Kinetics of selenite adsorption on hydroxyaluminum- and hydroxyaluminosilicate-montmorillonite complexes

A lack of understanding about the selenite adsorption behavior on hydroxyaluminum (HyA)- and hydroxyaluminosilicate (HAS)-interlayered phyllosilicates led us to conduct the present study. The kinetics of selenite adsorption on montmorillonite (Mt), HyA(OH/Al = 2.0)-Mt, HAS1(OH/Al = 2.0; Si/Al = 0.24)-Mt, and HAS2(OH/Al = 2.0; Si/Al = 0.48)-Mt were studied at pH 4.5, with an initial selenite concentration of 0.025 mM, a clay concentration of 0.5 g L(-1), temperatures of 288, 298, 308, and 318 K, and background electrolyte concentration of 10(-2) M NaNO3. Of the six different kinetic models tested, the second-order rate equation best described the kinetic data obtained for the initial fast reaction (5-30 min) followed by a slow reaction (30-180 min) in the adsorption systems. Elevated temperatures brought about a substantial increase in the rate constants. Compared with Mt, different HyA/HAS-Mts had 2 to 21 times higher rate constants for the fast reaction and up to five times higher rate constants for the slow reaction. Silication of HyA-Mt to form HAS1-Mt and HAS2-Mt substantially lowered the rate constants for both the fast and slow reactions. For the fast reaction, Mt had the highest activation energy and HyA-Mt had the lowest activation energy (around four times lower than Mt); silication increased the activation energy of selenite adsorption on the HAS-Mts. The pre-exponential factor, an index of the frequency of selenite collision with the clay surface, was remarkably lower for the HyA/HAS-Mts in comparison with Mt. The data obtained in the present study are of fundamental significance in understanding the role of Al interlayering and coating and silication of Al polymers on expansible phyllosilicates in influencing the dynamics of Se in soil and related environments.