Electrochemical study of orpiment (As2S3) dissolution in a NaOH solution

The chemical dissolution mechanism of natural orpiment polycrystals in a 1 M NaOH solution has been investigated using a combination of spectroscopic, chromatographic and electrochemical methods. Two different types of electrodes were used for characterization of dissolution process by open-circuit potential (OCP) measurements, linear sweep (LSV) and cyclic voltammetry (CV). With one electrode, particles of yellow As2S3 were mechanically immobilized on the surface of the paraffin-impregnated graphite electrode (PIGE) and with the other electrode, As2S3 particles were homogeneously dispersed in a carbon paste electrode (CPE). In addition, leached solution species were characterized by (a) UV–vis spectroscopy, (b) ion chromatography and (c) CV using a platinum disc electrode. Arsenite and sulfide ions were detected during the initial solubilization step of orpiment followed by proposed thioarsenite and pentavalent arsenic species. Arsenate and thioarsenates species are minor components of the leaching solution. In addition, the oxidation of orpiment in alkaline media was investigated. The chemical properties of arsenic have an important role in the dissolution process and the chemical speciation over time.

Research Highlights
► Dissolution of As2S3 (a covalently bonded solid) in a sodium hydroxide solution occurs due to the polarizing effect of As3+ cations which distorts the electron cloud around the adsorbed hydroxide ions on orpiment surface.
► The dissolution of orpiment leads to the formation of As(V) (arsenates and thioarsenates) despite the fact that a non-oxidative chemical dissolution occurs.
► A hydrolysis mechanism describes the early stages of dissolution leading to the arsenite and sufide ions formation.
► A chemical dissolution is investigated with a combination of electrochemical methods.
► In addition, the orpiment oxidation is studied in sodium hydroxide media.