Roles of organic compounds in metal transport during water/rock reactions

Effects of organic acids on geochemical and biogeochemical processes are quantified by incorporating data from theoretical and analytical studies. In the theoretical studies, the temperature dependence of thermodynamic properties for metal complexes with dicarboxylic acid ligands and carboxylate chelating agents is estimated from regression and correlation methods. In cases for some metal-dicarboxylate complexes, their standard partial molal entropies () were derived from regression of experimental stability constants available from measurements at various temperatures with the revised Helgeson-Kirkham-Flowers equation of state. In cases for some dicarboxylate complexes and multiple carboxylate chelates where measured data are not available, is estimated from correlations between standard partial molal entropies of association () and standard partial molal entropies of aqueous cations (). The correlations appear to be cation charge independent, which allows predictions for numerous complexes. The slopes of such correlations for chelates appear to be systematically dependent on numbers of carboxyl groups in the chelating agents. This permits prediction of based on molecular structures for myriad complexes where experimental data are limited.; The predicted thermodynamic data can be used to evaluate numerous geochemical and biogeochemical processes. Examples of speciation calculation for trace elements in oil-field brines and soil solution in the presence of carboxylic acids were demonstrated. It was found that types of organic acids and metal cations affected the speciation to different extents. Among dicarboxylic acids, oxalic and malonic acids affect the speciation of cations more than succinic, glutaric and adipic acids at similar concentrations. In soil solution speciation, less than 0.1 ppm of ethylenediaminetetraacetic acid (EDTA) can dominate the speciation of trace elements.; In addition to relatively low molecular weight organic acids, colloidal and insoluble organic matter may be involved in metal transport in soils. The effects of less soluble organic matter on Pb transport were investigated by sequential extraction of Pb associated with various soil components including organic matter. Small proportions of Pb were found associated with organic matter, whereas Fe and Mn oxides and hydroxides appeared to accommodate a majority of Pb in the soils.; To fully understand the transport of Pb in soils and to track path of smelter Pb mobility in soils, Pb elemental and isotopic compositions in bulk soils were determined. Variations of Pb concentrations and isotopic ratios at different sample locations are used to evaluate the contribution of the smelter to Pb in soils. Lead concentrations in surface soils varied from 1500 ppm at locations less than 2 km from the smelter to 100 ppm at locations greater than 2 km from the smelter. In addition, the surface soils near the smelter were found to have similar isotopic compositions as those of the smelter Pb. With depth, Pb concentrations decrease in asymptotic fashions reaching background concentrations of 15–70 ppm depending on sample locations. Although Pb concentrations decrease exponentially with depth in all of the locations, patterns of isotopic variation with depth differ according to distance from the smelter and Pb concentrations in bulk soils. At asymptotic depths, 80–90%, 30–40% and less than 10% of the Pb could be derived from the smelter for the samples at less than 1.5 km, 2–4 km and over 4 km from the smelter respectively. Isotopic interpretation of Pb sources allows estimating rates of Pb transport in soils as 1.3–1.6 cm yr