Microbial reductive dechlorination of trichloroethylene (TCE) in groundwater can be stimulated by adding of electron donors. However, side reactions such as Fe (III) reduction competes with this reaction. This study was set-up to relate the inhibition of microbial TCE dechlorination to the quantity and quality (mineralogy) of Fe (III) in the substrate and to calibrate a substrate extraction procedure for testing bioavailable Fe (III) in sediments. Batch experiments were set-up with identical inoculum (KB-1 culture) and liquid medium composition, and adding either 1) variable amounts of ferrihydrite or 2) 14 different Fe (III) minerals coated onto or mixed in with quartz sand (at constant total Fe) at a stoichiometric excess Fe (III) over electron donor. Increasing amounts of ferrihydrite significantly increased the time for complete TCE degradation from 8 days (control sand) to 28 days (excess Fe). Acid extractable Fe (II) increased and magnetite formed during incubation, confirming Fe (III) reduction. At constant total Fe in the sand, TCE dechlorination time varied with Fe mineralogy between 8 days (no Fe added) to >120 days (Fe-containing bentonite). In general, poorly crystalline Fe (III) minerals inhibited TCE dechlorination whereas crystalline Fe (III) minerals such as goethite or hematite had no effect. The TCE inhibition time was positively correlated to the Fe (II) determined after 122 days and to the surface area of the Fe (III) minerals. Only a fraction of total Fe (III) is reduced, likely because of solubility constraints and/or coating of Fe (III) minerals by Fe (II) minerals. Iron extraction tests based on Fe (III) reduction using NH2OH(.)HCl predict the competitive inhibition of TCE degradation in these model systems. This study shows that Fe mineralogy rather that total Fe content determines the competitive inhibition of TCE dechlorination.
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