Redox behavior and chemical species of arsenic in acidic aqueous system

Arsenic (As) removal from smelting acidic wastewater is an urgent task. The most common method is oxidation of trivalent As(III) to pentavalent As(V) subsequently precipitated by ferric (Fe(III)) salts. Foundations of redox behavior and chemical species are of great importance for understanding As removal. In this work, cyclic voltammetry (CV) and UV-Vis spectroscopy were used for laboratory observation; meanwhile HSC and MINTEQ software were employed for theoretical analyses. It is found that As(III) oxidation, a multiple electron transfer reaction, is diffusion-controlled. The oxidation over-potential is very high (about 0.9 V) in sulfuric acid solutions (pH 1.0). In addition, Fe(III)-As(V) complexes are evidenced by UV-Vis spectra and chemical species analyses in series of Fe(III)-As(V)-H2SO4-H2O solutions. Therefore, the Fe(III) and As(V) species distribution against pH values are determined and a new phi-pH diagram with inclusion of Fe-As complexes is consequently compiled based on thermodynamic data predicted by other researchers.