Simultaneous effect of pH, temperature, ionic strength, and initial concentration on the retention of lead on illite

Sorption of metal ions by clay minerals is a complex process involving different mechanisms, and controlled by different variables that can interact. The aim of this work was to study the retention mechanisms of Pb2+ ions on illite. We obtained Pb2+ sorption isotherms at constant pH, sorption edges as a function of pH, sorption isotherms at 5, 25 and 45 °C (pH = 5.5), and response surfaces of the simultaneous effect of pH, initial concentration, ionic strength, and temperature on the retention of Pb2+ ions on illite. Surface topography and atomic composition of illite samples doped with Pb2+ were studied by scanning electron microscopy and X-ray fluorescence microanalysis. Below pH 4.5, sorption of Pb2+ ions on illite is via ion exchange with H3O+ and Na+ ions that saturated the exchange sites, where the exchange with Na+ is the main mechanism between pH 3.5 and 4.5. Al3 + ions whose concentration in solution increases with decreasing pH can also compete with Pb2+ ions. For pH above 5.0, proton stoichiometry was higher than 1.0, and the effect of ionic strength on the amount of Pb2+ retained decreased with pH; in fact, at pH 6.8, the amount adsorbed was practically 100% and independent of ionic strength. The pronounced decrease in the intensity of the OH-stretching band for Pb-illite sample at pH 7.0 suggests that Pb2+ ions had lost the inner hydration sphere. The analysis of images obtained by SEM from 3 mM Pb2+ solutions with and without illite showed particles with the same morphology, which indicate a homogenous lead precipitation from the solution. For pH values higher than 4.5, increase in the temperature favored the retention of Pb2+ ions.