Anaerobic Biostimulation for the In Situ Precipitation and Long-Term Sequestration of Metal Sulfides

Abstract : The precipitation of dissolved metals in groundwater under anaerobic conditions has been the subject of numerous studies, but there has been very limited research on the mineralogy and the stability of these precipitates under variable geochemical conditions. Examining the mineralogy and establishing the rate of dissolution of precipitates under aerobic conditions is the subject of this SERDP supported research. A field research site was established at Site No. ST-65, a former refueling area at the Avon Park Air Force Range in Florida. Site ST-65 had high concentrations of arsenic (As) in shallow groundwater within a sandy, naturally anaerobic aquifer. Sediment and groundwater were collected from the area with the highest concentration of As in groundwater (1800 parts per billion [ppb]) and were used to construct four flow-through columns for a bench-scale laboratory study conducted at Princeton University. In these experiments, indigenous sulfate reducing bacteria were stimulated with injections of sodium lactate, ethanol, ferrous iron and sulfate over a period of several months under strictly anaerobic conditions. Analysis of the sediments from one column indicated the presence of As-bearing sulfides, including arsenopyrite and realgar, at ten times the concentration of the natural sediments. Aerobic water was then passed through one of these columns for 115 days. Analyses of the dissolved As concentrations over this time when compared to the total As in the analyzed sediments suggested that only 2% of the sequestered As was dissolved after 116 pore volumes of aerobic groundwater had passed through the column. Analyses of the As concentration of sediments in 15 mL column experiments leached by the same water over the same time period, which was equivalent to 11,600 pore volumes, yielded an As loss rate which agreed within error with the results of the larger column experiment.