Some Structural and Electronic Features of the Interaction of Phosphate with Metal−Humic Complexes

Previous studies demonstrated the formation of stable phosphate−metal−humic complexes in solution. These studies, however, indicated that the proportion of complexed metal that intervenes in phosphate fixation is rather low. In this study we investigate the possible structural and electronic features of the binding site involved in phosphate fixation in metal−humic complexes that could explain this fact. To this end, we have studied phosphate−metal−humic complexes involving Fe(III), Al(III), and Zn(II) using three complementary techniques: infrared spectroscopy (FTIR), fluorescence, and molecular modeling. The FTIR study indicated that, in the case of those complexes involving Fe and Zn phosphate, fixation is associated with a stabilization of the metal−carboxylate bond. In the case of Al this effect is less clear. This effect of phosphate fixation on the characteristics of the metal−humic binding site was also supported by the results obtained in the Fluorescence study, which showed significant changes in the quenching effect normally associated with metal complexation in humic substances upon phosphate fixation. Finally, the molecular modeling study revealed that the stability of phosphate−metal−humic complexes is inversely related to the stability of the metal−humic interaction. This result could explain why only a relatively low proportion of humic complexed metal is involved in phosphate fixation.