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 mgg(-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 mgg(-1)min(-1) for the unmodified adsorbent. The overall sorption rate k, increased only slightly from 5.1x10(-2) to 9.7x10(-2)gmg(-1)min(-1) for phosphate-modified adsorbent, 3.8x10(-2) to 5.4x10(-2)gmg(-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 500mg/L of Pb(2+) in solution as 19 and 10.68 kJmol(-1) for phosphate-modified and unmodified adsorbents, respectively. The positive values of both DeltaH degrees and DeltaS degrees obtained suggest an endothermic reaction and in increase in randomness at the solid-liquid interface during the adsorption of Pb(2+) onto the adsorbents. DeltaG degrees 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.