KINETICS, SURFACE CHEMISTRY, AND STRUCTURAL EVOLUTION OF MICROBIALLY MEDIATED SULFIDE MINERAL DISSOLUTION

Abstract

The effects of different microbial populations on the oxidative dissolution of sulfide minerals at 37°C and pH 1.5 were examined over a period of 22 days. Samples of pyrite, marcasite, and arsenopyrite were exposed to a sulfur-oxidizing isolate (Thiobacillus caldus), an iron-oxidizing isolate (Ferroplasma acidarmanus), and a mixed enrichment culture containing T. caldus, F. acidarmanus, and Leptospirillum ferrooxidans. Changes in chemical speciation of the mineral surface products were monitored by Raman spectroscopy over the course of the experiment, structural evolution was examined with scanning electron microscopy, and the total soluble iron was used as a measure of the dissolution rate.In the case of all three minerals, an increase in dissolution rate was observed only in the presence of iron-oxidizing microorganisms (i.e., F. acidarmanus or the enrichment culture). The chemical speciation at the mineral surface in the presence of these iron-oxidizing species is indistinguishable from that of abiotic control reactions under the same conditions; both are dominated by elemental sulfur. In contrast, experiments with T. caldus indicate that the quantity of elemental sulfur on the mineral surface is <1% of the amount observed on samples exposed to the F. acidarmanus culture. It is surprising that removal of the elemental sulfur from the mineral surface by the sulfur-oxidizing species is not accompanied by an increase in the dissolution rate of the mineral. This finding suggests that although elemental sulfur forms on the surface during oxidative dissolution, it does not passivate the mineral surface.