Arsenite and Arsenate Adsorption on Ferrihydrite:  Surface Charge Reduction and Net OH- Release Stoichiometry

Arsenite [As(III)] and arsenate [As(V)] are highly toxic inorganic arsenic species that represent a potential threat to the environment and human health. Iron oxides including poorly crystalline oxides, e.g., ferrihydrite, play a significant role in controlling dissolved As concentration and limit the mobility and bioavailability of As(III) and As(V). Adsorption occurs by ligand exchange of the As species for OH2 and OH- in the coordination spheres of surface structural Fe atoms. The objective of this study was to evaluate H+/OH- release stoichiometry and changes in surface charge properties of the adsorbent during the adsorption of arsenite and arsenate on ferrihydrite in the pH range of 4−10. This information, which is not directly accessible through spectroscopic studies, provides important clues to bonding mechanism. While arsenate adsorption resulted in the net release of OH- at pH 4.6 and 9.2, arsenite adsorption resulted in net OH- release at pH 9.2 and net H+ release at pH 4.6. The amount of H+ or OH- release per mole of adsorbed As varied with the As surface coverage, indicating that different mechanisms of arsenic adsorption predominate at low versus high coverage. The experimentally observed surface charge reduction and net OH- release stoichiometry were compared with the theoretical stoichiometry of the surface adsorption reactions that might occur. The results provide evidence that during arsenite adsorption at low pH, i.e., pH 4.6, the oxygen of the Fe−O−As bond remained partially protonated as Fe−O(H)−As. There is evidence that the monodentate bonding mechanism might play an increasing role during arsenate adsorption on ferrihydrite with increasing pH (at pH > 8). The results of this study have provided ancillary evidence to support the experimentally observed reduced adsorption of arsenite at low pH and of arsenate at high pH.