Kinetics of selenite desorption by phosphate from hydroxyaluminum- and hydroxyaluminosilicate-montmorillonite complexes

There exists a clear knowledge gap about the kinetics and mechanisms of desorption of metals and metalloids from soils and clays, even after repeated emphasis on its importance for a better understanding of the mobility, transport and fate of metals and metalloids in natural environments. In this study, we investigated the phosphate-induced desorption kinetics of preadsorbed selenite from montmorillonite (Mt) and its complexes with hydroxyaluminum (HyA-Mt) and hydroxyaluminosilicate with Si/Al molar ratios of 0.24 (HAS1-Mt) and 0.48 (HAS2-Mt). The mole fraction of preadsorbed selenite released in 24 h from different clays followed the trends of Mt≫HAS2-Mt>HAS1-Mt≫HyA-Mt with a significant increase with elevated phosphate concentration in the desorbing solution. In contrast, the amount of selenite adsorbed per unit mass of different clays followed a clear cut opposite trend of HyA-Mt≫HAS1-Mt>HAS2-Mt≫Mt. Such results clearly indicate that HyA- and HAS-interlayering and coatings on Mt not only augmented its selenite adsorption affinity and capacity, but also increased the adsorption bond strength. Silication in HyA not only reduced the selenite adsorption affinity and capacity, but also weakened the adsorption bond strength. Desorption kinetics of selenite from the clays was best described by the power-function model which was closely followed by both parabolic diffusion and Elovich models. The rates of the mole fraction of selenite desorption by phosphate from different clay systems followed the order of Mt>HAS2-Mt>HAS1-Mt>HyA-Mt with a significant increase with an increased level of phosphate in the desorbing solution. This establishes a significant role of HyA/HAS-interlayering and coating on Mt in slowing down the rate of phosphate-induced release of selenite. Thus, the findings of the present study merit vital environmental significance, which would help in better understanding the role of Al interlayering and coating and silication of Al polymers on expansible phyllosilicates in influencing Se mobility in soil and related environments.