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

Abstract Kaolinite dissolution rates at ambient temperature and pH 7.1–8.5 were measured with the isotope tracer method. A rare Si isotope 29Si was introduced to the experimental solutions, which reacted with Georgia kaolinite (KGa-1b) composed of mostly 28Si. Reaction rates were tracked by 29Si/28Si 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 × 10−14 mol (kaol) s−1 m−2 consistently fitted the 29Si/28Si 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.