J.Cent.South Univ.Technol.(2010) 17: 289-294

DOI:10.1007/s1771-010-0044-0                                                                                                                    

Thermodynamic study on reaction path of Hg(Ⅱ) with S(Ⅱ) in solution

CHAI Li-yuan(柴立元), WANG Qing-wei(王庆伟), WANG Yun-yan(王云燕),

LI Qing-zhu(李青竹), YANG Zhi-hui(杨志辉), SHU Yu-de(舒余德)

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: The mercury sulfidation experiments were conducted in the pH range from 1 to 13. The results show that Hg(Ⅱ) reacted with equimolar S(Ⅱ) has the lowest remained Hg(Ⅱ) concentration (9.7 mg/L) at pH 1.0 and the highest remained concentration   (940.8 mg/L) at pH 13.0. Meanwhile, the changes of pH values were monitored exactly, which reveal that solution pH values change when mixing the same pH value solutions of HgCl₂ and Na₂S. In order to explain the phenomena and determine the reaction paths of Hg(Ⅱ) reacting with S(Ⅱ) in the solution, the concerned thermodynamics was studied. Species of S(Ⅱ)-H₂O system and Hg(Ⅱ)-H₂O system at different pH values were calculated, and then the species distribution diagrams of S(Ⅱ)-H₂O system, Hg(Ⅱ)-H₂O system and Hg(Ⅱ)-Cl^--OH^--H₂O system were drawn. Combining the experimental data and thermodynamic  calculation, the mechanism of Hg(Ⅱ) reacting with S(Ⅱ) was deduced. The results indicate that different species of S(Ⅱ) and Hg(Ⅱ) have the diverse reaction paths to form HgS precipitate at different pH values and the standard Gibbs free energies changeof those equations are also calculated, which can provide a guidance for mercury-containing wastewater treatment with Na₂S.

Key words: mercury species; Na₂S; thermodynamics; reaction path; wastewater treatment;

1 Introduction

Mercury pollution has been recognized as a global environmental problem because of its volatility and toxicity [1]. It can cause impairment of pulmonary function and kidney, chest pain and dyspnoea [2]. Mercury and its compounds have been widely employed in fungicides, dental preparations, electrical apparatus, industrial control instruments, dry batteries, paints, pharmaceuticals, explosives, metal plating, pesticides, pulp and paper industry [3-4].

Removal of mercury ions from wastewater is usually performed using varieties of traditional procedures viz. chemistry precipitation [5], electrolytic method [6], metal deoxidization [7], ion exchange [8], adsorption [9], reverse osmosis [10], solvent extraction [11], microorganism [12] and other procedures [13]. Among these methods, chemical precipitation with Na₂S and NaHS is by far the most widely used process to remove mercury from wastewater. The main advantages of sulfidation treatment are high rate of mercury removal even at low pH, and low reactor detention time required due to the high reactivity of sulfides. So, the use of sulfides to precipitate mercury(Ⅱ) as HgS from industrial effluents is important [14-15]. Moreover, HgS is polymorphic and exists as the common red (α) form known as cinnabar and the black metastable (β) form, metacinnabar, and both forms are highly insoluble in aqueous solutions with a large K_sp value (4×10^-53) [16-17].

In the past two decades, a great deal of attention has been paid to understanding the optimal parameters of S(Ⅱ) to treat mercury-containing wastewater. In these studies, the precipitation of metals was investigated to make them qualify the Integrated Wastewater Discharge Standard [14]. However, the mechanism of mercury treated by sulfidation agents was not further studied. The researchers usually use the equation Hg²⁺+ S^2-→HgS  to describe this mechanism. However, this equation does not occur in the practical process because the species of Hg and S in the solution was not considered. Generally, pH value of solution is one of the important parameters governing the treatment of wastewater containing mercury using Na₂S, Hg(Ⅱ) and S(Ⅱ) species were changed as pH value of solution varied. Various species of Hg(Ⅱ) and S(Ⅱ) have different reaction paths to   form HgS precipitate, which directly affects the mercury removal efficiency. The objectives of this work were to investigate the reaction path of aqueous solution containing HgCl₂ treated by Na₂S at different pH values through thermodynamic calculation and experimental data.

2 Thermodynamics

2.1 Distribution of S(Ⅱ) species in S(Ⅱ)-H₂O system at different pH values

Total S(Ⅱ) concentration (c_t(S^2-)) in Na₂S solution consists of c(S^2-), c(HS^-) and c(H₂S).

c_t(S^2-)=c(S^2-)+c(HS^-)+c(H₂S)

S^2-+H⁺HS^-  K₁=10^-14.15

HS^-+H⁺H₂S  K₂=10^-6.88

Herein:

Those equations describe the measured concentrations of c(S^2-), c(HS^-) and c(H₂S) as function of the total S(Ⅱ) concentration. Order α₀=c(H₂S)/c_t(S^2-), α₁=c(HS^-)/c_t(S^2-), α₂=c(S^2-)/c_t(S^2-). The proportion of H₂S, HS^- and S^2- in Na₂S solution at different pH values is shown in Fig.1.

Fig.1 Proportions (α) of S^2-, HS^- and H₂S in solution at different pH values

Fig.1 depicts that the dominant S(Ⅱ) species in the solution is S^2- only at pH＞13.0, HS^- at pH 6-13 and H₂S at pH＜8, both HS^- and H₂S species exist between pH 6 and pH 8. At pH 12, the percentage of S^2- is only 0.71%. H₂S evaporation will occur as gaseous hydrogen sulfide from solution when H₂S concentration in aqueous solution exceeds the solubility of 69.4 mmol/L at 40 ℃.

2.2 Distribution of Hg(Ⅱ) species in Hg(Ⅱ)-H₂O system at different pH values

The main species of Hg(Ⅱ) that exist in solution are Hg²⁺, HgOH⁺, Hg(OH)₂ and Hg(OH)₃^- at different pH values. The primary reaction of those species converting into each other and the equilibrium constants [17] can be described as follows:

Hg²⁺+OH^- HgOH⁺   K₃=10^10.6

HgOH⁺+OH^-Hg(OH)₂  K₄=10^11.2

Hg(OH)₂+OH^-Hg(OH)₃^-  K₅=10^-0.9

In dilute solution, mercury species distributing at different pH values are calculated according to the above equilibrium constants, let α′₀, α′₁, α′₂ and α′₃ represent the percentages of Hg²⁺, HgOH⁺, Hg(OH)₂ and Hg(OH)₃^-, respectively. The calculated results are shown in Fig.2.

α′₀= 1/[1+K₃c(OH^-)+K₃K₄c²(OH^-)+K₃K₄K₅c³(OH^-)]

α′₁=α′₀K₃c(OH^-)

α′₂=α′₀K₃K₄c²(OH^-)

α′₃=α′₀K₃K₄K₅c³(OH^-)

Fig.2 Proportions (α′) of Hg²⁺, HgOH⁺, Hg(OH)₂ and Hg(OH)₃^- at different pH values

It can be seen from Fig.2 that the dominant mercury species in the solution is Hg²⁺ at pH＜3.0, Hg(OH)₂ at 5.0＜pH＜15.0 and both of these species between pH 3.0 and 5.0. In addition, small amount of HgOH⁺ is also present between pH 2 and 6. At pH 3.0, percentages of Hg²⁺, Hg(OH)^- and Hg(OH)₂ are 49.30%, 19.61% and 1.09%, respectively. When pH value is higher than 15.0, Hg(OH)₃^- is the predominant species in the solution.

Hg(OH)₂ precipitates as HgO in solution when Hg(OH)₂ reaches the saturated concentration of 2.4×10^-4 mol/L (K₆=2.4×10^-4) [18]. Fig.3 shows the calculated results of the solution containing 100.0 mg/L Hg(Ⅱ) by using the following equations and the equili- brium constants [17]:

Hg(OH)₂(aq)Hg(OH)₂(s)  K₆=10^-3.76

Hg(OH)₂[HgOH⁺][OH^-]  K₇=10^-11.2

Hg(OH)₂[Hg²⁺][OH^-]₂  K₈=10^-21.8

Hg(OH)₂+OH^-Hg(OH)^3-  K₅=10^-0.9

As shown in Fig.3, Hg(Ⅱ) exists in water-soluble form in the solution at pH＜3.2 and then  transforms into HgO precipitation at pH 3.2. The content of Hg(OH)₂ is saturated with 48.13 mg/L Hg(Ⅱ) in the solution in pH range of 3.2-13.0. If pH value is greater than 13, HgO precipitate is re-dissolved as Hg(OH)₃^-. At pH＞15, HgO precipitate completely transforms into Hg(OH)₃^- and the concentration of Hg(Ⅱ) in solution reaches up to 100.0 mg/L. Accordingly, it is impossible to precipitate the mercury only by adjusting the pH value of the solution when the concentration of Hg(Ⅱ) is lower than 48.13 mg/L.

Fig.3 Concentrations of Hg(Ⅱ), Hg²⁺, HgOH⁺, Hg(OH)₂ and Hg(OH)₃^- at different pH values (initial Hg(Ⅱ) concentration of 100 mg/L)

3 Experimental

3.1 Materials and analytical methods

HgCl₂, Na₂S·9H₂O, KI, Na₂S₂O₃, K₂CrO₄, Zn(CH₃COO)₂ were purchased from Sinopharm Chemical Reagent Co., Ltd. in Shanghai. All reagents used in this work were analytical reagent grade. Wastewater containing 0.24 mmol/L Hg(Ⅱ) was prepared by dissolving 0.065 0 g solid HgCl₂. The Na₂S solution was prepared by dissolving Na₂S·9H₂O crystals. Prior to use, Na₂S·9H₂O crystals were washed with distilled water to remove oxidized sulfur species from the surface of the crystals. S(Ⅱ) concentration in Na₂S solution was standardized to 925.25 mg/L by a standard iodimetric method [19], 1 mol/L NaOH and 1 mol/L HCl solutions were used to adjust the solution pH value. All solutions were prepared in distilled water. Glassware was washed with detergent, rinsed with tap water, soaked with HNO₃ (50%, volume fraction) for 24 h, and then rinsed with distilled water prior to drying. The pH value was measured with a digital ORION-420 pH meter calibrated with pH 4.01 and 6.86 standard buffers. Mercury was determined by a QM201 mercury analyzer (cold vapor atomic absorption spectrometer) using 25% SnCl₂ reductant in 20% HCl.

3.2 Experimental method

Before mixing 150 mL synthetic wastewater containing Hg(Ⅱ) 0.24 mmol/L with 7.80 mL Na₂S solution according to molar ratio of S(Ⅱ) to Hg(Ⅱ) of 1:1, the two solutions were adjusted to the same pH values of 1, 3, 5, 7, 9, 11 and 13 by HCl or NaOH. Thereafter, the reaction was carried out in water bath temperature was kept at 25 ℃. Remained mercury concentration and pH value in treated water were detected every 2 min in initial 20 min, every 5 min between 20-40 min and every 10 min between 40-  120 min.

4 Results

For the mercury sulfidation experiments, the remained mercury concentration with time at different pH values is shown in Fig.4.

Fig.4 Dependence of remained mercury concentration on time at different pH values (initial Hg(Ⅱ) concentration of 48.13 mg/L)

Mercury solution of 48.13 mg/L cannot be removed by adjusting pH value via forming HgS precipitate. The results in Fig.4 clearly show that the reaction between Hg(Ⅱ) and S(Ⅱ) has the fastest reaction rate at pH 1.0 and the remained mercury concentration is 9.7 mg/L. The reaction rates are identical at pH 5.0 and 7.0, the remained mercury concentration are 16.68 and 3.78 mg/L, respectively, and this reaction rates were slightly higher than those at pH 3.0. The remained mercury concentration increased from 206.79 to 940.8 mg/L when the initial aqueous solution pH value increased from 9.0 to 13.0.

Accurate measurement of pH value is of primary importance for interpretation of aqueous chemical speciation. The changes of pH values before and after reaction are shown in Table 1. The results reveal that pH values of solution change after mixing the same pH value of HgCl₂ and Na₂S solutions. In acidic condition, the pH value of the mixed solution decreases, when the initial solutions pH value is 5.0, after reaction the pH value decreases to 3.24; at the initial pH 3, after reaction the mixed solution pH is 2.73; at the initial pH 1.0, the amount of H⁺ released from the reaction process is small relative to the strong acidic solution, so the decreased pH value is only 0.1. However, in neutral and alkaline condition, the pH value of the mixed solution increases, at the initial pH 7, the mixed solution pH value increase to 8.01; at initial pH 9, the pH of solution after reaction is 10.08, at pH 11 and 13, comparing to the OH^- concentration in the solution the liberated OH^- from the reaction process can be ignored, so the increased pH value is neglected.

5 Discussion

5.1 Distribution of Hg(Ⅱ) species in Hg(Ⅱ)-Cl^-- OH^--H₂O system at different pH values

In the previous experiments, all the solution pH values were adjusted by HCl or NaOH. The accumulation of Cl^- and OH^- may affect the species of Hg(Ⅱ) in Hg(Ⅱ)-Cl^--OH^--H₂O system. The species of Hg(Ⅱ) in 0.24 mmol/L HgCl₂ solution were calculated using thermodynamic data shown as the following equations [16] considering the accumulation of Cl^- and OH^-, e.g., Cl^- concentration in HgCl₂ solution     (0.24 mmol/L, pH=1) was nearly 0.1 mol/L. The results are depicted in Fig.5.

Hg²⁺+Cl^-HgCl⁺  K₉=10^6.67

HgCl⁺+Cl^-HgCl₂  K₁₀=10^6.44

HgCl₂+Cl^-HgCl₃^-  K₁₁=10^0.87

HgCl₃^-+Cl^-HgCl₄^2-  K₁₂=10^1.15

Hg²⁺+OH^-HgOH⁺  K₃=10^10.6

HgOH⁺+OH^-Hg(OH)₂  K₄=10^11.2

Hg(OH)₂+OH^-Hg(OH)₃^-  K₅=10^-0.9

Fig.5 Thermodynamics of Hg(Ⅱ)-Cl^--OH^--H₂O system (initial Hg(Ⅱ) concentration of 48.13 mg/L)

Comparing Fig.2 with Fig.5, it can be concluded that the species of Hg(Ⅱ) changes obviously, which is attributed to the accumulation of Cl^- and OH^-. In pH range of 0-3, the dominant mercury species are HgCl₄^2-, HgCl₃^- and HgCl₂ in the presence of Cl^- and OH^- (Fig.5) instead of Hg²⁺ and HgOH⁺ in the absence of Cl^- (Fig.2). Correspondingly, the dominant species is HgCl₂ at pH 3-5, HgCl₂ and Hg(OH)₂ at pH 5-8, Hg(OH)₂ at pH 8-12, and Hg(OH)₃^- at pH＞15. Moreover, both Hg(OH)₂ and Hg(OH)₃^- exist at pH 12-15.

5.2 Reaction paths of Hg(Ⅱ) with S(Ⅱ)

In 0.24 mmol/L HgCl₂ solution, mercury species are HgCl₄^2-, HgCl₃^-, HgCl₂, Hg(OH)₂ and Hg(OH)₃^-; while in Na₂S solution, S(Ⅱ) species are H₂S, HS^- and S^2- at different pH values. The potential reactive routes of Hg(Ⅱ) reacted with S(Ⅱ) to form HgS precipitate are shown as the following equations and the  values of those reactions are calculated using the reported standard Gibbs energies [17].

HgCl₄^2-+H₂SHgS+2H⁺+4Cl^-

=-90.922 kJ/mol                        (1)

HgCl₃^-+H₂SHgS+2H⁺+3Cl^-

= -96.487 kJ/mol                                  (2)

HgCl₂+H₂SHgS+2H⁺+2Cl^-

= -100.940 kJ/mol                                (3)

HgCl₂+HS^-HgS+H⁺+2Cl^-

=-121.750 kJ/mol                                (4)

Hg(OH)₂+H₂SHgS+2H₂O

=-211.059 kJ/mol                                 (5)

Hg(OH)₂+HS^-HgS+H₂O+OH^-

=-151.999 kJ/mol                                 (6)

Hg(OH)₂+S^2-HgS+2OH^-

=7.750 kJ/mol                                    (7)

Hg(OH)₃^-+HS^-HgS+H₂O+2OH^-

=-157.137 kJ/mol                                 (8)

Hg(OH)₃^-+S^2-HgS+3OH^-

=2.613 kJ/mol                                     (9)

When solution pH value is lower than zero, species of Hg(Ⅱ) and S(Ⅱ) in solution are HgCl₄^2- and H₂S, respectively. The reaction path can be described using Eq.(1).

In pH range of 0-3, species of Hg(Ⅱ) in solution are HgCl₄^2-, HgCl₃^- and HgCl₂, and species of S(Ⅱ) is H₂S. The potential reaction paths can be described using Eqs.(1)-(3). The  of the above three reactions are nearly equal, so the chemical reaction path depends on the species of Hg(Ⅱ). For instance, in the pH range of 0-1, the dominant species of Hg(Ⅱ) is HgCl₄^2- and the chemical reaction path could be Eq.(1). At pH 1-2, the priority of the above three reactions is similar. At pH 2-3, the species of Hg(Ⅱ) is HgCl₂ and the chemical reaction path could be Eq.(3).

In pH range of 3-5, the dominant species of Hg(Ⅱ) and S(Ⅱ) are HgCl₂ and H₂S, respectively. The reaction path could be Eq.(3). H⁺ liberated from H₂S results in the decreased pH value in the mixed solution, which is consistent with the experimental data (Table 1).

Table 1 Comparison of pH values before and after reaction

In pH range of 5-6, the dominant species of Hg(Ⅱ) is HgCl₂ and the dominant species of S(Ⅱ) are H₂S and HS^-, the  values of Eqs.(3) and (4) are similar; the chemical reaction path should be determined by the concentration of H₂S. Therefore, the chemical reaction path could be Eq.(3) according to Fig.1.

In pH range of 6-8, the species of Hg(Ⅱ) are Hg(OH)₂ and HgCl₂. The proportion of H₂S and HS^- in Na₂S solution is similar. Eq.(5) is most likely to occur because the  of Eq.(5) is minimum among Eqs.(3)-(6).  of reaction between Hg(OH)₂ with H₂S and HS^- is less than that of reaction between HgCl₂ with H₂S and HS^-. Thus, Hg(OH)₂ is easier to react with H₂S, HS^- than HgCl₂. In the initial solutions of pH 7, pH value in the mixed solution increases under no stirred condition. Accordingly, the reaction paths are Eqs.(5) and (6). The main species of S(Ⅱ) changes to HS^- when pH value of Na₂S solution approaches 8.0. As a result, the dominant reaction path is Eq.(6).

When the initial pH value ranges from 8 to 12, the dominant reaction path is Eq.(6). Because free OH^- from Hg(OH)₂ molecule enters the mixed solution, resulting in pH value increased. However, the reaction of Eq.(6) is more difficult to carry out because the OH^- concentration increases along with the enhancement of system pH value, the higher remained mercury concentrations are determined in mixed solution, which is consistent with experimental results (Fig.4).

In pH range of 12-15, the species of Hg(Ⅱ) are Hg(OH)₂ and Hg(OH)₃^-, and species of S(Ⅱ) are HS^- and S^2-. The potential reaction paths are Eqs.(6)-(9). Since the  values of Eq.(7) and Eq.(9) are more than 0, both reactions cannot carry out spontaneously. Although the  of Eq.(8) is less than 0, Hg(OH)₃^- and S^2- are electronegative and mutually exclusive between the particles, so the reaction is also difficult to carry out. Correspondingly, the reaction process is Eq.(6) in pH range of 12-15. So in the strongly basic solution mercury is difficult to be removed by Na₂S, which is confirmed by experimental results (Fig.4).

6 Conclusions

(1) The optimal pH value is 5-9 for wastewater containing mercury treated by Na₂S because HgS and HgO precipitates can be formed simultaneously in this pH range.

(2) Thermodynamic calculation results indicate that S(Ⅱ) and Hg(Ⅱ) have different species in the solution of different pH values, so S(Ⅱ) and Hg(Ⅱ) have the various reaction routes to form HgS precipitate. The  of Eq.(5) is minimum among Eqs.(1)-(9), which suggests that Eq.(5) is the most efficient reaction path for mercury-containing wastewater treated by Na₂S.

(3) The pH value of mixed solution changes after mixing HgCl₂ and Na₂S solution of the same pH value. The mixed solution pH decreases in acidic condition and increases in neutral and alkaline condition.

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Foundation item: Project(50925417) supported by China National Funds for Distinguished Young Scientists; Project(50830301) supported by the Key Project of the National Natural Science Foundation of China; Project(308019) supported by the Key Science and Technical Project of Ministry of Science and Technology of China; Project(2007BAC25B01) supported by the National Key Project of Science and Technical Supporting Programs Funded by Ministry of Science and Technology of China during the 11th Five-Year Plan; Project(08JJ3020) supported by the Natural Science Foundation of Hunan Province, China

Received date: 2009-08-24; Accepted date: 2009-12-19

Corresponding author: WANG Yun-yan, PhD; Tel: +86-731-88830875; E-mail: wyy@mail.csu.edu.cn

(Edited by YANG You-ping)