GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 248-5
Presentation Time: 9:00 AM-6:30 PM


MILLER, Brian1, LEVITT, Eric2, SAFFARI GHANDEHARI, Shahrzad2, LEE, Ming-Kuo2, SAUNDERS, James A.2, UDDIN, Ashraf2, BILLOR, M. Zeki3, REDWINE, Jim4, MARKS, Justin4 and WANG, Luxin2, (1)Geosciences, Auburn University, Auburn, AL 36849; Department of Geosciences, Auburn University, 210 Petrie Hall, Auburn, AL 36849, (2)Department of Geosciences, Auburn University, 210 Petrie Hall, Auburn, AL 36849, (3)Geology Department, Auburn University, Auburn, AL 36849, (4)Anchor QEA, Pensacola, FL 36830,

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 Kα emission. XRD spectra of solids show patterns most closely matching pyrite and arsenian pyrite (FeAs0.026S1.974) 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.