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Selective and Effective Reduction of Gold Ions from Acidic HCl− NaClO3 Leachate with Oxalic Acid Lei Xiong,†,‡ Shaobo Shen,*,†,‡ Longhui Liu,†,‡ and lifeng Zhang†,‡ †

Beijing Key Lab of Green Recycling and Extraction of Metals, Beijing 100083, China School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China

‡

ABSTRACT: The reduction of gold ions from HCl−NaClO3 leaching solution with oxalic acid was performed. It was found that, for 50 mL of gold-containing leaching filtrate, 20 mL of 6% (w/v) oxalic acid was required to achieve a selective and effective reduction of gold ions. The reaction temperature and time were found to have significant effects on gold reduction, and the optimal values were determined to be 85 °C and 2 h, respectively. The reduction of gold increased with increasing agitation speed and speed of addition of oxalic acid, and the optimal values were 180 rpm and 1.740 mL/min or more, respectively. Under the optimal conditions, the reductions of Au3+, Cu2+, Fe3+, Pb2+, Ag+, and Si4+ were 95.56%, 0.66%, 2.33%, 1.40%, 0.70%, and 0.50%, respectively. The appropriate solution acidity and solution potential, which are two important conditions for achieving the selective reduction of gold ions, were about 623 mV and 2.3−3.3 M, respectively.

1. INTRODUCTION We previously reported a fast and effective method for gold leaching from a desulfurized gold ore using acidic sodium chlorate solution at low temperature.1 An acidic goldcontaining chloride filtrate collected from the leaching experiments was thus obtained. The concentrations of gold and hydrochloric acid from the leachate were 3.43 ppm and ∼3.33 M, respectively. The gold ions existed mainly in the form of chloroauric ([AuCl4−]) ions.1 The reduction of Au(III) to nanoparticles of metallic Au has been reported using reducing agents such as ascorbic acid,2 Sn(II),3 glycerol,4 hydrogen peroxide,5 glucose,6 trisodium citrate,7 and dimethylamine borane.8 These reducing agents are either too expensive or have to be used in alkaline solutions and are not suitable for use in reducing chloroauric ion in strongly acidic chloride solution. To separate gold and other metal ions in the leachate, four reducing agents, namely, sodium sulfite, hydrazine, ferrous sulfate, and oxalic acid, were considered and tested for the selective reduction of gold ions in the leachate. It was found that, when sodium sulfite (Na2SO3) was used as the reducing agent, toxic SO2 gas was released by the reaction, which is harmful to the operating environment. In addition, small amounts of Pb2+ contained in the leachate could also be precipitated in the form of PbSO4, which could affect the purity of the gold. The reduction speed was too low when ferrous sulfate (FeSO4) was used as the reducing agent. Moreover, too much ferrous sulfate was required for the reduction, which could also affect the purity of the gold. Although hydrazine (N2H4·H2O) was able to reduce Au3+ in the leachate, Cu2+, Fe3+, and Pb2+ in the filtrate were also reduced simultaneously. Moreover, hydrazine is flammable and toxic. It can explode when intensive heat is released on an industrial scale. It is also more expensive and difficult to transport or store than the other reagents considered. Therefore, sodium sulfite, ferrous sulfate, and hydrazine are not suitable for the reduction of gold in the present filtrate. We found that oxalic acid is environmentally friendly for the gold reduction. AL-Thabaiti et al. reported that © 2014 American Chemical Society

the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB) can accelerate the reduction of chloroauric ([AuCl4−]) ions by oxalic acid to prepare Au nanoparticles.9 To the best of our knowledge, except for that publication,9 no systematic study of the reduction of chloroauric ([AuCl4−]) ion from real HCl−NaClO3 leachates of gold ores by oxalic acid has ever been reported. Aside from Au3+ ions, other ions such as Cu2+, Fe3+, Pb2+, and Ag+ are also found in the leachate. To avoid the formation of CuC2O4 precipitate and reduce neutralization process costs, the leachate was directly employed in the reduction experiments without pH adjustment. Thus, a comprehensive investigation of gold reduction with oxalic acid was performed in this work.

2. MATERIALS AND METHODS 2.1. Materials. About 10 L of gold-containing filtrate was obtained by leaching a gold ore rocket with a mixed solution composed of sodium chlorate, sodium chloride, and hydrochloric acid under various conditions as described in the literature.1 The contents of gold and sulfur from the gold ore sample were 55.7 g/t and 11.67 wt %, respectively. Before being leached, the gold ore powder was roasted in O2 to remove sulfur at 650 °C. The chemical composition of the leaching filtrate is listed in Table 1. The gold concentration from the leaching filtrate was 3.43 ppm. Oxalic acid (H2C2O4) and hydrochloric acid (HCl), both of analytical grade, and deionized water were used in this work. Argon gas with a purity of 99.99% (v/v) was used in this work. 2.2. Procedures. 2.2.1. Gold Reduction with Oxalic Acid Solution. Unless specified otherwise, the reduction conditions were as follows: A 50 mL sample of leaching filtrate was transferred into a 250 mL flask as shown in Figure 1. Argon gas Received: Revised: Accepted: Published: 16672

July 23, 2014 September 29, 2014 September 30, 2014 September 30, 2014 dx.doi.org/10.1021/ie5029446 | Ind. Eng. Chem. Res. 2014, 53, 16672−16677

Industrial & Engineering Chemistry Research

Article

250 rpm. The variation in the oxidation−reduction potential (ORP) of the leachate with the volume of H2C2O4 added was investigated. The measurement of the solution ORP was conducted with an ORP electrode at room temperature (23 °C). When the solution ORP was measured, the solution agitation was ceased until a stable ORP value could be recorded. A platinum wire electrode and a Ag,AgCl/KCl (222 mV vs SHE) electrode was used to measure the solution ORP. The solution potential (in mV) relative to a standard hydrogen electrode (SHE) is denoted as Eh (vs SHE), which was obtained according to the measured ORP value (in mV) plus 222 mV.

Table 1. Main Chemical Composition of the Leaching Filtratea component

concentration (mg/L)

component

concentration (mg/L)

Au Cu Fe

3.43 453.00 3709.57

Pb Ag Si

87.97 1.96 25.73

a

HCl concentration was about 3.3 M.

3. RESULTS AND DISCUSSION 3.1. Feasibility Analysis of Selective Reduction of Gold Ions. The mechanism of gold dissolution in chloride solutions involves the initial formation of a gold(I) chloride species (AuCl) on the surface of metallic gold. This species reacts further with chloride ion to form the dissolved species AuCl2−, which, in turn, is oxidized to AuCl4−.10,11 At 1.0 M NaCl concentration and potentials higher than 950 mV (vs SHE), AuCl4− was found to be the stable gold−chloride complex in solution10 to pH = 10. The reduction of AuCl4− complex in acidic aqueous chloride solutions can be represented by anodic half-reactions as follows12 Figure 1. Experimental apparatus for the selective reduction of gold ions.

AuCl4 − + 3e− = Au + 4Cl−,

(C0V0 − CtVt ) × 102 C0V0

(2)

This implies that, when AuCl4− ions are present in an acidic chloride solution, the reductant in the solution should have a standard reduction potential of less than 1.002 V for the formation of metallic Au. For oxalic acid, the standard reduction potential in acidic solution is12

was passed through the filtrate at a flow rate of 200 mL/min for 20 min to remove the chlorine dissolved in the filtrate. Then, the solution temperature was raised to 85 °C using a hot water bath. Both the solution and the water bath were agitated gently with magnetic and mechanical agitation, respectively. Then, 20 mL of 6% (w/v) H2C2O4 solution was added to the flask at 0.87 mL/min using a peristaltic pump while the magnetic agitation speed was kept at 250 rpm. When all 20 mL of the H2C2O4 solution had been added and up to 2.5 h had passed, the flask reactor was removed from the water bath. The solution was immediately filtered through a Whatman GF-A membrane with a pore diameter of 0.45 μm. The filtrate was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES; SPECTRO ARCOS EOP, SPECTRO Analytical Instruments GmbH, Kleve, Germany) to measure the concentration of Au. The percentage of gold ions reduced was calculated as Au reduced (%) =

φ10 = 1.002 V

2CO2 + 2H+ + 2e− = H 2C2O4 ,

φ20 = − 0.481 V

(3)

This indicates that oxalic acid in an acidic aqueous solution had the capability to reduce AuCl4− to the corresponding metallic Au under standard conditions. However, Cu2+, Fe3+, Pb2+, and Ag+ were also present in the acidic chloride solution (Table 1). These ions also had a possibility of being reduced by oxalic acid under standard conditions. For the specific conditions employed in this work, a feasibility prediction of the selective reduction of AuCl4− to Au was conducted based on ORP (oxidation−reduction potential) measurements of the real solution and an Eh−pH diagram. The specific conditions for drawing the Eh−pH diagram were as follows: The concentrations of Au3+, Cu2+, Fe3+, Pb2+, and Ag+ ions in the leaching solutions as measured by ICP-AES are listed in Table 1. The concentration of Cl− in the leaching solution was about 3.3 M. The solution pH values measured by a pH meter were between −1.00 and −2.00 in this case. The solution potential (in mV) relative to SHE was 1123 mV. Thus, Au3+, Cu2+, Fe3+, Pb2+, and Ag+ were the main ions in the leachate. The reaction temperatures used in this work for reducing Au3+ with oxalic acid ranged from 23 to 95 °C. AuCl4− is the stable Au3+ species in solutions with high concentrations of hydrochloric acid.10,13,14 Although AuCl4− can be reduced to AuCl2− by oxalic acid at room temperature, AuCl2− is unstable and can be decomposed at elevated temperature15 according to the reaction

(1)

where C0 and Ct are the concentrations of gold (in mg/L) from the sample solutions before and after reduction with H2C2O4, respectively. V0 and Vt are the volumes (in mL) of the sample solutions before and after reduction with H2C2O4, respectively. 2.2.2. Variation of the ORP of Sample Solution with H2C2O4 Added. A 50 mL sample of leaching filtrate was transferred into an Erlenmeyer flask with a volume of 250 mL. Argon gas was passed through the filtrate at a flow rate of 200 mL/min for 20 min to remove the chlorine dissolved in the filtrate. The solution potential (in mV) relative to a standard hydrogen electrode (SHE) at this time was 1123 mV. Then, 40 mL of 6% (w/v) H2C2O4 was gradually added to the flask from a titration tube while the magnetic agitation speed was kept at

3AuCl 2− = 2Au(s) + AuCl4 − + 2Cl− 16673

(4)

dx.doi.org/10.1021/ie5029446 | Ind. Eng. Chem. Res. 2014, 53, 16672−16677


Article

CuCl+ is the stable Cu2+ species in solutions with high concentrations of hydrochloric acid.16 CuCl+ can be reduced to CuCl32− by oxalic acid at room temperature, but CuCl32− is not stable and can be decomposed into CuCl2− and Cl− at elevated temperatures.17,18 Thus, CuCl2− is the more stable Cu+ species during the reduction process. AgCl2− is the stable Ag+ species in solutions with high concentrations of hydrochloric acid.19 PbCl2 is the stable Pb2+ species in solutions with high concentrations of hydrochloric acid.20 FeCl2+ is the stable Fe3+ species in solutions with high concentrations of hydrochloric acid.21 FeCl+ is the stable Fe2+ species in the solutions with high concentrations of hydrochloric acid.22 Based on the above literature reports, it was assumed that soluble AuCl4−, AuCl2−, CuCl+, CuCl2−, FeCl2+, FeCl+, PbCl2, and AgCl2− species were present during the reduction process. The equations of interest for this system include the following12 AuCl 2− + e− = Au + 2Cl−,

φ50 = 1.15 V

φ60 = 0.771 V

(6)

Fe2 + + 2e− = Fe,

φ70 = −0.44 V

(7)

Fe2 + + Cl− = FeCl+,

AgCl2− + e− = Ag + 2Cl−,

K8 = 102.13

φ230 = 0.50 V

The final two relevant equations are O2 + 4H+ + 4e− = 2H 2O, 2H+ + 2e− = H 2 ,

φ240 = 1.229 V

φ250 = 0 V

(23)

12

(24) (25)

Based on the values of φ01−φ025 and the concentrations of AuCl4−, AuCl2−, CuCl+, CuCl2−, FeCl2+, FeCl+, PbCl2, AgCl2−, and Cl− in the solution obtained after the addition of 20 mL of oxalic solution, the Eh−pH diagram of the metal−Cl−H2O system for the specific case was obtained, as shown in Figure 2A. It was assumed that AuCl4−, CuCl+, and FeCl2+ in the initial solution were converted completely to AuCl2−, CuCl2−, and FeCl+, respectively, during the stepwise reductions of Au3+, Cu2+, and Fe3+, respectively. Thus, the concentrations of AuCl4−, CuCl+, and FeCl2+ were equal to those of AuCl2−, CuCl2−, and FeCl+, respectively, during the stepwise reduction. When 20 mL of 6% (w/v) oxalic acid was added to the leaching

(5)

Fe3 + + e− = Fe2 +,

Fe3 + + 2Cl− = FeCl 2+,

Based on eqs 21 and 22, the following equation was derived

(8)

K 9 = 100.36

(9)

Based on eqs 6−8, the following equation was deduced FeCl 2+ + 3e− = Fe + 3Cl−,

φ100 = − 0.0782 V

(10)

Based on eqs 7 and 9, the following equation was deduced FeCl+ + 2e− = Fe + Cl−,

φ110 = − 0.451 V

(11)

For Cu species, the following equations are relevant Cu 2 + + e− = Cu+,

Cu+ + e− = Cu,

12

φ120 = 0.159 V

(12)

φ130 = 0.520 V

Cu 2 + + Cl− = CuCl+,

(13)

K14 = 105.5

Cu+ + 2Cl− = CuCl 2−,

(14)

K15 = 102.44

(15)

Based on eqs 12−14, the following equation was deduced CuCl+ + 2e− = Cu + Cl−,

φ160 = 0.502 V

(16)

Based on eqs 13 and 15, the following equation was deduced. CuCl 2− + e− = Cu + 2Cl−,

φ170 = 0.376 V

For Pb species, the following equations are relevant 2+

Pb

−

+ 2e = Pb,

φ180

Pb2 + + 2Cl− = PbCl 2,

(17) 12

= −0.126 V

(18)

K19 = 102.44

(19)

Figure 2. (A) Eh−pH diagram of metal−Cl−H2O system: 20 mL of 6% (w/v) H2C2O4 was added to 50 mL of Au-containing leaching filtrate. The ion concentrations of Au, Fe, Cu, Pb, Ag, and HCl in 70 mL of the mixed solution were assumed to be as follows: [AuCl4−] = 1.74 × 10−5 M, [FeCl2+] = [FeCl+] = 4.74 × 10−2 M, [CuCl+] = [CuCl2−] = 5.09 × 10−3 M, [PbCl2] = 3.03 × 10−4 M, [AgCl2−] = 1.30 × 10−5 M, [HCL] = 2.36M, 25 °C. Legend: 1, O2 → H2O; 2, [AuCl4−] → Au; 3, [AuCl2−] → Au; 4, [CuCl+] → Cu; 5, [CuCl2−] → Cu; 6, [AgCl2−] → Ag; 7, H2O → H2; 8, [FeCl2+] → Fe; 9, PbCl2 → Pb; 10, FeCl+ → Fe; 11, CO2 → H2C2O4. (B) Variation of the solution Eh value with the volume of H2C2O4 added at 25 °C: A solution containing 6% (w/v) H2C2O4 was added gradually to 50 mL of Au-containing leaching filtrate. The ion concentrations of Au, Fe, Cu, Pb, Ag, and HCl from the filtrate were 1.74 × 10−5, 6.65 × 10−2, 7.13 × 10−3, 4.2 × 10−4, 1.8 × 10−5, and 3.3 M, respectively.

Based on eqs 18 and 19, the following equation was derived PbCl 2 + 2e− = Pb + 2Cl−,

φ200 = −0.198 V

For Ag species, the following equations are relevant Ag + + e− = Ag,

φ210 = 0.799 V

Ag + + 2Cl− = AgCl2−,

K 22 = 105.04

(20) 12

(21) (22) 16674



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solution, the solution oxidation−reduction potential was 623 mV (Figure 2B), and only AuCl4− and AuCl2− could be reduced in that case (Figure 2A). Thus, it is possible to selectively reduce gold ions with oxalic acid. 3.2. Effect of Oxalic Acid Amount. The effect of oxalic acid (H2C2O4) addition on gold reduction is presented in Figure 3. The gold reduction increased markedly from 65.4% to

Figure 5. Effect of time on gold reduction (50 mL of original Aucontaining leaching solution, 250 rpm, 20 mL of 60 g/L oxalic acid, speed of oxalic acid addition of 0.870 mL/min).

3.5. Effect of Speed of Addition of H2C2O4. The effect of the speed of H2C2O4 addition on gold reduction is presented in Figure 6. The gold reduction increased with increasing speed of Figure 3. Effect of oxalic acid amount on gold reduction (50 mL of original Au-containing leaching solution, 85 °C, 2.5 h, 250 rpm, speed of oxalic acid addition of 0.870 mL/min).

93.3% when the volume of H2C2O4 added was increased from 10 to 20 mL, and then the extent of reduction increased slowly from 93.3% to a plateau value 95.3% when the H2C2O4 volume was increased from 20 to 30 mL. Thus, the optimal volume was determined to be 20 mL. The optimal amount of oxalic acid required to reduce 50 mL of gold-containing solution was 1.2 g, considering that the concentration of oxalic acid was 6.0% (w/ v). 3.3. Effect of Temperature. The effect of temperature on gold reduction is presented in Figure 4. The gold reduction

Figure 6. Effect of speed of addition of H2C2O4 on gold reduction (50 mL of original Au-containing leaching solution, 85 °C, 2.5 h, 250 rpm, 20 mL of 60 g/L oxalic acid.).

H2C2O4 addition in the range of 0.174−0.870 mL/min and reached a plateau value of 97.6% in the range of 0.870−1.740 mL/min. The greater the speed of addition, the shorter the gold reduction time. Thus, the optimal addition speed was 1.740 mL/min or more. 3.6. Effect of Agitation Speed. The effect of the agitation speed on gold reduction is presented in Figure 7. The gold reduction increased with increasing agitation speed in the range Figure 4. Effect of temperature on gold reduction (50 mL of original Au-containing leaching solution, 2.5 h, 250 rpm, 20 mL of 60 g/L oxalic acid, speed of oxalic acid addition of 0.870 mL/min).

increased significantly from 81.9% to 97.6% when the temperature was increased from 25 to 85 °C, and then the extent of reduction decreased slightly from 97.6% to 96.8% when the temperature was increased from 85 to 95 °C. 3.4. Effect of Time. The effect of time on gold reduction is presented in Figure 5. The gold reduction increased significantly with time for a certain temperature in the time range of 0.5−2 h and reached a plateau in the range of 2−3 h, irrespective of the temperature. Thus, the optimal reaction time was determined to be 2 h.

Figure 7. Effect of agitation speed on gold reduction (50 mL of original Au-containing leaching solution, 85 °C, 2.5 h, 20 mL of 60 g/ L oxalic acid, speed of addition of oxalic acid of 0.870 mL/min). 16675



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achieve a selective and effective reduction of gold ions. The solution ORP at 25 °C was 623 mV in this case. The reaction temperature and time had significant effects on gold reduction, and the optimal temperature and time were found to be 85 °C and 2 h, respectively. The reduction of gold increased with increasing agitation speed and speed of addition of oxalic acid. The optimal agitation speed and speed of addition of oxalic acid were found to be 180 rpm and 1.740 mL/min or more, respectively. Under the optimal conditions, the reductions of Au3+, Cu2+, Fe3+, Pb2+, Ag+, and Si4+ were 95.56%, 0.66%, 2.33%, 1.40%, 0.70%, and 0.50%, respectively.

of 0−180 rpm and reached a plateau value of 97.6% in the range of 180−250 rpm. Thus, the optimal agitation speed was determined to be 180 rpm. 3.7. Selectivity of Metal Reduction. The extents of reduction of Au3+, Cu2+, Fe3+, Pb2+, Ag+, and Si4+ were found to be 95.56%, 0.66%, 2.33%, 1.40%, 0.70%, and 0.50%, respectively. Thus, Au3+ was selectively reduced with oxalic acid under the current conditions. This conclusion is in agreement with the thermodynamic calculations described in section 3.1. It is noticeable that the concentrations of Au3+, Cu2+, Fe3+, Pb2+, and Ag+ were 1.74 × 10−5, 7.13 × 10−3, 6.65 × 10−2, 4.2 × 10−4, and 1.8 × 10−5 M, respectively. Although the concentrations of Cu2+ and Fe3+ were much higher than that of Au3+ in the leaching filtrate, the reductions of Cu2+ and Fe3+ were even lower than that of Au3+. The reduction of Au3+ by oxalic acid in acidic HCl−NaClO3 solution was highly specific. Thus, an appropriate solution potential is an important condition for the selective reduction of gold ions. The appropriate solution potential was about 600 mV for the current leachate. 3.8. Discussion. The solubility products (K sp ) of Au2(C2O4)3, CuC2O4, FeC2O4·2H2O, PbC2O4, and Ag2C2O4 are 1 × 10−10, 4.43 × 10−10, 3.2 × 10−7, 4.8 × 10−10, and 5.4 × 10−12, respectively. Even though the concentrations of Cu2+, Fe3+, and Pb2+ were higher than that of Au3+ in the leaching filtrate, according to the reduction results, precipitates of CuC2O4, FeC2O4·2H2O, and PbC2O4 did not form to a great extent (Table 2). Perhaps the high acidity of the leaching filtrate inhibited the formation of C2O42− according to the equation23 H 2C2O4 = 2H+ + C2O4 2 − ,

K = 10−5.06

■

*Tel.: +86-10-62332525. Fax: +86-10-62332525. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 50874011) and the Programs for Innovative Research Team of Undergraduate Students at University of Science and Technology Beijing (Grants 14020118 and 14020367).

■

Table 2. Concentration Variations of Various Metals before and after Reduction with Oxalic Acida concentration (mg/L) before reduction

after reduction

reduction extent (%)

Au Cu Fe Pb Si Ag

3.43 453.00 3709.57 87.97 25.73 1.96

0.08 225.10 1811.66 43.37 12.80 0.97

95.56 0.66 2.33 1.40 0.50 0.70

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element

AUTHOR INFORMATION

Corresponding Author

a

Solution volumes were 50.0 and 100.0 mL before and after oxalic acid reduction, respectively. Reduction conditions: 85 °C, 2.5 h, 250 rpm, speed of addition of oxalic acid of 0.870 mL/min.

Thus, high solution acidity is an important condition for the selective reduction of gold ions. The HCl concentration from the original leachate was about 3.3 M. When 20 mL of H2C2O4 was added to 50 mL of the leachate, the solution HCl concentration became 2.35 M. Thus, the appropriate HCl concentration for the selective reduction of gold with oxalic acid was determined to be between 2.3 and 3.3 M.

4. CONCLUSIONS The reduction of gold ions from HCl−NaClO3 solutions with oxalic acid was conducted. The effects of various reaction conditions on gold reduction were studied. The following conclusions can be drawn from this research: For 50 mL of leaching filtrate, 20 mL of 6% (w/v) oxalic acid was required to 16676



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(13) Usher, A.; McPhail, D. C.; Brugger, J. A spectrophotometric study of aqueous Au(III) halide−hydroxide complexes at 25−80 °C. Geochim. Cosmochim. Acta 2009, 73, 3359−3380. (14) Chen, X.; Chu, W.; Chen, D.; Wu, Z.; Marcelli, A.; Wu, Z. Correlation between local structure and molar ratio of Au(III) complexes in aqueous solution: An XAS investigation. Chem. Geol. 2009, 268, 74−80. (15) Gammons, C. H.; Yu, Y.; Williams-Jones, A. E. The disproportionation of gold(I) chloride complexes at 25 to 200°C. Geochim. Cosmochim. Acta 1997, 61, 1971−1983. (16) Ramette, R. W.; Fan, G. Copper(II) Chloride Complex Equilibrium Constants. Inorg. Chem. 1983, 22, 3323−3326. (17) Brugger, J.; Etschmann, B.; Liu, W.; Testemale, D.; Hazemann, J. L.; Emerich, H.; van Beek, W.; Proux, O. An XAS study of the structure and thermodynamics of Cu(I) chloride complexes in brines up to high temperature (400 °C, 600 bar). Geochim. Cosmochim. Acta 2007, 71, 4920−4941. (18) Sherman, D. M. Complexation of Cu+ in hydrothermal NaCl brines: Ab initio molecular dynamics and energetics. Geochim. Cosmochim. Acta 2007, 71, 714−722. (19) Liu, X.; Lu, X.; Wang, R.; Zhou, H. Silver speciation in chloridecontaining hydrothermal solutions from first principles molecular dynamics simulations. Chem. Geol. 2012, 294−295, 103−112. (20) Luo, Y.; Millero, F. J. Stability constants for the formation of lead chloride complexes as a function of temperature and ionic strength. Geochim. Cosmochim. Acta 2007, 71, 326−334. (21) Strahm, U.; Patel, R. C.; Matljević, E. Thermodynamics and Kinetics of Aqueous Iron(III) Chloride Complexes Formation. J. Phys. Chem. 1979, 83, 1689−1695. (22) Heinrich, C. A.; Seward, T. M. A spectrophotometric study of aqueous iron(II) chloride complexing from 25 to 200 °C. Geochim. Cosmochim. Acta 1990, 54, 2207−2221. (23) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 90th ed.; CRC Press: Boca Raton, FL, 2009; pp 8−42.

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