Pyrite Biomineralization and Arsenic Sequestration at a Florida Industrial Site: Imaging and Geochemical Analysis

Abstract

A yearlong field-scale bioremediation experiment was conducted at a Florida industrial site, where groundwater in an unconfined aquifer was contaminated by an arsenic-based herbicide. The bioremediation technique stimulated the indigenous sulfate reducing bacteria (SRB) with a nutrient-rich slurry solution containing labile organic carbon, ferrous iron, sulfate, and fertilizer. This amendment induced sulfate reducing conditions and caused the coprecipitation and adsorption of the dissolved arsenic in biogenic pyrite. This research characterized the biogenic pyrite formed and assessed the spatial and temporal changes in groundwater chemistry during the project. Pyrite was characterized using multiple techniques including X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and electron microprobe analysis. These analyses confirmed the rapid formation of pyrite one week after the injection. The pyrite formed either as well-defined euhedral nano-crystals or as spherical aggregates (framboids) 1-50 µm in diameter. The electron microprobe analysis determined that the pyrite contained between 0.05-0.4 weight % of sequestered arsenic. The dissolved arsenic concentration in the water decreased from pre-injection levels of 300-500 ppb to below the site regulatory limit of 50 ppb (> 90% removal rate) during the initial six month period. The reactive transport of the injectant plume was investigated using a conservative chloride tracer and biomineralization of pyrite along two flow transects. The results show that the stimulated pyrite biomineralization accounted for more than 80% of overall arsenic removal and dilution caused less than 20% of concentration reduction. Saturation index calculations show that arsenian-pyrite iii quickly became oversaturated in targeted wells one week after injection of the solution and then remained mostly saturated during the one-year monitoring period, suggesting that the arsenic sequestration was effectively maintained by the stability of arsenian-pyrite. This research presents data showing through the amendment of a nutrient-rich solution, indigenous SBR can effectively sequester arsenic into pyrite at levels great enough to bring dissolved arsenic concentrations below the regulatory or perhaps even drinking water standards. Grants obtained from the National Science Foundation were integral to the execution of this research. I would like to thank the following people who generously gave their time, advice, and criticisms: Dr. Ming-Kuo Lee, Dr. James Saunders, Dr. Ashraf Uddin, and the entire faculty at the Auburn Department of Geosciences. A special thanks to the chair of my advising committee, Dr. Lee, who provided me with invaluable guidance and mentorship during my time at Auburn. He always had an open door and was more than willing to help with any problem or question I encountered. A thanks is also owed to Dr. Saunders for his guidance and hand in recruiting me to Auburn University. Another member of the Geosciences Department that was vital to my research was Dr. Bil...

Long-Term Arsenic Sequestration in Biogenic Pyrite from Contaminated Groundwater: Insights from Field and Laboratory Studies

“…Notably, the specific framboidal morphology of the arsenian pyrite observed in this study has been synthesized by Gartman and Luther [45] using ferrous iron and hydrogen sulfide. Their laboratory reaction engendered reducing conditions and the precipitation of arsenian pyrite framboids in less than 12 h, aligning with this study's rapid precipitation of arsenian pyrite after exposure to the injectate [29,45]. More work is needed to determine the exact redox and biological conditions for these specific morphological changes.…”

“…The scanning electron microscope (SEM) imaged gold-coated stubs and thin sections from injection well I-2 one week, two weeks, one month, three months, six months, and nine months following the injection. The pyrite grains throughout the experiment exhibited two morphologies: euhedral crystals 1-10 μm across and spherical aggregates of nanocrystalline pyrite (framboids) 5-30 μm in diameter (Figure 3a, Figures S5 and S6) [11,29]. Elemental mapping of these grains corroborates the EMP findings of the arsenian pyrite's heterogeneity (Figure 3b-d, Figures S7 and S8).…”

“…This study builds upon previous injection strategies at the same study site by Lee et al [11] and Wilson [29], who used two injection wells (located near the center of the contaminant plume) and eight monitoring wells (three of which were upgradient of the injection wells) to promote SRB-arsenic remediation. These previous studies were proof of concept, small field-scale investigations to determine whether the injection would have any success in this aquifer.…”

“…The solids were analyzed over 5-75 theta for an hour and for one second at each point using the DIFFRAC.EVA software (version 4.2.2.3). The software translates the spectra peaks to d-spacings, which can be matched to specific minerals in the database, allowing the study to identify the minerals that precipitated from SRB bioremediation [29].…”

“…where C pre-injection is the chloride concentration in the groundwater prior to the injection, C injectate is the concentration in the injection mixture (240 mg/L), and C mixed is the concentration during a particular sampling period, likely due to the mixing of the injectate and the groundwater [11,29]. In the two representative monitoring wells of M-1 and LH-2, the arsenic removal fraction plateaus to >0.80 and >0.90, respectively, after two months and continues to moderately increase over the remaining months.…”