Can Fe3+ and Al3+ ions serve as cationic bridges to facilitate the adsorption of anionic As(V) species on humic acids? A density functional theory study.

A computational chemistry investigation was undertaken to shed light on the facilitatory role played by Fe3+ and Al3+ cations in the adsorption of anionic As(V) species by humic acids through the formation of so-called cationic bridges. Geometric and energetic parameters were obtained using density functional theory at the B3LYP/6-31G(d,p) level in conjunction with the polarizable continuum model (to account for the influence of bulk water). We found that, despite their similar molecular geometries, the adsorption energies of the As(V) species AsO43- and H2AsO4- differ when Fe3+, FeOH2+, Al3+, and AlOH2+ participate in the bridge. We also found that effective adsorption of As(V) species by humic acids strongly depends on whether the considered cationic bridges are tightly coordinated by humic acids at the adsorption sites, as well as on the rigidity of these humic acid adsorption sites.