PYRITE BIOMINERALIZATION AND ARSENIC SEQUESTRATION AT A FLORIDA INDUSTRIAL SITE: IMAGING AND GEOCHEMICAL ANALYSIS

A year-long field scale bioremediation experiment was conducted at a Florida industrial site, where groundwater in an unconfined aquifer was contaminated by arsenic-based herbicide. The bioremediation technique stimulated the indigenous sulfate-reducing bacteria (SRB) with a nutrient-rich slurry solution containing labile organic carbon, ferrous iron, sulfate, and fertilizer. This amendment induced sulfate-reducing conditions and caused the co-precipitation and adsorption of the dissolved arsenic in biogenic pyrite. This research characterized the biogenic pyrite formed and assessed the spatial and temporal changes in groundwater chemistry during the project. Pyrite was characterized using multiple techniques including X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and electron microprobe analysis. These analyses confirmed the rapid formation of pyrite one week after the injection. The pyrite formed either as well-defined octahedral nano-crystals or as spherical aggregates (framboids) 1-50 µm in diameter. The electron microprobe analysis determined that the pyrite contained between 0.05-0.4 weight % of sequestered arsenic. The dissolved arsenic concentration in the water decreased from pre-injection levels of 300-500 ppb to below the site regulatory limit of 50 ppb (> 90% removal rate) during a six month period. The reactive transport of the injectant plume was investigated using a conservative chloride tracer and biomineralization of pyrite along two flow transects. The results show that the stimulated pyrite biomineralization accounted for more than 80% of overall arsenic removal and dilution caused less than 20% of concentration reduction. Saturation index calculations show that arsenian pyrite quickly became oversaturated in targeted wells in one week and then remained saturated during the one-year monitoring period, suggesting that the arsenic sequestration was effectively maintained by the stability of arsenian pyrite. This research presents data showing through the amendment of a nutrient-rich solution, indigenous SBR can effectively sequester arsenic into pyrite at levels great enough to bring dissolved arsenic concentrations below the regulatory or even drinking water standards.