Extraction of Fe(III) from hydrochloric acid solutions using Amberlite XAD-7 resin impregnated with trioctylphosphine oxide (Cyanex 921)

Hydrometallurgy 98 (2009) 257–266

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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t

Extraction of Fe(III) from hydrochloric acid solutions using Amberlite XAD-7 resin impregnated with trioctylphosphine oxide (Cyanex 921) R. Navarro a,⁎, V. Gallardo a, I. Saucedo a, E. Guibal b a b

Universidad de Guanajuato, Instituto de Investigaciones Científicas, Cerro de la Venada s/n, Pueblito de Rocha, C.P. 36040 Guanajuato, Gto., Mexico Ecole des Mines d'Alès, Laboratoire Génie de l'Environnement Industriel, 6 Avenue de Clavières, F-30319 Alès Cedex, France

a r t i c l e

i n f o

Article history: Received 12 September 2008 Received in revised form 14 April 2009 Accepted 21 May 2009 Available online 27 May 2009 Keywords: Cyanex 921 Trioctylphosphine oxide Extractant impregnated resin Fe(III) Sorption Desorption Kinetics Diffusion Solvation mechanism

a b s t r a c t Ferric ions were efficiently removed from HCl solutions using Amberlite XAD-7 resin impregnated with trioctylphosphine oxide (Cyanex 921). Iron was removed under the form HFeCl4 through direct binding on the resin or by extraction with Cyanex 921 involving a solvation mechanism. High concentrations of HCl and intermediary extractant loadings were required for maximum sorption efficiency and rationale use of the extractant. At intermediary extractant loading (in the range 300–450 mg Cyanex 921 g− 1) the maximum sorption capacity increased with extractant loading. Maximum sorption capacity slightly increased with temperature, the reaction is endothermic and the enthalpy change was found close to − 30.8 kJ mol− 1. Sorption isotherms were fitted with the Langmuir equation and maximum sorption capacity reached values as high as 20–22 mg Fe g− 1 in 3 M HCl solutions. Despite the good fit of experimental data with the pseudo second-order rate equation, sorption kinetics was controlled by the resistance to intraparticle diffusion. The intraparticle diffusion coefficient (De) varying in the range 1.2 × 10− 11–4.7 × 10− 10 m2 min− 1 was found to increase with metal concentration and with temperature, while varying the extractant loading it reached a maximum at a loading close to 453 mg Cyanex 921 g− 1. The desorption of Fe(III) can be achieved using 0.1 M solutions of nitric acid, sulfuric acid, sodium sulfate and even water, maintaining high efficiencies for sorption and desorption for at least 5 cycles. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The regulations in a number of countries are imposing the recovery of valuable materials from waste before the material can be disposed of. The recovery and the recycling of metals from secondary resources (i.e., waste material, used catalysts and batteries and so on) is thus a key point for a sustainable growth. Two ways are usually cited for the recovery of metals from waste materials: the wet route (hydrometallurgy) and the dry route (pyrometallurgy) (Brooks, 1991). In the case of hydrometallurgy, the waste material is leached and the metals can be recovered from leachates using conventional processes: precipitation (with the drawback of poor selectivity) (Navarro et al., 2007a), resins and adsorbents (more or less competitive when applied to dilute acidic solutions for base metals) (Bassi et al., 1999; Trochimczuk, 2002), membrane processes (usually expensive) (Ortega et al., 2007), electrowinning (Rudnik and Nikiel, 2007), liquid/liquid extraction (preferable in the case of concentrated solutions) (Almela et al., 1998; Mantuano et al., 2006; Ribeiro et al., 2004; Salgado et al., 2003). The major drawback for liquid/

⁎ Corresponding author. Tel./fax: +52 473 7327468. E-mail address: [email protected] (R. Navarro). 0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2009.05.009

liquid extraction consists in the loss of expensive and sometimes toxic solvents, for this reason impregnated resins have been considered an alternative process that combines the easy management of solid sorbents and the high efficiency and selectivity of extractants (Cortina et al., 1995; Juang, 1999; Kabay et al., 1998; Kabay et al., 2003; Shiau and Juang, 1998; Warshawski, 1981). The extractant impregnated resins (EIR) can be prepared by a series of processes including the direct incorporation of the extractant during the synthesis of the resin. However, the most common process consists in the impregnation of the resin with the extractant previously diluted in an appropriate solvent. The solvent will be removed by evaporation after complete wetting of the resin with the impregnation solution. A number of extractants and resins have been used over the last decades for the preparation of EIR; the selection of the couple extractant/resin should take into account the properties of the support (i.e., hydrophobicity, porous characteristics…) and the properties of the extractant (i.e., its affinity for the target metals, its polarity…). Phosphine derivatives have proved to be very efficient for the binding of metal cations through a series of mechanisms including ion exchange or solvation (Alguacil and Alonso, 2004; Martinez and Alguacil, 2000; Navarro et al., 2007b; Pawar and Dhadke, 2005; Pawar et al., 2002; Remya and Reddy, 2004; Rodriguez et al., 2005). On the other hand, Amberlite resins are supports

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commonly used for the preparation of EIR (Koshima, 1986; Tewari and Singh, 2002, Bourget et al., 2005; Cortina et al., 1998; Hinojosa Reyes et al., 2001; Merdivan et al., 2001); they have been designed with different characteristics of porosity (pore volume and pore size) and with different surface characteristics, being hydrophobicity controlled by the type of polymer (acrylic-ester, or styrene-divinyl benzene polymers). This work is part of a series of studies dealing with the immobilization of Cyanex 921 (trioctyl phosphine oxide, TOPO) in Amberlite XAD-7 resin for the preparation of sorbents tailored for Zn(II) (Navarro et al., 2007b) and Cd(II) recovery (Navarro et al., 2008b). These metals are generally found in the leachates of batteries together with Fe(III) (Mantuano et al., 2006). The present work evaluates the impact of HCl concentration on Fe(III) recovery as a function of extractant loading, sorption isotherms are established in order to evaluate the maximum sorption capacity and the thermodynamic characteristics of the sorption system. The kinetics is discussed in order to evaluate the limiting steps. The kinetic profiles are compared for different experimental conditions (temperature, metal concentration and extractant loading). Finally the recycling of the resin is tested over 5 cycles using water, sodium sulfate, sulfuric acid and nitric acid solutions as eluents. Knowing the characteristics of the EIR for Fe(III) recovery in conjunction with the sorption properties of the EIR for Cd(II) and Zn(II) will make possible anticipating the possibility for separating these metals.

2.2. Resin impregnation In the present work the extractant was immobilized on the resin by a physical technique. Different processes may be used for the physical impregnation of the resin including (i) the wet method, (ii) the dry method, (iii) the impregnation in the presence of a modifying agent, or (iv) the dynamic method (Wu et al., 1999). Previous studies have shown that the dry method increases the stability of the extractant on the resin. The dry impregnation of the resin was actually performed by contact of 5 g of conditioned Amberlite XAD-7 with 25 mL of ketone for 24 h (Hinojosa Reyes et al., 2001). Varying amounts of Cyanex 921 diluted in ketone (0.5 M) were added to resin slurry for 24 h, under agitation. The solvent was then slowly removed by evaporation in a roto-vapor. The amount of extractant immobilized on the resin (qCyanex 921) was quantified by the following procedure. A known amount of impregnated resin (250 mg) was mixed with methanol (5 mL) for 24 h to remove the extractant by dissolving. The washing treatment was repeated once. The solvent was finally separated from the resin, which was dried at 100 °C for 24 h. The mass difference (MCyanex 921) between impregnated (MXAD-7/Cyanex 921) and washed resin (MXAD-7) was used to calculate the amount of extractant immobilized on the EIR: qCyanex 921 =

MXAD
7=Cyanex 921 − MXAD
7 : MXAD
7=Cyanex 921

ð1Þ

The experimental procedure allowed the preparation of EIR containing 28 mg extractant g− 1 up to 652 mg extractant g− 1.

2. Materials and methods 2.1. Materials

2.3. Sorption procedures Amberlite XAD-7 was supplied by Sigma-Aldrich (Saint-Louis, U.S.A.). This is a polyacrylic acid ester type resin ([CH2–CH(COOR)–]n). The physical characteristics of the resin are summarized in Table 1 (Navarro et al., 2007b). Amberlite XAD-7 can be considered as a nonionic, moderately hydrophilic macroporous polymer. The resin was conditioned by the supplier with NaCl and Na2CO3 to retard bacterial growth. It was necessary to clean it to remove salts and monomeric material present on the resin. The resin was therefore put into contact with ketone for 24 h at 25 °C. After filtration under vacuum to remove excess ketone, the resin was rinsed with de-mineralized water. It was then washed with nitric acid (0.1 M) for 24 h. The resin was filtered under vacuum and then rinsed with de-mineralized water to constant pH. Finally, it was put into contact with ketone for 12 h before being filtered under vacuum and dried in a roto-vapor at 80 °C. Cyanex 921 was supplied by Cytec (Canada) in the form of a white solid, with higher solubility in aromatic diluents than aliphatic solvents; it is the trioctyl phosphine oxide extractant (TOPO). The chemical structure is (C8H17)3PfO, it is a neutral organo-phosphorous extractant with a solvating activity. The molecular weight is 386 g mol− 1. Due to the length of aliphatic chains the solubility of the extractant in water is relatively low, below 1.5 mg L− 1, and even below in acidic solutions. Other reagents (salts, acids…) were analytical grade and supplied by KEM (Mexico). Standard metal solutions were supplied by Perkin Elmer (U.S.A.).

Table 1 Physical properties of Amberlite XAD-7. Particle size Superficial area Resin porosity Pore size (mean value) Pore volume Skeletal density

20/60 mesh–250/850 µm 450 m2 g− 1 0.55 85–90 Å 0.97–1.14 cm3 g− 1 1.24 g cm− 3

Fe(III) solutions were prepared in HCl solutions of different concentrations (0.5–8 M) with metal concentrations ranging between 20 and 150 mg Fe L− 1. The sorption experiments were performed by mixing the resin with Fe(III) solutions for 24 h with a solid/liquid ratio fixed to m / V = 4 g L− 1 (m: mass of sorbent, V: volume of solution). The contact was operated on a reciprocal shaker (Cole Parmer 51502) with an agitation speed of 150 movements per minute at constant temperature. After filtration the samples were analyzed by atomic absorption spectrometry (AAS Perkin Elmer 3110). The amount of metal adsorbed (q, mg Fe g− 1) was calculated by the mass balance equation: q = V(C0 − Ceq) / m, where C0 and Ceq (mg Fe L− 1) are the initial and equilibrium Fe(III) concentrations, respectively. Several sorption kinetic experiments were performed by contact under agitation of a fixed amount of EIR (loading in the range 115– 575 mg extractant g− 1) with a fixed volume (m /V: 4 g L− 1) of 3 M HCl solution containing varying concentrations (in the range 60–140 mg Fe (III) L− 1). Temperature was varied in the range 10–40 °C. Samples were collected at fixed times and analyzed for determination of residual concentrations. 2.4. Desorption and recycling procedures For the study of Fe(III) desorption, an amount of 100 mg of EIR (extractant loading: 453 mg extractant g− 1) was mixed with 25 mL of Fe(III) solution (3 M HCl solution, initial metal concentration: 40 mg Fe L− 1) for 24 h. The residual concentration measured by AAS after filtration served to determine the amount of metal bound to the resin. The metal-loaded resin was mixed for 24 h with 25 mL of a series of eluents: water, 0.1 M Na2SO4, 0.1 M HNO3, 0.1 M H2SO4. After filtration the concentration in the eluate was determined by AAS in order to obtain the amount of Fe desorbed from the resin and to calculate the desorption yield. For the evaluation of sorption/desorption cycles, the same procedure was used for 5 cycles. Several desorption kinetic experiments were performed by collecting at fixed times samples of the

R. Navarro et al. / Hydrometallurgy 98 (2009) 257–266

259

115, 145, 196, 294, 444 and 575 mg Cyanex 921 g− 1, respectively. In the case of the highest extractant loading (i.e., 652 mg g− 1), the experimental curve perfectly superimposed the 575 mg Cyanex 921 g− 1 curve. This is a first indication that an excess of extractant does not improve sorption efficiency of the EIR. 3.2. Interpretation of sorption mechanism The efficiency of sorption was improved when increasing HCl concentration and Cyanex 921 loading as shown in Fig. 1(a). The positive effect of increasing HCl concentration is probably related to the change in metal speciation. Fig. 2 shows the species distribution diagram of Fe(III) with varying HCl concentrations. The diagram has been established using the consecutive stability constants cited by Bjerrum and Lukes (1986), derived from Gamlen and Jordan (1953), appearing in Eqs. (2.a)–(2.d): −

3þ

2þ

þ Cl ⇆ FeCl

Fe

2þ

FeCl

þ

−

þ

þ Cl ⇆ FeCl2 −

FeCl2 þ Cl ⇆ FeCl3 −

−

FeCl3 þ Cl ⇆ FeCl4

K1 ¼ 30

ð2:aÞ

K2 ¼ 4:5

ð2:bÞ

K3 ¼ 0:73

ð2:cÞ

K4 ¼ 0:0105

ð2:dÞ

In order to take into account the ionic strength influence, these formation constants were determined as combined constants as a function of complex concentrations and chloride activity: Fig. 1. Influence of Cyanex 921 loading and HCl concentration on Fe(III) sorption using EIR — (a) sorption efficiency versus HCl concentration; (b) distribution ratio versus extractant loading (log–log plot) (C0: 20 mg Fe L− 1; T: 20 °C).

solution containing the appropriate eluent in contact with metalloaded resin (metal binding from 3 M HCl solutions). 3. Results and discussion 3.1. Influence of extractant loading and HCl concentration on Fe(III) recovery A series of EIR loaded with increasing amounts of Cyanex 921 has been prepared and tested for the recovery of Fe(III) (C0: 20 mg Fe L− 1) from HCl solutions. The concentration of HCl was varied between 0.5 M and 8 M. Fig. 1(a) shows the sorption efficiency of the resin for selected experimental conditions. The first evidence is that increasing the concentration of HCl improved significantly the efficiency of Fe(III) removal. Trochimczuk (2002) also observed a strong increase in the sorption of Fe(III) by phosphorus-containing resins when increasing HCl concentration. At low extractant loading (i.e., 28 mg Cyanex 921 g− 1) maximum sorption efficiency was obtained for 6 M HCl; when increasing HCl concentration above 7 M metal recovery tended to decrease. This curve was very close to that obtained with Amberlite XAD-7 (free of extractant): the optimum of Fe(III) extraction was obtained close to 7 M HCl concentration. At low Cyanex 921 loading (below 100 mg g− 1), Fe(III) extraction remained negligible when HCl concentrations were below 3 M. Above 3 M sorption efficiency sharply increased; however, Fe(III) recovery did not exceed 70% (except for EIR loaded with 68 mg Cyanex 921 g− 1 at 8 M HCl). Complete metal recovery (under selected experimental conditions) was only observed when extractant loading in the EIR reached 115 mg g− 1. For these high loadings, a minimum HCl concentration was required to reach sorption efficiencies greater than 95%. This limit concentration significantly decreased when extractant loadings increased: 7 M, 6 M, 5 M, 4 M, 3 M and 2 M for extractant loadings of

Ki =

½FeCli  ½FeCli  = ½FeCli − 1 aCl − ½FeCli − 1 γFHCl CHCl

ði = 1 N 4Þ:

ð3Þ

In this case, as a first approximation, the authors considered the ratio of activity coefficients of consecutive iron chloro-complexes (γFeCli/γFeCli − 1) as a constant included in the combined stability constant values (Ki). The original combined formulation of Gamlen and Jordan (1953) was retained in order to employ the reported Ki value. The chloride activity (aCl− = γ±HClCHCl) was estimated by the mean molar activity coefficient of HCl (γ±HCl), and the values at different molar HCl concentration (CHCl) were calculated from Robinson and Stokes (1970). The obtained species distribution diagram of Fe(III) in HCl (Fig. 2) was used to explain the Fe(III) extraction behavior. The recovery of Fe(III) by extractant-free resin may be explained by several hypotheses including an interaction between HCl and carbonyl group of the resin (polyacrylic acid ester, [CH2–CH(COOR)–]n). In the case of gold sorption, at high HCl concentration, Villaescusa et al. (1992) suggested a mechanism involving the hydrolysis of polyacrylic acid ester functions that are converted into RCOOH functions, which in turn are

Fig. 2. Species distribution diagram of Fe(III) as a function of HCl concentration.

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R. Navarro et al. / Hydrometallurgy 98 (2009) 257–266

able to bind Au(III) through ion pair formation (HAuCl4). In the case of nanofiltration membrane systems, Navarro et al. (2008a) observed that the immersion of polyamide membranes ([R–NH–(CfO)R]n) in acidic solutions resulted in changes of chemical nature and charge of the membranes: the modification was caused by the partial hydrolysis of the polymers. The mineral acids are retained by the polymer matrix of the membrane, involving interactions such as fO··H+X−. In the case of XAD-7 resin, similar behavior can be expected: HCl is extracted by the resin through interaction of oxygen of polyacrylic matrix (fO··H+Cl−). Chloride ions can be exchanged with the anionic complex according to: fO···H+FeCl− 4 . In the same way, Koshima (1986) reported the sorption of a series of trivalent metals, including Fe(III) on Amberlite XAD-7 resins in highly acidic solutions and concluded that the metals were adsorbed by an interaction between protonated oxygen atoms of polyacrylate and chloro-complexes to form fO··H+MCl− 4 . Similar reactions have been proposed by Laatikainen and Paatero (2005) for the Au(III) extraction on Amberlite XAD-7 in HCl solution. They have proposed two mechanisms for the binding of gold chloro-complexes on the resin: (i) first the protonation of carbonyl groups by HCl, followed by the exchange with tetrachloroauric acid; or alternatively (ii) hydrophobic interactions with adsorbable complexes of low charge. However, they conclude that the protonation mechanism (also observed in mild acidic conditions) is not sufficient to explain the high sorption levels obtained in their study. They consider that hydrophobic interactions may be more likely to justify the high levels on gold sorption they observed. Similar hydrophobic interactions may be involved in the extraction of FeCl− 4 , preferentially to other Fe(III) species. The tetrachloroferrate anion has a tetrahedral structure and it does not contain water ligands thus being the most hydrophobic of the Fe(III) chloro-complexes. The hydrophobicity and the absence of dipole moment have been proposed as crucial factor in the distribution of the complex between aqueous and organic phases (Laatikainen and Paatero, 2005). The interaction of HCl with carboxylic groups on the resin can be described by the following reaction: þ

−

R þ H þ Cl ⇆ RHCl

ð4Þ

Where R and RHCl represent the resin under the neutral form and loaded with HCl, respectively. The predominant form of the resin depends on HCl concentration in the solution: however, it appears difficult obtaining the equilibrium constants due to the complexity in quantifying the amount of HCl adsorbed on the resin. Indeed, the presence of HCl in the aqueous phase filling the pores of the resin makes the differentiation of the two types of acids difficult (i.e., sorbed and immobilized in the aqueous phase). Different reactions may be involved in Fe(III) extraction by the freeextractant resin, depending on the distribution of Fe(III) species (predominant species) (Fig. 2). At low HCl concentration (below 2 M), Fe(III) extraction remained negligible due to the predominance of cationic species (Fig. 1(a)). For HCl concentration ranging between 2 M and 7 M, extraction efficiency linearly increased with the molar fraction of FeCl− 4 . In this range of HCl concentration, FeCl3 is the predominant Fe(III) species; the extraction reaction could be described by Eqs. (5) and (6), depending on the predominant form of the resin. However, Eq. (5) appears more appropriate for describing the increase in the efficiency of metal extraction with the increase of HCl concentration, shown in Fig. 1(a). þ

−

FeCl3 þ R þ H þ Cl ⇆ RHFeCl4

ð5Þ

FeCl3 þ RHCl ⇆ RHFeCl4

ð6Þ

When HCl concentration reaches 8 M, FeCl− 4 is the predominant Fe(III) species in the solution and the extraction reaction could be described by Eqs. (7) and (8), depending on the form of the resin. At this concentration the resin is probably under the form RHCl and

the decrease in the extraction efficiency with the increase of HCl concentrations probably confirms that Eq. (8) best describes the mechanism involved in Fe(III) extraction. −

þ

FeCl4 þ R þ H ⇆ RHFeCl4 −

ð7Þ −

FeCl4 þ RHCl ⇆ RHFeCl4 þ Cl

ð8Þ

This is probably between 7 M and 8 M concentrations that the change in the predominance of R and RHCl can be observed. Considering now the extraction of Fe(III) with the resin impregnated with Cyanex 921 (Fig. 1(a)), the sorption efficiency remained negligible when HCl concentration was below 2 M, especially at low Cyanex 921 loading. Metal binding was only significant with high extractant loadings. This may be explained considering that at low HCl, the positive species (Fe3+, FeCl2+ and FeCl+ 2 ) are the predominant Fe(III) species. These species are poorly reactive with Cyanex 921, which is a neutral organo-phosphorous extractant, well known for extracting metal ions by solvation mechanism. Only metal species present under neutral form or forming ion pairs can be bound to this extractant (Alguacil and Alonso, 2004; Meera et al., 2001; Saji et al., 1998). When HCl concentration is 2 M, the neutral species (FeCl3) is the predominant form and the anionic form (FeCl− 4 ) began to appear (Fig. 2). At low extractant loading sorption efficiency remained negligible but progressively increased with increasing extractant loading. The efficiency of the process increased with increasing HCl concentration, and with the increase of the molar fraction of FeCl− 4 . This species can form ion pairs with protons (HFeCl4) that can be subsequently bound to the extractant. These results are consistent with those obtained by several authors in the extraction of Fe(III) in HCl media using different solvating extractants. El Dessouky et al. (2008) investigated Fe(III) extraction using tributylphosphate (TBP) and Cyanex 921 (L) in kerosene from chloride medium. The extracted species were found to be HFeCl4·2TBP and HFeCl4·L, respectively. Alguacil and Alonso (2000) studied Fe(III) transport from HCl solutions using Cyanex 921 as the carrier in a supported liquid membrane, concluding that iron was transported as HFeCl4·L2. Saji et al. (1998) investigated Fe(III) extraction from HCl + NaCl solutions using Cyanex 923 (a mixture of four trialkylphosphine oxides, TRPO) that exhibits extraction properties similar to that of TOPO. They concluded that Fe(III) is extracted as HFeCl4·2TRPO. Gupta et al. (2002) also reported the extraction of Fe(III) from HCl solutions using Cyanex 923 (TRPO) in toluene; they concluded that the metal was extracted under the form: HFeCl4·2TRPO. These results confirm that Fe(III) is extracted from HCl solutions under the form HFeCl4 associated with a variable number of extractant molecules. The preferential extraction of this species (over other metal species) can be explained by its higher hydrophobicity and the absence of dipole moment for tetrachloroferrate anion. With the objective of identifying the number of molecules of Cyanex 921 involved in the extraction process, the method of slope analysis was used by plotting (in log–log format) the distribution ratio (D, L kg− 1) versus extractant loading for each of the HCl concentration (Fig. 1(b)). The log D varied in the range 0–4.4. It progressively increased with increasing both extractant loading and HCl concentration. The figures do not show a linear trend: changes in the slopes were observed when varying the amount of Cyanex 921 immobilized on the resin. This indicates that various reactions may be involved in the extraction process. However, all these curves follow two general trends characterized by slopes close to 0 and 3, for low and high concentrations of Cyanex 921, respectively. For low concentrations of Cyanex 921 (i.e., qcyanex 921 b100 mg g− 1), the distribution ratio remained almost constant (slope close to 0). This can be explained by the preferential extraction of Fe(III) on the polymer matrix of the resin: the distribution coefficient increased with HCl concentration according to Fig. 1(a) and Eq. (5).

R. Navarro et al. / Hydrometallurgy 98 (2009) 257–266

For higher Cyanex 921 loadings (qcyanex 921 N100 mg g− 1), the slopes of the log–log plot D = f(qcyanex 921) were significantly greater. At low HCl concentration (b2.0 M), the weak sorption of Fe(III) on the polymer matrix suggests that most of the metal was recovered through the extraction of Fe(III) by Cyanex 921. The slope approached 3, indicating that three molecules of the extractant were involved in Fe(III) extraction (FeCl+ 2 being the predominant metal species), according to the following equation (Fig. 2): þ FeCl2

þ

−

þ 3L þ H þ 2Cl ⇆ HFeCl4 L3

ð9Þ

where L represents Cyanex 921 and HFeCl4L3 the tetrachloroferric acid solvated by three molecules of extractant. For concentrations ranging between 2 M and 8 M, the predominant species is FeCl3. At 2 M HCl concentration, the slope was close to 3 and the extraction mechanism is described by: þ

−

FeCl3 þ 3L þ H þ Cl ⇆ HFeCl4 L3

ð10Þ

For HCl concentration increasing (above 3 M) the slope progressively decreased; probably due to an increased contribution of the polymer matrix of the resin in the removal of Fe(III) (according to Eq. (5)). It is interesting to observe that at high extractant loading (i.e., greater than 0.8 mol kg− 1) the distribution ratio tended to stabilize. This should be taken as an indicative tendency since the residual values were close to the analytical limits under selected experimental conditions. In the case of Cd(II) and Zn(II) extraction using similar systems, metal recovery proceeded through the extraction of species such as H2MCl4Lx, where x varies between 2 and 4 for Cd(II), and between 2 and 3 for Zn(II) (Navarro et al., 2007b, 2008b). In order to confirm these hypotheses, the distribution ratio was plotted (log–log plot) against the activity of HCl (aHCl) (aHCl = aH+ = aCl− = γ±HClCHCl); which represents the activity of ions H+ and Cl−, estimated by the mean molar activity coefficient of HCl (γ±HCl) at different molar HCl concentrations (CHCl). The slope of these figures (not shown) gives the number of ions H+ and Cl− involved in the Fe(III) extraction process. The slope changed indicating that various reactions were involved in metal extraction. However, for HCl concentrations in the range 1–6 M, most of the curves showed linear trends with a slope varying between 1.3 and 1.8. This means that equilibrium was mainly controlled by the extraction of FeCl3 by Cyanex 921 immobilized in the resin with the contribution of one ion H+ and one ion Cl− (Eqs. (4) and (9), slope: 2). For lower HCl concentrations (i.e., b2 M), the slopes increased; however, the limited number of experimental data does not allow proposing a clear and definitive interpretation of the mechanism, though Eq. (9), (slope: 3) appears to be the most probable reaction. On the other hand, the extraction of HCl by Cyanex 921 is a secondary reaction that may significantly impact the extraction of metal ions in HCl solutions (due to the competitor effect they may impart). Various studies in liquid–liquid extraction have shown the extraction of HCl by solvating extractants. Martinez et al. (1996, 1997) described the extraction of HCl by Cyanex 923 (in decane) and Cyanex 921 (in xylene), and they suggested that in both cases the acid was extracted under the form HClL. Alguacil and López (1996) also commented that HCl extraction by Cyanex 923 (in toluene and decane) was operated by extraction under the form HClL; however, at high acid concentration they suggested that the acid could be extracted under the form (HCl)2L. Barroso et al. (1997) gave similar interpretation for the extraction of HCl by Cyanex 925. Unfortunately, there is no literature available on the equilibrium constants regarding the extraction of HCl by EIRs. However, as a first approximation for HCl extraction by EIRs systems, HCl extraction studies have been performed using Cyanex 921 without solvent, assuming that the extractant is adsorbed on the resin surface, free of organic solvents. This study has suggested that the predominant species of Cyanex 921 were L, (HCl)L and (HCl)2L in the following HCl

261

concentration ranges: b2 M, 2–8 M and N8 M, respectively (Navarro et al., 2008b). Considering HClL as the predominant species in the 2–8 M HCl concentration range, Eq. (10) can be changed to Eq. (11): 2 molecules of HCl can be released. According to this hypothesis, a slope close to −4 (for D versus aHCl, in log–log plot) was expected, contrary to experimental data that gave a slope in the range 1.3–1.8. þ

FeCl3 þ 3HClL ⇆ HFeCl4 L3 þ 2H þ 2Cl

−

ð11Þ

The extraction of hydrochloric acid by Cyanex 921-impregnated resins was reported as a competitive reaction in the case of Zn(II) (Navarro et al., 2007b) and Cd(II) (Navarro et al., 2008b) extraction at high HCl concentrations. However, for Fe(III) extraction, HCl extraction did not significantly influence metal extraction even at 8 M HCl concentration. Gupta et al. (2002) investigated the extraction of Fe(III) from HCl solutions with Cyanex 923 in toluene. They obtained results that are consistent with the data obtained in the present study: extraction efficiency only decreased at very high HCl concentration (95% in 10 M HCl solutions). This probably means that HCl is part of the solvated species that were extracted: HFeCl4(HClL)3 or HFeCl4L(HClL)2, or that HCl is not extracted as the species HClL with a well-defined stoichiometry but by mechanisms involving non-stoichiometric intermolecular interactions such as those involved in homogeneous solutions or in reverse micelles (Regel-Rosocka and Szymanowsky, 2005). The value of the slopes obtained from experimental data in the present study could thus proceed from the contribution of different reactions involved in the extraction mechanism, mainly those described by Eqs. (5) and (10). To summarize, the extraction process of Fe(III) from HCl solutions on the resin XAD-7 impregnated with Cyanex 921 proceeds from a series of simultaneous reactions, which relative contributions depend on experimental conditions: (i) formation of different Fe(III) chloro-complexes in the aqueous phase, (ii) HCl extraction on the polymer matrix of the resin, (iii) HCl extraction by Cyanex 921, (iv) Fe(III) extraction under the form HFeCl4 on the polymer matrix, and (v) Fe(III) extraction by Cyanex 921 under the form HFeCl4L3, probably with some molecules of HCl being part of the solvate. The complexity of the system makes the identification of the extracted species and the equilibrium constant complex. It is thus difficult to accurately model the extraction processes involved in Fe(III) extraction. 3.3. Sorption isotherms Sorption isotherms have been established for different experimental conditions, varying extractant loading in order to evaluate the stoichiometric ratio between the extractant and the metal at saturation of the EIR, and temperature in order to evaluate thermodynamic parameters. The concentration of HCl was fixed to 3 M, an intermediary concentration of HCl representative of industrial effluents and corresponding to a poor effect of the support (resin) on Fe(III) binding. In all cases, the curves were characterized by a sharp increase of the sorption capacity at low metal concentration followed by a plateau corresponding to the saturation of the EIR. This shape is representative of a Langmuir-type isotherm represented by the equation:

q=

qm bCeq 1 + bCeq

ð12Þ

where q is the sorption capacity (mg Fe g− 1, or mmol Fe g− 1) in equilibrium with Ceq (mg Fe L− 1, or mmol Fe L− 1), qm is the sorption capacity at saturation of the monolayer coverage (mg Fe g− 1, or mmol Fe g− 1), b is the affinity coefficient (L mg− 1, or L mmol− 1). The parameters of the model have been obtained by non-linear regression in order to diminish the statistics bias, compared to the linearization/linear

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regression method (Kinniburgh, 1986). The lines in Figs. 3 and 4 show the modeling of experimental data with the Langmuir equation using the parameter values summarized in Table 2. 3.3.1. Influence of extractant loading on sorption isotherm Fig. 3 confirms that the sorption capacity increased with increasing the amount of extractant immobilized in the EIR. Table 2 summarizes the parameters of the Langmuir equation and more specifically the maximum sorption capacity: when increasing the extractant loading from 293 to 453 mg Cyanex 921 g− 1, the maximum sorption capacity increased from 10.6 to 17.3 mg Fe g− 1. Based on molar units these saturation values correspond to molar ratio Cyanex 921/Fe close to 4.0 and 3.8 for loadings of 293 and 453 mg Cyanex 921 g− 1, respectively. 3.3.2. Influence of temperature on sorption isotherm Sorption isotherms were established at temperatures varying between 10 and 40 °C (Fig. 4(a)). Though the variations are not very marked, a slight increase in sorption capacity can be observed, especially at 40 °C. Table 2 summarizes the values of the parameters of the Langmuir equation; the maximum sorption capacity remained stable between 10 and 30 °C and increased by 20% when increasing the temperature to 40 °C. The affinity coefficient b decreased with temperature increase. The distribution ratio (D, L g− 1) is defined here as initial slope of the sorption isotherm, this also corresponds to the product of qm and b (Tien, 1994):

lim

Ceq Y0

q Ceq

! = bqm = D:

ð13Þ

The distribution ratio depends on the temperature and can be correlated through the Arrhenius plot to the reciprocal of temperature in order to determine the heat of adsorption (ΔH°, enthalpy change, kJ mol− 1) of the sorption process (Barroso et al., 1997; Jia et al., 2002; Martinez et al., 1997; Stumm and Morgan, 1996):

Fig. 4. Influence of temperature on Fe(III) sorption isotherm (a) and thermodynamics of Fe(III) sorption isotherm (Arrhenius plot of ln D versus the reciprocal of temperature) (b). ([HCl)]: 3 M; qcya: 453 mg Cyanex 921 g− 1).

Δ lnD −ΔH o = R Δð1 = T Þ

3.4. Sorption and desorption kinetics

ð14Þ

ln D vs. 1 / T is shown in Fig. 4(b); the slope is close to 3700. The application of Eq. (8) to experimental results gives an enthalpy change of − 30.75 kJ mol− 1. The enthalpy change is lower than the values reached in the case of Au(III) extraction from HCl solutions using Cyanex 921 (i.e., ΔH° = −44.6 kJ mol− 1) (Martinez et al., 1997), or using Cyanex 925 (i.e., ΔH° = −45.9 kJ mol− 1) (Barroso et al., 1997).

The kinetics of sorption can be controlled by several steps corresponding to diffusion mechanisms or proper reaction rate control (Tien, 1994). Since the different mechanisms can be mixed and both contributing to the control of sorption rate, it is generally required using complex systems of equations in order to evaluate the kinetic parameters and the diffusion coefficients. It is not systematically possible solving these systems due to the incomplete knowledge of the characteristics of the sorbents or binding mechanisms (dispersion in the size of the sorbent particle, multiple reactions involving different metal species). A more simple approach is thus generally sufficient to determine the controlling steps and to evaluate the impact of experimental parameters on these diffusion coefficients. For these reasons the kinetics was modeled using simple equations independently calculated.

Table 2 Sorption isotherms — modeling of experimental data with Langmuir equation.

Fig. 3. Sorption isotherms — influence of extractant loading ([HCl]: 3 M; T: 20 °C).

T (°C)

qcya (mg/g)

qm (mg Fe g− 1)

b (L mg− 1)

Estimated variance

D (L g− 1)

N (Cya/Fe) (mol/mol)

10 20 30 40 20

453 453 453 453 293

18.1 17.3 18.5 22.3 10.6

1.74 1.03 0.76 0.38 0.48

1.24 0.55 2.36 0.15 0.59

31.5 17.8 14.1 8.5 5.1

3.62 3.79 3.54 2.94 4.01

N (Cya/Fe): molar ratio between Cyanex 921 and Fe(III) at saturation of the EIR.

R. Navarro et al. / Hydrometallurgy 98 (2009) 257–266

The external film diffusion can be experimentally approximated by a simple equation, developed by McKay and Allen, assuming the film diffusion to be the controlling step (Guibal et al., 1999; Gupta et al., 2006): Cðt Þ C0

=

  1 ωD 1 + ωD A + exp − kf t 1 + ωD 1 + ωD mD V

ð15Þ

where, kf is the external diffusion coefficient (m s− 1), A is the total external surface area available for contact with the solution (m2), V is the volume of solution (m3), ω is the sorbent dosage (4 g L− 1) and D is the distribution ratio obtained from the Langmuir constants (D = bqm; here for extractant loading of 453 mg Cyanex 921 g− 1: 17.8 and 8.5 L g− 1 for T: 20 °C and T: 40 °C, respectively). Plotting ln(C(t) / C0 − 1 / (1 + ωD)) = φ(C(t)) versus t gives the external diffusion parameter kf by determination of the slope of the linear part. Since the resistance to external diffusion is suspected to be predominant only in the first minutes of the reaction (and the resistance to intraparticle diffusion being thus negligible) the determination of the slope of the linear part was limited to the first 15 min of contact.       h i Cðt Þ 1 1 + ωD A ωD − = − kf t + ln : u Cðt Þ = ln C0 1 + ωD ωD V 1 + ωD ð16Þ The ratio A/V was determined using the equation: A 6ω 6ω  = =  ρ V d p p;app 1 − ep dp ρp

ð17Þ

where εp is the porosity of EIR particles (εp = 0.55), dp the mean diameter of EIR (dp = 0.55 mm), ρp and ρp,app are the true and apparent densities of EIR respectively. The apparent density (ρp,app = 1.00 g mL− 1) was obtained taking into account the skeletal density of the resin (ρp = 1.24 g mL− 1), the porosity of the resin and the amount of extractant immobilized in the resin (qCyanex 921: 453 mg g− 1 EIR), assuming that the free volume was partially occupied by the extractant and that resin volume remained constant. The apparent density of the dry resin (raw resin not impregnated) was 0.558 g mL− 1, after hydration the density approached 1.05 g mL− 1. Several tests were carried out by dropping EIR into HCl solutions (of varying concentration and varying density) and testing the flotation of sorbent particles. This experimental approach was consistent with the values obtained by calculation. According to these assumptions, the outer surface of the adsorbent per unit volume of the particle-free suspension (A/V) was close to 43 m− 1. The intraparticle diffusion coefficient (De, effective diffusivity, m2 min− 1) was determined using Crank's equation, assuming the solid to be initially free of metal, and the kinetics to be controlled by intraparticle diffusion resistance (Crank, 1975):   2 ∞ 6α ðα + 1Þ exp −Dre2qn t X qt = 1− qeq 9 + 9α + q2n α 2 n=1

263

however, the PSOE systematically fitted much better experimental data than PFOE. For this reason, only the PSOE was maintained. The pseudo second-order equation described by Ho (2006) was used for modeling. This simplified model assumes the reaction rate to be of the second order relative to metal concentration both in the solution and sorbed at the surface of the sorbent. The development of this model leads to the equation: Pseudo second
order equation: qt =

q2∞ k2 t : 1 + q∞ k2 t

ð21Þ

Where k2 is the pseudo second-order rate constant (g mg− 1 min− 1). The parameters q∞ and k2 are pseudo-constants, depending on experimental conditions; they were obtained after linearization of the equation, according to: t 1 1 = + t: qt q∞ k2 q2∞

ð22Þ

In order to verify the effect of conditioning of the resin on the sorption kinetics, conditioning times of 1 day and 15 days were tested and compared to a resin tested without conditioning. HCl solutions at the target concentration were used to conditioning the resin. The results (not shown) demonstrated that the conditioning in this case did not improve the kinetics of the process; kinetic profiles were almost superimposed (Additional material, available in the Electronic additional material). 3.4.1. Effect of Fe(III) concentration Fig. 5 shows the kinetic profiles obtained varying initial Fe(III) concentrations. The equilibrium was reached within 12 h under selected experimental conditions. The lines represent the modeling of experimental data using the intraparticle diffusion model and the intraparticle diffusion coefficients included in Table 3. The coefficient increased by one order of magnitude when doubling initial metal concentration (in the range 1.2× 10− 11–47 × 10− 11 m2 min− 1). Nestle and Kimmich (1996) demonstrated by NMR-imaging techniques that in the case of the sorption of metal ions using alginate gels the intraparticle diffusion coefficient may follow a Langmuir-type trend with reference to metal concentration. These values of the intraparticle diffusion coefficient are consistent with those values cited by Navarro et al. (2007b) in the case of Zn(II) sorption (De: 2.6 × 10− 11 m2 min− 1) and for Cd(II) sorption (Navarro et al., 2008b) (De: 2.2 × 10− 11 m2 min− 1) from HCl solutions using similar EIR. The film diffusion coefficient was much less affected by increasing metal concentration: between 60 and 140 mg Fe L− 1, the film diffusion coefficient only decreased from 7.7 × 10− 4 to 2.2 × 10− 4 m min− 1. The Biot number, Bi (dimensionless),

ð18Þ

qt and qeq are the concentrations of the metal in the resin at time t and equilibrium, respectively, and qn non-zero roots of the equation: tan qn =

with

3qn 3 + αq2n

qeq 1 = : VC0 1+α

ð19Þ ð20Þ

A simple approach was also adopted for the evaluation of the reaction rates. Pseudo first-order (PFOE, the so-called Lagergren equation) and pseudo second-order (PSOE) equations were tested;

Fig. 5. Fe(III) sorption kinetics — influence of initial metal concentration ([HCl)]: 3 M; qcya: 453 mg Cyanex 921 g− 1; T: 20 °C).

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R. Navarro et al. / Hydrometallurgy 98 (2009) 257–266

Table 3 Diffusion coefficients — influence of experimental parameters. Experimental parameters

Film diffusion

Intraparticle diffusion

qcya (mg/g)

T (°C)

C0 (mg Fe L− 1)

kf × 104 (m2 min− 1)

R2

De × 1011 (m2 min− 1)

Resid.

453 453 453 453 115 196 575

20 20 20 40 20 20 20 

60 100 140 100 100 100 100

7.7 2.9 2.2 10.9 n.d. n.d. n.d.

0.978 0.958 0.979 0.900 n.d. n.d. n.d.

1.2 8.3 46.6 11.1 7.6 8.2 3.8

0.04 0.16 0.08 0.10 0.11 0.16 0.02

n P

Resid.:

j=1

qexp ðtj Þ qeq

2

−

qcalc ðtj Þ qeq

: n.d.: not determined.

which serves to determine the predominant controlling diffusion mechanism, was determined using the following equation (Tien, 1994; Weber and DiGiano, 1996): kf dp Bi = De

ð23Þ

where dp is the mean particle diameter (m). Here, the Biot number was found to be much higher than 200. A Biot number greater than 1 indicates that the intraparticle mass transfer resistance is predominating over film diffusion for kinetic control. The pseudo second-order equation fitted well experimental data as shown in Table 4, as indicated by the values of the correlation coefficient. The calculated equilibrium sorption capacities (q∞) were close to the experimental values and the kinetic parameter slightly varied (in the range 1.3 × 10− 3–3.5 × 10− 3 g mg− 1 min− 1) with changing initial metal concentration. 3.4.2. Effect of Cyanex 921 loading With EIR the loading of the resin is also a critical parameter for kinetics. Four different loadings have been compared with similar experimental conditions. Fig. 6 shows the kinetic profiles obtained increasing extractant loading from 115 to 575 mg Cyanex 921 g− 1. Equilibrium was reached between 8 and 12 h. Tables 3 and 4 summarize the values of the model parameters for diffusion models and pseudo second-order rate equation, respectively. The intraparticle diffusion coefficient slightly increased from extractant loadings increasing from 115 to 453 mg Cyanex 921 g− 1, before decreasing at high extractant loading (i.e., 575 mg Cyanex 921 g− 1). The film diffusion coefficients have not been determined, since the Langmuir parameters were not available at all extractant loadings. As expected from the section testing the impact of extractant loading (Figs. 1(a) and 3), the sorption capacity at equilibrium calculated using the pseudo second-order equation increased with extractant loading (Table 4). Conversely, the rate parameter remained remarkably constant (varying in the range 1.1 × 10− 3–1.3 × 10− 3 g mg− 1 min− 1).

Fig. 6. Fe(III) sorption kinetics — influence of Cyanex 921 loading ([HCl)]: 3 M; C0(Fe): 100 mg Fe L− 1; T: 20 °C).

3.4.3. Effect of temperature The comparison of sorption isotherms for different temperatures has shown the positive effect of high temperatures on maximum sorption capacities. The impact of increasing temperature has also been tested on sorption kinetics. Fig. 7 shows that the kinetic profiles followed the same trends. Both film diffusion and intraparticle diffusion coefficients were increased at a temperature of 40 °C. While the increase of the temperature slightly increased sorption capacity, the rate parameter defined by the pseudo second-order equation was doubled when doubling the temperature. The increase of temperature reduced the resistance to diffusion (both film and intraparticle diffusion) and improved reaction rate. This is consistent with the enhancement of sorption performance observed at equilibrium. 3.4.4. General comment on the kinetic modeling Both the intraparticle diffusion model and the pseudo second-order equation fitted well experimental data. However, studies of solvent extraction of Fe(III) from HCl solutions by Cyanex 921 (El Dessouky et al., 2008) or Cyanex 923 (Saji et al., 1998, Gupta et al., 2002) have demonstrated that extraction kinetics (under fast agitation) is very fast (equilibrium was attained within 2–5 min). Moreover, Alguacil and Alonso (2000) reported a fast chemical reaction at the interface in the Fe(III) transport using a supported liquid membrane containing Cyanex 921. This means that sorption kinetics is probably not controlled by the reaction rate (despite the good correlation obtained in the fitting of experimental data with the pseudo second-order rate equation). This hypothesis would be confirmed by investigating the impact of resin particle size on the sorption kinetics. Serarols et al. (2001) carried

Table 4 Parameters of the pseudo second-order equation model. Experimental conditions

Pseudo second-order equation

qcya (mg/g)

T (°C)

C0 (mg Fe L− 1)

qeq (mg Fe g− 1)

k2 × 103 (g mg− 1 min− 1)

q∞ (mg Fe g− 1)

R2

453 453 453 453 115 196 575

20 20 20 40 20 20 20

60 100 140 100 100 100 100

14.2 18.5 17.9 22.4 11.1 15.0 21.6

2.2 1.3 3.5 2.8 1.1 1.2 1.2

14.4 19.0 18.0 22.5 11.9 15.7 22.3

1.000 0.998 1.000 1.000 0.991 0.996 1.000

Fig. 7. Fe(III) sorption kinetics — influence of temperature ([HCl)]: 3 M; C0(Fe): 100 mg Fe L− 1; qcya: 453 mg Cyanex 921 g− 1).

R. Navarro et al. / Hydrometallurgy 98 (2009) 257–266

out Zn(II) sorption on Amberlite XAD-2 resin impregnated with DEHPA (di-2-ethylhexyl phosphoric acid) using different metal concentrations and 2 sizes of particles. They concluded that with low-size particles the effective diffusion coefficient and the mass transfer coefficient slightly increased with metal concentration. In the case of large-size particles more complex phenomena are involved in the control of the kinetics: at low metal concentration (i.e., below 130 mg L− 1) the film mass transfer resistance was the limiting step while at high metal concentration the kinetics was controlled by the intraparticle diffusion. In the present work the calculation of Biot number allowed concluding that film mass transfer was not the ratelimiting step. It is thus probable that despite the good fits obtained with the pseudo second-order rate equation the kinetic profiles are controlled by the resistance to intraparticle diffusion. 3.4.5. Desorption kinetics Fig. 8 compares the kinetic profiles for the sorption and the desorption of Fe(III) under comparable experimental conditions (same m/V ratio, desorption was operated using water as the eluent). The desorption process seemed to be much faster than the sorption step. The kinetics of desorption is characterized by a sharp initial increase of the amount of Fe(III) released: 90% of the total amount of Fe(III) desorbed was recovered within the first 30 min. A significant change in the slope of the curve was observed at longer contact time with a slow step characterized by a linear slope. After 8 h 99% of desorbable Fe(III) was removed. These results clearly indicate that the sorption and the desorption processes were not controlled by the same mechanisms. The faster kinetics of desorption may also indicate that different diffusing species were involved in the sorption and the desorption steps. 3.5. Desorption and recycling Several eluents were tested including neutral solutions (i.e., water and sodium sulfate) and acidic solutions (sulfuric acid and nitric acid). The desorbed EIR were tested for a series of 4 other cycles. Table 5 summarizes the data obtained with the four eluents for 5 cycles. The desorption was close to 99% and higher, whatever the number of cycles with neutral solutions and acidic media. Regardless of the type of eluent (among neutral and acidic agents) the sorption capacity progressively decreased, though remaining higher than 90%. The average values for sorption efficiency over the 5 cycles were in the range 94–95%, regardless of the type of eluent. For desorption the average values were higher than 98.5%.

Fig. 8. Comparison of sorption and desorption kinetic profiles (adsorption: [HCl)]: 3 M; C0(Fe): 60 mg Fe L− 1; qcya: 196 mg Cyanex 921 g− 1; desorption: H2O).

265

Table 5 Sorption (S) and desorption (D) efficiencies (%) over 5 cycles. Cycle 1 Na2SO4 H2SO4 HNO3 H2O

Cycle 2

Cycle 3

Cycle 4

Cycle 5

S

D

S

D

S

D

S

D

S

D

96.5 96.6 96.6 96.6

94.6 95.6 93.8 98.7

95.2 95.2 94.2 94.8

98.9 98.2 99.9 98.7

94.8 95.0 94.3 94.3

99.8 98.5 99.8 100

93.5 95.2 92.5 92.8

99.4 99.9 99.2 99.9

94.4 93.8 91.6 90.5

99.8 99.6 99.6 99.7

Due to the weak effect of the type of eluent on sorption and desorption steps (over the 5 cycles) water would probably be more appropriate for Fe(III) recovery avoiding the use of chemicals. The high efficiency of these eluents can be explained by the destabilization of chloro-complexes of Fe(III) that were bound on the EIR. The absence of chloride anions leads to a change in the speciation of ferric species, which in turn contributes to make free ferric ions releasable. The reversibility is thus probably related to a change in the speciation of Fe(III) in the solution. 4. Conclusions Cyanex 921-impregnated resins revealed highly efficient for Fe (III) recovery from HCl solutions. Fe(III) sorption efficiency increased with HCl concentration. Though the extractant-free resin (Amberlite XAD-7) can adsorb Fe(III) when HCl concentration exceeded 3 M, the impregnation of the resin substantially improved resin sorption capacity. The extraction mechanism is controlled by several parameters such as the speciation of Fe(III) (chloro-complexes), the extraction of HCl by both the resin and the extractant. Metal extraction proceeds through direct binding on the resin (HFeCl4) or by extraction with Cyanex 921, under the form HFeCl4L3, probably some molecules of HCl being part of the solvate. The Langmuir equation fitted well experimental data and the maximum sorption capacity increased with extractant loading up to 22 mg Fe g− 1 in 3 M HCl concentration. However, it sounded that an excess of extractant in the resin above 450 mg Cyanex 921 g− 1 did not contribute to improve sorption capacity: a better rationale use of the extractant can be obtained at intermediary loading, probably due to steric hindrance when the EIR is saturated with extractant. The thermodynamics of the sorption process showed an enthalpy change close to − 30.8 kJ mol− 1 and an entropy change close to − 22.8 J mol− 1 T− 1. The study of the kinetics showed that the resistance to intraparticle diffusion had a greater impact on the control of sorption kinetics than the resistance to film diffusion (at least under selected experimental conditions (agitation velocity and so on …)). The intraparticle diffusion increased with metal concentration and temperature. However, varying extractant loading resulted in a discontinuous trend for the variation of the intraparticle diffusion rate that reached a maximum close to 453 mg Cyanex 921 g− 1. Though the pseudo second-order equation fitted well kinetic profiles (high correlation values), the fast kinetics observed in liquid/liquid system suggest that the contribution of reaction rate should be marginal compared to the resistance to intraparticle diffusion. Optical stereomicroscopy showed the formation of aggregates at the surface of the EIR when the resins were loaded with high concentrations of Cyanex 921. This could explain some of the discontinuities observed in the behavior of the EIR when increasing extractant loading. This is currently under investigation. The metal bound to the EIR was efficiently desorbed by a series of eluents, and the recycling was shown efficient for at least 5 sorption/ desorption cycles using 0.1 M solutions of sulfuric acid, nitric acid, sodium sulfate and even water. The desorption efficiency remained constant over the 5 cycles, though the sorption efficiency slightly decreased (maintaining above 90% at the fifth cycle).

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