Determination of kinetics and equilibrium of Pb/Na exchange on clinoptilolite

Ion exchange kinetics and equilibria of lead on Na-pretreated clynoptilolite have been studied using batch method by means of electrometric and polarographic techniques. Equivalent fraction of lead ions exchanged with sodium has been determined by measuring time dependent Na+ ion concentration with sodium ion selective electrode (SISE). Electrode readings could be taken for a time interval of 15 s at early time of exchange. Time dependence of pH during exchange process has been followed simultaneously. Time dependent exchange curves showed a sharp increase at the beginning followed by a slower one for longer times until reaching equilibrium. The McKay equation for isotopic exchange kinetics has been applied to describe experimental results both in terms of out- and in-going ions. The film and particle diffusion coefficients evaluated from the McKay constants are not significantly affected by initial solution concentrations studied in the range of 1 x 10(-4) to 1 x 10(-4) M. Kinetic curves have been successfully modeled using the diffusion coefficients. An increase in the pH values at low concentrations below 10(-3) M Pb can be attributed to contribution of H+/Na+ exchange whereas the decrease of pH is indicative of the uptake of hydroxyl ions to form lead hydroxide clusters into clinoptilolite at higher concentrations. Total amount of Pb adsorbed and exchanged at equilibrium condition has been determined from polarographic measurements and compared with those found for exchanged Pb. Equilibrium data have been analyzed and modeled in terms of Freundlich and Langnmir adsorption isotherms. Thermodynamic parameters for transition state have been evaluated from temperature dependent kinetic results. (C) 2005 Elsevier B.V. All rights reserved.