Specific ion effect on the point of zero charge of α-alumina and on the adsorption of 3,4-dihydroxybenzoic acid onto α-alumina surface

In the scope of adsorption of simple organic acid at the mineral oxide–water interface that represent natural ecosystem, adsorption of 3,4-dihydroxybenzoic acid onto α-alumina is explored in the presence of different salts at pH 5 and 7. The adsorption isotherms showed a critical salt concentration (in the present system it is 0.025 mM) that demarcates positive and negative effects of different salts on the adsorption densities of 3,4-dihydroxybenzoic acid onto α-alumina surface. The theoretical origin of specific ion effects below and above the critical salt concentration is likely to be different. Nevertheless, at higher salt concentration the polarizability of ions is reasonably accounted for the specific ion effects.