Bioremediation of Arsenic Contaminated Groundwater in Northwest Florida: Mineralogy, Geochemistry, and Microbiology Changes

Abstract

This research demonstrates that the stimulation of natural sulfate-reducing bacteria (SRB) in groundwater can precipitate biogenic pyrite nanoparticles that play an active role in sequestering dissolved arsenic from contaminated groundwater. This process is executed through the sorption and co-precipitation of arsenic onto biogenic pyrites. This pilot experiment is one of the first applications of this bioremediation technology at the field scale. At an industrial site where the groundwater within a shallow unconfined aquifer is contaminated by arsenic, a nutrient solution was injected to stimulate the activity and metabolism of SRB. The injection of biodegradable organic carbon, ferrous iron, sulfate, and fertilizers successfully stimulated SRB metabolism one week after the initial injection. The microscopic, X-ray, and electron microprobe analyses have confirmed the bio-mineralization of pyrite. Over time, these pyrite nanoparticles grew to form 1 to 10 µm in diameter, and formed euhedral crystals and spherical aggregates that contain 0.05 to 0.4 wt. % arsenic. The data suggest that the biogenic pyrite formed at the site has an excellent capacity to sequester arsenic to their iron sulfide solids. Arsenic sequestration in pyrite through adsorption and co-precipitation led to dissolved arsenic concentrations decreasing from initial concentrations of 0.3-0.5 mg/L to below the regulatory clean-up standard for the site of 0.05 mg/L just weeks after the injection. This study also demonstrated that the iron sulfide biominerals remain stable in the aquifer even after organic carbon from the injection is exhausted during fluctuations of redox conditions. The ongoing stability of the biominerals allowed for sequestration of arsenic to continue for months after their main growth stage. The results of this experiment have implications for the future remediation of both natural and industrial sites where high arsenic levels contaminate local water supplies. In addition to the potential of this remediation tool, it is also possible that the application of this technique can be down-sized and optimized to inexpensively treat contaminated aquifers in which groundwater is withdrawn from individual wells for drinking, cooking, and irrigation uses, , which remains a concerning human health problem throughout the world today.