Study on the Leaching of Mercuric Oxide with Thiosulfate Solutions

Mercury is receiving more concern due to its high mobility and high toxicity to human health and the environment. Restrictive legislations and world-wide efforts have been made on mercury control, especially the release and disposal of mercury-contaminated wastes. This paper describes a novel technology for detoxifying mercury-containing solid wastes with thiosulfate salts. Various parameters which may potentially influence mercury extraction from mercuric oxide with the thiosulfate leaching system—including reagent concentration, solution pH, and temperature—have been examined. The experimental results show that virtually all mercuric oxide can rapidly dissolve into the thiosulfate leaching system under optimized experimental conditions, indicating that thiosulfate is an effective lixivant for recovering mercury from mercury-containing solid waste.


Introduction
Mercury has attracted great public concern due to its high hazards and adverse effects to both human health and the environment.Toxicology studies have proven that mercury-both inorganic and organic-can cause serious health effects, though organic mercury is generally more toxic [1][2][3][4].However, mercury is highly mobile, and mercury contamination can become much more widespread than that of other heavy metals [5,6].Mercury may undergo complex physical, chemical, and biological transformations in the environment-e.g., the atmospheric transport of Hg 0 , the photochemical oxidation and subsequent deposition of mercury on water and land, and the methylation of Hg 2+ by reducing bacteria in anoxic habitats and its absorption and accumulation by organisms, which results in high mercury concentration in fish and chronic low level exposure to humans through the food chain [7,8].More than two thousand people were affected and 1043 died due to Minamata Disease caused by mercury pollution from a local chemical plant in Japan in the late 1970s [9].
Mercury emission may come from natural, anthropogenic, and reemitted resources.Typical natural mercury emissions include volcanic emission, forest fires, and degassing from mercury mineral deposits.Re-emitted mercury resources, such as those mercury-contaminated sites or systems, are also important in the global mercury emission cycle [10].Anthropogenic mercury emission mainly results from coal and oil combustion, urban discharge (municipal and medical wastes), and the mercury mining/mineral processing industry [11].The extensive exploration of cinnabar ore in the Wanshan area (Guizhou, China) has resulted in severe mercury pollution to the local aquatic system and land [12].In artisanal gold mining, mercury is used to extract gold from ore, which always results in severe mercury releases to the local environment (surrounding rocks, soils, and sediments) [13,14].It has been reported that ionic forms of mercury are strongly adsorbed by soils and sediments and are desorbed slowly.Most mercury ions are absorbed by organic matter (mainly fulvic and humic acids) in acidic soils.As a result, the following mercury compounds are commonly found in mercury-contaminated solid wastes: Hg 2 Cl 2 , HgO/Hg(OH) 2 , HgS, and methyl-mercury compounds such as CH 3 HgCl and CH 3 HgOH [15].
To detoxify mercury-containing wastes, technologies such as amalgamation, adsorption, precipitation (sulfides/selenides), thermal treatment, and stabilization/solidification through vitrification or chemical formation have been developed [16].For example, elemental mercury can also react with elemental sulfur to form mercury sulfide (HgS), which is kinetically stable at room temperature [15].A more stable and insoluble compound is HgSe, also known as tiemannite.The technologies and processes using HgS or HgSe for mercury stabilization are commonly used in HgO-containing wastes and solid wastes containing a large amount of mercury [17].In adsorption treatments, adsorbents (mainly activated carbons) are used to stabilize Hg 0 [18].Thermal desorption, retorting, and roasting-which all employ heating (at low or high temperatures)-have been commonly used to treat mercury contaminated soil, sediments, and other solid wastes [19][20][21].For stabilization/solidification processes, the vitrification process is a high-temperature treatment technology designed to immobilize contaminants by incorporating them into a vitrified end product [22][23][24][25].For solid mercury-contaminated wastes, water-based technologies (or leaching technologies) such as soil washing that use both physical and chemical separation methods to reduce contaminant concentrations in wastes have also been commonly used [26].These processes can be used independently or in conjunction with other treatment technologies.Chemicals such as HCl, HNO 3 , and H 2 O 2 have been used as lixivants for the remediation of mercury-contaminated sediments [27].Thiosulfate salts are also suggested as the leaching reagent for the extraction of mercury from mercury-containing soil.Issaro et al. [28] reported that 50% ± 5% of mercury can be extracted from contaminated soil after leaching with 0.01 M Na 2 S 2 O 3 for about 24 h.Lu et al. [29] examined the effect of thiosulfate on mercury removal, and reported that Na 2 S 2 O 3 can be used as a chemical aid for improving trace mercury removal.However, since mercury may be present in various forms in contaminated wastes, detailed information on leaching behavior and mechanisms of different mercury components in the thiosulfate leaching system is lacking.As one of the typical inorganic mercuric compounds, mercuric oxide was chosen for research in this paper.The leaching behavior of HgO in thiosulfate solutions under different experimental conditions (temperature, thiosulfate concentration, liquid-solid ratio, and initial pH) was examined, and the potential use of sodium thiosulfate for the remediation of mercury-contaminated wastes is discussed based on experimental results.
Elemental mercury is virtually insoluble in water (50 µg/L at 298 K), while mercuric oxide (in the form of Montroydite in nature)-a kind of alkaline oxide-has a solubility of 15.8 mg/L in water at 298 K, and has good solubility in diluted hydrochloric acid, dilute nitric acid, and alkaline cyanide.The aqueous speciation and coordination of Hg have been well-documented [30][31][32][33][34][35][36].The oxidation states of Hg in aqueous systems are 0, +1, and +2, of which Hg (2+) is the most stable in typical aerated water.Mercury speciation and coordination in the aquatic system without the presence of other strongly complex ligands is mainly controlled by hydrolysis reactions.The Eh-pH diagram of the Hg-H 2 O system at room temperature has been calculated by many researchers [37].Basically, at low pH, Hg 2+ is octahedrally coordinated by six water molecules, forming the hex-aquaion Hg(H 2 O) 6 2+ , with Hg-O bond lengths of 2.34-2.41Å[33].As the pH increases, Hg 2+ hydrolyzes to form HgOH + and Hg(OH) 2 , of which two Hg-O bonds are shortened to distances of 2.00-2.10Å, while the remaining bonds are lengthened to approximately 2.50 Å.The distorted octahedral coordination is indicative of the tendency for Hg (2+) to form mononuclear linear, double-coordinated complexes.When sulfur species are present, mercury sulfide (HgS) is stable in a wide range of pH at low potentials.At high potentials, sulfur will occur as more stable species, such as HSO 4 − (low pH) and SO 4 2− (high pH) [30,38].If sulfur occurs as non-stable species, such as in the form of thiosulfate, complex reactions between Hg 2+ and S 2 O 3 2− may happen in aqueous solution, and the complexing reactions can be represented as follows [39]: the complexes can only form conditionally, depending on the solution potential, pH, temperature, and concentrations of relevant species.

Experimental
Reagent grade mercuric oxide (particle size: P80-16 µm), sodium thiosulfate, sulfuric acid, and sodium hydroxide were used as received from local suppliers.De-ionized water was used in all experiments.Leaching tests were carried out in a sealed beaker, and mixing was provided by a mechanical agitator with glass impellers.Temperature was controlled with a water bath.Equilibrium pH was adjusted by direct addition of concentrated H 2 SO 4 solution (50% v/v) or NaOH solution (2 mol/L).The solution pH and potential were measured with a glass pH probe during leaching.For each test, a designated amount of mercuric oxide was leached in 500 mL solution, and 10 mL of slurry were taken out for sampling at different time intervals.The slurry was filtered through a conical separator with filter paper.The mercury content in the aqueous sample was analyzed by flame atomic absorption spectrophotometer.

Effect of pH
Tests of leaching HgO in Na 2 S 2 O 3 solutions under different initial pH were first conducted at indoor temperature (20 ± 1 • C).In these tests, 0.4 g HgO was leached in 500 mL aqueous solution with 0.01 molar Na 2 S 2 O 3 .Variations of solution pH and mercury extraction at different leaching times are shown in Figures 1 and 2, respectively.In all three cases, the solution pH increases from the initial pH to around 11.5, and then keeps nearly constant after that.These results are consistent with reports of the dissolution chemistry of mercury species in thiosulfate solutions in the literature [33].The reaction of HgO and Na 2 S 2 O 3 in aqueous solution can be represented as follows: where n may be 2 or 3, depending on the Na 2 S 2 O 3 concentration.The Gibbs values of Equation ( 3) with ligand numbers of 2 and 3 were also calculated, and the results were −22.05 kJ/mol and −36.32 kJ/mol, respectively, indicating that the complex reactions are spontaneous.The dissolution of HgO in the Na 2 S 2 O 3 solution results in the release of OH -ions, which leads to an increase of pH when leaching begins until equilibrium is established (about 5-10 min under the experimental conditions).Accordingly, the dissolution of HgO results in increasing Hg concentration in the solution with increasing leaching time.It was also observed that for different initial pH, higher Hg extraction is obtained with initial pH 9.67 than those of initial pH 7.00 and initial pH 10.96 at the condition of Na 2 S 2 O 3 concentration 0.01 M.This phenomenon is probably related to the stability of thiosulfate.The potential decomposition reactions at low and high pH are expressed below: At alkaline conditions, According to Reactions (1) and ( 2), to dissolve 0.4 g HgO entirely, at least 5.5 × 10 −3 mole Na 2 S 2 O 3 will be needed to form the thiosulfate complex, theoretically.In other words, the total amount of Na 2 S 2 O 3 (0.01 M) used in these tests is merely enough for complex reactions.As a result, the mercury extraction under the defined experimental conditions is sensitive to the change of initial pH in this case.Further tests indicate that when a high concentration of Na 2 S 2 O 3 (0.1 M) is used for leaching, the initial solution pH exhibits little effect on the leaching rate.

Effect of Na2S2O3 Concentration
To determine the effect of Na2S2O3 on HgO dissolution, tests were carried out with 0.

Effect of Na2S2O3 Concentration
To determine the effect of Na2S2O3 on HgO dissolution, tests were carried out with 0.4 g HgO in 500 mL solutions with different concentrations of Na2S2O3.For all tests, the pH was uncontrolled during leaching.Similar to the tests above, the solution was initially around 7.0 and reached constant (around 11.5) after 5 min.Variations of Hg extraction with time are shown in Figure 3.At low Na2S2O3 concentration (0.01 mol/L), the Hg extraction increases relatively slowly with increasing leaching time, and the final Hg extraction is only 70% after leaching for 30 min.At a higher concentration of Na2S2O3 (0.05 mol/L), the dissolution rate of HgO increases significantly-the Hg extraction reaches about 95% after leaching for 5 min and then keeps nearly stable after that.When the concentration of Na2S2O3 in solution further increases to 0.1 mol/L, the HgO dissolution rate is slightly higher than that of 0.05 mol/L Na2S2O3 in solution, and virtually all HgO dissolves within 5 min.

Effect of Na 2 S 2 O 3 Concentration
To determine the effect of Na

Effect of Temperature
To determine the effect of temperature on HgO leaching with Na2S2O3, tests were carried out with 0.4 g HgO in 500 mL 0.01 mol/L Na2S2O3 solutions.For all tests, the pH was uncontrolled during leaching.Variations of Hg extraction with time are shown in Figure 4.It shows that the initial leaching rate of HgO increases with an increase of temperature, when temperature varies from 293 K to 323 K.The final Hg extraction after 40 min at the higher temperature (303 K) is close to 95%, much higher than that at 293 K (about 70%).However, the difference of final Hg extractions at 303 K, 313 K, and 323 K is insignificant.Considering the potential effect of the decomposition of S2O3 2− during leaching, a simple estimation of the initial leaching rate of HgO under different temperatures was conducted.Assuming that Hg Hg HgO Hg max Hg max where kT is the initial leaching rate of HgO at temperature T (K); XHgO is the fraction of unreacted HgO; t is leaching time (min); CHg is the molar concentration of dissolved Hg in leachate (mol/L); and CHg-max is the maximum molar concentration of dissolved Hg in leachate (mol/L).
The Arrhenius equation in terms of the reaction rate constant kT can be written in Equation 7.Here the ΔE is the apparent activation energy for the leaching Reaction (3), and A is the constant.If the leaching rate in the first two min is used for the calculation, the plot of log kT vs. 1000/T is shown in Figure 5, and a straight line is obtained when the temperature varies from 293 K to 323 K.The negative value of the line's slope indicates that leaching Reaction (3) is endothermic, and increasing temperature promotes the leaching of HgO with Na2S2O3 between 293 K and 323 K.The apparent activation energy can be calculated according to the slope of the line, giving a value of 23.3 kJ/mol, indicating that the initial leaching of HgO in Na2S2O3 solution is under mixed control and both the diffusion of the lixivant S2O3 2− and the complexing reaction between mercury and S2O3 2− exhibit effects on the dissolution of HgO.

Effect of Temperature
To determine the effect of temperature on HgO leaching with Na 2 S 2 O 3 , tests were carried out with 0.4 g HgO in 500 mL 0.01 mol/L Na 2 S 2 O 3 solutions.For all tests, the pH was uncontrolled during leaching.Variations of Hg extraction with time are shown in Figure 4.It shows that the initial leaching rate of HgO increases with an increase of temperature, when temperature varies from 293 K to 323 K.The final Hg extraction after 40 min at the higher temperature (303 K) is close to 95%, much higher than that at 293 K (about 70%).However, the difference of final Hg extractions at 303 K, 313 K, and 323 K is insignificant.Considering the potential effect of the decomposition of S 2 O 3 2− during leaching, a simple estimation of the initial leaching rate of HgO under different temperatures was conducted.
Assuming that where k T is the initial leaching rate of HgO at temperature T (K); X HgO is the fraction of unreacted HgO; t is leaching time (min); C Hg is the molar concentration of dissolved Hg in leachate (mol/L); and C Hg-max is the maximum molar concentration of dissolved Hg in leachate (mol/L).
The Arrhenius equation in terms of the reaction rate constant k T can be written in Equation (7).Here the ∆E is the apparent activation energy for the leaching Reaction (3), and A is the constant.If the leaching rate in the first two min is used for the calculation, the plot of log k T vs. 1000/T is shown in Figure 5, and a straight line is obtained when the temperature varies from 293 K to 323 K.The negative value of the line's slope indicates that leaching Reaction (3) is endothermic, and increasing temperature promotes the leaching of HgO with Na 2 S 2 O 3 between 293 K and 323 K.The apparent activation energy can be calculated according to the slope of the line, giving a value of 23.

Conclusions
The leaching behavior of HgO in Na2S2O3 solutions was investigated in the paper.Effects of initial pH, Na2S2O3 concentration, and temperature on the dissolution rate of HgO in Na2S2O3 solution were examined.The test results indicate that the solution pH exhibits a significant effect on HgO leaching when the molar ratio of Na2S2O3 to total mercury is relatively low (e.g., less than 3).An increase of Na2S2O3 concentration may potentially increase the dissolution of HgO due to the increase of the molar ratio of Na2S2O3 to mercury.Increasing temperature can slightly increase the dissolution rate of HgO in Na2S2O3 solutions when the temperature varies from 293 K to 323 K.The apparent activation energy can be calculated according to the slope of the line, giving a value of 23.3 kJ/mol, and the initial leaching of HgO in Na2S2O3 solution is under mixed control.Under optimum experimental conditions, virtually all mercuric oxide can dissolve in Na2S2O3 solutions within five minutes, indicating that the thiosulfate leaching system is an effective way to detoxify mercury solid wastes containing mercuric oxide.

Conclusions
The leaching behavior of HgO in Na2S2O3 solutions was investigated in the paper.Effects of initial pH, Na2S2O3 concentration, and temperature on the dissolution rate of HgO in Na2S2O3 solution were examined.The test results indicate that the solution pH exhibits a significant effect on HgO leaching when the molar ratio of Na2S2O3 to total mercury is relatively low (e.g., less than 3).An increase of Na2S2O3 concentration may potentially increase the dissolution of HgO due to the increase of the molar ratio of Na2S2O3 to mercury.Increasing temperature can slightly increase the dissolution rate of HgO in Na2S2O3 solutions when the temperature varies from 293 K to 323 K.The apparent activation energy can be calculated according to the slope of the line, giving a value of 23.3 kJ/mol, and the initial leaching of HgO in Na2S2O3 solution is under mixed control.Under optimum experimental conditions, virtually all mercuric oxide can dissolve in Na2S2O3 solutions within five minutes, indicating that the thiosulfate leaching system is an effective way to detoxify mercury solid wastes containing mercuric oxide.

Conclusions
The leaching behavior of HgO in Na 2 S 2 O 3 solutions was investigated in the paper.Effects of initial pH, Na 2 S 2 O 3 concentration, and temperature on the dissolution rate of HgO in Na 2 S 2 O 3 solution were examined.The test results indicate that the solution pH exhibits a significant effect on HgO leaching when the molar ratio of Na 2 S 2 O 3 to total mercury is relatively low (e.g., less than 3).An increase of Na 2 S 2 O 3 concentration may potentially increase the dissolution of HgO due to the increase of the molar ratio of Na 2 S 2 O 3 to mercury.Increasing temperature can slightly increase the dissolution rate of HgO in Na 2 S 2 O 3 solutions when the temperature varies from 293 K to 323 K.The apparent activation energy can be calculated according to the slope of the line, giving a value of 23.3 kJ/mol, and the initial leaching of HgO in Na 2 S 2 O 3 solution is under mixed control.Under optimum experimental conditions, virtually all mercuric oxide can dissolve in Na 2 S 2 O 3 solutions within five minutes, indicating that the thiosulfate leaching system is an effective way to detoxify mercury solid wastes containing mercuric oxide.
4 g HgO in 500 mL solutions with different concentrations of Na2S2O3.For all tests, the pH was uncontrolled during leaching.Similar to the tests above, the solution was initially around 7.0 and reached constant (around 11.5) after 5 min.Variations of Hg extraction with time are shown in Figure 3.At low Na2S2O3 concentration (0.01 mol/L), the Hg extraction increases relatively slowly with increasing leaching time, and the final Hg extraction is only 70% after leaching for 30 min.At a higher concentration of Na2S2O3 (0.05 mol/L), the dissolution rate of HgO increases significantly-the Hg extraction reaches about 95% after leaching for 5 min and then keeps nearly stable after that.When the concentration of Na2S2O3 in solution further increases to 0.1 mol/L, the HgO dissolution rate is slightly higher than that of 0.05 mol/L Na2S2O3 in solution, and virtually all HgO dissolves within 5 min.
2 S 2 O 3 on HgO dissolution, tests were carried out with 0.4 g HgO in 500 mL solutions with different concentrations of Na 2 S 2 O 3 .For all tests, the pH was uncontrolled during leaching.Similar to the tests above, the solution was initially around 7.0 and reached constant (around 11.5) after 5 min.Variations of Hg extraction with time are shown in Figure 3.At low Na 2 S 2 O 3 concentration (0.01 mol/L), the Hg extraction increases relatively slowly with increasing leaching time, and the final Hg extraction is only 70% after leaching for 30 min.At a higher concentration of Na 2 S 2 O 3 (0.05 mol/L), the dissolution rate of HgO increases significantly-the Hg extraction reaches about 95% after leaching for 5 min and then keeps nearly stable after that.When the concentration of Na 2 S 2 O 3 in solution further increases to 0.1 mol/L, the HgO dissolution rate is slightly higher than that of 0.05 mol/L Na 2 S 2 O 3 in solution, and virtually all HgO dissolves within 5 min.
3 kJ/mol, indicating that the initial leaching of HgO in Na 2 S 2 O 3 solution is under mixed control and both the diffusion of the lixivant S 2 O 3 2− and the complexing reaction between mercury and S 2 O 3 2− exhibit effects on the dissolution of HgO.

Acknowledgements:
The authors thank Mr. Yan Fu for his generous help on the project.The work was supported by the National Natural Science Foundation of China [grant number 51374054].

Acknowledgements:
The authors thank Mr. Yan Fu for his generous help on the project.The work was supported by the National Natural Science Foundation of China [grant number 51374054].