Impacts of enhanced microbial-photoreductive and suppressed dark microbial reductive dissolution on the mobility of As and Fe in flooded tailing soils with zinc sulfide

Semiconducting minerals are ubiquitous in soil and their association with microbes often affects the environment. In this study, the impacts of zinc sulfide (ZnS, used as a model compound for semiconducting sphalerite) on As/Fe mobility of flooded tailing soils under dark and intermittent illumination conditions were elucidated for the first time. Microbial reductive dissolution of As(V) and Fe(III) was more pronounced under intermittent illumination than under dark conditions. In ZnS-amended soils, release of As(III) and Fe(II) was 1.3 and 1.7 times higher, respectively, under intermittent illumination than the highest concentrations released from soils amended with acetate alone under dark conditions (12741.1 ± 714.3 μg/L and 37.9 ± 2.3 mg/L, respectively). However, under dark conditions in ZnS-amended soil, the release of As(III) and Fe(II) was 0.8 and 0.7 times that of the highest concentrations released from soils amended with acetate under dark conditions, respectively. Treatment with ZnS potentially decreased the abundance of several metal-reducing bacteria (e.g., Bacillus, Geobacter, Clostridium, and Desulfitobacterium), which resulted in lower As/Fe reduction than for the acetate alone treatment under dark conditions. However, under intermittent illumination, the excited mineral photoholes were scavenged by humic/fulvic acids, and photoelectrons were synchronously separated and participated in the microbial electron chain. The fortification provided by the photoelectrons subsequently boosted As/Fe reduction, even though there was a lower abundance of metal-reducing bacteria. Hence, this study provides an in-depth understanding of the impacts of semiconducting minerals on the fates of metal pollutants, microbial diversity, and the bioavailability of dissolved organic matter in flooded soils.