The influence of pyrite grain size on the final oxygen isotope difference between sulphate and water in aerobic pyrite oxidation experiments†

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Abstract
Oxidation experiments with different pyrite grain sizes (63–100, 100–140, 140–180 μm) were carried out to investigate the oxygen and sulphur isotope composition of sulphate produced under aerobic acid conditions, which may help to understand oxidation mechanisms and to interpret data from natural sites. Long-term experiments with grain size 63–100 μm showed that constant δ 18OSO4 values were not achieved before 100 days. The final oxygen isotope difference between water and sulphate indicates that a small proportion of molecular oxygen is incorporated into sulphate even in the later course of the oxidation due to sulphite oxidation by molecular oxygen. However, most of the sulphate oxygen derives from water. Similar δ 18OSO4 values from experiments with grain sizes 63–100, 100–140, and 140–180 μ m indicate similar oxidation mechanisms for all three grain sizes. These results differed from previous results of identical experiments with grain size18OSO4 values were obtained. We propose that the greater proportion of molecular oxygen in sulphate from oxidised fine-grained pyrite is caused by an intensified adsorption of molecular oxygen on sulphur sites of ultrafine pyrite particles. Hence, the formation of sulphate from the (initial) reaction on sulphur sites of pyrite and from sulphite oxidation should be more dominant if ultrafine material is present. The δ 34SSO4 values (2.0–2.7) obtained from experiments with the coarser grain sizes agreed with the δ 34S value of pyrite (2.4), whereas sulphur isotopes of sulphate obtained from previous experiments with fine-grained pyrite showed an initial 32S enrichment compared with pyrite. Due to the lack of δ 34SSO4 values from the beginning of the experiments with coarser grain sizes, it remains speculative that sulphur isotopes indicate at least initial differences in oxidation mechanisms between fine and coarser pyrite grain sizes.

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