Abiotic Reaction and Diffusion of Chlorinated Solvents in Fe2+ Bearing Intact Limestones and Mudstones
Final Report Abstract
The goal of this project was to determine the rate and extent of abiotic perchloroethylene (PCE) and trichloroethene (TCE) transformation using pyrite containing sedimentary rocks from southwest Germany. Initial experiments did not show any of the expected transformation products, such as vinyl chloride or acetylene. However, significant amounts of similar compounds e.g., methane, ethene, ethane, propane and butane were observed along with low molecular weight organic acids (LMWOAs), such as acetate. To prevent misinterpretation of transformation pathways, further tests, without chlorinated ethenes, were conducted focusing on C1-C4 gases and LMWOAs released from the selected sedimentary rock types. These experiments involved pulverization of samples using a zirconium oxide planet ball mill followed by wet and dry batch incubation experiments and longterm diffusion tests with intact rock subsamples. During ball milling, all carbonate rocks yielded methane (CH4), ethane (C2H6), ethene (C2H4), propane (C3H8), butane (C4H10), CO2, and hydrogen (H2). Sandstones only yielded H2, CH4, and CO2. Stable carbon isotope signatures of these products indicate that volatiles were thermogenic (i.e., abiotic) in origin, probably due to cracking of organic matter (kerogen). However, H2 was also observed, suggesting that it was produced mechanochemically. Samples from ball milling used in subsequent wet incubations stored at 55 °C for up to 112 days showed reproducibly many of the same gasses as observed in dry ball milling. Notably, hydrogen was observed as well, which was possibly produced by mechanochemical reactions of water on freshly broken mineral surfaces. Regarding LMWOAs, formate, acetate, and citrate, were detected in all samples, except for the sandstone, suggesting that the material came from the kerogen, either through chemical processes or was previously trapped. Thermal desorption done for selected carbonate samples yielded SO2 and CS2, indicating the presence of poorly crystalline Fe- and Mg-phases and sulfide induced reactions. Taken together, these data suggest that hydrocarbons and organic acids were artificially generated, suggesting that batch experiments with processed rocks do not accurately reflect natural conditions. Regarding abiotic dechlorination, these data suggest that organic acids and reduced gasses (e.g., ethene) could be due to sample processing techniques.
