Adsorption of Pb(II) on palygorskite from aqueous solution: Effects of pH, ionic strength and temperature

Palygorskite was characterized by XRD, FTIR and acid-base titration. The fitting of surface charge density using FITEQL resulted in an excellent fitting of experimental data and following intrinsic equilibrium constants: log K-XH = 2.08, log K-AL-OH2+(int) = 5.46, and log K-Al-O-(int) = -8.58. The total density of =Al-OH and =XNa were 9.76 x 10(-5) mol/g and 1.36 x 10(-4) mol/g, respectively. The adsorption of Pb(II) from aqueous solution on palygorskite was studied as a function of pH, ionic strength and temperature. Adsorption of Pb(H) on palygorskite was strongly dependent on pH, and weakly dependent on ionic strength. Two different adsorption mechanisms were proposed: (1) exchange reaction with little or no dependence on pH (i.e., =X2Pb0) at low pH; and (2) strong pH-dependent surface complexation reactions (=Al-OPb+) at neutral or alkaline conditions. The adsorption and desorption isotherms indicated that Pb(II) adsorption to palygorskite was reversible. The adsorption isotherms were simulated well by the Langmuir model. XPS analysis also indicated the adsorption species of Pb(II) to be =X2Pb0 at low pH and predominantly =Al-OPb+ at higher pH. (C) 2009 Elsevier B.V. All rights reserved.