OXIDATION OF ARSENOPYRITE BY AIR AND AIR-SATURATED, DISTILLED WATER, AND IMPLICATIONS FOR MECHANISM OF OXIDATION

Arsenic and sulphur, and possibly iron, exist in multiple oxidation st ates in the near-surface of unoxidized arsenopyrite. X-ray photoelectr on spectroscopy (XPS) of an unoxidized surface reveals that sulphur is present as disulphide (S-2(2-), 78 At.%), monosulphide (S2-, 15%), an d as polysulphide (S-n(2-), where 2 < n < 8). As1- predominates, but 1 5% As-0 is also observed. Most iron is present as Fe2+ bonded to arsen ic and sulphur [Fe(II)-(As-S)], but some Fe3+ may be present, and bond ed to As-S [Fe(III)-(As-S)]. Exposure for twenty-five hours to the atm osphere reveals development of Fe(III)-oxyhydroxide species at the sur face, with binding energies similar to goethite. Approximately one qua rter of the arsenic is present as As5+, As3+, and As1+, although As1- predominates. Minor polysulphides and thiosulphate are produced during oxidation, Surfaces reacted with air-saturated, distilled water for e ight hours undergo extensive oxidation. Fe(III)-oxyhydroxides are the dominant Fe surface species. As5+, As3+, and As1+ are as abundant as A s1-, and sulphate is detectable on the surface. The binding energies a nd proportions of the oxygen-bearing species of the O(ls) spectra are effectively the same in the air-oxidized and water-reacted experiments . The striking similarities suggest strongly that the nature of the sp ecies produced, and their rates of formation, are the same during reac tion with air and with air-saturated distilled water. The mechanisms b y which oxygen near-surface species are produced may well be similar i n both media. Arsenic is the most readily oxidized species, and sulphu r is most slowly oxidized during reaction with the atmosphere. Reactio n of arsenopyrite with air-saturated, distilled water oxidizes As1- an d Fe2+ at similar rates, and much more rapidly than sulphur is oxidize d. Auger depth profiles of oxidized arsenopyrite demonstrate that As d iffuses from the interior of the mineral to the surface during oxidati on. Diffusion of As to the oxidized surface, combined with the observe d production of large amounts of As3+ and As5+ in the near-surface, pr omotes rapid, selective leaching of arsenites and arsenates, Arsenites (AsO33-) are particularly toxic to biota, and pose a potential risk t o water quality where arsenopyrite is abundant.