Extraction of Gold(III) from Hydrochloric Acid into Various Ionic Liquids

Oct 12, 2015 - The extraction of gold(III) from 0.10 mol dm–3 hydrochloric acid into various aprotic ionic liquids (ILs) was investigated at 25 °C...
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Extraction of Gold(III) from Hydrochloric Acid into Various Ionic Liquids: Relationship between Extraction Efficiency and Aqueous Solubility of Ionic Liquids Shoichi Katsuta,* Yuta Watanabe, Yusuke Araki, and Yoshihiro Kudo Department of Chemistry, Graduate School of Science, Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263-8522, Japan ABSTRACT: The extraction of gold(III) from 0.10 mol dm−3 hydrochloric acid into various aprotic ionic liquids (ILs) was investigated at 25 °C. Eleven kinds of ILs were examined as the extraction solvents; the cations were 1-butyl-3-methylimidazolium ([BMIm]+), 1-hexyl-3-methylimidazolium, 1-methyl-3-octylimidazolium, 1-butyl-2,3-dimethylimidazolium, 1-butyl-1-methylpyrrolidinium, and methyltrioctylammonium; the anions were tetrafluoroborate, hexafluorophosphate, bis(trifluoromethanesulfonyl)amide ([NTf 2 ] − ), and bis(pentafluoroethanesulfonyl)amide. The metal was extracted as anionic tetrachloroaurate ([AuCl4]−) into all the nonprotic ILs with high distribution ratios (>100). The distribution ratio of gold(III) differs largely (480-fold at most) depending on the IL species and also depends on the amount of extracted gold(III). These dependences of the distribution ratio were quantitatively explained as functions of the solubility product of the IL used, where we considered the equilibrium model comprising the ion pair extraction of [AuCl4]− with the IL cation in the aqueous phase and the ion exchange of [AuCl4]− with the IL anion in the IL phase. From a practical purpose, the back extraction of gold(III) from an IL ([BMIm][NTf2]) phase was also examined; nearly quantitative stripping was possible using acidic thiourea solutions. KEYWORDS: Liquid−liquid extraction, Ionic liquids, Gold(III), Tetrachloroaurate, Solubility product

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INTRODUCTION Liquid−liquid extraction has been used as a simple and convenient method for recovering, removing, or concentrating metal ions in aqueous solutions. However, the use of organic solvents that are generally volatile, flammable, and harmful has been a long pending problem. Recently, ionic liquids (ILs) have attracted increasing attention as replacements for conventional organic solvents because they are almost nonvolatile and hence nonflammable and have low toxicity.1−4 One of the striking features of ILs as extraction solvents is that they can extract ionic species by themselves, the mechanism of which is generally considered to be ion exchange.5−13 The extractability of ILs for ionic species are very much dependent on the kind of IL, although until recently there was no quantitative explanation for the “solvent effect” of ILs on the extraction efficiency. We recently proposed an equilibrium model for the extraction of phenolate anions such as picrate by ILs.14,15 In this model, the target anion in the aqueous phase is extracted together with the IL cation in the aqueous phase and also extracted by exchange with the IL anion in the IL phase. We derived theoretical equations expressing the distribution ratio (D) of the target anion as a function of the aqueous solubility product (Ksp) and the extraction equilibrium constant of the IL. The dependences of D on the amount of the extracted anion and on the kind of IL were explained on the basis of the © XXXX American Chemical Society

equations. More recently, we have reported that the model is applicable to the extraction of organic mono- and divalent cations by ILs.16 Gold(III) is one of the major targets to which the solvent extraction technique is applied as a separation method. The extraction of gold(III) in aqueous chloride media is often conducted by using high molecular weight amines or tertiary amines diluted in nonpolar organic solvents, where gold(III) is extracted in the form of anionic tetrachloroaurate ([AuCl4]−) together with cationic protonated amines.17−20 Recently, several researchers have studied the extraction of gold(III) by ILs.21−25 Dupont et al.24 applied our model to the extraction of tetrahalogenoaurate ions ([AuCl4]− and [AuBr4]−) by some trialkyl(2-ethoxy-2-oxoethyl)ammonium-based ILs and discussed the extraction efficiency based on the extraction equilibrium constants. In this study, we investigated the extraction behavior of gold(III) from 0.10 mol dm−3 hydrochloric acid into various Special Issue: Ionic Liquids at the Interface of Chemistry and Engineering Received: August 30, 2015 Revised: October 8, 2015

A

DOI: 10.1021/acssuschemeng.5b00976 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. ILs Used in This Work

deionized water. The yield was almost quantitative (96%). [BMIm][NTf2] was prepared in a similar manner from aqueous solutions of 2.9 mol dm−3 [BMIm]Cl and 3.6 mol dm−3 Li[NTf2] (Kanto Chemical Co., Inc.; 99.7% purity) in a yield of 95%. The purities of the products were checked by atomic absorption spectrophotometry for alkali metal ions and ion-selective potentiometry for Cl; mass fractions w(Na) < 9 × 10−7 and w(Cl) < 7 × 10−7 for [BMIm][PF6]; w(Li) < 7 × 10−8 and w(Cl) < 2 × 10−6 for [BMIm][NTf2]. [HMIm][PF6] (for synthesis grade), [HMIm][NTf2] (high purity grade), [MOIm][BF4] (for synthesis grade), [MOIm][PF6] (for synthesis grade), and [MOIm][NTf2] (for synthesis grade) were purchased from Merck KGaA; they were washed three times with deionized water just prior to use. A special grade reagent of [BMPyr][NTf2] (w(H2O) = 3 × 10−6, w(Li) < 2 × 10−7, w(Na) < 1 × 10−7, w(F) < 1 × 10−6, and w(Cl) = 1 × 10−6) supplied from Kanto Chemical Co. was used as received. [MOIm][BETI],15 [BM2Im][NTf2],26 and [MTOA][NTf2]27 were the same as used

hydrophobic aprotic ILs at 298 K in order to understand the solvent effect of ILs on the extractability of tetrachloroaurate(III). Eleven kinds of ILs were used, and their full names, abbreviations, and structural formulas are listed in Table 1. The results obtained are discussed based on the theoretical models proposed previously. Here, we have also determined the aciddissociation constant of bis(trifluoromethanesulfonyl)amide acid (H[NTf2]) in water and the solubility of each IL in 0.10 mol dm−3 hydrochloric acid, which are used for the discussion.

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EXPERIMENTAL SECTION

Materials. [BMIm][PF6] was prepared by mixing equal volumes of aqueous solutions of 4.1 mol dm−3 [BMIm]Cl (Merck KGaA, Darmstadt, Germany; for synthesis grade) and 4.8 mol dm−3 Na[PF6] (Sigma-Aldrich Co., St. Louis, MO; 98% purity); the IL phase separated from the aqueous phase was washed five times with B

DOI: 10.1021/acssuschemeng.5b00976 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering previously. Standard aqueous solutions of gold(III) (HAuCl4 in 1 mol dm−3 HCl) were purchased as 1000 ppm atomic absorption standards from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) or prepared from H[AuCl4]·4H2O (for analysis grade) purchased from Kojima Chemicals. Co., Ltd. (Saitama, Japan) or Takeda Chemicals Co., Ltd. (Osaka, Japan). Dichloromethane of guaranteed grade (Kanto Chemical Co.) was purified by distillation. H[NTf2] (1,1,1-trifluoroN-[(trifluoromethyl)sulfonyl]methanesulfonamide; 99.0% purity), hydrochloric acid (ultrapure grade), sodium picrate monohydrate (extrapure grade), and thiourea (extra-pure grade) were purchased from Kanto Chemical Co. and used without further purification. Water was distilled and further deionized with a Milli-Q Lab system (Millipore Co., Billerica, MA). Determination of Aqueous Acid-Dissociation Constant of H[NTf2]. The pH values of aqueous solutions of H[NTf2] (5.0 × 10−2 and 1.0 × 10−1 mol dm−3) were measured in a thermostatic water bath at (298.15 ± 0.05) K with a Sartorius Docu-pH+ meter equipped with a glass electrode. For an acidic aqueous solution of an acid HA, the following equations are valid: [H+] = [A−] (charge balance); [HA]T = [HA] + [A−] (mass balance), where [HA]T denotes the total concentration of HA. The equilibrium concentrations of [NTf2]− ([A−]) and H[NTf2] ([HA]) were calculated based on these equations, where the concentration of H+ was calculated as [H+] = 10−pH/γH+ by using the γH+ value (activity coefficient of H+) reported in the literature.28 The acid-dissociation constant defined as Ka = [H+][A−]/[HA] was determined. Determination of Solubilities of ILs in 0.10 mol dm−3 Hydrochloric Acid. The solubility of each IL in 0.10 mol dm−3 hydrochloric acid was determined by measuring the concentrations of the cation in the aqueous solutions saturated with the IL at (298.15 ± 0.05) K. For the ILs except for [MTOA][NTf2], an aliquot of the ILsaturated aqueous solution was transferred into a stoppered glass tube, to which sodium picrate and sodium hydroxide were added so that their concentrations became 5 × 10−2 mol dm−3 and 1 × 10−2 mol dm−3, respectively. Dichloromethane, whose volume was equal to that of the aqueous solution, was further added, and the biphasic mixture was stirred for 30 min. The picrate concentration in the dichloromethane phase was determined spectrophotometrically (λmax = 367.8 nm, ε = 1.70 × 104 mol−1 dm3 cm−1 for [BMIm]+;14 λmax = 367.0 nm, ε = 1.70 × 104 mol−1 dm3 cm−1 for [HMIm]+;29 λmax = 367.8 nm, ε = 1.72 × 104 mol−1 dm3 cm−1 for [MOIm]+;14 λmax = 375.0 nm, ε = 1.81 × 104 mol−1 dm3 cm−1 for [BM2Im]+;15 λmax = 374.8 nm, ε = 1.69 × 104 mol−1 dm3 cm−1 for [BMPyr]+.14 For [MTOA][NTf2], which has a very low solubility, the concentration of [MTOA]+ in the ILsaturated aqueous solution was determined by the method of Taguchi et al.30 Forward Extraction of Gold(III). An aqueous solution containing gold(III) (4.1 × 10−6−1.1 × 10−2 mol dm−3) and HCl (0.10 mol dm−3) was prepared and allowed to stand for more than 24 h at room temperature. The aqueous solution was placed in a stoppered test tube together with a water-saturated IL. Here, the volume of the viscous IL phase was accurately evaluated from the mass by using the densities of wet (water saturated) or dry ILs; 1.3550 g cm−3 for [BMIm][PF6] (wet),31 1.4311 g cm−3 for [BMIm][NTf2] (wet),31 1.2937 g cm−3 for [HMIm][PF6] (dry),32 1.370 g cm−3 for [HMIm][NTf2] (dry),33 1.0832 g cm−3 for [MOIm][BF4] (wet),13 1.2323 g cm−3 for [MOIm][PF6] (wet),15 1.3276 g cm−3 for [MOIm][NTf2] (wet),13 1.3966 for [MOIm][BETI] (wet),15 1.4159 g cm−3 for [BM2Im][NTf2] (wet),15 1.3919 g cm−3 for [BMPyr][NTf2] (wet),31 and 1.1113 g cm−3 for [MTOA][NTf2] (wet).27 The volume ratio of the IL phase to the aqueous phase was adjusted to 0.83−0.10, where the volume change of the IL phase upon its dissolution into the aqueous phase was corrected by using the solubility data (Table 2). The test tube containing the biphasic mixture was mechanically shaken for 1 h at 250 strokes per min in a thermostatic chamber at (298.2 ± 0.2) K; the shaking time (1 h) was preliminary confirmed to be sufficient to establish the extraction equilibrium. After the phases were separated by centrifugation at 3000 rpm, the tube was allowed to stand for 15 min in a thermostatic water bath at (298.15 ± 0.05) K. The concentration of gold(III) in the aqueous phase was determined with a Hitachi Z-

Table 2. Solubilities and Solubility Products of ILs in 0.10 mol dm−3 Hydrochloric Acid at 298.2 K IL [BMIm][PF6] [BMIm][NTf2] [HMIm][PF6] [HMIm][NTf2] [MOIm][BF4] [MOIm][PF6] [MOIm][NTf2] [MOIm][BETI] [BM2Im][NTf2] [BMPyr][NTf2] [MTOA][NTf2]

solubilitya (mol dm−3) (7.67 ± 0.01) (1.99 ± 0.05) (2.84 ± 0.03) (6.01 ± 0.02) (6.4 ± 0.1) (9.30 ± 0.09) (1.898 ± 0.005) (3.67 ± 0.07) (1.29 ± 0.02) (1.82 ± 0.02) (5.49 ± 0.03)

× × × × × × × × × × ×

10−2 10−2 10−2 10−3 10−2 10−3 10−3 10−4 10−2 10−2 10−5

Ksp 5.88 3.5 8.07 3.2 4.1 8.65 3.2 1.35 1.5 2.9 2.7

× × × × × × × × × × ×

10−3 10−4b 10−4 10−5b 10−3 10−5 10−6b 10−7 10−4b 10−4b 10−9b

a

Average and standard error values of the IL cation concentrations measured in the 0.10 mol dm−3 hydrochloric acid solutions saturated with ILs. bCalculated by considering protonation of [NTf2]−.

5000 polarized Zeeman atomic absorption spectrophotometer in the graphite furnace mode. The concentration of gold(III) in the IL phase was calculated from the aqueous concentrations before and after extraction. The distribution ratio of gold(III), D, was calculated as the ratio of the molar concentration in the IL phase to that in the aqueous phase. For some ILs, the aqueous concentration of the IL cation was also measured in the same manner as described above. Back Extraction of Gold(III). A [BMIm][NTf2] solution containing gold(III) (5.0 × 10−3 mol dm−3) was prepared by forward extraction of gold(III) from 0.10 mol dm−3 hydrochloric acid into [BMIm][NTf2]. The IL solution and an aqueous solution containing thiourea (0.10 or 1.0 mol dm−3) and HCl (0, 0.10, or 1.0 mol dm−3) were placed in a test tube, where the volume ratio of the IL phase to the aqueous phase was adjusted to 0.10. The test tube was mechanically shaken for 1 h at (298.2 ± 0.2) K. After centrifugation, the concentration of gold(III) in the aqueous phase was determined by atomic absorption spectrophotometry in the graphite furnace mode.

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RESULTS AND DISCUSSION Properties of ILs in Hydrochloric Acid. From three independent measurements, the Ka value of H[NTf2] was determined to be 0.70 ± 0.04 mol dm−3 (pKa 0.16). The Ka value is somewhat larger than the literature value, 0.02 mol dm−3 (pKa 1.7).34 The degree of dissociation (αHA) of H[NTf2] can be calculated by αHA = Ka/(Ka + [H+]); the αHA value is 0.88 in 0.10 mol dm−3 hydrochloric acid, indicating that only a small portion of [NTf2]− is protonated. From the higher acidity of H[BETI] than H[NTf2] in the gas phase,35 the protonation of [BETI]− was regarded as negligibly small. The protonation of [BF4]− and [PF6]− was also not considered in this study. The solubilities of ILs in 0.10 mol dm−3 hydrochloric acid were determined by measuring the concentration of the IL cation (C+) in the IL-saturated hydrochloric acid, and are shown in Table 2. Assuming that ion association of C+ with the IL anion (A−) is negligible, the values of the solubility product, Ksp = [C+][A−], was calculated as the square of solubility. When A− is partially protonated, the solubility product values obtained by this calculation are conditional solubility products in 0.10 mol dm−3 hydrochloric acid, i.e., Ksp′ = [C+]([A−] + [HA]). For the [NTf2]−-based ILs whose Ka value is known, the real Ksp values were calculated by considering the protonation of [NTf2]−. The Ksp values are also listed in Table 2. The solubilities in 0.10 mol dm−3 hydrochloric acid are all slightly higher than those in water reported previously,15 which is interpreted by the effect of ionic strength on the activity coefficients of C+ and A−. C

DOI: 10.1021/acssuschemeng.5b00976 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Table 3. Concentrations of Au(III) and IL Cations (C+) in the Aqueous Phase and Distribution Ratio of Au(III) in IL/0.10 mol dm−3 Hydrochloric Acid Systems at 298.2 K equilib. conc. in the aq phase (mmol dm−3) initial conc. of Au(III) in the aq phase (mmol dm−3) IL = [BMIm][PF6] 10.5 4.14 2.11 1.10 0.418 IL = [BMIm][NTf2] 9.96 4.11 2.15 1.08 0.513 0.246 0.123 0.0619 IL = [HMIm][PF6] 9.35 4.29 1.67 0.670 0.264 IL = [HMIm][NTf2] 10.4 4.19 1.73 0.682 0.278 IL = [MOIm][BF4] 10.2 4.36 1.86 IL = [MOIm][PF6]

IL/aq volume ratioa

Au(III)

C

+

equilib. conc. in the aq phase (mmol dm−3) initial conc. of Au(III) in the aq phase (mmol dm−3)

log D

0.0838 0.0840 0.0840 0.0840 0.0841

0.0895 0.0358 0.0179 0.00966 0.00393

68.6 72.1 73.9 72.2 73.9

3.143 3.135 3.145 3.127 3.097

0.0944 0.0947 0.0943 0.0943 0.0946 0.101 0.100 0.100

0.829 0.279 0.116 0.0596 0.0266 0.0129 0.00568 0.00305

15.3 17.7 18.7 19.3 20.5 19.8 20.0 20.3

2.067 2.161 2.271 2.257 2.287 2.254 2.313 2.286

0.0935 0.0932 0.0932 0.0932 0.0934

0.0198 0.00685 0.00252 0.000811 0.000345

22.3 25.8 26.7 27.6 28.4

3.701 3.827 3.852 3.947 3.912

0.0982 0.0986 0.0987 0.0983 0.0983

0.404 0.105 0.0365 0.0135 0.00562

0.0833 0.0833 0.0836

0.00174 0.000728 0.000314

IL =

IL =

IL =

2.73 4.26 5.05 5.52 5.84

IL =

2.403 2.594 2.671 2.702 2.694

IL =

4.846 4.857 4.850

a

Extraction Equilibria of Gold(III) in IL/Hydrochloric Acid Biphasic Systems. In Table 3, the log D values of gold(III) obtained at the different initial concentrations are summarized together with the equilibrium concentrations of gold(III) and C+ in the aqueous phase. In most of the systems, the D value of gold(III) is nearly independent of its initial concentration; however, in a few of the systems, a decrease of D with an increase of the initial concentration of gold(III) is observed. In response to the decrease of D, the concentration of C+ in the aqueous phase also decreases, which means that the concentration of A− in the aqueous phase increases according to the solubility product principle. In addition, although all the D values are more than 100, the dependence of D on the kind of IL is remarkable; for example, the D value for [MOIm][BF4] is about 480-fold higher than that for [BMPyr][NTf2]. Such effects of the concentration of gold(III) and the kind of IL on the extractability of gold(III) are discussed on the basis of the theory14,15 that was applied previously to the distribution of phenolate anions in the IL/water biphasic systems. In 0.10 mol dm−3 hydrochloric acid, [AuCl4]− is the predominant species of gold(III).36,37 We observed that the

9.42 3.88 1.62 0.634 0.259 [MOIm][NTf2] 8.20 2.71 0.640 0.259 0.0621 [MOIm][BETI] 0.0251 0.00985 0.00414 [BM2Im][NTf2] 0.494 0.197 0.0790 [BMPyr][NTf2] 0.629 0.257 0.102 0.0414 0.0161 [MTOA][NTf2] 0.963 0.522 0.230 0.143

IL/aq volume ratioa

Au(III)

C+

log D

0.0973 0.0971 0.0966 0.0968 0.0974

0.00960 0.00284 0.000957 0.000398 0.000169

5.33 7.42 8.01 8.98 8.95

4.003 4.148 4.244 4.216 4.197

0.0988 0.0982 0.0996 0.100 0.0995

0.200 0.0317 0.00559 0.00184 0.000356

0.534 1.14 1.63 1.86 1.80

2.607 2.934 3.057 3.145 3.241

0.101 0.101 0.101

0.00152 0.000546 0.000242

2.185 2.227 2.202

0.101 0.101 0.100

0.0150 0.00595 0.00268

2.501 2.505 2.453

0.0946 0.0945 0.0946 0.0946 0.0947

0.0405 0.0162 0.00635 0.00275 0.00102

2.186 2.197 2.202 2.171 2.191

0.105 0.103 0.104 0.100

0.00275 0.000800 0.000229 0.0000864

0.00767 0.0128 0.0215 0.0308

3.524 3.801 3.984 4.218

Dissolution of IL to the aqueous phase corrected.

UV−vis absorption spectrum of gold(III) extracted in [BMIm][NTf2] (λmax = 322.5 nm, ε = 4.78 × 103 mol−1 dm3 cm−1) is similar to that in 0.10 mol dm−3 hydrochloric acid (λmax = 313.5 nm, ε = 5.42 × 103 mol−1 dm3 cm−1). In this study, we supposed that gold(III) is extracted as [AuCl4]− (in the following abbreviated by T− (target anion)) into each IL through the following equilibrium reactions. Ion exchange extraction: T− W + A−IL ⇄ T−IL + A− W

(1)

Ion pair extraction: T− W + C+ W ⇄ T−IL + C+IL

(2)

where the subscripts W and IL represent the species in the aqueous and IL phases, respectively. These equations are based on the fact that the aqueous concentrations of A− and C+ increase and decrease, respectively, with an increase of the amount of gold(III) extracted. In addition, there is a dissolution equilibrium of IL in the aqueous phase. C+IL + A−IL ⇄ C+ W + A− W D

(3)

DOI: 10.1021/acssuschemeng.5b00976 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering The equilibrium constants for eqs 1−3 are, respectively, defined as follows. Kex ‐ IE =

Kex ‐ IP =

log D = log Kex ‐ IP + log

[T−]IL [A−]W [T−]W

(4)

[T−]IL [T ]W [C+]W

(5)

−

K sp = [C+]W [A−]W

2

(13)

eqs 12 and 13 are essentially the same, because they can be converted to each other by the relationship Ksp = Kex‑IE/Kex‑IP. When the protonation of A− is negligible, eqs 12 and 13 are transformed into the following equations, respectively. log D = log Kex ‐ IE

(6)

− log

where Kex‑IE and Kex‑IP are the ion exchange extraction constant and the ion pair extraction constant, respectively. These equations are based on the assumption of dilute solutions for C+ and A− in the aqueous phase and for T− in both the phase. The concentrations of C+ and A− in the IL phase are not included on the right side of eqs 4−6 because their activities are regarded as constant. It should be noted that these equilibrium constants are not independent of each other; there is a relationship Ksp = Kex‑IE/Kex‑IP. For the aqueous phases before and after extraction, the charge balance equations are written as follows.

Δ[T−]W + {Δ[T−]W 2 + 4K sp}1/2 2

(14)

log D = log Kex ‐ IP + log

−Δ[T−]W + {Δ[T−]W 2 + 4K sp}1/2

(15) 2 eqs 14 and 15 are equal to those derived for the distribution of phenolate anions in IL/water biphasic systems.14 To confirm the validity of eqs 12 and 13, the log D values in some IL/0.10 mol dm−3 hydrochloric acid biphasic systems are plotted against the log Δ[T−]W values in Figure 1. The solid

Before extraction: [H+]W,init = [Cl−]W,init + [T−]W,init

−Δ[T−]W + {Δ[T−]W 2 + 4(1 + [H+]W /K a)K sp}1/2

(7)

After extraction: [H+]W + [C+]W = [Cl−]W + [A−]W + [T−]W

(8)

where the subscript “init” denote the initial concentration. It was experimentally confirmed that Cl− was not detectably extracted into the IL phase in the IL/0.10 mol dm −3 hydrochloric acid biphasic systems, indicating that [Cl−]W,init = [Cl−]W. When the extraction of H+ is negligible and the protonation of A− occurs in the aqueous phase, [H+]W,init = [H+]W + [HA]W. Therefore, the following equation is derived from eqs 7 and 8. Δ[T−]W = [A−]W + [HA]W − [C+]W

Figure 1. Distribution ratio of [AuCl4]− in IL/0.10 mol dm−3 hydrochloric acid biphasic systems as a function of the difference between the initial and equilibrium concentrations of [AuCl4]− in the aqueous phase (Δ[T−]W). The solid lines are the regression curves based on eqs 12 and 13.

(9)

where Δ[T−]W represents [T−]W,init − [T−]W, namely, the difference between the initial and equilibrium concentrations of T− in the aqueous phase. Substituting the definitions of Ksp and Ka into eq 9 yields the following equations expressing [C+]W and [A−]W. [C+]W =

lines in Figure 1 were obtained by nonlinear least-squares fitting (KaleidaGraph, Synergy Software) of eq 12 or eq 13, where the fitting parameter was Kex‑IE or Kex‑IP, respectively. The lines are in good agreement with the experimental points, supporting the assumption that gold(III) is extracted as a monovalent anion [AuCl4]−. The lower log D value in the higher initial concentration of gold(III) can be quantitatively explained by these equations. The Kex‑IE and Kex‑IP values determined in this way are summarized in Table 4. Interestingly, the Kex‑IE values for the ILs with the same A− but different C+ are nearly constant. The same is true for the Kex‑IP values for the ILs with the same C+ but different A−. These results are ascribable to the fact that C+ and A− have no direct contribution to the ion exchange extraction and the ion pair extraction of [AuCl4]−, respectively (see eqs 1 and 2). It is also suggested that the solvation energies of [AuCl4]− are similar in the ILs used in this study. IL Dependence of Distribution Ratio. According to eq 12 or 13, when Δ[T−]W ≪ 2(1 + [H+]W/Ka)1/2Ksp1/2, namely in a dilute condition of T−, the D value is almost independent

−Δ[T−]W + {Δ[T−]W 2 + 4(1 + [H+]W /K a)K sp}1/2 2

(10) [A−]W =

Δ[T−]W + {Δ[T−]W 2 + 4(1 + [H+]W /K a)K sp}1/2 2(1 + [H+]W /K a)

(11) −

+

Substituting eqs 11 and 10 for [A ]W and [C ]W in eqs 4 and 5, respectively, the distribution ratio of gold(III), D = [T−]IL/ [T−]W, is expressed by the following equations. log D = log Kex ‐ IE − log

Δ[T−]W + {Δ[T−]W 2 + 4(1 + [H+]W /K a)K sp}1/2 2(1 + [H+]W /K a)

(12) E

DOI: 10.1021/acssuschemeng.5b00976 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 4. Distribution Ratios of Gold(III) Ion at Infinite Dilution and Extraction Equilibrium Constants (Molarity Scale) in IL/0.10 mol dm−3 Hydrochloric Acid Systems at 298.2 K

a

IL

log D0

[BMIm][PF6] [BMIm][NTf2] [HMIm][PF6] [HMIm][NTf2] [MOIm][BF4] [MOIm][PF6] [MOIm][NTf2] [MOIm][BETI] [BM2Im][NTf2] [BMPyr][NTf2] [MTOA][NTf2]

3.15 2.26 3.87 2.723 4.868 4.24 3.20 2.21 2.48 2.191 4.71

log Kex‑IPa 4.26 3.96 5.42 4.944 6.063 6.27 5.92 5.65 4.37 3.932 8.97

± ± ± ± ± ± ± ± ± ± ±

log Kex‑IEa

0.01 0.02 0.03 0.006 0.007 0.01 0.02 0.01 0.02 0.006 0.04

2.03 0.50 2.33 0.444 3.673 2.21 0.42 −1.22 0.54 0.392 0.39

± ± ± ± ± ± ± ± ± ± ±

0.01 0.02 0.03 0.006 0.007 0.01 0.02 0.01 0.02 0.006 0.04

Figure 2. Relationship between log D0 and log Ksp1/2 for [NTf2]−based ILs (red filled circles) and [PF6]−-based ILs (red open circles) with cations. The numbers of ILs correspond to those in Table 1. The broken lines are the regression lines with a slope of −1.

Values after “ ± ” denote standard errors.

of the Δ[T−]W value. In this case, the distribution ratio (D0) is expressed by the following approximate equations. log D0 = log Kex − IE − log K sp1/2 + log(1 + [H+]W /K a)1/2 log D0 = log Kex − IP + log

K sp1/2

+

1/2

+ log(1 + [H ]W /K a) +

(16) (17) 1/2

where the third term in the right side, log(1 + [H ]W/Ka) , is zero when the protonation of A− is negligible; even in the case of the [NTf2]−-based ILs, the term is practically negligible (0.03 in 0.10 mol dm−3 hydrochloric acid). The D0 values calculated by using these equations are shown in Table 4. From eqs 16 and 17, the following expectations are provided concerning the solvent effect of IL on the D0 value: (1) When the kind of C+ is varied for a given A−, the D0 value is in inverse proportion to the Ksp1/2 value because the Kex‑IE value in eq 16 is nearly constant. (2) When the kind of A− is varied for a given C+, the D0 value is in direct proportion to the Ksp1/2 value because the Kex‑IP value in eq 17 is nearly constant. Consequently, the D0 value is greater when using the IL composed of more hydrophobic C+ and more hydrophilic A− and the variation of D0 with the kind of C+ or A− can be quantitatively expected on the basis of the Ksp value of the IL. To verify the validity of these expectations, the plots of log D0 vs log Ksp1/2 are shown for [NTf2]−-based or [PF6]−-based ILs with different cations (Figure 2) and for the [MOIm]+based ILs with different anions (Figure 3). In Figure 3, there appears to be almost a linear relationship with a slope of +1 for the [MOIm]+-based ILs with different anions, which supports the above expectation (2). In Figure 2, according to the expectation (1), a good linear relationship with a slope of −1 is observed for the [NTf2]−-based ILs or the [PF6]−-based ILs, respectively. In our previous study on the distribution of picrate anion in the [NTf2]−-based IL/water biphasic systems,15 a similar result was obtained in the log D0 vs log Ksp1/2 plot; however, the slope −1 line for the ILs with 1,3-dialkylimidazolium cations showed a larger intercept than that for the ILs with other aprotic cations. This result was interpreted in terms of the hydrogen bond interaction between the picrate anion and the 1,3-dialkylimidazolium cation in the IL phase owing to the acidic nature of C(2) hydrogen atom of the cation. In the present study, such differences between the cation types are not observed; it thus appears that the interaction of [AuCl4]− with the 1,3-dialkylimidazolium cation is too weak to contribute significantly to the distribution ratio of [AuCl4]−. It was also

Figure 3. Relationship between log D0 and log Ksp1/2 for [MOIm]+based ILs with different anions. The numbers of ILs correspond to those in Table 1. The broken line is the regression line with a slope of +1.

reported previously15 that the picrate anion is well extracted by the ILs with protic cations such as 1-butylpyrrolidinium ([BPyrH]+), probably due to the strong hydrogen bond interaction between the picrate anion and the protic cations. On the other hand, as we reported previously,23 gold(III) is extracted as electrically neutral species by protic ILs. The gold(III) extraction from 0.10 mol dm−3 hydrochloric acid into the protic ILs are much lower than that into the aprotic ILs having similar chemical structures; e.g., log D0 = 1.20 for [BPyrH][NTf2]23 and 2.19 for [BMPyr][NTf2]. At this stage, it is not clear why the extraction behavior of gold(III) is so different between the protic and aprotic ILs. Stripping of Gold(III) from IL Phase. In the case of ILs that are not volatile, the back extraction of gold from the IL phase into an aqueous medium is important to recover the metal. It was reported that acidic thiourea solutions is effective for the purpose of stripping gold even in the case of using ILs as extraction solvents.25 Thus, in this study, back extraction of gold(III) from [BMIm][NTf2] was briefly examined by using aqueous or hydrochloric acid solutions of thiourea as stripping solutions. When the volume ratio of the stripping solution to the IL solution was 10, the back extraction percentages of gold(III) were as follows: 71.7% for 0.10 mol dm−3 thiourea; 91.9% for 0.10 mol dm−3 thiourea and 0.10 mol dm−3 HCl; F

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ACS Sustainable Chemistry & Engineering 74.2% for 1.0 mol dm−3 thiourea; 93.6% for 1.0 mol dm−3 thiourea and 1.0 mol dm−3 HCl. Nearly quantitative stripping was achieved with the solution containing 1.0 mol dm−3 thiourea and 1.0 mol dm−3 HCl.

Replacements for Traditional Organic Solvents and Their Applications towards “Green Chemistry” in Separation Processes. In Green Industrial Applications of Ionic Liquids; Rogers, R. D., Seddon, K. R., Volkov, S., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2003; pp 137−156. (3) Han, X.; Armstrong, D. W. Ionic Liquids in Separation. Acc. Chem. Res. 2007, 40, 1079−1086. (4) Oppermann, S.; Stein, F.; Kragl, U. Ionic Liquids for Two-phase Systems and Their Application for Purification, Extraction and Biocatalysis. Appl. Microbiol. Biotechnol. 2011, 89, 493−499. (5) Dai, S.; Ju, Y. H.; Barnes, C. E. Solvent Extraction of Strontium Nitrate by a Crown Ether Using Room-Temperature Ionic Liquids. J. Chem. Soc., Dalton Trans. 1999, 1201−1202. (6) Visser, A. E.; Swatloski, R. P.; Griffin, S. T.; Hartman, D. H.; Rogers, R. D. Liquid/Liquid Extraction of Metal Ions in Room Temperature Ionic Liquids. Sep. Sci. Technol. 2001, 36, 785−804. (7) Carda-Broch, S.; Berthod, A.; Armstrong, D. W. Solvent Properties of the 1-Butyl-3-methylimidazolium Hexafluorophosphate Ionic Liquid. Anal. Bioanal. Chem. 2003, 375, 191−199. (8) Vidal, S. T. M.; Correia, M. J. N.; Marques, M. M.; Ismael, M. R.; Reis, M. T. A. Studies on the Use of Ionic Liquids as Potential Extractants of Phenolic Compounds and Metal Ions. Sep. Sci. Technol. 2005, 39, 2155−2169. (9) Khachatryan, K. S.; Smirnova, S. V.; Torocheshnikova, I. I.; Shvedene, N. V.; Formanovsky, A. A.; Pletnev, I. V. Solvent Extraction and Extraction-Voltammetric Determination of Phenols Using Room Temperature Ionic Liquid. Anal. Bioanal. Chem. 2005, 381, 464−470. (10) Vijayaraghavan, R.; Vedaraman, N.; Surianarayanan, M.; MacFarlane, D. R. Extraction and Recovery of Azo Dyes into an Ionic Liquid. Talanta 2006, 69, 1059−1062. (11) Pei, Y. C.; Wang, J. J.; Xuan, X. P.; Fan, J.; Fan, M. Factors Affecting Ionic Liquids Based Removal of Anionic Dyes from Water. Environ. Sci. Technol. 2007, 41, 5090−5095. (12) de los Rios, A. P.; Hernandez-Fernandez, F. J.; Lozano, L. J.; Sanchez, S.; Moreno, J. I.; Godinez, C. Removal of Metal Ions from Aqueous Solutions by Extraction with Ionic Liquids. J. Chem. Eng. Data 2010, 55, 605−608. (13) Katsuta, S.; Nakamura, K.; Kudo, Y.; Takeda, Y.; Kato, H. Partition Behavior of Chlorophenols and Nitrophenols between Hydrophobic Ionic Liquids and Water. J. Chem. Eng. Data 2011, 56, 4083−4089. (14) Katsuta, S.; Nakamura, K.; Kudo, Y.; Takeda, Y. Mechanisms and Rules of Anion Partition into Ionic Liquids: Phenolate Ions in Ionic Liquid/Water Biphasic Systems. J. Phys. Chem. B 2012, 116, 852−859. (15) Watanabe, Y.; Katsuta, S. Distribution of a Monovalent Anion in Various Ionic Liquid/Water Biphasic Systems: Relationship of the Distribution Ratio of Picrate Ions with the Aqueous Solubility of Ionic Liquids. J. Chem. Eng. Data 2014, 59, 696−701. (16) Hamamoto, T.; Okai, M.; Katsuta, S. The Laws Governing Ionic Liquid Extraction of Cations: Partition of 1-Ethylpyridinium Monocation and Paraquat Dication in Ionic Liquid/Water Biphasic Systems. J. Phys. Chem. B 2015, 119, 6317−6325. (17) Nicol, M. J.; Fleming, C. A.; Paul, R. L. The Chemistry of the Extraction of Gold. In The Extractive Metallurgy of Gold; Stanley, G. G., Ed.; The Southern African Institute of Mining and Metallurgy: Johannesburg, South Africa, 1987; pp 831−905. (18) Martinez, S.; Sastre, A. M.; Alguacil, F. J. Solvent Extraction of Gold(III) by the Chloride Salt of the Tertiary Amine Hostarex A327. Estimation of the Interaction Coefficient between AuCl4− and H+. Hydrometallurgy 1999, 52, 63−70. (19) Kolekar, S. S.; Anuse, M. A. Rapid Solvent Extraction of Gold(III) with High Molecular Weight Amine from Organic Acid Solution. Gold Bull. 2001, 34, 50−55. (20) Kejun, L.; Yen, W. T.; Shibayama, A.; Miyazaki, T.; Fujita, T. Gold Extraction from Thiosulfate Solution Using Trioctylmethylammonium Chloride. Hydrometallurgy 2004, 73, 41−53. (21) Papaiconomou, N.; Vite, G.; Goujon, N.; Leveque, J.-M.; Billard, I. Efficient Removal of Gold Complexes from Water by Precipitation

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CONCLUSIONS In this study, it was found that the equilibrium theory we developed for the partition of organic ions can be applied to the partition of gold(III) ([AuCl4]−) in various aprotic IL/0.10 mol dm−3 hydrochloric acid biphasic systems. The following is the summary. (1) The extraction of [AuCl4]− from the hydrochloric acid phase into the IL one can be accounted for by both the models of ion pair extraction with the IL cation (C+) in the aqueous phase and of ion exchange with the IL anion (A−) in the IL phase. The distribution ratio of [AuCl4]− (T−) is expressed by a theoretical equation as a function of the amount of decrease of the aqueous concentration of T− upon extraction (Δ[T−]W), the aqueous solubility product of IL (Ksp), and the equilibrium constant of ion exchange extraction (Kex‑IE) or that of ion pair extraction (Kex‑IP). In the case of A− = [NTf2]− that forms a protonated form (HA) in the acidic aqueous solution, the distribution ratio is also a function of the aqueous pH and the dissociation constant of HA (Ka); however, the effect of the protonation is only a little in the 0.10 mol dm−3 hydrochloric acid solution. (2) The distribution ratio of T− is nearly constant when Δ[T−]W ≪ 2(1 + [H+]W/Ka)1/2Ksp1/2. The limiting distribution ratio (D0) is greater for the IL having more hydrophobic C+ and more hydrophilic A−. The D0 value is inversely proportional to Ksp1/2 in the variation of C+, and proportional to Ksp1/2 in the variation of A−. The regularities hold for various ILs including the ILs with 1,3-dialkylimidazolium cations that possess slightly acidic nature. This suggests the specific interaction (hydrogen bonding) of the 1,3dialkylimidazolium cations with [AuCl4]− is negligibly weak. Protic ILs, however, are exceptional because gold(III) is extracted as the neutral chloride complexes by the protic ILs. Although the extractability of gold(III) from hydrochloric acid to an IL depends greatly on the kind of IL, it is possible to quantitatively estimate the extraction efficiency of any aprotic IL from the Ksp value. The knowledge gained from this study will be useful in the application of ILs as extraction solvents for metal ions.

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AUTHOR INFORMATION

Corresponding Author

*S. Katsuta. Tel.: +81-43-290-2781. Fax: +81-43-290-2874. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by a grant-in-aid for scientific research (No. 26410145) from the Ministry of Education, Culture, Sports, Science and Technology of Japan and by a research grant from the Futaba Electronics Memorial Foundation.

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REFERENCES

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H

DOI: 10.1021/acssuschemeng.5b00976 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX