Characterization of Pb, Cu, and Cd adsorption on particulate organic matter in soil

Evidence exists for significant metal enrichment in particulate organic matter (POM) compared to other soil constituents, but the relevant mechanisms are poorly understood. In the present study, adsorption of the heavy metals Pb, Cu, and Cd on a loamy soil and on POM separated from this soil was investigated. The adsorption kinetic data can be well described with a pseudo-second order model, whereas the equilibrium data are well fitted by a Langmuir model. Adsorption isotherms and kinetics data, in addition to the influence of pH on metal adsorption, showed that POM had a much higher adsorption capacity for Pb, Cu, and Cd compared to the whole soil. Ionic strength effects on metal adsorption, Fourier transform infrared spectroscopy, x-ray absorption spectroscopy x-ray absorption spectroscopy including x-ray absorption near-edge structure and extended x-ray absorption fine-structure spectroscopy were employed to elucidate the adsorption mechanisms. The results suggested that Pb and Cu adsorption on POM was mainly through inner-sphere complexes with carboxyl and hydroxyl groups. Cadmium was possibly adsorbed via outer-sphere complexation, indicated by the influence of ionic strength on Cd adsorption.