The process of sulfide oxidation in some acid sulfate soil materials

The process of sulfide oxidation in acid sulfate soils (ASS) is complex, involving the formation of numerous oxidation products. In this study the sulfide oxidation process was examined in 2 ASS materials over a period of 36 days using laboratory incubation experiments. Both ASS materials experienced substantial sulfide oxidation and acidification during incubation. The oxidation of pyrite was the primary cause of acidification in these ASS materials. Although a decrease in magnetic susceptibility (χ) over the initial 4 days of incubation suggested the rapid oxidation of ferromagnetic iron monosulfide greigite (Fe₃S₄), the total acid volatile sulfur (S _AV) fraction increased in concentration by an order of magnitude over the initial 8 days of incubation. Oxygen (O₂) concentration profiles indicated the presence of anoxic conditions in the centre of the incubating materials even after 16 days of exposure to the atmosphere enabling S _AV formation to occur. The oxidation of the S_AV fraction did not result in substantial acidification. A large proportion of the water-soluble iron released by sulfide oxidation was precipitated as iron oxides and hydroxides. Sulfate (SO₄^2-) was the dominant sulfur species produced from sulfide oxidation in both ASS materials, although water-soluble SO₄^2- was a poor indicator of the extent of sulfide oxidation. The sulfoxyanion intermediates, thiosulfate (S ₂O₃^2-) and tetrathionate (S₄O ₆^2-), were detected only in the early stages of incubation, with minimal amounts being detected after the initial 4 days. The relative abundance of these 2 intermediate sulfur species appeared to be dependent on the soil pH, with S₄O₆^2- dominating S₂O₃^2- in the more acidic ASS material (i.e. pH <6) as has been observed in previous studies. The diminishing presence of sulfoxyanion intermediates as oxidation progressed was indicative that ferric ion (Fe³⁺) and bacterial catalysis were driving the oxidation processes. While these sulfoxyanion intermediates only constituted a small percentage of the reduced inorganic sulfur (RIS) fraction, they accounted for up to 9.3% of the total soluble sulfur fraction. Elemental sulfur (S⁰) was not an important sulfide oxidation product in the ASS materials examined in this study.