Pyrite in acid sulfate soils: transformation and inhibition of its oxidation by application of natural materials

Worldwide estimate of acid sulfate soils is 12–14 million ha. In Malaysia, they occur mainly along the coastal plains of Peninsular Malaysia and Sarawak. The soils are characterized by very low pH and high amounts of exchangeable Al, resulting from oxidation of pyrite when the soils are drained. This paper explains the transformation of pyrite when the soils undergo weathering due to drainage. Soil samples containing pyrite from Cg-horizons of selected acid sulfate soils in Peninsular Malaysia were incubated in the laboratory. Some of the samples were treated with phosphate and peat so as to prevent the oxidation of pyrite. The changes in pyrite morphology were studied by SEM-EDAX. At the end of incubation, water-soluble metals were determined. The result showed that pyrite in the unoxidized state had Fe to S ratio of 1:2, indicating the absence of accessory elements (As, Cd, etc.,) in the pyrite structure. This means that when this pyrite disintegrates, there is no danger of heavy metal pollution. As the weathering continues, pyrite dissolves quickly forming jarosite. Phosphate treatment reduced pyrite oxidation. The amounts of Al and Fe in the water-soluble form were large which could be toxic to plants and aquatic life.