Size distribution, complexing capacity, and stability of phosphate-metal-humic complexes

Size distribution, maximum complexing ability, and stability constants for phosphate-metal-humic (PO43--M-HA) complexes involving two trivalent (Fe and Al) and five divalent metal (M) bridges (Zn, Cu, Mn, Ca, and Mg) were investigated at the pH values 4, 6, and 8. Results highlighted the existing competition between metal-humic acid (M-HA) aggregation and the formation of PO43--M-HA complexes. However, the fact that only a very low fraction of complexed metal is involved in PO43- fixation seems to be related to the existence of specific electronic and/or steric requirements in the binding site in the metal-humic complex. The importance of the ionic form of phosphate (H2PO4- or HPO42-) and the involvement of phenolic and especially carboxylic groups in the phosphate binding are discussed. Finally, the order of stability obtained for PO43--M-HA complexes was similar to that of M-HA complexes. This result suggests that PO43--M-HA might play a significant role in the dynamics of phosphorus in certain soil types.