The effect of SO4 on the ferrihydrite adsorption of Co, Pb and Cd: ternary complexes and site heterogeneity

Ferrihydrite adsorption of Co, Pb and Cd is enhanced in the presence of sulfate. This effect, which has been observed previously for Cu and Zn, is not predicted by the diffuse layer model using adsorption constants derived from single sorbate systems. However, by including ternary surface complexes with the stoichiometry ≡FeOHMeSO4, where ≡Fe is an Fe on the ferrihydrite surface and Me is Co, Pb, or Cd, the effect of SO4 on cation adsorption is accurately predicted for the range of cation, ferrihydrite and SO4 concentrations studied. Surface site heterogeneity for Co, Pb and Cd adsorption in the absence of SO4 also was considered. While Co adsorption could be described with a 2-site model (with site densities of 5×10−3 and 2×10−1 mol mol Fe−1) Pb adsorption required a third site with a higher affinity and a site density of 3.5×10−4 mol molFe−1. Cadmium adsorption was reasonably well described with a 2-site model but showed some evidence for a third higher affinity site. Recommended values of adsorption constants for use in SO4-rich environments, such as acid mine drainage, are presented.