ARSENIC SEQUESTRATION BY NATURAL BACTERIAL SULFATE REDUCTION: A FIELD-SCALE BIOSTIMULATION EXPERIMENT

Arsenic contaminated groundwater of both natural and anthropogenic origin is a concern worldwide. Previous studies have shown that indigenous bacteria are capable of controlling arsenic mobilization within aquifers, and that arsenic can be sequestered by iron sulfide solid phases via co-precipitation and sorption under sulfate reducing conditions. Manipulating the activity of sulfate reducing bacteria to facilitate formation of biogenic iron sulfides in the subsurface has been proposed as an in-situ approach to arsenic groundwater remediation.

A field-scale experiment is in progress at a contaminated industrial site in northwest Florida, where bacterial sulfate reduction has been stimulated by injection of labile organic carbon, ferrous sulfate, and fertilizer into the aquifer. ICP-MS analysis of groundwater shows arsenic levels falling by as much as 87% over a four month period following the onset of sulfate reduction. Black sludge-like biogenic precipitates are sampled from the bottoms of injection and adjacent monitoring wells starting one week after injection. Samples were stored frozen and dried via vacuum desiccation. X-ray powder diffraction spectra were obtained with a Bruker D2 PHASER, and X-ray fluorescence spectra with a Bruker Tracer IV-SD. All XRF spectra exhibit prominent Fe, As, and S K_α emission. XRD spectra of solids show patterns most closely matching pyrite and arsenian pyrite (FeAs_0.026S_1.974) as early as a week after injection. Solids from one well display a characteristic amorphous arsenian pyrite XRD signature after 7 days, and well defined crystalline peaks within 14 days. Polished sections of solids collected at two months were prepared and found to contain abundant pyrite framboids 2-20 µm dia.

Our research shows that immobilization of arsenic by Fe sulfides can be effected in-situ by stimulating bacterial sulfate reduction, and that these phases are crystalline and stabilized shortly after precipitation. Iron sulfide solids will be analyzed via ICP-MS to confirm and quantify their arsenic sequestration capacity, SEM-EDAX and XANES employed to investigate the nature of arsenic incorporation and its speciation in biogenic pyrite. The experiment will continue to evaluate long-term stability and persistence of arsenic sorption and co-precipitation post sulfate reduction.