GEOCHEMISTRY OF GROUNDWATER AND NATURALLY OCCURRING PYRITE IN UPHAPEE CREEK, MACON CO., ALABAMA, AND A COMPARISON WITH BIOMINERALIZED PYRITE IN AN INDUSTRIAL SITE, FLORIDA

Naturally occurring pyrite has been developed at a natural site in Uphapee Creek, Macon County, Alabama. The EMP analysis showed that the pyrite grains contain 0.2-0.92 wt% arsenic (As) in it. The SEM analysis showed that naturally formed pyrite was developed in large (20-150 μm) euhedral shaped individual grains and aggregates. XRF analysis of the collected lignitized wood or organic material (collected from the wells during drilling) also showed the presence of a significant amount of As. An assumption was made that the groundwater is also contaminated with an elevated level of As. But from the ICP-MS analysis, detected As level was not high as expected and it was within the acceptable limit (<10 ppb). The groundwater was found to be Ca-SO₄ type. The water was not SO₄ limited, that can be a logical explanation for having a very low level (≈2 ppb) of As. Continuous sulfate reduction forming H₂S, and that reacting with the iron present in the water to form biogenic pyrite minerals. The groundwater was found to be moderately reducing (Eh= 46 to 173 mV), and nearly neutral (pH ≈ 6.5). Because of low As concentration in groundwater in this site no remediation technique was applied.

Florida industrial site located at Bay County, groundwater was severely contaminated with As and the concentration was 300-500 ppb. A bioremediation experiment was conducted to see the effectiveness of the technique. Indigenous sulfate-reducing bacteria (SRB) were stimulated using a nutrient-rich slurry solution containing organic carbon (molasses), ferrous sulfate, and fertilizer. The artificial introduction of strong sulfate-reducing condition caused the co-precipitation and adsorption of the dissolved As in biogenic pyrite. The analyses of the pyrite grains showed that it formed either as well-defined euhedral nano-crystals or as spherical aggregates (framboids) 1-50 μm in diameter, and contained between 0.05-0.4 wt % of sequestered As. The dissolved As concentration in the water decreased to less than 50 ppb that indicates at least 90% removal from initial concentration.