The goal of the proposed research is to examine the role of organic-silica and organic-aluminum complexes in feldspar dissolution. Organic acids have been implicated as important reactants in a number of weathering environments, and are suggested to accelerate feldspar dissolution, increase feldspar solubility, mobilize aluminum and silica in solution, and alter the equilibrium between the solution and the precipitated secondary phases. The precise mechanism by which organic acids influence feldspar dissolution, however, and their significance in varying surficial weathering processes, is not well established. In natural waters at circum-neutral to acidic pH, there appears to be four primary processes: proton hydrolysis, solution complexation of aluminum, solution complexation of silica, and surface coordination resulting in accelerated dissolution. Depending on the environmental conditions and the organic acid species present, the dominant mechanism may be one or a combination of any of the four possibilities. The approach for the proposed study is to combine three independent methodologies to investigate a simple feldspar-aqueous- -electrolyte system. Dissolution experiments will be done to quantify the effects of various organic-acid anions on dissolution kinetics as a function of pH, temperature, and solution composition. These experiments are designed to separate the parallel mechanisms of acidic hydrolysis, solution complexation, and surface interactions. Spectroscopic experiments will investigate the stability and formation kinetics of the solution complexes of aluminum and silica. Raman, FTIR, UV-VIS, C and Si methods will be used to gain information on bonding environment, stoichiometry, and stability as a function of pH and temperature, using the same solution compositions as in the dissolution experiments. These studies will be combined with molecular orbital modeling relationships of the organic-acid--metal combinations as a function of organic-acid functional-group chemistry. The results of the solution complexation studies and modeling will be used to calibrate large-scale molecular-mechanics models of silicate surfaces that will be used to test the hypothetical interactions derived from the dissolution experiments. This is a two-year study which will interface with ongoing studies of field locations at which silica-organic complexation has been identified as a significant weathering mechanism, and these field sites will be used to test possible models of interaction. In addition, the study will involve collaboration with Los Alamos National Laboratory in developing methods of spectroscopic analysis of organic-metal complexes in aqueous systems.
