Experimental study of the effect of pH on the kinetics of montmorillonite dissolution at 25 °C

The effect of pH on the kinetics of smectite (K-montmorillonite) dissolution was investigated at 25 °C in batch and stirred flow-through reactors over the pH range of 1–13.5, in KNO3 solutions. Dissolution rates were obtained based on the release of Si and Al at steady-state under far from equilibrium conditions. Dissolution was non-stoichiometric between pH 5 and 10, due to adsorption/precipitation of Al. Dissolution rates computed from batch and flow-through experiments were consistent, irrespective of the Si and Al concentrations. Sample pre-treatment and the interlayer cation do not affect the steady-state dissolution rate or stoichiometry of cation release. The rate dependence on pH can be described by:R(molm-2s-1)=10-12.30aH+0.40+10-14.37+10-13.05aOH-0.27The experimental results are consistent with a dissolution mechanism involving inward movement of a dissolution front from crystal edges. Consequently, normalization of the dissolution rates should take into account the reactive surface located on the smectite edges.