UNIDIRECTIONAL KAOLINITE DISSOLUTION RATES AT NEAR-EQUILIBRIUM USING SI ISOTOPE TRACERS

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.