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

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 μg/L) at pH 1.0 and the highest remained concentration (940.8 μg/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 change (ΔrGmΘ) of those equations are also calculated, which can provide a guidance for mercury-containing wastewater treatment with Na2S.