Some factors affecting the dissolution kinetics of anorthite at 25°C

Abstract

Batch dissolution experiments were conducted at 25°C to determine the effects of agitation, particle size, suspension density, wetting and drying cycles, drying temperature, sequential rinses, ionic strength, and the addition and removal of products on the rates of anorthite (An 93) dissolution. In general, the dissolution kinetics at constant pH were not zero order with respect to products in solution, and this nonlinear release persisted beyond the time when Ca and Si stoichiometric dissolution was reached. The failure to establish zero-order kinetics could not be attributed to the weathering of damaged surfaces or fine, broken particles. Leached layer depths, calculated from solution composition, ranged from 2.6–3.5 nm; but a Ca-depleted surface layer was observed by energy dispersive X-ray analysis only on the reaction fines. Agitation rate had a marked effect on dissolution rate, contrary to expectations based on a surface reaction control mechanism. Anorthite dissolution in the presence of cation- and anion-exchange resins produced zero-order kinetics at sustained high rates. We hypothesize that these linear rates were due to the continuous removal of Al from solution by the resins. Consistent with these results, there was no effect of added Ca or Si on the rate of reaction; but the addition of Al slowed the initial rate of reaction at pH 3.6 and 6.0 but not at pH 3.0. Long-term dissolution studies (up to 4.5 ys) resulted in final reaction rates over two hundred times slower than previously reported for feldspar dissolution. These data are consistent with the idea that the presence of Al in solution and the incorporation of Al into the hydrous silanol surface slow the rate of anorthite dissolution and are important factors affecting the rate over all time periods. The addition of KCl slowed the dissolution rate either through competitive exchange with structural Ca or adsorbed H, or by blocking the polymerization reactions at the surface.