Kinetic and thermodynamic studies of the adsorption of lead (II) ions onto phosphate-modified kaolinite clay

This study is on the kinetics and thermodynamics of the adsorption of Pb 2+ onto phosphate-modified and unmodified kaolinite clay obtained from Ubulu-Ukwu in Delta State of Nigeria. Increasing initial Pb 2+ concentration increased the rate of Pb 2+ adsorbed with increase in initial Pb 2+ concentration from 300 to 1000 mg/L. Increasing Pb 2+ concentration also increased the initial sorption rate h, from 1.404 to 13.11 mg g −1 min −1 for phosphate-modified kaolinite clay and 1.04–3.48 for unmodified kaolinite clay as Pb 2+ concentration. Increase in temperature was found to increase the initial sorption rate of Pb 2+ adsorption onto phosphate-modified adsorbent from 3.940 to 8.85 and 2.55 to 4.16 mg g −1 min −1 for the unmodified adsorbent. The overall sorption rate k, increased only slightly from 5.1 × 10 −2 to 9.7 × 10 −2 g mg −1 min −1 for phosphate-modified adsorbent, 3.8 × 10 −2 to 5.4 × 10 −2 g mg −1 min −1 for unmodified adsorbent. The adsorption reaction on both adsorbents was found to be chemically activated reaction and endothermic with energy of activation, E, at 500 mg/L of Pb 2+ in solution as 19 and 10.68 kJ mol −1 for phosphate-modified and unmodified adsorbents, respectively. The positive values of both Δ H° and Δ S° obtained suggest an endothermic reaction and in increase in randomness at the solid–liquid interface during the adsorption of Pb 2+ onto the adsorbents. Δ G° values obtained were all negative indicating a spontaneous adsorption process. The presence of Cd 2+ decreased both initial sorption rate and the amount of Pb 2+ adsorbed on phosphate-modified and unmodified adsorbents at equilibrium. The adsorption process follows a pseudo-second-order reaction scheme.