| Abstract: |
Successfully modeling the surface charge of goethite and anion adsorption on goethite using a surface complexation model (SCM) alone cannot verify the assumptions of this model. In this study, the assumptions of 2-pK triple layer model (TLM) and two-site 1-pK basic stern model (BSM) were assessed with respect to their ability to interpret both the proton–anion adsorption ratios of dimethylarsinate (DMA), monomethylarsonate (MMA), and arsenate and their effect on the ζ-potential. The proton–DMA adsorption ratio is around 0.9 at pH 4.25 and 1.1 at pH 6.75 at DMA surface coverage ranging from 0 to 2 μmol m−2, and the ζ-potential is independent of DMA adsorption at these two pH values. The proton–MMA adsorption ratio increases to 1.5 at pH 4 and 2.1 at pH 6.75 as the MMA surface coverage decreases to 0.5 μmol m−2. The ζ-potential is less dependent on MMA adsorption at a surface coverage range of 0 to 1.8 μmol m−2, and it then decreases with a further increase in the MMA surface coverage at pH 4 and 6.75. The proton–arsenate adsorption ratio decreases to 2 as the arsenate surface coverage approaches zero, and the ζ-potential decreases linearly with the increasing arsenate surface coverage at pH 4 and 6.75. Neither the 2-pK TLM nor the 1-pK BSM give a consistent interpretation of both the proton–arsenic adsorption ratio and the effect of arsenic on the ζ-potential. The results suggest that the 1-pK MUSIC model in which each type of surface hydroxyls has its own intrinsic proton-affinity constant and only one type of surface hydroxyls is involved in DMA, MMA, and arsenate adsorption is preferably pursued. The protonation degree of reactive hydroxyls estimated from proton–arsenic adsorption ratios is 0.2 at pH 4 and 0 at pH 6.75 in 0.001 M NaNO3. |
