ELECTROCHEMICAL STUDY OF ORPIMENT (AS2S3) AND REALGAR (AS2S2) IN ACIDIC MEDIUM

Electrochemical characterizations of high purity natural crystals of o rpiment (As2S3) and realgar (As2S2) were performed in the presence of a bacteria-free, acidic bacterial growth medium for Thiobacillus ferro oxidans, name M2. A carbon-paste electrode (CPE), composed of a mixtur e of pure mineral grains with graphite powder and silicon oil as a bin der was used for the experiments. Reproducibility of the mineral surfa ces was assured by monitoring the open-circuit potential (OCP) of the initial CPE-mineral-electrolyte interface. The voltammetric study reve aled that the oxidation processes for both of these arsenic-bearing mi nerals involves two steps: an initial oxidation of the sulfide mineral to H3AsO3 and HSO4, followed by the oxidation of H3AsO3 to H3AsO4. Th e reduction processes for the two minerals, however, differ. While the reduction of orpiment occurs in a dual step process, for realgar, the process is achieved in a single step.