Adsorption of lead at variable pH onto a natural porous medium: Modeling of batch and column experiments

The surface complexation approach has been applied to describe the adsorpti on of lead and proton onto a heterogeneous natural porous medium (red pozzo lan) at constant ionic strength (0.1 mol L-1). Acid-base titration experime nts were used to determine the minimum number of sites needed to describe t he surface heterogeneity. Lead adsorption tests at several pH and total lea d concentration were used to complete the model structure, whose adjustable parameters (site concentrations and apparent formation constants) were det ermined by nonlinear multivariate regression of titration and adsorption da ta. The final model represents the acid-base properties of the surface by t he presence of two amphoteric sites, SOH and TOH, and a monoprotic one, MOH ; whereas lead adsorption is considered only onto SOH and MOH sites. The mo del allows a good representation of the experimental behavior in the whole experimental range (pH = 4-10 for titration, pH = 3.5-6.0 and total lead co ncentration from 8.8 x 10(-5) to 7.3 x 10(-4) mol L-1 for adsorption experi ments). Theoretical surface speciation shows that lead adsorption occurs mo stly onto site MOH at low pH level and on both SOH and MOH sites at higher values.:The model was independently validated by simulating Ph and pH break through experiments performed in small chromatrographic columns. Experiment al breakthroughs are well predicted by an advection-dispersion transport mo del coupled with the chemical equilibrium routine (IMPACT), without any par ameter adjustment. A large spreading of Pb and pH breakthroughs was calcula ted and experimentally observed, even in the absence of any kinetic effect. That spreading is explained in terms of the concomitant presence of compet itive adsorption and surface heterogeneity.