close. Since the sorbent dosage

(SD: 0.4 g EIR L−1) (T: 20 ◦C; CHCl: 0.5 M; v: 150 rpm; C0: 25 mg Pd L−1; lines representthe modeling of experimental data using the intraparticle diffusion coefficients ofthe Crank equation reported in Table 3).

is constant the external surface area available for reaction is

Fig. 8. Pd(II) uptake kinetics. Influence of metal concentration (CHCl: 0.5 M; v:150 rpm; T: 20 ◦C; qCyphos IL-101: 401 mg IL g−1; SD: 0.4 g EIR L−1; lines represent themodeling of experimental data using the intraparticle diffusion coefficients of theCrank equation reported in Table 3).

R. Navarro et al. / Chemical Engineering Journal 185– 186 (2012) 226– 235 233

Table 3Intraparticle diffusion coefficient (calculated with the Crank’s equation; CHCl: 0.5 M).

T (◦C) v (rpm) C0 (mg Pd L−1) qCyphos IL-101 (mg IL g−1 EIR) SD (g EIR L−1) De × 1011 (m2 min−1)

20 150 25 106 0.4 10.520 150 25 207 0.4 6.420 150 25 401 0.4 1.620 150 25 207 1.07 6.820 150 25 106 6.45 20.020 150 15 401 0.4 1.04

saishisatgsoieioictsoibcmcsfthghT

F2rfi

20 150 35 401

20 250 25 401

40 150 25 401

orption using Cyphos IL-101 immobilized in alginate capsules [36],nd for Cd(II) [30] and Au(III) [34] sorption using Cyphos IL-101mmobilized in Amberlite XAD-7 resins. Actually, in a previoustudy, Amberlite XAD-7 resins impregnated with Cyphos IL-101ave been extensively characterized for textural properties, includ-

ng superficial surface area (SSA, m2 g−1), pore volume, and poreize [30]. Increasing IL loading resulted in a drastic decrease of SSAnd pore volume but an increase of the size of the pores (becausehe smallest pores were progressively filled with the IL inducing therowing of the size of detectable pores by BET analysis). Progres-ively filling the pores of the resins with the IL makes mass transferf the metal less efficient since the diffusivity of the metal in waters higher than in the solvent phase. This can, at least partially,xplain the decrease in mass transfer properties of the EIR whenncreasing IL loading. The comparison of kinetics profiles changingnly IL loading is difficult since the increase of IL loading resultsn higher sorption capacity and lower residual concentration. Thehange in the concentration gradient between aqueous phase andhe center of the EIR makes more complex the accurate compari-on of kinetic performance. For this reason, a complementary seriesf experiments was performed adjusting sorbent dosage for eachmpregnated resin (with different IL loading) to insure compara-le levels of residual concentration) (Fig. 7). Hence, the gradientoncentration (solution/center of the particle) remains approxi-ately equivalent for the different experiments. Initial slope of the

urves strongly increased with diminishing the IL loading (higherorbent dosage) due, at least partially, to a greater external sur-ace area. Additionally, based on previous comments, it is expectedhat experiments performed with lower IL loading (higher SD) willave faster mass transfer since these experiments correspond to

reater specific surface area, higher pore volume and consequentlyigher mobility of metal ions in the internal porous network.his is confirmed by the comparison of the diffusion coefficients

ig. 9. Pd(II) uptake kinetics. Influence of temperature (T: 20 ◦C; CHCl: 0.5 M; C0:5 mg Pd L−1; qCyphos IL-101: 401 mg IL g−1 EIR; SD: 0.4 g EIR L−1; v: 150 rpm; linesepresent the modeling of experimental data using the intraparticle diffusion coef-cients of the Crank equation reported in Table 3).

0.4 2.20.4 1.70.4 4.9

that increased from 1.6 × 10−11 to 2.0 × 10−10 m2 min−1, underselected experimental conditions. Actually, the diffusion coef-ficient followed an exponential trend versus the SSA (i.e., De

(10−11 m2 min−1) = 0.98 exp[0.0125 SSA (m2 g−1)]).Effect of metal concentration: Metal concentration was var-

ied between 15 and 35 mg Pd L−1, Fig. 8 shows that the kineticprofiles were very similar in this concentration range (homo-thetic variation). The intraparticle diffusion coefficient linearlyincreased with initial metal concentration from 1.04 × 10−11 to2.2 × 10−11 m2 min−1. Despite the positive effect of increasing theconcentration gradient, this parameter (at least in this concentra-tion range) hardly affects the resistance to intraparticle diffusion.In the case of Zn(II) sorption with the same EIR, Gallardo et al.[32] observed a slight decrease of intraparticle diffusion coefficientwith increasing metal concentration. The same trend was observedfor Cd(II) sorption using the same EIR [30]. On the contrary, forAu(III), Navarro et al. [34] concluded that the intraparticle diffu-sion coefficient increased with increasing metal concentration. Inthe case of Pd(II) sorption using Cyphos IL-101 immobilized inalginate capsules, the coefficient De remained almost unchangedbetween 10 and 30 mg Pd L−1 (varying between 0.7 × 10−11 and0.9 × 10−11 m2 min−1).

Effect of temperature: Fig. 9 compares the kinetic profiles for20 and 40 ◦C, under comparable experimental conditions. Sorptionisotherms showed a limited impact of temperature on equilibrium.This is confirmed on this experiment: the residual concentra-tion was hardly changed by increasing temperature. The initialslope of the curve increases with temperature and the equilib-rium is reached significantly earlier at 40 ◦C than at 20 ◦C (around24–30 h versus 72–96 h). The enhancement of mass transfer is con-firmed by the variation of the intraparticle diffusion coefficient thatwas increased from 1.6 × 10−11 to 4.9 × 10−11 m2 min−1. Similarimprovement was observed for Cd(II) sorption using the same EIR[30]. Among the possible reasons for this improvement of masstransfer the most probable is the effect of temperature on the vis-cosity of the IL in the porous network. According the technical noteof Cytec for Cyphos IL-101, the introduction of a solvent in theIL and/or the increase of temperature gives to the IL a water-likebehavior in terms of viscosity. Previous hypotheses concerning thedecrease in diffusion properties with high IL loading pointed outthe impact of filling porous network with a liquid inducing lowerdiffusivity (compared to water). Decreasing solvent viscosity con-tributes to improving diffusivity of the metal in this phase, whichin turn improves metal mass transfer.

3.5. Metal desorption and resin recycling

Metal desorption and resin recycling are key parameters in theevaluation of the sorption process with EIRs. Taking into account

the cost of the EIRs it is important to demonstrate the possibil-ity to re-use the sorbent. Additionally, the second objective to thedesorption step is to contribute to the enhancement of the concen-trating effect of the sorption process (his happens when metal is

234 R. Navarro et al. / Chemical Engineering

F2

cotuo

diFtasdrastbaimaiewam

sTdswdue

ig. 10. Metal desorption and resin recycling (qCyphos IL-101: 401 mg IL g−1 EIR; T:0 ◦C; SD: 0.4 g EIR L−1. Extraction conditions: CHCl: 0.5 M; C0: 25 mg Pd L−1).

ompletely recovered in a minimal volume of solution). In this partf the work, the objective was essentially to evaluate the possibilityo remove the metal (choosing the appropriate eluent) and to re-se the EIR, complementary studies would be necessary to optimizeperating conditions for reaching optimum concentrating effect.

Based on previous experience on the desorption of PGMs usingifferent sorbents, two eluents were selected for in-deep test-

ng: nitric acid (5 M) and thiourea (1 M in 0.5 M HCl solution).ig. 10 shows the test of EIR recycling for a series of five sorp-ion/desorption cycles. With nitric acid metal was completelydsorbed (experimental conditions correspond to an excess oforbent) while the desorption efficiency tended to progressivelyecrease down to 76%. Two successive desorption steps were car-ied out to evaluate the possibility to remove complementarymounts of metal. The amount of metal released at the secondtep does not exceed 2.3%, indicating that the amount of metalhat remained on the EIR was probably tightly bound to the sor-ent. This means that the resin will progressively saturate leaving

limiting number of reactive groups for metal binding. Thioureas most efficient than nitric acid in terms of metal desorption and

ore specifically in terms of resin recycling. However, the recoverynd valorization of Pd from eluate will be much more complicatedn the presence of a ligand such as thiourea compared to a simplelution with nitric acid. So, for further application, the selectionill have to take into account not only the efficiency of the process

nd the life cycle of the resin but also the possibility to valorize theetal.The second panel of Fig. 10 shows that thiourea (in the HCl

olution) significantly improved the global efficiency of the EIR.hree washing steps (with 1 M HCl solution) were operated afteresorption in order to remove the thiourea from the EIR. Metalorption remained complete (at the exception of the third cycle

hen the sorption efficiency slightly decreased to 98% (probablyue to an incomplete washing of the EIR: the presence of resid-al traces of thiourea may contribute to decreasing the sorptionfficiency of the sorbent). In this case, again the second desorption

Journal 185– 186 (2012) 226– 235

step does not bring significant improvement in the desorption effi-ciency (amounts released were down to 1.6%). Contrary to nitricacid solutions the desorption remained complete over five cycles.Thiourea (1 M) in acidic conditions (0.5 M HCl solutions) revealsthe most efficient eluent allowing EIR recycling for a least fivecycles, at the expense of a careful washing of the EIR betweendesorption and sorption steps to prevent its release during the sorp-tion step (the binding of the metal with thiourea in the solutionreduces the availability of the metal for binding to IL). Thioureahas been frequently used for PGMs desorption from loaded sor-bents [5,7,9,13,15,36]. For example Fujiwara et al. [7] tested HCl,thiourea/HCl, NaOH and KCN/NaOH for Pt(IV), Pd(II) and Au(III) des-orption from lysine-modified cross-linked chitosan. They obtainedthe best results using thiourea in acidic conditions and potas-sium cyanide in alkaline media. The desorption efficiency increaseswith concentration (of the complexing agent and the acid/alkalineagent). In the case of Cyphos IL-101 immobilized in alginate cap-sules, Vincent et al. [36] also tested nitric acid and thiourea for Pd(II)desorption, the desorption exceeded 90% but both sorption anddesorption decreased with the number of cycles. The immobiliza-tion of the IL in the biopolymer capsule induces some restrictionsin the performance of the process, probably due to limitations inthe transfers of the eluent, the metal and the relevant complexes.

4. Conclusion

The immobilization of Cyphos IL-101 in the porous network ofAmberlite XAD-7 allows conferring to the resin interesting sorp-tion properties for Pd(II) in HCl solutions. With sorption capacitiesas high as 70 mg Pd g−1 EIR, the EIR has higher sorption per-formance in 0.5 M HCl solutions to those obtained with someconventional synthetic resins. The interaction occurs through thebinding of chloro-anionic palladate species (PdCl3− and/or PdCl42−)to phosphonium cation of the IL (i.e., R3R′P+) through chloride ionexchange. A fraction of the IL remains bound to the resin surfacebecoming inactive; however, a 1:1 molar ratio (Pd/IL) is obtainedat saturation of the EIR. SEM–EDX analysis shows that the IL ishomogeneously distributed in the resin and that all reactive groupsremain accessible (at saturation the distribution of the metal ishomogeneous in the particle).

Uptake kinetics are controlled by the resistance to intraparticlediffusion and the effect of film diffusion can be considered negli-gible. Temperature, affecting IL viscosity in the porous network ofthe resin, reduces resistance to intraparticle diffusion. IL loadingalso influences mass transfer performance: increasing the loadingof the EIR reduces the specific surface area and pore volume of theEIR since the filling of the porous network limits the diffusivity ofmetal ions in the solvent phase (compared to their diffusivity inthe aqueous phase). The intraparticle diffusion coefficient variesbetween 1 × 10−11 and 2 × 10−10 m2 min−1. This is consistent withthe values obtained with the same EIR for the sorption of othermetal ions.

Metal desorption can be operated using thiourea (1 M, in 0.5 MHCl solution) preferentially to nitric acid (5 M). Metal removal iscomplete and the EIR can be reused for at least five cycles, main-taining constant sorption and desorption efficiencies. However, therecycling of the resin requires an extensive acidic washing after des-orption to remove any trace of thiourea that could compete withthe EIR for binding to Pd(II) at the next sorption step.

Acknowledgments

Authors thank the University of Guanajuato (CIAI 2010, 122/10)for financial support. Cytec (Canada) is acknowledged for the giftof Cyphos IL-101 sample. Authors acknowledge J.-M. Taulemesse

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R. Navarro et al. / Chemical Engine

Centre des Matériaux de Grande Diffusion at Ecole des Mines’Alès) for SEM–EDX analyses.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.cej.2012.01.090.

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