Kinetics of lead adsorption/desorption on goethite: Residence time effect

Sorption/desorption reactions at the mineral/water interface will control s olution concentrations of trace elements and their bioavailability in natur al environments. While there has been a great deal of research examining tr ace element sorption reactions on soil and soil constituent surfaces, there is relatively little research examining desorption reactions and the effec t of residence time. In this study, we examined the influence of sorption d ensity and residence time on the sorption/desorption kinetics of Pb on goet hite, Lead sorption was rapid and nearly complete in less than 1 h, with es sentially 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 dif fusion equation. At all sorption densities investigated, desorption rate co efficients and the quantity of Pb desorbed were greater for the short-term experiments (5 days). However, statistical analyses indicated that these di fferences were not statistically significant. It is suggested that an insuf ficient desorption period caused by the high affinity of Pb for the goethit e surface or the slow diffusion of Pb(OH)(+) into crystal defects may be re sponsible for the absence of a residence time effect. Further studies are n ecessary to determine if the observed trends are real or are artifacts of t he experiment.