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Controlling geological and hydrogeological processes in an arsenic contaminated aquifer on the Red River flood plain,Vietnam 总被引：1，自引：0，他引：1
Flemming Larsen Nhan Quy Pham Nhan Duc Dang Dieke Postma Søren Jessen Viet Hung Pham Thao Bach Nguyen Huy Duc Trieu Luu Thi Tran Hoan Nguyen Julie Chambon Hoan Van Nguyen Dang Hoang Ha Nguyen Thi Hue Mai Thanh Duc Jens Christian Refsgaard 《Applied Geochemistry》2008
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Atmospheric carbon dioxide is widely studied using records of CO2 mixing ratio, δ13C and δ18O. However, the number and variability of sources and sinks prevents these alone from uniquely defining the budget. Carbon dioxide having a mass of 47 u (principally 13C18O16O) provides an additional constraint. In particular, the mass 47 anomaly (Δ47) can distinguish between CO2 produced by high temperature combustion processes vs. low temperature respiratory processes. Δ47 is defined as the abundance of mass 47 isotopologues in excess of that expected for a random distribution of isotopes, where random distribution means that the abundance of an isotopologue is the product of abundances of the isotopes it is composed of and is calculated based on the measured 13C and 18O values. In this study, we estimate the δ13C (vs. VPDB), δ18O (vs. VSMOW), δ47, and Δ47 values of CO2 from car exhaust and from human breath, by constructing ‘Keeling plots’ using samples that are mixtures of ambient air and CO2 from these sources. δ47 is defined as , where is the R47 value for a hypothetical CO2 whose δ13CVPDB = 0, δ18OVSMOW = 0, and Δ47 = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO2] from 383 to 404 μmol mol−1, in δ13C and δ18O from −9.2 to −10.2‰ and from 40.6 to 41.9‰, respectively, in δ47 from 32.5 to 33.9‰, and in Δ47 from 0.73 to 0.96‰. Air sampled at varying distances from a car exhaust pipe was enriched in a combustion source having a composition, as determined by a ‘Keeling plot’ intercept, of −24.4 ± 0.2‰ for δ13C (similar to the δ13C of local gasoline), δ18O of 29.9 ± 0.4‰, δ47 of 6.6 ± 0.6‰, and Δ47 of 0.41 ± 0.03‰. Both δ18O and Δ47 values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at∼200 °C between CO2 and water generated by combustion of gasoline-air mixtures. Samples of CO2 from human breath were found to have δ13C and δ18O values broadly similar to those of car exhaust-air mixtures, −22.3 ± 0.2 and 34.3 ± 0.3‰, respectively, and δ47 of 13.4 ± 0.4‰. Δ47 in human breath was 0.76 ± 0.03‰, similar to that of ambient Pasadena air and higher than that of the car exhaust signature. 相似文献
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The kinetics of iodide (I−) and molecular iodine (I2) oxidation by the manganese oxide mineral birnessite (δ-MnO2) was investigated over the pH range 4.5-6.25. I− oxidation to iodate proceeded as a two-step reaction through an I2 intermediate. The rate of the reaction varied with both pH and birnessite concentration, with faster oxidation occurring at lower pH and higher birnessite concentration. The disappearance of I− from solution was first order with respect to I− concentration, pH, and birnessite concentration, such that −d[I−]/dt = k[I−][H+][MnO2], where k, the third order rate constant, is equal to 1.08 ± 0.06 × 107 M−2 h−1. The data are consistent with the formation of an inner sphere I− surface complex as the first step of the reaction, and the adsorption of I− exhibited significant pH dependence. Both I2, and to a lesser extent, sorbed to birnessite. The results indicate that iodine transport in mildly acidic groundwater systems may not be conservative. Because of the higher adsorption of the oxidized I species I2 and , as well as the biophilic nature of I2, redox transformations of iodine must be taken into account when predicting I transport in aquifers and watersheds. 相似文献