Decomposition of clay minerals in model experiments and in soils: Possible mechanisms, rates, and diagnostics (analysis of literature)

The analysis of model experiments on the dissolution of clay minerals showed that, beginning from a certain moment, this process reaches a steady state and proceeds at a constant rate. The minimum dissolution rate was observed in a neutral environment, where this value varied in the range from n × 10−14-n × 10−12 mol/(m2 s). Under acidic and alkaline conditions, this value increased to n × 10−12 or n × 10−10 mol/(m2s) for most clay minerals. The first stage of the dissolution mechanism involved the formation of protonated (in an acidic environment) and deprotonated (in an alkaline environment) complexes, which destabilized and polarized metal-oxygen (or metal-hydroxyl) bonds in the crystal lattice. At the second stage, the rupture of Si-O and Al-O bonds and the release of these components into the solution occurred at a specific concentration of these complexes, and this stage largely controlled the dissolution rate of the mineral. The presence of organic ligands forming mononuclear polydentate complexes on the surface of the mineral particles at the same solution pH increased the dissolution rate of the minerals by several times and sometimes by an order of magnitude proportionally to the concentration of these complexes on the surface of the particles. It was found that the dissolution rates of kaolinite, illite, and smectite in the podzolic horizon of loamy podzolic soil calculated from the losses of clay minerals in the soil profile with consideration for the soil age exceeded the corresponding values obtained in model laboratory experiments at the same pH values by several orders of magnitude. The revealed differences could be related to the long-term functioning of biota in native soils and the existing uncertainties in the assessment of the active surface of mineral particles.