FIELD AND LABORATORY INVESTIGATIONS OF GROUNDWATER ARSENIC SEQUESTRATION IN BIOGENIC PYRITE AT AN INDUSTRIAL SITE IN FLORIDA

Pumping groundwater from arsenic (As)-contaminated aquifers has exposed millions of people around the world to high and potentially deadly doses of the contaminant. Moreover, developing countries lack the resources and technology to remove this hazardous substance. Previous studies have investigated cost-effective techniques to remove groundwater arsenic by stimulating sulfate-reducing bacteria (SRB) to form As-sequestering pyrite. This study intends to improve upon these past methods to demonstrate the effectiveness of SRB As remediation at an industrial site in Florida.

The study has devised an injection scheme to sequester As for one year: A ferrous sulfate and molasses mixture that will stimulate SRB was injected into eleven wells located hydrologically upgradient of the plume so that the injectate travels downgradient for full-scale remediation. From June 2018 to June 2019, groundwater samples and newly precipitated solids were collected weekly for the first four weeks, monthly for the first three months, and quarterly for the remaining months. X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses, electron microprobe analysis, scanning electron microscope (SEM) imaging, along with geochemical calculations of arsenic speciation and mineral saturation state, have determined that As has been sequestered in the form of arsenian pyrite. Sulfur isotope compositions of dissolved sulfate were measured to fingerprint SRB activity in treated groundwater. Additionally, geospatial and time series data demonstrate that As concentrations have decreased throughout the site over the twelve months.

From these analyses, the study has reduced As concentrations to below the site’s regulatory standard of 0.05 mg/L, as the SRB-stimulating injection engenders the formation of arsenian pyrite. Given these promising results, this study is within reach to optimize an affordable As-remediating procedure for industry and developing communities to reduce As to safe standards.