Arsenopyrite weathering in acid rain: Arsenic transfer and environmental implications

Arsenopyrite is widely distributed and weathers readily in the nature, releases As and pollutes the surrounding environment. Acid rain is acidic in nature as contains sulfur oxides (SO_x) and nitrogen oxides (NO_x), and is a typical hazardous material to human. When arsenopyrite encounters acid rain, their interaction effect may aggregate environmental degradation. In this work, the weathering behavior of arsenopyrite in simulated acid rain was studied using the electrochemical techniques and surface analysis. Cyclic voltammetry and Raman and XPS confirmed that FeAsS was oxidized to Fe²⁺, AsO₃^3- and S⁰ at the initial phase, then, Fe²⁺ was converted to Fe³⁺, S⁰ transformed to SO₃^2- and ultimately to SO₄^2-, and AsO₃^3- to AsO₄^3- with the accumulation of H⁺. Polarization curve revealed higher temperature or higher acidity of acid rain increased the weathering trend and rate of arsenopyrite, and electrochemical impedance spectroscopic measurements showed the causes behind this to be smaller resistance and greater capacitance at the double layer and passivation film. Arsenopyrite weathering rate and temperature has a relationship: lnk = -3824.8/T + 10.305, via a transition state with activation enthalpy 29.37 kJ mol^-1 and activation entropy - 167.40 J mol^-1 K^-1. This study provides a rapid and quantitative in-situ electrochemical method for arsenopyrite weathering and an improved understanding of arsenopyrite weathering in acid rain condition. The results have powerful implications for the remediation and management of As-bearing sites affected by mining activities in acid rain area.