Abstract:
Steady-state dissolution and precipitation rates of a pure highly crystalline kaolinite were determined as a function of chemical affinity and aqueous Si and Al concentrations using a mixed flow reactor at 150°C, 40 bars, and a pH of 2, 6.8, and 7.8. Dissolution experiments, performed in solutions undersaturated with respect to all possible solid phases, exhibited stoichiometric Al and Si release. For chemical affinities (A) ranging between 2 and 10 kcal/mol, measured dissolution rates are an inverse function of aqueous Al concentration, and of aqueous Al and silica concentrations at acidic and alkaline pH, respectively. As equilibrium is approached (A =< 2 kcal/mol), these rates become increasingly controlled by chemical affinity. The variation of dissolution/crystallization rates with chemical affinity and aqueous Al and Si concentrations are described using a coupled transition state theory (TST)/Langmuir adsorption model by adopting the concept that reaction rate is controlled by the decomposition of a silica rich/aluminum deficient precursor complex. This complex is formed by the exchange of three hydrogen ions for one aluminum atom at the kaolinite surface. At alkaline conditions, this reaction competes with precursor complex condensation and/or aqueous silica adsorption forming unreactive siloxane groups. Taking account of the precursor complex formation reactions, equations were derived to describe kaolinite dissolution and crystallization rates over the full range of chemical affinity and aqueous Al and Si concentrations at both acidic and alkaline pH values. Near to equilibrium, kaolinite dissolution and precipitation rates predicted by these equations are considerably lower than those obtained from the extrapolation of far from equilibrium rates using previous rate laws also derived from TST but based on incorrect precursor complex formation reactions. The lower rates found in the present study are consistent with those deduced from petrographic observations in sedimentary basins.