Equilibrium and spectroscopic studies of lead retention in smectite1

Abstract

Heavy metal pollution of soils and water is a serious environmental problem. Sorption onto solid surfaces from aqueous solutions is an important process influencing transport and accumulation of heavy metals in the environment. Lead (Pb) sorption in smectite SWy‐2 was investigated by equilibrium sorption studies, coupled with spectroscopic methods. The isotherm and pH‐edge of lead sorption in the smectite were measured in batch experiments. The sorption isotherms under uncontrolled pH were best‐fitted with the Langmuir equation, while those with a fixed pH 5.5 were best‐fitted with the Freundlich 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 the low pH range. The effects of associated nitrate and perchlorate anions were significant only for the Pb sorption at high Pb concentration (>2 mM). FTIR and XRD spectroscopies were performed with oriented clay samples. In the infrared spectra, apeak with a maximum near 1398 cm‐1 and a shoulder centered near 1470 cm‐1 was significant for the samples treated with >1 mM Pb at pH >5.6. This peak may be composed of several IR bands, including an OH bending band from Pb hydroxides and CO3 2‐ stretching bands from Pb carbonates. The basal spacing of Pb‐treated smectites increased with increasing Pb content in the clay and correlated well with the intensity of OH bending band (around 1621 cm‐1) of adsorbed water. The increase of basal spacing at low pH (pH <3.3) was poorly correlated with the intensity of water OH bending band. The comprehensive studies provided many evidences for revealing the structure of various lead complexes on clay surfaces.