The Oxidation of Iron Sulphides in Soils in Relation to the Formation of Acid Sulphate Soils, and of Ochre Deposits in Field Drains

Abstract

Aerating pyritic soils causes acidification and the formation of acid sulphate soils, or cat-clay. The oxidation of pyrite in soils is associated with the deposition in tile drains of a form of ochre quite distinct from that formed by the action of filamentous iron bacteria. Pyrite-derived ochre results from the action of Thiobacillus ferrooxidans, which, below pH 3.5-40, catalyses the oxidation of Fea+ and pyrite. In soils less acid than c. pH 4, pyrite oxidizes relatively slowly by chemical reactions to Fe*+ and SO:-. Under these conditions iron enters the drains as Fe'f and is there oxidized by T. fewooxidans and deposited as hydrated ferric oxide. Once the soil becomes acid enough for T. fenoojcidans to multiply, the rate at which pyrite oxidizes increases several-fold, and at c. pH 3 iron appears in the drainage water in the ferric form. Liming seems to decrease the rate of oxidation.

Anoxic versus oxic sample pretreatment: Effects on the speciation of sulfur and iron in well-aerated and wetland soils as assessed by X-ray absorption near-edge spectroscopy (XANES)

“…These forms are conventionally assigned to inorganic sulfides, such as FeS, pyrrhotite, or pyrite (see compilation in Prietzel et al, 2003;Goh et al, 2006; Table 2). In the mineral soil horizons, the decrease of peaks at energies<2472 eV was associated with an increase of the peak at 2482.5 eV representing oxidized S(VI+), which can be easily explained by oxidation of sulfide to sulfate (Bloomfield, 1972;Engler and Patrick, 1973;Wieder et al, 1985;Maher et al, 2004;Ward et al, 2004;Bottrell et al, 2007). Accordingly, the mean oxidation state of sulfur in the mineral soil was strongly increased after oxic compared to anoxic sample pretreatment (Fig.…”

Geochemical indicators of preservation status and site deterioration at Star Carr

“…To aid interpretation, the ÀLog10 of the Sulphate:Sulphide Ratio has been calculated. Positive values for this index represent an excess of Sulphate indicating oxidised conditions probably enhanced by the sulphide oxidising bacteria T. ferrooxidans (Bloomfield, 1972;Arkesteyn, 1980), whilst negative values show an excess of Sulphide and indicate reducing conditions probably caused by sulphate reducing bacteria such as Desulfovibrio disulfuricans (Holden et al 2006). …”

“…To aid interpretation, the ÀLog10 of the Iron III:Iron II Ratio has been calculated. Positive values for this index represent an excess of Iron III indicating oxidised conditions probably caused by the iron oxidising bacteria Thiobacillus ferrooxidans (Bloomfield, 1972;Arkesteyn, 1980), whilst negative values show an excess of Iron II and indicate reducing conditions.…”

Treatments to Combat Pyrite Oxidation in Coal Mine Spoil

“…The rate limiting step in the oxidation pathway can be either the bacterial oxidation of ferrous to ferric ions, or the actual ferric ion oxidation of pyrite, or the surface area of the pyrite available for reaction with the ferric ions (Garrels and Thompson 1960, Bloomfield 1972, van Breemen 1973.…”