Decomposition rate of Na-montmorillonite in aqueous suspension

A study has been carried out to investigate the rate of decomposition of a sodium-montmorillonite in aqueous suspension. The variation of the electrical conductivity, σ , of the suspension as a function of the time, t , was recorded at the constant temperature of 25 ± 0.5 °C. A first detectable rapid step was followed by a very slow step which tends to reach a constant value of the electrical conductivity (σ∞) after a long period of time. It is supposed that an initial diffusion-controlled mechanism governing the exchange between adsorbed Na+ ions and H+ ions in the solution and the subsequent hydrogen-induced hydrolysis are the main mechanisms representing the two steps respectively. Concerning the first relatively faster step, a linear relation between the electrical conductivity and the square root of time was in fact observed. On the other hand, the slow second step was described by a second-order kinetic equation which governs the rate of the whole process. It is therefore possible to calculate the specific rate k of the process. This allows us to quantify the extent and the rate of decomposition of the investigated clay. As revealed by the chemical analysis, the increase in the electrical conductivity of the suspension during time was related to release of Si, Al, Fe, Ca, and Mg, besides Na+ ions as a result of the clay mineral decomposition.