Sorption of cadmium on palygorskite, sepiolite and calcite: Equilibria and organic ligand affected kinetics

Quantity, affinity and rate of heavy metals retention by soil constituents can greatly influence the mobility and bioavailability of these metals in soil environments. Equilibrium and kinetic studies of cadmium (Cd) sorption on palygorskite, sepiolite and calcite minerals were carried out. Effects of low molecular weight organic acid anions, acetate and citrate, and siderophore desferrioxamine B (DFOB) on the Cd sorption rates were also investigated. Langmuir and Freundlich isotherms adequately fitted the equilibrium sorption data with r2 values >0.89. Among the minerals studied, sepiolite showed the highest sorption capacity and chemical affinity for Cd. Uptake of Cd by the minerals was initially rapid and then slowly continued until approached a pseudo equilibrium. The time-dependent Cd sorption data were well-described by pseudo second-order (0.98 < r2 < 1), Elovich (0.84 < r2 < 0.99) and power function (0.80 < r2 < 0.98) kinetic models. Sepiolite and calcite possessed the highest and the lowest Cd sorption rate values, respectively. Acetate and citrate ligands generally induced decreases in the Cd retention rates; however, the inhibitory effect of citrate was more pronounced. The DFOB siderophore, on the other hand, enhanced the rate of Cd uptake by the sorbents.