Biogeochemistry of sulfate reducing bioreactors: how design parameters influence microbial consortia and metal precipitation

Abstract

The release of mining influenced waters (MIW) and associated mitigation is a global economic liability in excess of a hundred billion dollars worldwide. The treatment of MIW is necessary to protect ecological and human health and well being. To that end, passive sulfate reducing bioreactors (SRBRs) provide a viable, cost-effective treatment option for nonoperational and remote sites. Despite implementation over the past two decades, these systems are typically designed and operated through best practice assumptions with limited fundamental understanding of “black box” microbial and geochemical mechanisms driving the desired metal immobilization. The research presented investigates geomicrobial interactions in lignocellulose-based SRBRs with a focus on the influence of organic substrate, inoculum, and resulting microbial ecological selection on metal immobilization. Fundamental processes were assessed at both the laboratory and pilot scales using regionally acquired zinc-rich MIW to better understand the complex feedback between geochemical and syntrophically-driven biological processes. The constructs developed at these scales were then applied to samples derived from a field-scale system that had been in operation for more than a decade to gain further insights into metal immobilization and ecological processes at environmentally relevant scales. Ecological and geochemical results presented herein at laboratory, pilot, and field scales contribute to the established body of research on SRBRs by providing unprecedented resolution of microbially and geochemically-mediated metal precipitation mechanisms. Collectively, these results demonstrate that labile reactive substrates high in alfalfa content impart a selective bias for bacterial communities resulting in higher biogenic sulfide evolution and enhanced zinc removal. In contrast, recalcitrant substrates dominated by woody plant debris selected less effective microbial communities with respect to metal immobilization. Environmental perturbations did not affect the community structure in these columns as they were stable both over time and with depth. Despite this association with substrate, spatial and temporal shifts in metal precipitation regimes within the characterized columns were largely dictated by inorganic ligand availability rather than microbial profiles. Synchrotron analyses assessed biotic and abiotic effects of inoculation treatments on precipitate form and stability revealing that live inocula resulted in higher proportions of crystalline sphalerite and amorphous zinc-sulfide that could be attributed to microbial activity. Insights from these laboratory and pilot-scale systems were then applied to understanding a mature, field-scale reactor. Differential abundance determined a region identified as having both a distinct, SRB enriched community and increased sulfide-bound metal that was distinct from remaining zones which in contrast were enriched with respect to methanogenic archaea. This highlights how horizontally heterogeneous flow regimes can influence and should be considered during the design of larger-scale systems as well as design considerations to encompass abiotic versus biotic immobilization mechanisms. Collectively, this research has implications for SRBR design with respect to organic substrate selection, inoculation, and ligand distribution. Specifically these include the deployment of substrates that are a combination of labile and recalcitrant carbon, inocula has abiotic contribution to metal removal, and that inorganic ligand distribution determines where metals precipitate. These results also highlight the importance of further inquiry into the quality of organic carbon released from SRBRs, -omics approaches to delineate processes responsible for performance, as well as the importance of even MIW distribution in field SRBRs.