Redox Processes of Iron and Sulfur Involved in the Formation of Acid Sulfate Soils

Abstract

Acid sulfate soils are here defined as soils that have acidified to soil pH values below 4 as a result of oxidation by Fe sulfides, mainly pyrite (cubic FeS2). Their unoxidized sulfidic precursors are called potential acid sulfate soils. Acid sulfate soils occur frequently in recent coastal plains, where pyritic tidal marsh sediments have been drained, naturally or by man, and exposed to the air. In the absence of sufficient neutralizing substances such as calcium carbonate, the sulfuric acid liberated during oxidation of pyrite may cause a drop in soil to 3–4 or sometimes lower. “Acid sulfate weathering” can be observed also in pyritic sediments of greater age, as well as in pyritic mine spoils. Although the typical characteristics of these soils must be attributed mainly to the chemical properties of S, Fe plays a major role too: (1) Fe is associated with the S both in reduced and oxidized conditions, (2) dissolved Fe(III) is an essential intermediate oxidant in the oxidation of pyrite S at low, and (3) the strong acidity formed upon oxidation of pyrite can be attributed both to the acidic character of sulfuric acid itself and to the strong affinity of Fe(III) for OH.