Selective Sulphide Precipitation of Heavy Metals from Acidic

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Selective Sulphide Precipitation of Heavy Metals from Acidic Polymetallic Aqueous Solution by Thioacetamide Mahdi Gharabaghi,*,‡ Mehdi Irannajad,*,† and Amir Reza Azadmehr*,† † ‡

Department of Mining & Metallurgical Engineering, Amirkabir University of Technology, Hafez St, Tehran, Iran School of Mining Engineering, University College of Engineering, University of Tehran, Tehran, Iran

bS Supporting Information ABSTRACT: The selective separation and recovery of copper, cadmium, zinc, and nickel from a polymetallic solution with sulphide precipitation using thioacetamide have been investigated. Selective metal sulphide precipitation was studied as a function of pH, contact time, and temperature. The results showed that it was possible to separate metals by accurately controlling the pH and temperature. Below pH 2.5, copper precipitation was complete. The cadmium, zinc, and nickel selective precipitations were performed at pH of 4, 5.5, and 7.5, respectively. Temperature also had important effects on the selective separation, and metals precipitation yields increased with increasing temperature. Thioacetamide hydrolysis kinetics and its activation energies in various conditions were calculated. The metal sulphide precipitates were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the results showed that the produced precipitates had high purity.

1. INTRODUCTION Zinc plant residues are characterized as hazardous wastes because of the high concentration of heavy metals. These wastes generate heavy metals contaminated waste waters that pose serious environmental risks if not treated. In addition they release extremely acidic leachates with high concentrations of sulfate and heavy metals such as copper, nickel, zinc, and cadmium. Cadmium exhibits toxicity at extremely low concentrations, and zinc, nickel, and copper are toxic at higher concentrations.14 One sustainable option to decrease potential environmental pollution is based on the recovery and reuse of heavy metals. The extraction of metals from hazardous wastes has coincidence with the legislation assigned to protect the environment in the world and can decrease unfavorable environmental impacts.5 Zinc, cadmium, nickel, and copper are presented as easily leachable compounds in the zinc plant residues. These metals can be recovered by hydrometallurgical treatment using sulphuric acid.6 After the leaching process, the solution includes zinc, cadmium, nickel, and copper ions, and recovery of valuable metals from acidic polymetallic solution is very important. However, the metals extraction from this polymetallic solution using solvent extraction or electrolysis is extremely difficult, and it is necessary to do other purification processes such as sulphide or hydroxide precipitation. Various types of the precipitation and extraction processes are described by Cheng and co-workers,7 Ismael and Carvalho8 and Flett.9 For example, nickel and cadmium can be recovered from the solution by cementation from zinc sulfate solution. Although cementation is an easily applied and cost-effective method, this process cannot be effectively used for selective metal extraction, and it is not applicable to metal recovery from zinc plant residue solution.10 Metal sulphide precipitation is an important process for removing metals from industrial wastewaters and effluent treatment. It has some advantages such as lower solubility of metal sulphide r 2011 American Chemical Society

precipitates, potential for selective metal removal, fast reaction rates, better settling properties, and potential for reuse of sulphide precipitates by smelting. In addition, it results in lower effluent concentrations and less interference from chelating agents. However, this process is very sensitive to the sulphide dose, and, because of the low solubility of the metal sulphides, it has a limitation because the dosing of sulphide is difficult to control.11 In this study, S2 concentration in the solution was controlled S2 by using thioacetamide as sulphide source and accurate pH adjustment. Unlike using other sulphide source such as Na2S, NaHS, and so forth, S2 concentration in solution could be controlled using thioacetamide. This point is the most important advantage of our method. Various sulphide sources like solid (FeS, CaS, Na2S), aqueous (Na2S, NaHS, (NH4)2S), or gaseous sulphide (H2S) can be used for metal sulphide precipitation.1115 There are some investigations regarding zinc,1618 cadmium,19,20 nickel,21,22 and copper23,24 removal or formation from aqueous solutions. Some researchers have studied mixed metals sulphide precipitation using different sulphide sources.11,2527 However, in most of the studies, the sulphide precipitation was applied as an effluent treatment method, and it was used for heavy metals removal from mine waters and industrial effluents.25,2731 A sustainable method should recycle heavy metals from solution. It is possible to separate metal ions by selective sulphide precipitation because of the different solubility products of the different metal sulphides. There is a paucity of available information on the sulphidation of two or three metals in solution, and to the authors’ knowledge, there is Received: August 16, 2011 Accepted: December 5, 2011 Revised: November 24, 2011 Published: December 05, 2011 954

dx.doi.org/10.1021/ie201832x | Ind. Eng. Chem. Res. 2012, 51, 954–963

Industrial & Engineering Chemistry Research

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no published study involving the selective sulphidation of four metals from polymetallic solution.

H2 S a HS þ Hþ

K p1 ¼

½HS ½Hþ ½H2 S

ð1Þ

2. FUNDAMENTALS OF METAL SULFIDES PRECIPITATION The thermodynamic equilibrium involved in metal sulphide precipitation can be expressed as follows:

HS

a S2 þ Hþ

K p2 ¼

½S2 ½Hþ ½HS

1

Zn(II)

23.8

2

Cd(II)

27.7a, 29b

3

Ni(II)

20.7a

4 a

Log KSP (metal sulfide)

metal sulphide

M2þ þ S2 a MSðsÞ

ð3Þ

M2þ þ HS a MSðsÞ þ Hþ

ð4Þ

a

The solubility product of heavy metal sulfides (MS), Ksp=[M2+][S2], is the product of ion concentrations and is called the ion

48.0a, 48.5c

Cu(II) b

pK2 ¼ 17:4

ð2Þ

Table 1. Solubility Products of Metal Sulfides no.

pK1 ¼ 6:99

c

Data from ref 41. Data from ref 51. Data from ref 52.

Figure 1. EhpH diagram for the reaction system. (a) Cadmiumsulfurwater system, (b) zincsulfurwater system, T = 25 °C. Drawn by Medusa software (Royal Institute of Technology, Sweden). 955

dx.doi.org/10.1021/ie201832x |Ind. Eng. Chem. Res. 2012, 51, 954–963


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product constant or the solubility product. The solubility products of the metal sulphide presented in the reaction system are shown in Table 1. As shown in Table 1, The solubility products of the metal sulphides give the possibility of selective precipitation because they have different solubility products and metal sulphides will precipitate in the order of increasing their solubility products.15,24 Table 1 portrays that the metal sulphides have very low solubility and the process is sensitive to the amount of sulphide. It has been stated that this method has a limitation because the dosing of sulphide is difficult to control.11 It is possible to overcome these difficulties by using controlled addition of sulphide ions. In our solution, the precipitation started with copper sulphide which has the lowest solubility (Table 1). The behavior of metals in the reaction system can be predicted from equilibrium data. The equilibrium chemistry of the reaction system was examined with the aid of EhpH diagrams constructed by the Medusa software. The diagrams were prepared

using a sulfur concentration of one mol and different metal concentrations (corresponding to metal concentrations in leach liquor). The EhpH diagrams for the cadmium and zinc are shown in Figure 1. Figure 1 indicated that the presence of S2 is necessary for formation of metal sulphides in the mixed reaction systems. To control sulphide concentration, thioacetamide was used as sulphide source. Thioacetamide was dissociated in the medium and released H2S. The amounts of dissociation were depended on the medium pH and temperature. So these variables were controlled continually in the process. The thermodynamic equilibrium in the thioacetamide dissociation is as follows: CH3 CSNH2 þ 2H2 O a CH3 COOH þ NH3 þ H2 S ð5Þ

Table 2. Chemical Analysis of the Head Sample no.

component

concentration, mol/L

1

Zn

0.192

2

Cd

0.056

3

Ni

0.021

4

Cu

0.01

H2 S a HS þ Hþ

ð6Þ

HS a S2 þ Hþ

ð7Þ

M2þ þ S2 f MSðsÞ

ð8Þ

Reaction 6 was excessively accelerated by rapid consumption of S2 ions so that it is thought to be a reversible reaction.

Figure 2. Effect of pH on copper sulphide precipitation (85 °C, 45 min, 500 rpm).

Figure 4. Effect of pH on cadmium sulphide precipitation (85 °C, 45 min, 500 rpm).

Figure 3. Effect of temperature on copper sulphide precipitation (pH = 2.5, 500 rpm). 956



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Figure 5. Effect of temperature on cadmium sulphide precipitation (pH = 4.5, 500 rpm).

In this paper, various aspects of metal sulphide precipitations were investigated, with particular emphasis on mixture pH, because the concentration of sulfur species is a strong function of pH.32

3. EXPERIMENTAL SECTION 3-1. Materials. Analytical reagents were used for the preparation of this solution. Different amounts of thioacetamide (CH3CSNH2) reagent grade from Merck were dissolved in a volume of deionized water for preparing sulphide medium, and temperature and pH were continuously controlled in all tests. Sodium acetate, ammonium acetate, H2SO4, and NaOH, were used for pH adjustments. CuSO4, CdSO4, ZnSO4, and NiSO4 (were used for preparation of synthetic solution. All reagents were purchased from Merck, Germany, and used without further purification. 3-2. Physical Instrument. The concentrations of metals in the solution after precipitation were determined by Unicom atomic absorption spectrometry (AAS). The precipitates were characterized by X-ray diffractometer (Philips, Xunique II) and scanning electron microscopy (Philips XL30). 3-3. Method. Experiments were carried out with a synthetic solution containing the composition shown in Table 2 at initial pH 1. The precipitation tests were performed with freshly prepared sulphide solution. A 50 mL portion of synthetic solution was put in a 500 mL glass reactor equipped with a reflux condenser, and the sulphide solution was added continually to the mixture. In the experiments, the solution was agitated with the aid of a magnetic stirrer at 500 rpm. The reaction temperature was maintained constant in a water bath. To prevent liquid loss by evaporation when the system was heated, the reactor was fitted with a reflux condenser. Thioacetamide was applied to polymetallic solution, and the effects of important factors on the selective metal precipitations were studied. This study was aimed at investigating the selective precipitation of four metals in a stirred tank reactor. The effects of temperature and pH on the selective precipitation were studied, and the solid precipitates were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). In the experiments, thioacetamide amounts were consumed in 45 min of reaction. At the end of the experiments, the content in

Figure 6. Effect pH on zinc sulphide precipitation (85 °C, 45 min, 500 rpm).

the reactor was filtered, and the sulphide precipitates were collected and dried at 40 oC before analysis. To investigate selective sulphide separation, two types of experiments were performed. In the first step, experiments were carried out either with one metal (Cu, Cd, Zn, Ni solely), or with a mixture of all metals (second step). pH and temperature were controlled continually in all cases.

4. RESULTS AND DISCUSSION 4-1. Single Metal Study. Single metal precipitations were the first step of our study. These experiments were performed to evaluate the precipitation of each metal at different conditions. Experiments were carried out with only one metal (Cu, Cd, Zn, Ni) and the effects of pH and temperature on metal precipitations were examined. 4-1-1. Copper Precipitation. The precipitation curves obtained under various pHs and temperatures are illustrated in Figures 2 and 3, respectively. From the course of precipitation curve of copper, it was obvious that the optimum pH for copper precipitation is approximately 2.5. 957



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Figure 7. Effect of temperature on zinc sulphide precipitation (pH = 5.5, 500 rpm).

it was enough S2 to precipitate bulk copper content (97%) as sulphide (Figure 3). H2S is a soluble gas in water, and its concentration decreased by increasing temperature. On the other side, the rate of thioacetamide hydrolysis increased by increasing reaction temperature. Further, at higher temperature, the rate of molecular collisions increased. In addition, S2 concentration increased by increasing the medium pH, because S2 concentration was related to H+ concentration (eq 6). So to control S2 concentration, it is necessary to control both pH and temperature. 4-1-2. Cadmium Precipitation. The most important variables in cadmium precipitation rate are initial pH and temperature. The initial reaction rates were independent of cadmium concentration. Ennaassia et al. (2002) investigated the cadmium removal from phosphoric acid solutions with Na2S. They measured the efficiency of the process in terms of the residual cadmium concentration, and found that the removal efficiency decreased with an increase in acidity level and an increase in temperature. The effects of medium pH on cadmium sulphide precipitation are shown in Figure 4. As can be seen in Figure 4, the cadmium sulphide precipitation increased by increasing pH and reached its maximum at pH 4.5. At this pH value, above 97% cadmium ions were precipitated as CdS. Shpiner et al. (2009) used Na2S to remove Cr and Cd from oil well produced water and found that nearly 100% chromium and cadmium were removed.30 The effects of temperature on the cadmium sulphide precipitation were investigated, and the results are presented in Figure 5. It can be seen that cadmium extraction efficiency increased with increasing temperature. After 45 min, cadmium extraction rose from 38% at 45 °C to 67% at 65 °C. However, the cadmium recovery was 87% and 98% at 75 and 85 °C, respectively. Over 99% cadmium was extracted as sulphide by optimization of temperature and other important factors.37 4-1-3. Zinc Precipitation. Zinc is usually chosen as a model metal for metal sulphide precipitation studies because of its relevance in the environment, and also because it has simple chemistry. Peters et al. (1984) found that the zinc precipitation reaction was very fast, and they removed 99.7% of zinc content using this method.38 From the effects of pH on the zinc precipitation, it was found that >95% zinc was effectively precipitated at pH 5.5 (Figure 6). The zinc precipitation profile in Figure 7 shows a clear relationship of increasing zinc precipitation rate and increasing

Figure 8. Effect pH on nickel sulphide precipitation (85 °C, 45 min, 500 rpm).

It is clearly observed that about 95% of copper content was precipitated at pH 2.5, and it reached to 97% at pH 3. It was also observed that the pH had significant effects on the other metals removal from aqueous solution, and the elimination efficiency was increased by increasing solution pH.33,34 The optimum pH for copper precipitation was dependent on the amount of S2 in the system and the solubility product of copper. Theoretically, increasing temperature can promote the reaction rate and result in metal precipitation occurring more quickly. Roy and Srivastava (2007) suggested that the precipitation rates and morphologies of metal sulphide were affected by the reaction temperature since particle growth was favored at lower temperatures, but nucleation began to predominate at higher temperatures.35 In Cao and co-workers study, increasing the temperature in the precipitation reactor increased the removal efficiency, with the highest removal occurring at 60 °C.36 Ennaassia et al. (2002) found that the removal efficiency decreased with an increase in temperature.19 As shown in Figure 3, copper sulphide precipitation increased by increasing the temperature and reached its maximum at 85 °C. The results showed that sulphide precipitation of copper strongly depended on the amount of H2S from thioacetamide dissociation. At 85 °C 958



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Figure 9. Effect of temperature on nickel sulphide precipitation (pH = 7.5, 500 rpm).

Table 3. Result of Selective Metal Precipitation (T = 85 °C, 500 rpm)

4-2. Selective Metal Precipitation. With the aim of studying the potential for selective precipitation of metals, sequential precipitation of four metals was performed. The metals were mixed at pH 1, at 85 °C, and then thioacetamide was added slowly. The pH of the solution played a key role on selective separation, so we controlled pH continuously to obtain a selective separation. To recognize the effect of pH, precipitation studies were carried out at different pHs (17.5). Copper was precipitated at first, and cadmium was precipitated after copper. A single metal study showed that zinc and nickel were not precipitated at low pH. Zinc started to precipitate when the cadmium concentration was already very low. Like in the single metal precipitation, copper was precipitated at lower pH than cadmium, and zinc and nickel were precipitated at the final stages at pH 7.5. The results are shown in Table 3. In the four metals selective separations, the medium pH and thioacetamide concentration were controlled simultaneously. Thioacetamide concentration played an important role during cadmium and zinc selective separation. The percent of metal precipitations as a function of solution pH is shown in Table 3. It is clearly observed that Cd, Zn, and Ni were not precipitated during copper separation. The selectivity as high as 90% for copper was obtained. It was shown that the zinc was precipitated at an extent lower than 5% at pH 3.5 during cadmium separation and cadmium extraction was 85.12%. As shown in single metal studies, nickel was not precipitated at pH below 4. Zinc precipitation selectivity was 83% and 8% of nickel was precipitated during zinc separation. These results regarding copper precipitation are agreed with the results obtained from copper separation.23,40 For the cadmium precipitation, similar results were found by Ennaassia and co-workers who have investigated cadmium removal from phosphoric acid solutions using Na2S.19 In addition, some authors have reported the sulphide precipitation of zinc and nickel.36,39,41 Medium pH had significant effects in the precipitation process because the rate of S2 production was faster at high pH levels. In addition, the success of selective precipitation of metals as sulphide solid strongly depended on the quantity of H2S production from thioacetamide hydrolysis which was dependent on both medium pH and temperature. In the higher pH, more thioacetamide was dissociated; therefore, the amount of S2 was higher at higher pH.

metal precipitation as sulphide (%) pH

Cu

2.0

90.43

0

0

0

2.5

97.29

9.37

0

0

3.0

51.62

0

0

3.5

85.12

4.06

0

4.0 4.5

96.38

12.23 61.35

0 0

83.62

8

5.0

Cd

Zn

Ni

5.2

51.82

6.0

67.53

6.5

79.64

7.0

85.82

7.5

94.7

temperature from 25 to 85 °C. The effects of temperature on the zinc extraction results are supported by results in the literatures.36,37 When the system temperature was raised from 45 to 65 °C, the zinc extraction increased from 41% to 70% in 45 min. 99% zinc was extracted at the end of reaction at 85 °C. Higher temperatures also shorten the contact times required to € reach equilibrium. Similar results were found by Ozverdi and Erdem (2006) who have investigated metal adsorption by pyrite and synthetic iron sulphide.39 4-1-4. Nickel Precipitation. The effects of pH on nickel extraction from solution were investigated in the range of pH 37.5, and the results obtained are presented in Figure 8. After the slow initial stage, a linear dependence of nickel precipitation rate was noted. The pH required for precipitating 97% of nickel values from the solution was about 7.5. To quantify the nickel precipitation batch experiments were performed at different temperatures. As can be seen in Figure 9, only 61% of Ni was extracted at 45 °C, and it rose to 98% at 85 °C. As shown in Table 2, Log Ksp of metal sulphides had a direct relationship with suitable pH for metal separation. The Log Ksp of metal sulphides increased by increasing optimum pH for each metal precipitation. 959



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Table 4. Activation Energies for Single Metal Precipitation metal

activation energy (kJ/mol)

k0 (S1)

Cu

22.63

4.402 103

Cd

20.78

2.406 103

Zn

19.68

1.669 103

Ni

21.92

3.498 103

forth are utilized, colloid size particles are formed during the reaction. Formation of colloid size particles is unfriendly to filtration process. In our reaction system, thioacetamide dissociation produced an electrolytic solution (reaction 5) and decreased amounts of colloid size particle in the medium. This process removed filtration problems encountered in the sulphide precipitation systems. 4-2-1. Temperature Effects. Metal sulphides have very low solubility products and fast reaction kinetics. The temperature had great effects on the precipitation efficiency. The thioacetamide hydrolysis was increased by increasing reaction temperature. By increasing thioacetamide dissociation, further S2 was released in the reaction medium. Process temperature affects the initial precipitation rate and overall extent, indicating the metal precipitation by thioacetamide was chemical reaction controlled. At lower temperatures the time required could be correspondingly longer. Knowledge of the effect of temperature upon the rate of metal sulphide precipitation is of value in the calculation of suitable conditions for obtaining complete precipitation of metals. In a fluidsolid reaction system, the reaction rate is controlled either by the diffusion of reactant or by the rate of the chemical reaction at the surface.42 If the chemical reaction controls the rate, there must be a linear relation between the 1/T and the Ln (slope) of temperature effect lines (K). To investigate the effects of temperature on the precipitation kinetics, the Arrhenius equation was used. By using the Arrhenius equation, kr = k0 eEa/RT, a plot of ln k versus1/T is a straight line with a slope of Ea/R and an intercept of ln k0. A plot of ln k versus1/T for all single metals study was obtained (Figure 10ad). This figure shows the Arrhenius temperature dependence, and the activation energies were calculated as shown in Table 4. These results propose that the thioacetamide hydrolysis is controlled by surface chemical reactions. This assertion is based on the strong dependence of precipitation rate and extent on temperature. These values of the activation energies also prove that the process is controlled by chemical reaction control. This value of activation energy clearly suggests chemical reaction control for the process43,44 as this value is consistent with the values obtained in other fluidsolid reaction systems.4547 4-3. Effect of Excess Sulphide. The single metal tests showed that, when the metal-to-sulphide molar ratio was more than 1, the metals precipitated immediately and sulphide remained in the system for the duration of the experiment. In the mixed system, when excess sulphide was used, the selectivity decreased. For examples, during cadmium precipitation, when more sulphide was added, more zinc was coprecipitated. It is very important to take into account that increasing thioacetamide addition did not significantly influence the coprecipitation of Cd, Zn, and Ni during copper precipitation. During the copper precipitation, excess sulphide concentration had little effects of the other metal coprecipitation. This was because significant differences between copper Ksp and other metals Ksp (Table 1). In the cadmium precipitation, the difference between cadmium Ksp and zinc Ksp

Figure 10. Arrhenius plot for the single metals precipitation. (A) Copper, (B) Cadmium, (C) Zinc, (D) Ni.

Selective sulphide precipitation using thioacetamide had another benefit. When sulphide sources like Na2S, NaHS, and so 960



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Table 5. Chemical Analysis of the Precipitates component (%) precipitate

Figure 11. SEM images of sulphide precipites. (A) CuS particles, (B) EDX analysis of CuS particles, (C) EDX analysis of CuS particles, (D) CdS rods, (E) CdS rods, (F) one rod, (G) EDX analysis of CdS, (H) EDX analysis of CdS.

was lower; therefore, when more sulphide was added, excess sulphide affected on the zinc ions presented in the system, and more zinc was coprecipitated. We concluded that the excess sulphide decreased the selectivity, and it is probable that increasing addition rate will play more a important role during cadmium or zinc separation from solution. When excess sulphide was used, while the final concentration of metals measured in solution was constant, the remaining

Cu

Cd

Zn

Ni

S

purity

copper sulphide

63.6

2.9

0

0

33.4

CuS = 96

cadmium sulphide

0

74.4

3.3

0

21.9

CdS = 94

zinc sulphide

0

0

62.7

4.1

33.1

ZnS = 93.5

nickel sulphide

0

0

0.7

63.8

35.3

NiS = 98

sulphide in solution was significantly less than expected from the stoichiometry. Because of sulphide oxidation, the free sulphide in the solution decreased over time. In addition, in excess sulphide concentration different metal polysulphide complexes may form. The pH of solution was remained constant during the metals polysulphide complexes formation, indicating that protons were neither consumed nor released during this time. This situation was an evidence for above hypothesis. In addition, the excess of sulphide can cause redissolution of the formed precipitates.20 For copper precipitation using various bisulphide concentrations48 have verified a similar phenomenon.By increasing the medium pH at excess sulphide concentration various poly sulphide forms may be formed. In addition, it is stated that for ZnS precipitation the stoichiometric amount of sulphide should be added, since the excess dosage results in the solubilization of zinc, possibly because of the formation of soluble zinc complexes such as Zn(HS)+ and Zn(HS)2, ZnS(HS)22, and Zn(OH)(HS)23.15,49 4-4. Characterization of the Precipitates. The precipitates from the selective metals separation were subjected to mineralogical and SEM analysis. Regarding the obtained results, copper was precipitated as CuS (covellite) and cadmium was precipitated as CdS. Zinc and nickel were precipitated as ZnS and NiS, respectively. Similar results were obtained in the Sampaio and co-workers' study.50 SEM images of metal sulphide particles (formed in the four metals selective precipitation) are shown in Figure 11ah. The results of energy dispersive X-ray (EDX) analysis revealed that sulphide precipitated was highly pure, and X-ray fluorescence (XRF) analysis supported this idea (Table 5). As shown in Figure11a, the CuS formed as nanosized spherical particles. From the EDX pattern (Figures 11b and 11c) of this precipitate, it was proved that this precipitate had high purity. Cadmium was precipitated as rods at micrometer size (Figures 11d11e). To examine the composition of these rods, we have analyzed one of them (Figure 11f) using EDX. The results in the Figures 11g and 11h showed that these rods were composed of cadmium and sulphide. The SEM and EDX analysises for zinc and nickel sulphides also showed that these precipitates had high purity.

’ CONCLUSION This study indicated the possibility of selective precipitation of metals from solution containing four metals. The following conclusions can be drawn from this study in which thioacetamide was used for selective metal sulphide precipitation from aqueous solution. Sulphide ion produced by hydrolysis of thioacetamide and its concentration was related to the medium pH. Metals sulphide precipitation was pH and temperature dependent. The metal precipitation increased with increasing temperature, and the best temperature for selective separation was 85 °C. In the 961



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selective metal precipitation maximum extraction yields were 90% and 85% for copper and cadmium, respectively. Copper was continuously separated from solution at 85 °C, from pH 1 to 2.5. Cadmium, zinc, and nickel were precipitated at pHs of 3.5, 5, and 7.5, respectively. The characterization showed the copper precipitates as CuS (covellite) at pHs 2 and 3 and zinc and ZnS (spharelite) at pHs 3 and 4. CdS and NiS were the main cadmium and nickel which formed during the process. The results of this study showed that the sulphide precipitation by thioacetamide has potential for recovery of heavy metals from wastewaters. Mining operation effluents and industrial wastewaters usually contain high metal concentrations, and they can be treated by this method as sulphide precipitates. In addition, this method can be applied to the treatment of acid mine drainage. The results of this study are important from both environmental and economic points of view in the treatment and recovery of metals from wastewaters.

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’ ASSOCIATED CONTENT

bS

Supporting Information. Graphics about the selective metal precipitation at various pHs. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (M.G.); [email protected] (M.I.); [email protected] (A.R.A.).

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Selective Sulphide Precipitation of Heavy Metals from Acidic

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