Molecular Orbital Theory Study on Surface Complex Structures of Phosphates to Iron Hydroxides:  Calculation of Vibrational Frequencies and Adsorption Energies

Quantum mechanical calculations were applied to resolve controversies about phosphate surface complexes on iron hydroxides. Six possible surface complexes were modeled:  deprotonated, monoprotonated, and diprotonated versions of bridging bidentate and monodentate complexes. The calculated frequencies were compared to experimental IR frequency data (Persson et al. J. Colloid Interface Sci. 1996, 177, 263−275; Arai and Sparks J. Colloid Interface Sci. 2001, 241, 317−326.). This study suggests that the surface complexes change depending on pH. Four possible species are a diprotonated bidentate complex at pH 4−6, either a deprotonated bidentate or a monoprotonated monodentate complex at pH 7.5−7.9, and a deprotonated monodentate complex at pH 12.8. In addition, reaction energies were calculated for adsorption from aqueous solution to determine relative stability to form a monoprotonated monodentate complex and a deprotonated bidentate complex. According to these results, the monoprotonated monodentate complex should be favored. Vibrational frequencies of the monoprotonated monodentate and deprotonated bidentate complexes were analyzed with electronic effects on the Fe−OP and H−OP bonds.