Kinetics of Lead Adsorption/Desorption on Goethite: Residence Time Effect

Sorption/desorption reactions at the mineral/water interface will control solution concentrations of trace elements and their bioavailability in natural environments. While there has been a great deal of research examining trace element sorption reactions on soil and soil constituent surfaces, there is relatively little research examining desorption reactions and the effect of residence time. In this study, we examined the influence of sorption density and residence time on the sorption/desorption kinetics of Pb on goethite. Lead sorption was rapid and nearly complete in less than 1 h, with essentially no change in the quantity of Pb sorbed over the 12-week sorption period. Desorption of Pb was slow and was modeled best by the parabolic diffusion equation. At all sorption densities investigated, desorption rate coefficients and the quantity of Pb desorbed were greater for the short-term experiments (5 days). However, statistical analyses indicated that these differences were not statistically significant. It is suggested that an insufficient desorption period caused by the high affinity of Pb for the goethite surface or the slow diffusion of Pb (OH)+ into crystal defects may be responsible for the absence of a residence time effect. Further studies are necessary to determine if the observed trends are real or are artifacts of the experiment.