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MICROCOSM STUDY OF DEGRADATION OF CHLORINATED SOLVENTS ON SYNTHETIC GREEN RUST MINERALS
SU, C. AND R. T. WILKIN. MICROCOSM STUDY OF DEGRADATION OF CHLORINATED SOLVENTS ON SYNTHETIC GREEN RUST MINERALS. Presented at Battelle 6th Conference on Chlorinated Solvents and Recalcitrant Compounds, Monterey, CA, May 19 - 22, 2008.
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Green rust minerals contain ferrous ion in their structure that can potentially serve as a chemical reductant for degradation of chlorinated solvents. Green rusts are found in zerovalent iron based permeable reactive barriers and in certain soil and sediments. Some previous laboratory studies show substantial chemical reduction of trichloroethene (TCE) by green rusts; other studies show slow chemical reduction of trichloroethene by green rusts. There is a need for additional study to understand the extent, kinetics, and mechanisms of green rust interactions with chlorinated solvents. In the microcosm study, synthetic carbonate green rust (CGR) or sulfate green rust (SGR) (0.25 g) was reacted with 60 mL of TCE (2 to 10 mg L-1) without headspace in serum bottles at 23 ± 1°C on a shaker at 100 rpm in the dark for up to 30 days. At preset time intervals, serum bottles were sacrificially sampled for determination of TCE and its degradation products (dichloroethene isomers, vinyl chloride, chloride, acetylene, ethene, and ethane). Dissolved iron, boron, and silicon were measured by Inductively Coupled Plasma – Optical Emission Spectrometry. In some batches, ferrous sulfate was added to SGR suspensions before TCE addition to bring about a total dissolved Fe(II) concentration at 1 mM to test the effect of surface sorbed Fe(II) on TCE degradation. In other batches, CuSO4 was added at 0.5 to 10 mM to green rust suspensions with or without a Trizma buffer (pH 8) before TCE addition to test if there is a catalytical effect of added copper ions. TCE was not reduced substantially by CGR and SGR. The addition of Fe(II) did not enhance TCE degradation. The addition of Cu(II) only slightly increased TCE degradation rate. Additional experiments are being performed to compare the degradation of carbon tetrachloride by green rust with that of TCE. The microcosms study results will be discussed with regard to the different reactivities of chlorinated solvents with the green rust minerals. Characterization of solid phases was performed by X-ray diffraction and scanning electron microscopy. When SGR suspension was not buffered, added Cu(II) was reduced to Cu(I) to form Cu2O and CuFe(III)O2 at pH 4-5 in the presence of TCE; whereas in Trizma buffer (pH 8), SGR reduced Cu(II) to Cu(0) (final pH > 5.4). The results show that Cu(II) ions are reduced much faster than chlorinated solvents by green rust.