Unidirectional kaolinite dissolution rates at near-equilibrium and near-neutral pH conditions

Kaolinite dissolution rates at ambient temperature and pH 7.1-8.5 were measured with the isotope tracer method. A rare Si isotope Si-29 was introduced to the experimental solutions, which reacted with Georgia kaolinite (KGa-1b) composed of mostly Si-28. Reaction rates were tracked by Si-29/Si-28 ratios of reacted solutions. The 16 batch experiments were designed with a grid of solutions that ranged from near saturation to supersaturation with respect to kaolinite. An average dissolution rate (unidirectional) of 5.4 +/- 1.6 x 10(-14) mol (kaol) s(-1) m(-2) consistently fitted the Si-29/Si-28 ratios for all 16 experiments, indicating the dissolution rates were independent of pH in near-neutral pH waters and independent from the levels of departure from equilibrium. In other words, it appears that the dissolution reaction mechanisms do not change across from the kaolinite-undersaturated to kaolinite-supersaturated solutions near-equilibrium. The near-equilibrium kaolinite dissolution rates in this study are a new type of rates-unidirectional rates (from the isotope tracer method), which differ from all near-equilibrium kaolinite dissolution rates in the literature that are based upon Si or Al concentrations and are net rates (dissolution minus precipitation rates). Kaolinite dissolution was non-stoichiometric in all experiments. The Si and Al concentrations were sometimes systematic but more often erratic, resulting from the precipitation of Al-Si secondary phases. The experimental solutions were grossly supersaturated with respect to gibbsite, allophanes, and imogolites. This confirms our hypothesis that the scatter and conflicts of near-equilibrium data are caused by unaccounted-for secondary phase precipitation, but the isotope tracer method successfully circumvents this experimental pitfall. Most natural waters are supersaturated with clays and are near-neutral pH. Our experimental rates are more applicable to the studies of natural waters than the majority rates available in the literature, which have mostly been measured at far-from-equilibrium, acidic pH, and high temperatures.