Surface coverage effects on the desorption kinetics of selenite from a hydroxyaluminum–montmorillonite complex

Information on the desorption of metals and metalloids from soils and clays are essential for a better understanding of their mobility, transport, and fate in natural environments. We investigated nitrate-, phosphate-, and citrate-induced desorption kinetics of preadsorbed selenite (presented as Se henceforth) from a hydroxyaluminum–montmorillonite (HyA–Mt) complex at three different surface coverages of 8%, 25%, and 69% of its Langmuir predicted adsorption maximum (262.61 mmole kg−1). Generally the mole fraction of preadsorbed Se released after the attainment of desorption equilibrium was significantly higher with increasing surface coverage. Desorption kinetics of Se from the clay was best described by the Elovich model. The Elovich model parameter β representing the rate of Se desorption increased as the surface coverage increased. Both kinetic data and mole fraction of Se released at desorption equilibrium supported the contention that adsorption bond strength progressively decreases with increasing surface coverage. Both citrate and phosphate remobilized Se at significantly faster rates than nitrate at any surface coverage level. Citrate showed a significantly faster rate of Se release than phosphate only at 8% surface coverage but not at 25% and 69% surface coverages, suggesting that differential ability of these two ligands to influence the kinetics of Se release was also surface coverage dependant. The findings of the present study would help better understand the consequences of different surface coverages on soil colloids by preadsorbed Se as well as the impacts of phosphate fertilization and rhizospheric processes in influencing Se mobility in soil and related environments.

Hydroxyaluminum–montmorillonites are common in acidic soils and sediments. Selenite adsorption bond strength decreased with increasing surface coverage. Selenite desorption by various desorbing agents followed the order: Citrate ⩾ Phosphate > Nitrate.Figure optionsDownload high-quality image (64 K)Download as PowerPoint slideResearch highlights
► Different extents of surface coverage by preadsorbed selenite (Se) on the hydroxyaluminum-montmorillonite (HyA-Mt) complex showed a substantial alteration of the subsequent Se desorption behavior from the complex.
► The bond strength of Se adsorption on the HyA-Mt appears to be progressively weaker with increasing surface coverage as revealed by larger mole fraction of Se desorption and faster desorption kinetics with higher levels of surface coverage.
► Citrate and phosphate caused much faster Se desorption from the HyA-Mt than nitrate at any given level of surface coverage.
► The findings are of fundamental significance to better understand the consequences of different levels of surface coverage on soil colloids by preadsorbed Se as well as the impacts of phosphate fertilization and rhizospheric processes on the mobility and fate of selenite in soil and related environments.