THE OXIDATION AND DISSOLUTION OF ARSENIC-BEARING SULFIDES

Abstract

We provide a comprehensive review of research conducted on the dissolution–oxidation of As-bearing sulfides including arsenopyrite, orpiment, realgar, tennantite, and the amorphous forms of As2S3 and AsS. In general, a rise in pH and the presence of Fe3+ or bacteria (or both) cause the oxidation rates of As-bearing sulfides to increase, except for arsenopyrite. Whereas oxidation of arsenopyrite by dissolved oxygen (DO) does not have significant dependence on pH, it appears to be accelerated with increasing DO. At the surface of arsenopyrite, reaction with oxygen even in ultra-high vacuum results in the formation of As species from As to As5+. The reaction of tennantite in aqueous solution at basic pH (pH 10–11) results in the formation of copper oxide–hydroxide and arsenic hydroxide, whereas in acidic solutions, the formation of elemental sulfur or arsenic oxide is observed. At high concentrations of bicarbonate and carbonate, As can be released from Fe oxyhydroxide surfaces back to the solution because of surface charge, or may also act as a competitor for both As5+ and As3+ during sorption reactions. Overgrowths of Fe3+ oxyhydroxide, scorodite, and FeAsO3 are observed as a result of oxidation in air and aqueous medium. In an acidic solution, an overgrowth of elemental sulfur is reported, in both abiotic and biotic conditions. The reaction of arsenopyrite and Acidithiobacillus ferrooxidans in the acidic medium salt solution results in a FePO4 overgrowth of 0.2 μm thick, which is promoted by bacterial production of Fe3+. In natural and mining environments, conditions of high pH may result in increasing acidity and rate of release of As and S. The formation of secondary mineral phases on the surfaces of As-bearing sulfides may retard the mobility of As and potentially decrease the acidity. Therefore, passivation of mineral surfaces with a mineral coating may be a reasonable strategy to minimize the affects of surface mining. In general, interactions with bacteria enhance the oxidation rates of As-bearing sulfide minerals, but at rates lower than those measured in the laboratory under optimum conditions. Continuing work on As-bearing sulfides is recommended to better understand the impacts of these minerals on the environment.