Kinetics, surface chemistry, and structural evolution of microbially mediated sulfide mineral dissolution

The effects of different microbial populations on the oxidative dissolution of sulfide minerals at 37 degreesC 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. ca ldus. F. acidarmanus, and Leptospirillum ferrooxidans dans. Changes in chem ical speciation of the mineral surface products were monitored by Raman spe ctroscopy over the course of the experiment. structural evolution was exami ned 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 obse rved only in the presence of iron-oxidizing microorganisms (i.e., F. acidar manus or the enrichment culture). The chemical speciation at the mineral su rface 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 indi cate 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 i s surprising that removal of the elemental sulfur from the mineral surface by the sulfur-oxidizing species is nor accompanied by an increase in the di ssolution rate of the mineral. This finding suggests that although elementa l sulfur forms on the surface during oxidative dissolution, it does not pas sivate the mineral surface. Copyright (C) 2001 Elsevier Science Ltd.