Assessment of Two Field-Scale Sulfate Reducing Bioreactors Using Sulfur Isotopes

Abstract

Sulfate-reducing bioreactors (SRBRs) have shown promise as a costeffective option in the passive remediation of acid mine drainage (AMD). While these systems do provide the necessary conditions for increased bacterial activity, little is known about the internal dynamics and the functional lifespan of the systems in field settings. To help address these issues, two field-scale bioreactors are being monitored using an array of sampling ports distributed at varying depths throughout the treatment cells. These internal monitoring ports are located in such a way as to observe 3-D trends in activity occurring within the system. Water samples collected from the ports, as well as samples from the AMD inflow and outflow, have been analyzed for δ 34 S of sulfate as well as standard chemical parameters. Preliminary results indicate that in both systems, bacterial sulfate reduction is occurring yet the degree of reduction is not uniform throughout the cells. Within each system, areas where only a limited amount of bacterial sulfate reduction has occurred are characterized by high concentrations of sulfate coupled with δ 34 S values only slightly different than the influent AMD. In contrast, low sulfate concentrations together with large δ 34 S fractionations are found in areas where extensive bacterial sulfate reduction has taken place. The observed range in fractionation values likely reflects the development of preferential flow paths and points of stagnation within the systems. This implies that not all of the reactive substrate put into a cell will contribute to AMD treatment. The results of this study provide information not typically attainable in smaller laboratory-scale studies and point to the need for further engineering of SRBRs to optimize fieldscale applications.

Sulfur Isotope Fractionation as an Indicator of Biogeochemical Processes in an AMD Passive Bioremediation System

“…Optimal conditions for sulfate reducing microorganisms comprise anaerobic, alkaline settings with moderate salt and metal content as well as the availability of suitable electron donors, which may be either simple organic compounds or molecular hydrogen (H2) [30][31][32]. Due to high spatial and temporal variability of environmental conditions in the bioreactor and wetland, zones of optimal BSR are therefore created, with their location and extent controlled by the distribution and nature of the microbial communities, availability of organic carbon, biochemical gradients, and overall local environmental conditions [15,16]. BSR-produced sulfide can then react with aqueous Fe(II) and/or with Fe(III)-bearing minerals and precipitate iron sulfides [5].…”

“…In the Tab-Simco bioreactor, biologically-mediated S cycling is expected to produce a diverse range of reactive S-species with different oxidation states ( Figure 2b) and also generate distinct and often large S isotope fractionations between various reactants and products [15,16,20]. For example, the microbially-mediated dissimilatory SO 4 2− reduction to H 2 S produces characteristic depleted …”

“…Overall, the magnitude of 34 S enrichment of the effluent SO 4 2− is an indication of the presence of bacterial sulfate reduction as the AMD was treated in the Tab-Simco bioreactor [15,16].…”

“…H 2 S [5,[16][17][18]. BSR activity (2CH 2 O + SO 4 2− → 2HCO 3¯+ H 2 S) is desirable for mitigating AMD because it removes SO 4 2− , raises the pH through bicarbonate alkalinity (HCO 3¯) production and yields H 2 S for precipitating sparingly soluble metal sulfides [17].…”

“…However, this approach cannot identify the degree of SO 4 2− removal by several competing processes that each can lead to a decrease in SO 4 2− concentration in the treated AMD effluent, including dilution due to mixing with low-SO 4 2− waters, precipitation of SO 4 2− -bearing nNP, and microbially-mediated SO 4 2− reduction [15,16]. Certainly, a better understanding of the BSR processes and the nature and distribution of biotic and abiotic pathways of S cycling and sequestration in a PBS could have major benefits on designing more effective bioremediation systems.…”