Kaolinite dissolution and precipitation kinetics at 22 °C and pH 4

Abstract

Dissolution and precipitation rates of low defect Georgia kaolinite (KGa-1b) as a function of Gibbs free energy of reaction (or reaction affinity) were measured at 22 °C and pH 4 in continuously stirred flowthrough reactors. Steady state dissolution experiments showed slightly incongruent dissolution, with a Si/Al ratio of about 1.12 that is attributed to the re-adsorption of Al on to the kaolinite surface. No inhibition of the kaolinite dissolution rate was apparent when dissolved aluminum was varied from 0 and 60 μM. The relationship between dissolution rates and the reaction affinity can be described well by a Transition State Theory (TST) rate formulation with a Temkin coefficient of 2 R diss ( mol / m 2 s ) = 1.15 × 10 - 13 1 - exp - Δ G 2 RT . Stopping of flow in a close to equilibrium dissolution experiment yielded a solubility constant for kaolinite at 22 °C of 10 7.57. Experiments on the precipitation kinetics of kaolinite showed a more complex behavior. One conducted using kaolinite seed that had previously undergone extensive dissolution under far from equilibrium conditions for 5 months showed a quasi-steady state precipitation rate for 105 h that was compatible with the TST expression above. After this initial period, however, precipitation rates decreased by an order of magnitude, and like other precipitation experiments conducted at higher supersaturation and without kaolinite seed subjected to extensive prior dissolution, could not be described with the TST law. The initial quasi-steady state rate is interpreted as growth on activated sites created by the dissolution process, but this reversible growth mechanism could not be maintained once these sites were filled. Long-term precipitation rates showed a linear dependence on solution saturation state that is generally consistent with a two-dimensional nucleation growth mechanism following the equation R ppt ( mol / m 2 s ) = 3.38 × 10 - 14 exp - 181776 T 2 ln Ω . Further analysis using Synchrotron Scanning Transmission X-ray Microscopy (STXM) in Total Electron Yield (TEY) mode of the material from the precipitation experiments showed spectra for newly precipitated material compatible with kaolinite. An idealized set of reactive transport simulations of the chemical weathering of albite to kaolinite using rate laws from (Hellmann R. and Tisserand D. (2006) Dissolution kinetics as a function of the Gibbs free energy of reaction: an experimental study based on albite feldspar. Geochim. Cosmochim. Acta 70(2) 1037–1052) and this study respectively indicate that while pore waters are likely to be close to equilibrium with respect to kaolinite at pH 4, significant kaolinite supersaturation may occur at higher pH if its precipitation rate is pH-dependent.