Effect of humic acid coating on arsenic adsorption on ferrihydrite-kaolinite mixed systems

Effect of humic acid coating on arsenic adsorption on ferrihydrite-kaolinite mixed systems M. Martin, E. Barberis, L. Celi, A. Violante, L.M. Kozak, P.M. Huang The reactivity of the colloids formed in soil environments results from complex interactions between clay minerals, (oxy)hydroxides and organic matter of soils. The adsorption/desorption equilibria and kinetics of anionic and cationic species on/from natural colloids could substantially differ compared with their interaction with pure minerals. The objective of this study was to investigate the arsenite and arsenate equilibria and the kinetics of arsenite adsorption on ferrihydrite-coated kaolinite as influenced by the presence of humic acid coverage. The Langmuir adsorption maximum and affinity of arsenite and arsenate adsorption on the kaolinite-ferrihydrite system was reduced by the presence of the humic acid. The enthalpy of adsorption on the kaolinite-ferrihydrite system, measured with isothermal titration calorimetry, was -24.4 kJ mol-1 for arsenite and -39.9 kJ mol-1 for arsenate, and was only slightly decreased by the humic acid coating. The kinetics of arsenite adsorption was substantially influenced by the presence of the humic coverage, resulting in the decrease in the rate constant, the Arrhenius heat of activation, and the pre-exponential factor (indicative of the collision frequency of the sorbate and the sorbent). The hampering effect of the humic acid on arsenic adsorption was attributable to the blocking by the organic ligand of the adsorbing sites, coupled with the steric hindrance of the large organic molecules, which restricted the arsenic to approach the surfaces.