Heavy metal interactions with phosphatic clay: Kinetic and equilibrium studies

A novel adsorbent, phosphate-immobilized zirconium pillared clay (P-Zr-PILC) was prepared by pillaring bentonite clay with Zr-polyhydroxy cations followed by phosphate immobilization using a batch reactor at pH 3.0. P-Zr-PILC was characterized by means of FTIR, XRD, TG, SEM, and potentiometric titrations. Adsorbent with increased surface negative charge can be used for the adsorption of Hg(II), Cd(II) and Co(II) from water and industry effluents. The effects of pH, contact time, initial concentration, adsorbent dose, and adsorption isotherm on Hg(II), Cd(II) and Co(II) adsorption onto P-Zr-PILC were evaluated using batch experiments. Maximum removal of 99.5%, 98.0%, and 97.2% for Hg(II), Cd(II), and Co(II) respectively was observed for an initial concentration of 5.0 mg L-1 at pH 6.0 and at an adsorbent dose of 2 g L-1. Equilibrium was achieved in 4 h. The equilibrium data were fitted to the Redlich-Peterson isotherm model when compared with Langmuir and Freundlich isotherm models. The experimental kinetic data were analyzed using pseudo-first-order and pseudo-second-order kinetic models and film-diffusion process was found to be the rate limiting step for the adsorption of Hg(II), Cd(II) and Co(II) ions onto P-Zr-PILC. Desorption experiments showed that 0.2 M HCl can effectively desorb the adsorbed heavy metal ions and reusability of the adsorbent was tested by several cycles of adsorption-desorption experiments. (C) 2012 Elsevier B.V. All rights reserved.