Kinetic and thermodynamic aspects of the adsorption of Pb^2+ and Cd^2+ ions on tripolyphosphate-modified kaolinite clay

Kaolinite clay obtained from Ubulu-Ukwu, Delta state in Nigeria was modified with tripolyphosphate reagent to obtain TPP-modified Kaolinite clay. The kinetics and thermodynamics of the adsorption of Pb^2+ and Cd^2+ by TPP-Kaolinite clay were studied. Increasing temperature and initial metal ion concentration increased the sorption capacity of the adsorbent. The rates of adsorption of both Pb^2+ and Cd^2+ increased with increasing temperature but decreased with increasing initial metal ion concentration and time. The pseudo-second-order initial sorption rates for the sorption of Pb^2+ were found to be higher than those of Cd^2+. Pseudo-first-order model was found to only describe well, the data obtained in the first 8 min of the adsorption process. The sorption of both metal ions was endothermic and spontaneous with (delta)H◦ values of +13.94 kJ mol^−1 and +24.93 kJ mol−^1 for Pb^2+ and Cd^2+, respectively. Activation energy values obtained were between +8 kJ mol−1 and +22 kJ mol^−1. These values suggest that the rate-controlling step in the adsorption of Pb^2+ and Cd^2+ by TPP-Kaolinite clay was diffusion-controlled. The sorption of metal ions from binary solutions of both metal ions at different initial metal ion concentrations reduced the initial sorption rates of the adsorption of Pb^2+ by TPP-Kaolinite clay and increased that for Cd2+. The adsorption capacity of TPP-Kaolinite clay for both metal ions was also decreased by simultaneous presence of both metal ions.