Lee, M.-K., J.A. Saunders, T. Wilson, E. Levitt, S.S. Ghandehari, P. Dhakal, J. Redwine, et al.
Bioremediation 23(1):1-21(2019)

Biogenic pyrite formed by stimulating indigenous sulfate-reducing bacteria (SRB) in a natural aquifer removed dissolved arsenic from contaminated under strongly reducing conditions. Biodegradable organic carbon, ferrous iron, sulfate, and fertilizer were injected into groundwater to stimulate SRB metabolism, which began ~1 week later. Microscopic, X-ray diffraction, X-ray fluorescence, and electron microprobe analyses confirm the bio-mineralization of pyrite and over time, pyrite nanoparticles grew to form well-formed crystals (1-10 µm in diameter) or spherical aggregates that contain 0.05-0.4 wt.% arsenic. Dissolved arsenic decreased from 0.3-0.5 mg/L to <0.05 mg/L in three downgradient wells weeks after injection. At its peak, total arsenic removal rates reached >90% and lasted for 6 months until the upgradient groundwater mixed with the aquifer. Groundwater with the most active bacterial sulfate reduction became enriched in 34S (2.02-4.00‰) compared to unaffected well water (0.40-0.61‰). One to three orders of magnitude increases in SRB cells were observed in treated wells for at least 2 months after injection. For full-scale remediation, the injection of the solution should start at positions hydrologically upgradient from the major plume and proceed downgradient. If needed, aquifers may be repeatedly amended with biodegradable organic carbon to reestablish the reducing conditions that favor arsenic sequestration.