Equilibrium and spectroscopic studies of lead retention in smectite

Heavy metal pollution of soils and water is a serious environmental problem . Sorption onto solid surfaces from aqueous solutions is an important proce ss influencing transport and accumulation of heavy metals in the environmen t. Lead (Pb) sorption in smectite SWy-2 was investigated by equilibrium sor ption studies, coupled with spectroscopic methods. The isotherm and pH-edge of lead sorption in the smectite were measured in batch experiments. The s orption isotherms under uncontrolled pH were best-fitted with the Langmuir equation, while those with a fixed pH 5.5 were best-fitted with the Freundl ich equation. The pH-edge of lead sorption shifted to a higher pH as the Pb concentration increased. The ionic strength only affected Pb sorption in t he low pH range. The effects of associated nitrate and perchlorate anions w ere significant only for the Pb sorption at high Pb concentration (>2 mM). FTIR and XRD spectroscopies were performed with oriented clay samples. In t he infrared spectra, a peak with a maximum near 1398 cm(-1) and a shoulder centered near 1470 cm(-1) was significant for the samples treated with >1 m M Pb at pH >5.6. This peak may be composed of several IR bands, including a n OH bending band from Pb hydroxides and CO32- stretching bands from Pb car bonates. The basal spacing of Pb-treated smectites increased with increasin g Pb content in the clay and correlated well with the intensity of OH bendi ng band (around 1621 cm(-1)) of adsorbed water. The increase of basal spaci ng at low pH (pH <3.3) was poorly correlated with the intensity of water OH bending band. The comprehensive studies provided many evidences for reveal ing the structure of various lead complexes on clay surfaces.