The response of partially oxidised acid sulfate soil materials to anoxia

Abstract

Four acid sulfate soil (ASS) materials were subjected to anoxia after varying periods of oxidation to determine the geochemical response of these types of soils to flooding. The response of the partially oxidised ASS materials to the exclusion of oxygen was variable. The rate of sulfide oxidation, acidification, and the production of soluble oxidation products such as sulfate, iron, and aluminium generally decreased markedly when subjected to anoxia. However, especially in the highly acidic ASS materials (i.e. pH <3.5), sulfide oxidation and acidification generally continued (albeit at much slower rates), most probably due to oxidation by Fe3+. Rapid sulfide re-formation occurred in the peat ASS material that had been oxidised for 63 days, with 0.47% reduced inorganic sulfur (SCR) formed over 60 days of anoxia. This substantial sulfide re-formation was accompanied by only a slight increase in pH. Minimal sulfide re-formation occurred in 2 of the ASS materials when placed in anoxic conditions, most likely due to a lack of readily available organic matter in these materials. The results show that the imposition of anoxic conditions on partially oxidised ASS materials is generally effective in decreasing the rates of further sulfide oxidation, acidification, and the production of soluble sulfide oxidation products. Biogeochemical sulfide formation consumes acidity; however, sulfide re-formation was ineffective in reversing acidification under the conditions of this experiment. The results indicate that the treatment of sites containing actual ASS materials by management strategies relying on oxygen exclusion need to be accompanied by other strategies that include acidty neutralisation or containment.