Microbial sulfidogenesis of arsenic in naturally contaminated wetland soil

Abstract

Microbial sulfidogenesis plays a potentially important role in As biogeochemistry within wetland soils, sediments, and aquifers. This study investigated the effects of microbial sulfidogenesis on As mineralogy in the As-enriched wetland soil found at the natural geochemical anomaly of the Smolotely-Líšnice (Czech Republic) historical gold district. The distribution and speciation of As as a function of soil depth, and the metabolic properties of microbial communities in different sulfidogenic domains were examined by bulk soil as well as pore water analyses, selective chemical extractions, S isotopes, and DNA extractions. Total solid-phase analyses and selective extractions of the soil samples below ∼40 cm showed that As (up to 1.16 g kg−1) and Fe(II) are coupled to S and TOC, and had accumulated to a considerable extent in the exterior parts of NOM fragments (up to 19 wt.% As). Microscale imaging and Raman spectroscopy revealed that As speciation in the NOM exteriors is a combination of realgar (α-As4S4), bonazziite (β-As4S4) and arsenian Fe sulfides, primarily greigite (Fe3S4) and framboidal pyrite (FeS2). The sulfide phases were depleted in the 34S isotope by 6.3–29.4‰ relative to pore water SO42−; thus implying their biologically induced formation. Microbial communities associated with sulfidogenic environments in NOM and bulk soil had variable compositions, although the dissimilative SO42− reduction was usually the main metabolic trait. Relatively low isotopic fractionation in sulfide-rich NOM fragments (6.3–11.6‰) compared to bulk soil (down to −26.1‰) revealed a reservoir effect that developed probably at mm-scales. This indicates formation of sulfide phases in highly localized environments depleted in aqueous SO42− due to strong microbial sulfidogenesis when compared to the transfer rate of the solutes. The very high proportion of fermenting microorganisms in sulfide-rich NOM fragments provided further evidence of strongly reducing conditions, which are a prerequisite for sulfide phase precipitation. We have shown that by the development of suitable conditions for sulfidogenesis, NOM fragments play an active role in As immobilization in an As-enriched wetland soil. Regarding the effectiveness of As sulfidogenesis in shallow wetland soils to remediate groundwater, our findings imply that As contamination may either be limited by the low content of labile organic matter or by the fast transfer of solutes (groundwater flow).