However, the differences in selective anionic
adsorption cannot be adequately explained by the bare ionic radii, hydrated
radii, stokes radii, diffusion coefficients or charge. For instance,
sulfate has a greater ionic radii as compared to fluoride, yet it can
form both inner-sphere and outer-sphere complexes on variably
charged Al- and Fe-(hydr)oxides [38]. The hydrated radius of anions explains
the adsorption behavior of anions to a small extent. The hydrated
ion radius hinders the ions to approach to close the interface, the ‘depletion
region’,which might either exceed or bemuch smaller (sometimes,
even negative) than the hydrated radius of the ions [39].