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(△r GmΘ)of those equations are also calculated,which can provide a guidance for mercury-containing wastewater treatment with Na2S.
The thermodynamics equilibrium principle was used to construct the diagrams for the concentration of complex ions(pc) vs pH,the distribution ratio of lead hydroxyl complex ions(αn) vs pH,and the conditional solubility product of Pb(OH)2 vs pH in the Pb2+-H2O system. The relationship between the equilibrium concentration of each kind of lead hydroxyl complex ions in equilibrium with Pb(OH)2(s) and pH value was shown in the system. The minimum solubility of lead is at the pH value of 10.096-10.997. The distribution ratio of each kind of the lead hydroxyl complex ions is determined as a function of the pH value and the total lead concentration([Pb]T). The diagram for the conditional solubility product,pKSP vs pH,shows that each kind of lead hydroxyl complex ions existing in the system is dependent upon an optimized pH value at the established concentration of [Pb]T,and that pKSP reaches the minimum at the pH value of 10.3-11.2. The results can provide a theoretical basis for removing lead ions from wastewater by the neutralization and hydrolyzation technology.
The effects of Na+ and Ca2+ in the purified water on the conductivity of zinc electrolyte and the current efficiency of zinc electrolysis were studied by the alternating current bridge method and the simulated electrolysis experiments,and the water quality index of reused water was established. The results show that the conductivity of the solution and the current efficiency decrease as these two kinds of positive ions are added in the electrolyte. The effect of Ca2+ is much more remarkable than that of Na+. ρ(Na+)≤ 8 g/L and ρ(Ca2+)≤20 mg/L are the quality indexes in the zinc electrolysis process and the concentrations of Na+ and Ca2+ in the purified water reused to the process should be less than the limited values,i.e. the water quality index of the purified water should be controlled by its reused amount.
