Electrolyte Anion Affinity and Its Effect on Oxyanion Adsorption on Goethite

Abstract

The influence of various types of background electrolytes (NaCl, NaNO3, and NaClO4) on the proton adsorption and on the adsorption of sulfate and phosphate on goethite have been studied. Below the PZC the proton adsorption on goethite decreases in the order Cl>NO3>ClO4. The decreasing proton adsorption affects the adsorption of oxyanions on goethite. Anion adsorption of strongly binding polyvalent anions is lower in the studied electrolytes in the order Cl<NO3<ClO4. The ion pair formation constants of monovalent electrolyte anions on the goethite surface are derived from proton adsorption data. It is shown that the derived ion pair formation constants enable the prediction of the effect of different electrolyte anions on adsorption of polyvalent anions. Especially at low oxyanion and high electrolyte concentrations the differences between the electrolytes influence the anion adsorption considerably. The effect is in principle not different for anions with a higher affinity for goethite, such as phosphate in comparison with sulfate, but the effect is only there if the particles are positively charged, which in the case of phosphate is only the case at relatively low P concentration and sufficiently low pH values.