ANION ADSORPTION BY GOETHITE AND GIBBSITE

TLDR

The adsorption envelope of anions on goethite and gibbsite varies with pH and the pKa of the anion’s conjugate acid, with specific adsorption limited by surface charge at fully dissociated pH and extending to alkaline pH when the acid is incompletely dissociated near its pKa. Anion adsorption occurs when a proton is removed from a surface hydroxyl, creating a vacant site for the anion. Specific adsorption from fully dissociated acids can neutralize the surface charge, while adsorption from weak acids can further negative the surface, shifting the zero‑point of charge to more acidic values and indicating a complex relationship between anion adsorption and hydroxyl release that also affects cation behavior.

Abstract

Summary The relationship between adsorption and pH, termed the adsorption envelope, varies with the p K a, of the conjugate acid of the anion, for several adsorbents in presence of an excess of specifically adsorbed anions. At pH values where the acid is fully dissociated, specific adsorption occurs only to the extent of the positive charge of the surface and little specific adsorption is found at pH values more alkaline than the zero point of charge (zpc). With incompletely dissociated acids, anion adsorption can also take place at pH values more alkaline than the zpc providing the pH is somewhere near a p K a value of the acid, where the energy required to abstract a proton from the acid is at a minimum. The proton is required for the removal of a surface OH which provides a site for the anion. Specific anion adsorption (i) from a fully dissociated acid can reduce the positive charge of the surface to zero and (ii) from a weak acid can make even a negative surface more negative. Thus specific adsorption shifts the zpc to more acid values. The relationship between anion adsorption and OH release is not necessarily simple. The ideas proposed for anion adsorption have implications for cation adsorption and incorporation in an oxide surface.