Kinetics and mechanisms of Pb(II) sorption and desorption at the aluminum oxide water interface

The fate of Pb in the environment is highly dependent on sorption and desorption reactions on solid surfaces. In this study Pb sorption and desorption kinetics on gamma-Al2O3 at pH 6.50, I = 0.1 M, and [Pb](initial) = 2 mM were investigated using both macroscopic and spectroscopic measurements. X-ray absorption fine structure (XAFS) spectroscopy revealed a Pb-Al bond distance of 3.40 Angstrom, consistent with an inner-sphere bidentate bonding mechanism. XAFS results show no change with time in the average local atomic structure surrounding the Pb and no indication of the formation of Pb surface precipitates. Adsorption kinetics were initially fast, resulting in 76% of the total sorption occurring within 15 min, followed by a slow continuous sorption reaction likely resulting from diffusion through micropores. Desorption at I = 0.1 M and pH 6.50 was studied using a cation-exchange resin as a sink for Pb(aq). Under these conditions, Pb desorption was 98% reversible within 3 days of incubation time. Furthermore, desorption and adsorption kinetics demonstrated similar trends: a fast reaction followed by a slow reaction. The use of spectroscopy combined with adsorption and desorption kinetic studies has revealed important information on the interaction between lead and aluminum (hydr)oxides. This information is valuable for predicting the fate of Pb in the environment.