Oxidation of pyrite (iron disulphide) following its application to low sulphate soils

Abstract

The present research was conducted to determine the function of biological oxidation in the production of sulphate sulphur when pyrite (FeS2) was applied to sulphur deficient soils. Before incubating under controlled conditions, the soil-pyrite mixtures were either not sterilized or were treated with a range of microbial inhibitors, a biocide 8.0 Mrad of gamma irradiation or autoclaving twice. Samples of pyrite were found to be free of other detectable sulphur substances and to contain close to 50% disulphide sulphur. They were characterized for particle size distribution and surface area g-1. The seven soils used ranged in pH from 5.1 to 8.8 and with one exception contained ≤ 5.2 µg sulphate sulphur g-1 soil. A water potential of the soil-pyrite mixtures of -9.83 J kg-1 (field capacity), was found to provide oxidizing redox potentials, and was used largely. Pyrite flotation concentrates (large particle size) were used chiefly. Aerobic incubation of the soil-pyrite mixtures was accomplished in a double container system, which was modified in a way which was found to maintain sterility throughout the incubation period and decrease water losses. An indirect atomic absorption method was established, for determining sulphate sulphur in the monocalcium phosphate solution extracts of incubated soils plus pyrite. Subsequently a faster AutoAnalyzer method was used. Gamma irradiating or autoclaving soils produced substances which suppressed barium sulphate precipitation. This was overcome by treating the extracts with carbon.The rates of oxidation to sulphate of pyrite, both in the absence and presence of soil, were lower at drier water potentials than -9.83 J kg-1. Mount Morgan pyrite oxidized faster than Mount Lyell pyrite of the same surface area g-l in an alkaline black earth, but in an acid sand there was no difference in their rates of oxidation.Pyrite flotation concentrates oxidized to sulphate a mean of 2.5 times faster in the presence of six soils tested, than in the absence of soil, each at a water potential of -9.83 J kg-1. The biological and nonbiological nature of the responsible soil factors was subsequently investigated.The application of medium and high concentrations of formaldehyde and mercuric chloride separately, appeared to show that oxidation of pyrite in six soils tested was exclusively biological. However, it would be expected that some nonbiological oxidation observed in pyrite only, would continue when it was in the presence of soils. The possibilities are discussed that formaldehyde and mercuric chloride might have reacted with sulphite and sulphide ions respectively, preventing the formation of sulphate.In an alkaline black earth, application of a high concentration of sodium azide in the absence and presence of gamma irradiation, showed 88% and 95% respectively, of the total oxidation of pyrite as biological. Comparable figures for N-ethylmaleimide were 57% and 63% respectively. Possible nonbiological interference by these two inhibitors is discussed. Application of a high concentration of iodoacetic acid in the absence of gamma irradiation showed 54% biological oxidation.Gamma irradiating the soil and pyrite separately fol lowed by aseptic mixing, demonstrated biological activity which oxidizes pyrite in the two soils tested, but not in the pyrite. Adding a non-sterile extract of one of these soils, an alkaline black earth, to the same soil and pyrite which had been previously gamma-irradiated, increased the production of sulphate sulphur to the level for non-irradiated soil and pyrite.In each of the seven soils, results of gamma irradiating after the soil, pyrite and water were added together, showed proportions of biological oxidation of pyrite ranging from 42% in a basalt soil to 83% in a podzol loam (mean 57%). The results of autoclaving for the same seven soils, also after the constituents were added together, showed proportions of biological oxidation of pyrite ranging from 47% in an alkaline black earth to 87% in the podzol loam (mean 69%). Results from the two physical methods of sterilization are thought more reliable than those from inhibitor studies, because with the former no chemicals are added which could interfere nonbiologically in the oxidation of pyrite. Results from the two physical methods of sterilization provide good evidence that pyrite applied to soils in laboratory incubations is oxidized biologically, by activity contained in the soils only, to the substantial extents of 40 - 90%, depending on the soil. Similar proportions of biological oxidation may occur when pyrite is applied in the field.The microbial activity which oxidized pyrite was resistant to very alkaline soil pH, and to air dry storage of soil for at least four years. Differences existed between soils in their comparative capacities to oxidize pyrite and elemental sulphur, which suggests that differing microbial activity oxidizes these two substances.