Role of Ligand Acidity in Chelating Adsorption and Desorption of

Article pubs.acs.org/IECR

Role of Ligand Acidity in Chelating Adsorption and Desorption of Metal Salts Katri Laatikainen,*,† Markku Laatikainen,‡ Catherine Branger,§ Erkki Paatero,‡ and Heli Sirén† †

Laboratory of Chemistry and ‡Laboratory of Industrial Chemistry, Lappeenranta University of Technology, P.O. Box 20, FI-53851 Lappeenranta, Finland § Laboratory MAPIEM - EA4323, SUD Toulon-Var University, ISITV BP56, 83162 La Valette du Var Cedex, France ABSTRACT: A method is proposed to establish useful operation conditions for solid separation of materials having different acidities. The effect of ligand acidity on competitive adsorption and desorption of metal salts was studied by using copper sulfate and sulfuric acid as model electrolytes. Adsorption equilibria with four commercial N-donor chelating adsorbents, WP-1 (functional group branched poly(ethyleneimine)), CuWRAM (functional group 2-(aminomethyl)pyridine), Reillex HP (functional group poly(4-vinylpyridine)), and Dowex M 4195 (functional group bis-2-(pyridylmethyl)amine) were measured using potentiometric and batch uptake methods. Moreover, formation of the complexes in the solid phase was studied with reflection UV−vis spectroscopy. Experimental data were correlated with a nonideal competitive adsorption (NICA) model, and the obtained parameters were used in mapping the operation conditions for the studied adsorbents. Acidity of the adsorbents was proved to increase in the order WP-1 < Reillex HP < CuWRAM < Dowex M 4195, and their apparent acidities increase with temperature. Acidity is shown to have a remarkable effect on conditions where metal adsorption and desorption can take place. Correlation of the acidity parameters and copper adsorption parameters by linear free energy relations is also discussed. According to the results, WP-1 is useful only for highly diluted copper solutions, and desorption of the adsorbed copper takes place readily with 0.1−1 M H2SO4. The exploitable acid concentration range of Reillex HP is identical. Due to the lower metal affinity, it is less suitable for adsorption at low copper concentrations. Dowex M 4195 has the widest operation range and is suitable for very acidic copper separations. The only disadvantage seems to be that complete desorption of copper with acid is impossible. At moderately acidic solutions, CuWRAM offers the best balance between adsorption and desorption, because copper desorbs with 5 M H2SO4 solution.

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INTRODUCTION Chelating separation materials consists of specific or universal organic chelating ligands anchored on solid supports. These materials include both ion-exchange resins and adsorbents with chelating ligands as functional group. In the literature, the definitions are somewhat confusing, and in many cases all such materials are called chelating ion exchangers even if some of them are adsorbents. The distinction between the two types is clarified in Figure 1. Iminodiacetic acid (IDA) (Figure 1A) and bis-(2-pyridylmethyl)amine (PMA) (Figure 1B) are good examples of functional groups of a chelating ion exchanger and a chelating adsorbent, respectively. In chelating ion exchangers the ligands are charged and the metal cations act at the same time as central atoms and as counterions. In the case of chelating adsorbents, the neutral ligands form charged

complexes with metals, and anions are coadsorbed as counterions. Chelating ligands act as donors of electrons and form coordinative bonds with a metal cation working as the central atom in metal chelates and complexes. When a molecule has two or more donor atoms in the structure and participates in a ring-closure reaction, it is called a chelating ligand.1 The electron acceptor (the metal cation) acts as a Lewis acid. Consequently, the electron donor, the chelating ligand, acts as a Lewis base. The interactions between metals and chelating separation materials depend on the metal properties, the solution, and the ligand functionality.2 Metal−ligand affinity is the most important factor in explaining the selectivity properties of different chelating materials and metals.3 Nevertheless, the donor centers contained in the ligands may also bind protons, and in such cases the system is a competitive one even in the presence of only a single metal. There are several theories to explain the metal−ligand interactions, and they can be found in standard textbooks.1,4−6 Chelating ligands form stable complexes with transition metals. Therefore, chelating separation materials are a reasonable choice in removal of transition metals from aqueous Received: Revised: Accepted: Published:

Figure 1. Chelating ion-exchange resin with iminodiacetic acid (IDA) groups (A) and chelating adsorbent with bis-2-(pyridylmethyl)amine groups (PMA) (B) (S = solid support). © 2012 American Chemical Society

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April 29, 2012 August 17, 2012 August 26, 2012 August 26, 2012 dx.doi.org/10.1021/ie301115u | Ind. Eng. Chem. Res. 2012, 51, 12310−12320

Industrial & Engineering Chemistry Research

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solutions.7 During last 40 years the use of chelating separation materials, e.g. in hydrometallurgical applications, has been actively studied, and many specifically tailored materials have been proposed for impurity removal and purification of electrolyte solutions. A wide selection of commercial chelating separation materials are available, and IDA,8−12 aminomethylphosphonate (AMPA),13−15 and PMA17−24 are examples of widely studied ligands used in chelating separation materials. Although removal of transition metals by chelating separation materials has been studied extensively, their practical, useful operation conditions have not been explored systematically. For instance, some of the functional groups used in chelating adsorbents are so basic that they must be used in the pH range where metal precipitation occurs and becomes a problem. On the other hand, desorption properties of metals from some materials are extremely difficult. The objective of this study is to show explicitly how the operation conditions of a given adsorbent depend on the acidity of the functional groups. Such knowledge is of utmost importance in the selection of adsorbents for a given application because the actual metal concentration and pH range may vary in very wide limits. Four commercial N-donor chelating adsorbents with different acidities of functional groups were chosen. WP-1 contains branched poly(ethyleneimine) (BPEI), CuWRAM contains 2-(aminomethyl)pyridine (AMP) groups, Reillex HP contains poly(4-vinylpyridine) (PVPy), and PMA is the functional group of Dowex M 4195. Copper sulfate and sulfuric acid were used as the adsorbing components. The proposed method is based on data measured using equilibrium experiments, and the kinetic effects are not considered here. The acid−base properties and metal adsorption equilibria for copper were measured using potentiometric titrations and batch uptake measurements. Furthermore, complex formation is discussed using the data measured with UV−vis spectroscopy. The experimental data are correlated using nonideal competitive adsorption model (NICA) reported by Kinniburgh et al.25

Table 1. Properties of WP-1, CuWRAM, Reillex HP, and Dowex M 4195. The Values Are for Unprotonated Adsorbents unless Indicated Otherwise physical property

WP-1a

CuWRAMb

Reillex HP

Dowex M 4195

average particle size, μm specific surface area, m2/g BJH pore volume, mL/g average pore size, nm density ρs, g/mL intraparticle porosity εp, nitrogen content, mmol/

224 ± 23

229 ± 51

250−595d

676 ± 191

187 ± 3

145 ± 2

90

0.53

0.57

n.d.

n.d.

8.7

15.7

n.d.

n.d.

0.84 0.47

075c 0.43

1.14d n.d.

1.15 1.18

3.8

2.9

5.8

7.1

a

d

n.d.e

Results from ref 28. bResults from ref 29. cProtonated adsorbent. From ref 30. en.d. = not determined.

d

our previous studies. The parameters for average particle size, specific surface area and density for Reillex HP are taken from literature.30 Nitrogen content of Reillex HP and results for Dowex M 4195 were measured in this study. The densities of the adsorbents were measured with a calibrated pycnometer (in-house instrument, LUT, Finland). The intraparticle porosity was calculated from the pore volume and the density of the dry solid. The total number of amine groups (nitrogen content) was determined by elemental analysis (Elemanrar Variomax C/N-analyzer). The electrolyte solutions were prepared using deionized water and reagent-grade electrolytes. The supporting ionic strength was 2 mol/L, and it was adjusted with reagent-grade Na2SO4. Methods. Potentiometric Titrations. Titration curves of WP-1, CuWRAM, Reillex HP, and Dowex M 4195 were measured by equilibration in separate batches. All materials were titrated at 25 °C and at a constant supporting ionic strength Is = 2 mol/L adjusted with Na2SO4. WP-1 and CuWRAM were titrated also at 60 °C, and acid titration of Dowex M 4195 was made also at 50 °C. A constant amount of adsorbent (about 0.2 g) was weighed in glass vials containing different concentrations of sulfuric acid and metal salt. The liquid volume of all samples was 10 mL. The samples were shaken for at least 2 days. After separation of the solids, the equilibrium solutions were then kept several hours at 25 °C before measurement of the equilibrium pH. All titrations were repeated at least twice and the difference between duplicate data points was less than 3%. The adsorbed amount of metals was calculated from the change in the solution concentration. Metal ion concentrations were determined by plasma emission spectroscopy (Iris Intrepid II XDL ICP-AES). Sodium interference was corrected by using the same amount of Na2SO4 in calibration samples and in the samples. All samples were analyzed at least twice and the duplicate determinations agreed within variation of 5%. The detection limit of the compounds with plasma emission spectroscopy was 0.1 mg/L. Spectroscopic Measurements. The absorption spectra of the metal complexes in WP-1, CuWRAM, Reillex HP, and Dowex M 4195 were measured with reflection UV−vis spectrometer (Cary 100) at room temperature, and for WP-1 and CuWRAM also at 60 °C. Samples were prepared by making a thin and homogeneous resin layer on a supporting

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EXPERIMENTAL SECTION Materials. WP-1 (at present marketed as WPGM) and CuWRAM (at present marketed as CuSelect) obtained from Purity Systems Inc. are silica-supported chelating adsorbents, in which polyamines (molecular weight around 1200) are covalently anchored on mesoporous silica.26 The former contains BPEI covalently bound on silica,27 while the latter has an anchored polyamine layer, which is further functionalized with 2-(aminomethyl)pyridyl groups.26 Dowex M 4195, which contains bis-2-(pyridylmethyl)amine groups has a macroporous poly(styrene-co-divinylbenzene) (PS-DVB) structure. Reillex HP is prepared by cross-linking of poly(4vinylpyridine) (PVPy) with DVB. The materials were obtained from Dow Chemical Co. and from Aldrich Chemical Co. Inc., respectively. Before measurements the adsorbents were treated consecutively with 2 M H2SO4, purified water (milli-Q), 4 M NH4OH, and purified water (milli-Q). This cycle was repeated 3 times and the adsorbents were then dried at 60 °C under vacuum. In addition, CuWRAM was extracted overnight with methanol before the acid−base treatment in order to remove the colored substances observed in preliminary experiments. Physical characteristics of the treated adsorbents are listed in Table 1. Results for WP-128 and CuWRAM29 have been measured in 12311

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Figure 2. (A) Adsorption isotherms of sulfuric acid by WP-1 (diamonds), CuWRAM (squares), Reillex HP (triangles), and Dowex M 4195 (circles) at T = 25 °C. (B) Adsorption isotherms of sulfuric acid by WP-1 (diamonds) and CuWRAM (squares) at T = 60 °C and Dowex M 4195 (circles) at T = 50 °C. Is = 2 mol/L. The solid lines have been calculated from eq 1 with parameter values given in Table 2.

Table 2. NICA Parameters for Sulfuric Acid and Copper Sulfate When Adsorbed in WP-1, CuWRAM, Reillex HP, and Dowex M 4195 at 25−60 °C and Ionic Strength of 2 mol/L WP-1

CuWRAM

p1 = 0.99 p2 = 0.54 ARD, % = 3.30 ω1 = 0.57 log h1 log h2 log h1 log h2

CuSO4

ω1 = 1

T = 25 °C

T = 60 °C

T = 25−60 °C

6.64 0.75 2.61 0.78 12.53 0.24 0.70 0.27

5.59 0.74 2.04 0.60 11.91 0.25 0.76 0.72

−56

−34

κ1 κ2 κ1 κ2

T = 25 °C

T = 60 °C

T = 25−60 °C

log κ1 h1

1.91 0.49

1.24 0.74

−11

log κ1 h1

5.57 0.16

3.02 0.16

−44

Reillex HP

Dowex M 4195

p = 0.92 ARD, % = 17.2 log κ1 h1 log κ1 h1

H2SO4 CuSO4

2.64 1.04 1.77 0.31

log κ1 h1 log κ1 h1

T = 25 °C

T = 50 °C

T = 25−50 °C

1.28 0.73 9.76 0.12

1.06 0.82

−16

interactions of the adsorbed components.25 The value of pk (0 < p ≤ 1) determines the width of the site strength distribution. A small numeric value means a wide distribution. The pk value is common for all adsorbing components and it is assumed independent of temperature. q* is the maximum adsorption capacity and cp means stoichiometric concentration of the acid or the metal salt in the solution. In other words, speciation of the electrolytes is ignored here. Moreover, the solution-side activity coefficients are assumed to be constants because of the high ionic strength used in all experiments. The fraction of sites in population k is represented by ωk and Σωk = 1. Calculations. The NICA parameters were estimated from the titration and sorption equilibrium data as described earlier.28 Experimental data reported here as well as some copper sulfate equilibrium uptake data reported elsewhere28,29 were used for parameter estimation. Goodness of the fit was evaluated by means of average relative error (ARD) defined in

wafer. The resins were separated from titration samples and washed with water before recording the spectra.

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THEORETICAL BASIS Adsorption of the acid and metal salt from sulfate solutions was correlated using the nonideal competitive adsorption (NICA) model reported by Kinniburgh et al.25 as described earlier.28,29,31 The adsorbents are assumed to contain one (in the cases of CuWRAM, Reillex HP and Dowex M 4195) or two (in the case of WP-1) site populations, and the NICA isotherm can be written as shown in eq 1 ⎡ ⎤ pk − 1 ⎛ hi , k ⎞ (κi , kci)hi ,k ⎣∑j (κj , kcj)hj ,k ⎦ ⎟⎟ qi = q* ∑ ωk ⎜⎜ (L p ⎝ hH , k ⎠ 1 + ⎡∑ (κj , kcj)hj ,k ⎤ k k=1 ⎣ j ⎦ L

= 1 or 2)

ΔHads (kJ/mol)

p = 0.83 ARD, % = 10.4

T = 25 °C

adsorbate

ΔHads (kJ/mol)

p = 0.89 ARD, % = 5.03

ω2 = 0.43

adsorbate H2SO4

ΔHads (kJ/mol)

(1)

In eq 1, κ is the affinity constant and the parameter hi depends on the binding stoichiometry and on lateral 12312



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Table 3. Dissociation Constants pKa and Step-Wise Stability Constants log K of Copper for the Soluble Chelating Ligands, BPEI (Functional Group in WP-1), AMP (Functional Group in CuWRAM), PVPy (Functional Group in Reillex HP), and PMA (Functional Group in Dowex M 4195). Values Have Been Collected from Literature Sources Shown below the Table

a

Reference 31. bReference 32. cReference 33. dReference 34. eReference 35.

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eq 2, where Ndp is number of data points and Y is the fitted quantity. ARD =

1 Ndp

Ndp

∑

Yj ,exp − Yj ,calc

j=1

Yj ,exp

RESULTS AND DISCUSSION Acid Binding Properties. Adsorption isotherms of sulfuric acid in WP-1, CuWRAM, Reillex HP, and Dowex M 4195 at 25, 50 (Dowex M 4195) and 60 °C (WP-1 and CuWRAM) are illustrated in Figure 2. The supporting ionic strength is 2 mol/L in all cases. Titration curves of WP-1,31 CuWRAM,29 and Dowex M 419528 have been reported previously, and only the titration curve for Reillex HP was measured in this study. The experimental data of CuWRAM, Reillex HP, and Dowex M 4195 were correlated with the one-site NICA model (eq 1 with L = 1). The two-site NICA model (eq 1 with L = 2) was found necessary for WP-1 because of the more complex protonation behavior. The calculated values are shown as solid lines, and the estimated parameters are given in Table 2. As can be seen from Figure 2A, the shapes of the acid adsorption isotherms differ significantly; the curve of WP-1 indicates the highest affinity, and that of Dowex M 4195, the lowest. Protonation behavior of Reillex HP is quite close to that of WP-1, and CuWRAM behaves similarly to Dowex M 4195. According to Figure 2, acidity of the studied adsorbents increases in the order WP-1 < Reillex HP < CuWRAM < Dowex M 4195. This trend is also reflected in the model parameters shown in Table 2, and the effective acid binding constant h1 log K1 for WP-1 is about 5 times the values of those of CuWRAM and Dowex M 4195. As discussed earlier,28,31 WP-1 contains primary, secondary, and tertiary amine groups. Acid is adsorbed to primary and part of secondary amine groups even at very low acid concentrations. On the other

× 100% (2)

Adsorption enthalpy ΔHads was calculated using eq 3, and the κ values were obtained at different temperatures. Here R is the gas constant, T is absolute temperature, and T0 = 298 K. ln κi(T ) = ln κi(T0) +

ΔHads, i ⎛ 1 1⎞ ⎜ − ⎟ R ⎝ T0 T⎠

(3)

The precipitation limit was estimated by assuming that precipitation starts with formation of basic copper sulfate as shown in eq 4.28 1 2− 3 SO4 (aq) + H 2O 4 2 1 3 3 ⇄ CuSO4 × Cu(OH)2 (s) + H+(aq) 4 4 2

Cu 2 +(aq) +

(4)

Precipitation curves for basic copper sulfate were measured experimentally at 25, 50, and 75 °C (not shown here), and the soluble product was calculated as described earlier.28 The values estimated at 25 and 60 °C are log Ks,Cu (25 °C) = −16.1 and log Ks,Cu (60 °C) = −15.7. 12313



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Figure 3. Adsorption isotherms of sulfuric acid by WP-1 in presence of copper sulfate at 25 °C (A) and 60 °C (B). cCu0 = 0 mol/L (diamonds), 0.008 M (squares), 0.02 M (triangles), and 0.0024 M (circles). Is = 2 mol/L. The solid lines are calculated values.

Figure 4. Adsorption isotherms of sulfuric acid by CuWRAM in the presence of copper sulfate at 25 °C (A) and 60 °C (B). cCu = 0 mol/L (diamonds), 0.004 M (squares), 0.02 M (triangles), and 0.08 M (circles). Is = 2 mol/L. The solid lines are calculated values.

Figure 5. Adsorption isotherms of sulfuric acid by Reillex HP (A) and Dowex M 4195 (B) in the presence of copper sulfate at 25 °C. cCu = 0 mol/L (diamonds), 0.15 M (squares), 0.40 M (triangles), and 0.08 M (circles). Is = 2 mol/L. The solid lines are calculated values.

For comparison, dissociation constants and stepwise stability constants of Cu2+ for the soluble chelating ligands BPEI, AMP, PVPy, and PMA are shown in Table 3. As shown in ref 31, protonation of WP-1 takes place qualitatively similarly to that of soluble BPEI. Apparent acidity of the soluble BPEI is, however, markedly lower (Table 3), and the difference is considered to stem from the structure of the anchored polymer and from interactions with the silica surface. A similar effect was

hand, the acid binding ability of the remaining secondary amine groups and tertiary amines is significantly lower due to steric and electrostatic hindrances. In the case of CuWRAM and Dowex M 4195, acid binds on the nitrogen atoms of both aliphatic amines and those of the pyridyl groups.28 The similarity of the ligand chemical structure in CuWRAM and Dowex M 4195 explains the close similarity in their protonation behavior. 12314



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effect of temperature on copper adsorption was studied. The data obtained with WP-1 and CuWRAM are compared, and the results are shown in Figures 3 and 4. The results show that the effect of increasing temperature is stronger in the case of CuWRAM. The binding enthalpy, ΔHads, estimated for the more basic sites of WP-1 was −34 kJ/mol, whereas McIntyre et al.37 have reported ΔHads = −48 kJ/mol for ethylenediamine which can be considered as the repeating unit of BPEI. ΔHads = −44 kJ/mol was estimated for CuWRAM using eq 2 and a onesite NICA-model. The value obtained by Garcia-Espana et al.38 for the copper−AMP complex is −49.6 kJ/mol. Correlation between the ligand acidity and metal binding ability is conventionally studied using linear free energy relations (LFER).36 This approach is straightforward when applied to monodentate ligands having only one dissociation constant and one stability constant. As far as the ligands of this study are concerned, the situation is more complicated. The LFER plot shown in Figure 6 was constructed using the

observed also for the other three ligands (Table 3); acidity order of the ligands is the same as in the case of adsorbents, but the level of acidity is lower. Sorption of the studied electrolytes by the supports has been neglected. PS-DVB of Dowex M-4195 and Reillex HP matrix is expected to have no significant affinity for electrolytes. Silica used in WP-1 and CuWRAM, on the other hand, has both Lewis and Brønstedt-type sites, but because of the efficient coating by the polyelectrolyte layer, these sites do not have important contributions in electrolyte sorption. The effect of temperature was also studied, and the acid adsorption isotherms of WP-1 and CuWRAM at 60 °C and of Dowex M 4195 at 50 °C are shown in Figure 2B. The calculated values are shown as solid lines, and the estimated parameters are given in Table 2. According to Figure 2 and Table 2, increasing temperature decreases affinity for the acid, indicating that acid binding in all three adsorbents is exothermic and that they become more acidic with increasing temperature. ΔHads is the most negative (−56 kJ/mol) for WP1 and the least negative for CuWRAM (−11 kJ/mol) and for Dowex M 4195 (−16 kJ/mol). Similarity of the latter values is in accordance with the ligand structures. Moreover, CuWRAM appears even more acidic than Dowex M 4195 when the temperature increases from 25 to 60 °C. This is probably due to more severe steric hindrances and the effect of neighboring charges with three nitrogen atoms in bis-2-(pyridylmethyl)amine compared to two nitrogen atoms of 2-(aminomethyl)pyridine. Competitive Adsorption of Sulfuric Acid and Copper Sulfate. Competitive binding of sulfuric acid and copper sulfate was studied using potentiometric titrations, and the results are shown as acid adsorption isotherms measured at different initial copper concentrations. The isotherms measured for WP-1, CuWRAM, Reillex HP, and Dowex M 4195 at 25 °C and Is = 2 mol/L are shown in Figures 3A, 4A and 5. The experimental data were correlated using the NICA model (eq 1) as discussed in the previous section. The calculated values are shown as solid lines, and the estimated parameters are given in Table 2. Interestingly, the effects of copper on the protonations of CuWRAM (Figure 4) and Dowex M 4195 (Figure 5B) are quite different, although their acidities were shown to be rather similar. This suggests a substantially higher stability of the copper complex with PMA than with AMP. The difference in the affinity constants (Table 2) is more than 4 orders of magnitude. However, the first stability constants of the two ligands are quite similar (Table 3). Only log K1 values are compared, because formation of higher complexes with the anchored ligands is highly improbable. It seems possible that the silica−polyamine composite structure of CuWRAM is responsible for its lower affinity. On the other hand, comparison between WP-1 (Figure 3A) and Reillex HP (Figure 5A) shows that aq much higher concentration of copper is needed with Reillex HP to obtain an effect similar to that with WP-1. The difference can once more be explained by the stability constants listed in Table 3 and the affinity constants in Table 2. In both cases, the values of the constants of WP-1 are about 10 orders of magnitude higher. In general, the NICA model correlates the data reasonably well. The affinity differences are clearly the result of the parameter values. It is also well-known36 that relatively high enthalphy changes are involved in the complex formation reactions; therefore, the

Figure 6. LFER plot for the free and anchored ligands.

following assumptions: average pKa and log K1,Cu were used for the water-soluble ligands (open symbols in Figure 6), while log K1,Cu was plotted against log K1,H for the adsorbents. Although the data are widely scattered, the free ligands, WP1 and CuWRAM, are located around a straight line with a slope of 1.8. Another interesting result is elucidated in Figure 6 where, except for the Dowex 4195-PMA pair, the numeric values of dissociation and stability/affinity constants are markedly smaller for the anchored ligands. Analysis of Copper Complexes in Chelating Adsorbents. The structures of copper complexes on WP-1, CuWRAM, Reillex HP, and Dowex M 4195 were investigated by absorption spectroscopy in the visible range. The absorption spectra of copper complexes formed at pH 3.5 (A) and pH 2.0 (B) are shown in Figure 7. Only the absorptions related to the d-electron transitions are shown. The intensity of chargetransfer bands located at shorter wavelengths are omitted. Structures of copper complexes with the WP-1 and CuWRAM have been studied in previous papers.29,31 At pH 3.5, CuWRAM and Reillex HP spectra are quite similar and very different from those of WP-1 and Dowex M 4195 (Figure 7). Figure 8 gives the ligand field stabilization energies (Ed−d) calculated from the absorption maximum wavelengths. The breaking of degeneracies of the d orbitals by the crystal field is thus more important for WP-1 than for the 12315



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more restricted than in free solutions. The variation of coordination numbers may thus be assumed to be of minor importance in the present systems. For example, both soluble and silica-supported BPEI ligands form tetragonally distorted octahedral complexes with copper, and the coordination number is about 4 in a wide range of conditions. 31 Furthermore, we have shown in our previous study that both soluble and silica-supported AMPs form 1:1 and 1:2 copper complexes with Cu2+ ions.29 In those cases structures of the metal complexes are known to be tetragonally distorted octahedral complexes.38 Romary et al.35 and Nishikawa and Tsuchida34 have shown that copper is involved in coordination complexes with soluble bis-(2-pyridylmethyl)amine. It forms soluble PVPy complexes with a maximum coordination number of 4. Copper uptake data for Dowex 4195, however, suggests that only one anchored PMA group is attached to the metal ion.28 The effect of temperature on copper complexation in CuWRAM has been reported earlier.29 The spectra presented earlier are compared with those measured in this study for WP1 at 25 and 60 °C. As can be seen from Figure 9, temperature

Figure 7. Absorption spectra of Cu2+ adsorbed in WP-1 (solid lines), CuWRAM (long-dashed lines), Reillex HP (dotted lines), and Dowex M 4195 (dash−dotted lines) at pH 3.5 (A) and pH 2.0 (B). T = 25 °C, and Is = 2 mol/L.

Figure 9. Absorption spectra of Cu2+ adsorbed in WP-1 at 25 °C (solid lines), CuWRAM at 25 °C (long-dashed lines), WP-1 at 60 °C (dotted lines), and CuWRAM at 60 °C (dash−dotted lines) at pH 3.5. Is = 2 mol/L.

does not affect the position of the absorption maximum of CuWRAM, whereas the maximum of WP-1 shifts slightly to higher wavelengths. This is probably due to the increase in apparent acidity with increasing temperature. As expected, the data are in correlation on the basis of the acid adsorption enthalpies (Table 2), and the effect is the strongest for WP-1. Comparison of the Operation Range of the Adsorbents. The data obtained in the previous sections were used to map useful operation ranges of each adsorbent. It is important to note that these calculations account only for equilibrium effects. Kinetic limitations possibly present are not considered here. Therefore, the following plots define the range of conditions where the materials may be used. The least acidic adsorbent, WP-1, has such a high affinity for protons that basic metal sulfate precipitate is a problem in the metal sulfate solutions. Dynamic column separation systems are especially difficult to execute. In a similar way, basic metal sulfate precipitation is a problem also for the Reillex HP adsorbent. The precipitation limit calculated for reaction 4 is indicated in the following figures as a vertical plane. The position depends only on the concentrations of the acid and

Figure 8. Correlation between the adsorbent acidity and the energy of the absorption maximum.

other supports. This is in good agreement with the high value of its stability constant K1,Cu. A significant variation in the shape of the absorption bands can be observed between pH 2.0 and 3.5 for WP-1 and Reillex HP, corresponding to an enlargement of the bands. This can be explained by the appearance of partly deprotonated forms of these resins with increasing pH. The broad absorption bands of WP-1 and Reillex HP at pH 3.5 are the superposition of at least two bands corresponding to different protonated forms of the supported ligands. Another factor that affects the position of the absorption maximum is the coordination number. However, the formation of higher complexes with covalently anchored ligands is much 12316



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Figure 10. Operation conditions calculated for the WP-1, CuWRAM, Reillex HP, and Dowex M 4195 at 25 °C using eqs 1 and 2. Precipitation limit is indicated by the vertical plane.

Figure 11. Operation conditions calculated for CuWRAM at 25 (A) and 60 °C (B). Precipitation limit is indicated by the vertical plane.

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on data measured using equilibrium experiments, and influence of ligand acidity on uptake kinetics is not considered. The effect of ligand acidity on equilibrium adsorption of sulfuric acid and copper sulfate was studied using four commercial N-donor chelating adsorbents, WP-1, CuWRAM, Reillex HP, and Dowex M 4195. Adsorption equilibria were measured using potentiometric and spectroscopic methods. Experimental data were correlated with a nonideal competitive adsorption (NICA) model. Finally, the calculated data were used to map operation conditions for the studied adsorbents. With the method proposed in this study, the useful operation conditions of adsorbents with different acidities can be easily established and compared. Acidity of the studied adsorbents increases in the order WP-1 < Reillex HP < CuWRAM < Dowex M 4195, and it has a marked effect on the conditions where copper can be adsorbed in the adsorbents and desorbed from the adsorbents. According to the results, the least acidic WP-1 is useful for adsorption from dilute copper solutions, and desorption of the copper takes place readily using 1 M H2SO4. Dowex M 4195 is useful in widest range of acid and copper concentrations and can adsorb copper even from very acidic solutions; however, desorption requires special treatment with 3.5 M NH4OH. Except for highly acidic solutions, CuWRAM is suitable for copper adsorption in a wide range of metal and acid concentrations, and copper can be desorbed from the adsorbent with 5 M H2SO4.

metal salt. Because precipitation may exclude adsorption from concentrated solutions, performance of the adsorbent at low concentrations becomes a critical factor. This means that the metal affinity should be sufficiently high, even at acid concentrations well above the precipitation limit. The next point to be considered is the performance in the whole adsorption−desorption cycle. For example, Dowex 4195 adsorbance of copper is effective in a very wide range of conditions, but according to Jones and Grinstead,39 desorption of copper needs a treatment with 3.5 M NH4OH. The contribution of these factors to copper uptake is illustrated in Figure 10. Finally, an increase in operation temperature has been shown to beneficially influence the actual metal separation process,40 and therefore, the operating ranges of CuWRAM at two temperatures are compared in Figure 11. According to results shown in Figure 10, the useful operation range of WP-1 and Reillex HP is rather narrow, allowing copper adsorption only from a neutral or slightly acidic solution. Optimal conditions for use of WP-1 obviously lie at acid and copper concentrations of 10−5 −10 −3 mol/L. At these conditions the copper uptake capacity is only slightly lower than the maximum capacity, and the formation of basic copper sulfate precipitations can be avoided. At higher copper concentrations the operation range is narrower; for example, if the copper concentration were to be 1 mmol/L, H2SO4 concentration would have to be around 10−4 mol/L. Above this limit, the copper capacity is decreasing dramatically because of competitive adsorption of the acid. Due to the low acidity of WP-1, copper can be desorbed easily using 0.1−1 mol/L H2SO4. Selection of the operation conditions for Reillex HP is even more complicated. As can be seen in Figure 10, copper uptake by Reillex HP is very low at low copper concentrations. Practical capacity is attained only at copper concentrations above 10−3 mol/L. However, basic copper sulfate precipitation is limited also to the use of Reillex HP and H2SO4 where concentrations have to be 10−4 and 10−5 mol/L, respectively, so that precipitations can be prevented. A solution of 1 mol/L H2SO4 has high enough concentration to desorb copper from the adsorbent. Figure 10 also shows that CuWRAM and Dowex M 4195 can be used in a much wider range of copper and H2SO4 concentrations than WP-1 and Reillex HP. For example, the copper adsorption capacity of Dowex M 4195 is about half of the maximum capacity, even at copper and acid concentrations of 10−7 and 1 mol/L, respectively. At the same time, however, desorption of copper from the adsorbents28,39 is more complicated; 5 M H2SO4 is needed for CuWRAM, and 3.5 M NH4OH, for Dowex M 4195. According to Figure 11, the useful concentration range becomes narrower the more the temperature increases. This is partly due to the lowered affinity of the adsorbent and to the shift of the precipitation limit in higher acid concentrations. This means that more careful adjustment of the operation conditions is needed to attain the highest possible maximum uptake capacity.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +358 40 182 3138. Fax: +358 5 621 2199. E-mail: Katri. Laatikainen@lut.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Research Foundation of Lappeenranta University of Technology is gratefully acknowledged. The authors are thankful to Ms. Anne Hyrkkänen and Ms. Krista Pussinen for assistance with the experimental and analytical work.

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c E hi H Is κ Ka Kn Ndp p q q* R T

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CONCLUSIONS The method for searching for favorable operation conditions of separation materials with different acidities was studied. the effect of ligand acidity on competitive adsorption and desorption of metal salts was studied by using copper sulfate and sulfuric acid as model electrolytes. The method was based

NOMENCLATURE solution concentration, mol/L energy, eV empirical parameter (eq 1), − enthalpy, J/mol ionic strength of the supporting electrolyte, mol/L affinity constant, L/mol apparent dissociation constant, mol/L stepwise stability constant, L/mol number of data points, − heterogeneity parameter (eq 1), − concentration in the polymer coil, mol/L total concentrations of the amine groups, mol/L gas constant, J/kmol temperature, K or °C

Greek letters

ωk fraction of amine groups in population k, − ρ density, kg/L Subscipts and superscripts

a acid i,j component 12318



Article

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k site population s salt

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ABBREVIATIONS BPEI, branched poly(ethyleneimine); CuWRAM (at present marketed as CuSelect), silica-supported AMP adsorbent; Dowex M 4195, PS-DVB-supported PMA adsorbent; Reillex HP, PS-DVB-supported PVPy adsorbent; NICA, nonideal competitive adsorption model; PMA, bis(2-pyridylmethyl)amine; PS-DVB, polystyrene−divinylbenzene copolymer; PVPy, poly(4-vinylpyridine); WP-1, silica-supported BPEI adsorbent

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REFERENCES

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(39) Jones, K. C.; Grinstead, R. R. Properties and Hydrometallurgical Applications of Two Chelating Ion Exchange Resins. Chem. Ind. 1977, 637−641. (40) Sirola, K.; Laatikainen, M.; Paatero, E. Effect of Temperature on Sorption of Metals by Silica-Supported 2-(Aminomethyl)pyridine. Part II: Sorption Dynamics. React. Funct. Polym. 2010, 70, 56−62.

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Role of Ligand Acidity in Chelating Adsorption and Desorption of

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