Kinetics of Kaolinite Precipitation and Dissolution under Circumneutral pH Conditions

The precipitation of clay minerals exerts an important control on pore water chemistry in a variety of subsurface environments, and may even control the extent of undersaturation with respect to primary dissolving phases like feldspar (Maher et al., 2005). It is essential, therefore, to quantify clay precipitation kinetics, particularly under the circumneutral pH conditions typically found in subsurface environments. We have conducted kinetic studies of Georgia KGA-1b kaolinite dissolution and precipitation rates at 25C and input pH values of 4 in continuously stirred flowthrough reactors as a function of the extent of undersaturation or supersaturation, dissolved silica and aluminum concentration, and pH. Preliminary results suggest a promotion of dissolution rates by dissolved silica. For example, dissolution rates determined close to equilibrium (log Q/Keq ~ -0.5), but with high input silica concentrations (~ 300 micromolar), are more rapid than rates determined far from equilibrium with no dissolved silica in the input solution. Dissolution rates at pH 4 and 25C are broadly comparable to rates determined by Nagy et al. (1991). The dissolution experiments also provide a direct measure of the solubility of the KGA-1b kaolinite, an essential prerequisite to precipitation experiments intended to quantify rates as a function of supersaturation. Scanning transmission X-ray microscopy (STXM) and XPS analyses at the Advanced Light Source are being used to characterize reaction products.