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The Impact of FeS Mineralogy on TCE Degradation
Citation:
He, Y. T., J. T. WILSON, AND R. T. WILKIN. The Impact of FeS Mineralogy on TCE Degradation. In Proceedings, 10th Annual International In Situ and On-Site Bioremediation Symposium, Baltimore, MD, May 05 - 09, 2009. Battelle Press, Columbus, OH, 25, (2009).
Impact/Purpose:
To determine if freeze drying can significantly alter original iron sulfide minerals, and also to determine if the mineral phase has an important impact on the reactivity to degrade TCE.
Description:
Iron- and sulfate-reducing conditions are often encountered in permeable reactive barrier (PRB) systems that are constructed to remove TCE from groundwater, which usually leads to the accumulation of FeS mineral phases in the matrix of the PRB. Poorly crystalline mackinawite (FeS) has been shown to play an important role in the removal of TCE in these engineered systems. In a column experiment simulating conditions in a PRB at Altus AFB in Oklahoma, USA, abiotic reaction of TCE with FeS could account for up to 50% of TCE removal in the PRB. However, the rate constant obtained in the column experiment with biogenic FeS was an order of magnitude more rapid than rates reported in the literature for TCE reaction with chemically synthesized FeS. The experiments reported in the literature were conducted with chemically synthesized FeS that had been freeze dried as part of its preparation. To resolve the discrepancy between our results with biogenic FeS and the reports in the literature, we conducted batch experiments with FeS that was chemically synthesized under conditions that corresponded to the geochemical conditions in the column experiment (final free sulfide concentrations ~5 mg/L). All operations were carried out in an oxygen free glove box. FeS was synthesized by mixing equimolar solutions of Fe(NH4)2(SO4)2 with NaHS. The reaction mixture was allowed to settle overnight. Then the clear supernatant was discarded, and the black suspension was transferred to a dialysis bag. The bag was washed with filtered tap water containing ~100 mg/L NaHS, for a total of seven washes over a time span of 3 days. The tap water had 78.4 mg/L TIC, and pH near 8. One set of experiments were conducted with chemically synthesized FeS kept in suspension throughout the experiment, and a second set of experiments were conducted using FeS that had been freeze dried, then resuspended in water. The pH and free sulfide concentrations were adjusted to equal the conditions in the first set of experiments. The first order rate constant for removal of TCE by FeS kept as a suspension in water after preparation was 0.78 per day per mole of FeS in contact with a liter of water. This rate is 20 to 40 times faster than the rate of degradation by FeS that had been freeze dried. Mineralogical characterization by XRD and XAS showed that the FeS in suspension remained as mackinawite (FeS) throughout the 56 days of the experiment, while iron in the freeze dried FeS changed over time from mackinawite (FeS) to lepidocrocite and eventually all of the iron was converted to goethite. This study shows that freeze drying might significantly alter the original iron sulfide minerals, and that the mineral phase has an important impact on the reactivity to degrade TCE.
