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In situ reaction rates from a field biostimulation experiment
Goldschmidt Conference Abstracts 2006
The isotope salt effect of dissolved minerals and the mineral/water interaction: Implications for low-T oxygen isotope fractionation between carbonate minerals and water B.-L. XU, Y.-F. ZHENG, G.-T. ZHOU CAS Kay Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China ([email protected], [email protected]) The question about the magnitude of O isotope fractionations between calcite and aragonite at thermodynamic equilibrium involves correct understanding of O isotope exchange and equilibrium between carbonates and water in laboratory experiments. Thus, the crucial issue is to acknowledge whether or not the isotope salt effect of dissolved minerals would occur on O isotope exchange between carbonate minerals and water. This dictates the option whether or not a correction for mineral/water interaction should be applied when calculating mineral–water 103 lna factors by an arithmetic combination of theoretical 103 lnb factors for mineral and water, respectively. This correction was introduced for the purpose of reconciling the differences between the theoretical and experimental data for minerals. It follows the common practice in theoretical physics to bring calculations in agreement with observations. Application of the mineral/water correction to calculation of theoretical calcite–water fractionation equations, however, results in the apparent discrepancy between the arithmetic calculation and the experimental results. To resolve this discrepancy, it is critical to take into account the isotopic salt effect of dissolved minerals in aqueous solutions that is different between calcite and other minerals. From the practice of theoretical calculations, the isotope salt effect of dissolved minerals corresponds to the correction for mineral/water interaction in the modified increment method. Validity of such correction is supported from excellent agreements between the theoretical predictions and new experimental results for: (1) cerussite–water system by slow precipitation experiment at low temperatures, (2) dolomite–water system by microbial experiments at 25–45 °C and chemical syntheses at 40–80 °C. Except for calcite, the isotope salt effect of dissolved minerals is significant for these carbonates and thus the correction for mineral/water interaction is necessary. This has not only testified validity of the assumptions used in the modified increment method, but also provides physico-chemical basis for application of the thermodynamic data to isotopic geothermometry and geochemical tracing. A great virtue of this approach is that it allows fractionation factors to be predicted for minerals for which no experimental data was available, based only on a knowledge of their crystal chemistry (crystal structure, chemical composition, cation valence and ionic radius). doi:10.1016/j.gca.2006.06.1346
A747
In situ reaction rates from a field biostimulation experiment Z. ZHENG, M.D. REEDER, C. ZHU Geological Sciences, Indiana University, Bloomington, IN 47405, USA It is well known that the rates of microbially catalyzed biogeochemical reactions in laboratory incubation studies are 2–5 orders of magnitude higher than in situ rates in aquifers. This discrepancy partly reflects the complexity of geological systems. Typically, redox sensitive constituents such as uranium (U) and technetium (Tc) are found at very low, yet hazardous concentrations, whereas other redox-sensitive elements such as sulfate and nitrate are much more abundant, and aqueous redox reactions in the aquifers occur in a complex solid matrix with different species and crystallinities of iron and manganese oxyhydroxides. Here, we report a push–pull biostimulation test and in situ chemical reaction rates in a groundwater system that are too complicated to be duplicated by laboratory experimental methods. Nutrients were added to the system to increase the activity of the native microbial population to such a degree that the competitive terminal electron acceptors were depleted through a sequence of microbially mediated redox reactions, allowing for the reduction of U(VI) and Tc(VII), NO3 , SO4 2 , and Fe and Mn oxyhydroxides. We will show the derived in situ rates and geochemical modeling to simulate the biogeochemical processes. doi:10.1016/j.gca.2006.06.1347
The isotope salt effect of dissolved minerals and the mineral/water interaction: Implications for low-T oxygen isotope fractionation between carbonate minerals and water B.-L. XU, Y.-F. ZHENG, G.-T. ZHOU CAS Kay Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China ([email protected], [email protected]) The question about the magnitude of O isotope fractionations between calcite and aragonite at thermodynamic equilibrium involves correct understanding of O isotope exchange and equilibrium between carbonates and water in laboratory experiments. Thus, the crucial issue is to acknowledge whether or not the isotope salt effect of dissolved minerals would occur on O isotope exchange between carbonate minerals and water. This dictates the option whether or not a correction for mineral/water interaction should be applied when calculating mineral–water 103 lna factors by an arithmetic combination of theoretical 103 lnb factors for mineral and water, respectively. This correction was introduced for the purpose of reconciling the differences between the theoretical and experimental data for minerals. It follows the common practice in theoretical physics to bring calculations in agreement with observations. Application of the mineral/water correction to calculation of theoretical calcite–water fractionation equations, however, results in the apparent discrepancy between the arithmetic calculation and the experimental results. To resolve this discrepancy, it is critical to take into account the isotopic salt effect of dissolved minerals in aqueous solutions that is different between calcite and other minerals. From the practice of theoretical calculations, the isotope salt effect of dissolved minerals corresponds to the correction for mineral/water interaction in the modified increment method. Validity of such correction is supported from excellent agreements between the theoretical predictions and new experimental results for: (1) cerussite–water system by slow precipitation experiment at low temperatures, (2) dolomite–water system by microbial experiments at 25–45 °C and chemical syntheses at 40–80 °C. Except for calcite, the isotope salt effect of dissolved minerals is significant for these carbonates and thus the correction for mineral/water interaction is necessary. This has not only testified validity of the assumptions used in the modified increment method, but also provides physico-chemical basis for application of the thermodynamic data to isotopic geothermometry and geochemical tracing. A great virtue of this approach is that it allows fractionation factors to be predicted for minerals for which no experimental data was available, based only on a knowledge of their crystal chemistry (crystal structure, chemical composition, cation valence and ionic radius). doi:10.1016/j.gca.2006.06.1346
A747
In situ reaction rates from a field biostimulation experiment Z. ZHENG, M.D. REEDER, C. ZHU Geological Sciences, Indiana University, Bloomington, IN 47405, USA It is well known that the rates of microbially catalyzed biogeochemical reactions in laboratory incubation studies are 2–5 orders of magnitude higher than in situ rates in aquifers. This discrepancy partly reflects the complexity of geological systems. Typically, redox sensitive constituents such as uranium (U) and technetium (Tc) are found at very low, yet hazardous concentrations, whereas other redox-sensitive elements such as sulfate and nitrate are much more abundant, and aqueous redox reactions in the aquifers occur in a complex solid matrix with different species and crystallinities of iron and manganese oxyhydroxides. Here, we report a push–pull biostimulation test and in situ chemical reaction rates in a groundwater system that are too complicated to be duplicated by laboratory experimental methods. Nutrients were added to the system to increase the activity of the native microbial population to such a degree that the competitive terminal electron acceptors were depleted through a sequence of microbially mediated redox reactions, allowing for the reduction of U(VI) and Tc(VII), NO3 , SO4 2 , and Fe and Mn oxyhydroxides. We will show the derived in situ rates and geochemical modeling to simulate the biogeochemical processes. doi:10.1016/j.gca.2006.06.1347