Environmental behavior of arsenic in soils: Theoretical

Thermodynamics data of As species were used to develop the solubility isotherms of As minerals and solution species. The effect of redox on the stability of As minerals and the distribution of As solution species was also simulated. In oxygenated and alkaline systems, Ca₃(AsO₄)₂ was the most stable As mineral followed by Mn₃(AsO₄)₂. It was postulated that in such environments Ca₃(AsO₄)₂ and Mn₃(AsO₄)₂ might precipitate. Arsenic(V) oxide was too soluble to become an important solid phase in the aqueous environment. Under reduced and acidic concitions (pe + pH < 8 and pH < 6), As(III) oxides were stable along with As sulfides. These minerals were unstable, above pe + pH of 8. In oxidized solutions (pe + pH > 8), arsenate species were important whereas in reduced systems (pe + pH < 8), As(III) species were in abundance. Arsine gas can only form if the system was very reduced and acidic. Below pH 2.2, H₃AsO₄° was in abundance. As pH increased, H₂AsO₄^- species was important, followed by AsO₄^3- above pH 12. The results of thermodynamic stability and solubility models have been discussed in reference to soil environment.