Field and laboratory investigations of groundwater arsenic sequestration in biogenic pyrite at an industrial site in Florida

Abstract

Pumping groundwater from arsenic-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 (As) 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 a new injection scheme to sequester As for nine months: A ferrous sulfate and molasses mixture was injected into eleven wells located hydrologically upgradient of the plume, allowing the injectate to travel downgradient for full-scale remediation. The injectate was designed to stimulate SRB reduction and engender the precipitation of arsenian pyrite. From June 2018 to March 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 (EMP) measurements, scanning electron microscope (SEM) imaging, and geochemical calculations of arsenic speciation and mineral saturation, have determined that As has been sequestered mainly in the form of arsenian pyrite. These analyses demonstrate the rapid precipitation of biogenic pyrite two weeks after the injection, with the pyrite forming as euhedral crystals and spherical aggregates (framboids) 1-30 μm in diameter. EMP results indicate the sequestration of 0.03-0.89 wt% As in the precipitated pyrite. Additionally, geospatial and time series data from GIS maps demonstrate that As concentrations have decreased throughout the site over nine 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 engendered and stabilized the formation of arsenian pyrite throughout the nine months. Moreover, the results confirm that >80% of the arsenic was removed by SRB reduction and arsenian pyrite sequestration, with <12% removed by dilution induced by injection. 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.