Abstract

The citrate-induced desorption kinetics of pre-adsorbed Se from montmorillonite (Mt) and its complexes with hydroxyaluminum (HyA-Mt) and hydroxyaluminosilicate (HAS-Mt) were studied. The mole fraction of pre-adsorbed Se released in 24 h from different clays followed the trends of Mt >> HAS-Mt > HyA-Mt with a significant increase with elevated citrate concentration in the desorbing solution. In contrast, the amount of Se adsorbed per unit mass of different clays followed a clear-cut opposite trend of HyA-Mt > HAS-Mt >> Mt. The Se desorption kinetics in different systems indicated multiple rate characteristics; where an initial fast reaction (0.25–2 h) was followed by a slow reaction (2–16 h). Of the six different kinetic models tested (zero-, first-, and second-order; power function, Elovich, and parabolic diffusion), the second-order rate equation showed the overall best fit to the fast and slow desorption kinetic data from the clays. Based on second-order rate constants, the rates of the mole fraction of Se desorption by citrate from different clay systems at 298 K followed the order Mt > HAS-Mt >> HyA-Mt. For both fast and slow reaction, the rates of desorption increased proportionally with the level of citrate. Replicate experiments conducted across a range of temperature (288–318 K) yielded Arrhenius parameters that followed the order HyA-Mt > HAS-Mt >> Mt. Considering that a lower mole fraction of Se desorption after a particular reaction period, slower desorption kinetics, and a greater activation energy of desorption are the indices of increased adsorption bond strength, the results clearly indicate that HyA- and HAS-interlayering and coatings on Mt not only augmented its Se adsorption affinity and capacity, but also increased the adsorption bond strength. Silication in HyA not only reduced the Se adsorption capacity, but also weakened the adsorption bond strength. This establishes a significant role of HyA/HAS-interlayering and coating on Mt in influencing the rate of citrate-induced release of Se. Reduction in surface-positive potential following citrate adsorption on the clay surface, a direct ligand exchange between Se and citrate, and structural dissolution are possible mechanisms responsible for citrate-induced Se desorption in the present study.

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