Fate of Metals in Microbially-Mediated Pyrite Oxidation in a carbonate-buffered sandstone aquifer in Wisconsin, USA

Abstract:

Metal-sulfide mineral oxidation in circumneutral pH environments is a relevant groundwater quality concern for Wisconsin’s sandstone and carbonate Cambrian-Ordovician aquifer system. This aquifer system has been observed to host varying abundances of metal-sulfide minerals, such as pyrite, across Wisconsin, USA. When these sulfide-bearing geologic units are exposed to oxygenated groundwater or earth-surface conditions, they can oxidize and generate acidity, releasing metals into the aqueous environment.

Although pyrite dissolution in the context of acid mine drainage has been extensively studied, the role of bacteria in mediating pyrite oxidation at circumneutral pH is not well understood. It has recently been shown that chemolithotrophic bacteria can accelerate neutral-pH, aerobic oxidation of both synthetic and specimen pyrite up to 10-fold relative to abiotic controls. However, to our knowledge, no previous studies have documented the influence of biological activity on pyrite oxidation with native groundwater, subsurface materials, and bacterium.

We carried out microcosm experiments with native geologic material of a glauconitic and dolomitic sandstone, and a quartz sandstone from WI, each containing about 3% by weight or less of pyrite, and native groundwater from those geologic units as the aqueous medium and the bacterial inoculum. Chemolitho- and electroautotrphic bacteria (>95% 16S rRNA identity) were enriched in the quartz sandstone microcosms, and enough pyrite was oxidized to overcome the buffering capacity of the groundwater. Negligible pyrite was oxidized in the abiotic controls, and that groundwater remained circumneutral. ICP-OES data and PHREEQC sorption model results will be presented addressing the difference of metal fate in the biotic vs abiotic microcosms and acidic vs circumneutral groundwater. These findings will shed light on the fate of metals from microbially-mediated sulfide oxidation in carbonate-buffered sandstone aquifers.