Pyrite oxidation in acid sulphate soils: The role of miroorganisms

Abstract

A study has been made of microbial processes in the oxidation of pyrite in aicd sulphate soil material. Such soils are formed during aeration of marine muds rich in pyrite (FeS2). Bacteria of the type ofThiobacillus ferrooxidans are mainly responsible for the oxidation of pyrite, causing a pronounced acidification of the soil. However, becauseThiobacillus ferrooxidans functions optimally at pH values bellow 4.0, its activity cannot explain the initial pH drop from approximately neutral to about 4. This was shown to be a non-biological process, in which bacteria play an insignificant part. AlthoughThiobacillus thioparus andThiobacillus thiooxidans were isolated from the acidifying soil, they did not stimulate oxidation of FeS2, but utilized reduced sulphur compounds, which are formed during the non-biological oxidation of FeS2. Ethylene-oxide-sterilized and dry-sterilized soil inoculated with pure cultures of mixtures of various thiobacilli or with freshly sampled acid sulphate soil soil did not acidify faster than sterile blanks.Thiobacillus thiooxians. Thiobacillus thioparus. Thiobacillus intermedius andThiobacillus perometabolis increased from about 104 to 105 cells/ml in media with FeS2 as energy source. However, FeS2 oxidation in the inoculated media was not faster than in sterile blanks. Attempts to isolate microorganisms other thanThiobacillus ferrooxidans, like metallogenium orLeptospirillum ferrooxidans, which might also be involved in the oxidation of FeS2 were not successful. Addition of CaCO3 to the soil prevented acidification but did not stop non-biological oxidation of FeS2.