Jarosite is an iron-hydroxysulfate mineral commonly found in acid mine drainage (AMD). For its strong adsorption and co-precipitation of heavy metals, jarosite is considered a potent scavenger for the contaminant in AMD contaminated-environment. Sulfate reducing bacteria (SRB) play an important role in the reductive dissolution of jarosite; however, the mechanism involved is yet to be elucidated. In this study, an indigenous SRB community enriched from Dabaoshan mine area was employed, and a series of batch experiments were performed to explore the mechanism for jarosite reduction by SRB. During which, different cultures, with and without dissolved sulfate; physical separation jarosite from the bacteria in culture by dialysis bag or not, were considered. Results indicated that the reduction of jarosite by SRB was via an indirect mechanism. In systems with dissolved sulfate, lactate was incompletely oxidized to acetate coupled with the reduction of SO₄^2- to S^2-, which subsequently reduce the Fe³⁺ bound in jarosite chemically, forming secondary minerals including vivianite, mackinawite and pyrite. In systems without dissolved sulfate, jarosite dissolution occurred prior to reduction, and similar secondary minerals formed as well. The extracellular polymeric substance secreted from SRB was demonstrated to be capable of facilitating the release of sulfate from the jarosite mineral to the solution. Structural sulfate in the solid phase may not be available for SRB respiration. Direct contact between SRB and jarosite is not necessary for mineral reduction to occur, though; wrapping jarosite into dialysis bag suppressed the reduction to a certain extend. Therefore, the mechanism of jarosite reduction by SRB is proposed as three key steps: (1) the microbial reduction of dissolved SO₄^2- to S^2- by SRB; (2) the chemical reduction of jarosite by S^2-; and (3) the formation of secondary precipitation of mackinawite, pyrite or vivianite.

This study was supported by the National Natural Science Foundation of China (Nos. 41330639, 41720104004).