acidsulfate

Acid sulfate is a term used in environmental science to describe soils and sediments that contain oxidizable sulfide minerals, most often pyrite, which can generate sulfuric acid when exposed to air. In practice, the phrase is commonly encountered as acid sulfate soils (ASS), a category used especially in coastal and deltaic landscapes where waterlogged, reducing conditions previously preserved sulfide minerals.

Formation occurs when sulfide minerals are buried in anoxic, water-saturated soils. Under these conditions pyrite (FeS2) forms and remains stable. If the soil is subsequently drained or dredged, oxygen penetrates the profile and oxidizes the sulfides: pyrite + oxygen + water → sulfuric acid + ferric oxide + acidity. The result is a dramatic pH decrease (often below 4) and the release of dissolved aluminum and other metals, which can mobilize through runoff.

Environmental impacts include soil structure degradation, reduced fertility for crops, drainage-water acidity affecting streams and aquatic life, and damage to infrastructure such as bridges and drainage tiles. Recovery and remediation require careful water management to maintain reduced conditions, prompt neutralization via liming or carbonate amendments, and sometimes wetland or backfilling strategies to slow oxidation.

Management practices aim to prevent drainage or to treat affected water. This includes applying controlled drainage and waterlogging, constructing oxidation ponds or wetlands to neutralize acidity, and monitoring soil pH and metal concentrations. Mapping and risk assessment are routinely used in land development, mining, and agriculture in susceptible regions.

Notes: The term acid sulfate is sometimes used interchangeably with acid sulfate soils, but in chemical nomenclature, it is not a standard compound name. See also: pyrite, acid mine drainage, sulfuric acid.