Long-term sequestration of nickel in mackinawite formed by Desulfovibrio capillatus upon Fe(III)-citrate reduction in the presence of thiosulfate

Abstract

In euxinic sediments, the reaction between iron and sulfides results in the formation of Fe(II)-sulfides, which are known to play a key role in trace metal sequestration. The present study investigates the sequestration of nickel during Fe(II)-sulfide formation mediated by the (thio)sulfate-reducing bacterium Desulfovibrio capillatus in the presence of soluble Fe(III)-citrate and thiosulfate as the terminal electron acceptor. XRD, HRTEM and Fe K-edge EXAFS data indicate that biogenic mackinawite (FeS) was the sole mineral formed in our experiments. These data also show that the kinetics of mackinawite crystal growth was significantly accelerated when nickel was present in the starting solution. In addition, the lack of detection of other Fe(II)-sulfides indicates inhibition of the mackinawite (FeS) to greigite (Fe3S4) and/or pyrite (FeS2) transformation that is likely related to (i) the efficiency of Desulfovibrio capillatus in reducing S(0) to H2S and (ii) the absence of O2 during the experiments. Finally, chemical analyses show that 98% of the nickel is associated with biogenic mackinawite and no release of nickel from mackinawite was observed after up to 10 months of incubation under anoxic conditions. This finding is consistent with Ni K-edge EXAFS data which show that Ni(II) substitutes for Fe(II) in the structure of biogenic mackinawite. This study shows that (thio)sulfate-reducing bacteria can efficiently promote the formation of mackinawite in euxinic sedimentary environments and that these Fe(II)-sulfides can act as efficient and long-term trapping minerals for nickel in such settings. Considering the capacity of mackinawite at incorporating other trace metals such as Mn, Co, Cu and Zn, this iron sulfide could also serve as a host for these elements as well. This study suggests that mackinawite likely plays a more important role in the biogeochemical cycles of Fe, S, and associated trace metals than considered up to now.