Size Distribution, Complexing Capacity, and Stability of Phosphate−Metal−Humic Complexes

Abstract

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.

Efficiency of a new strategy involving a new class of natural hetero‐ligand iron(III) chelates (Fe(III)‐NHL) to improve fruit tree growth in alkaline/calcareous soils

“…The presence of this ligand might impede the formation of intermolecular metal bridges among natural chelating molecules, thus allowing the stable complexation of a larger metal concentration as well as the formation of metal‐natural chelates of smaller size. We have observed these mechanisms in studies on the complexation of phosphate by metal‐humic complexes 12, 14, 15…”

“…In the case of humic‐related chelates or complexes, the problems of their low efficiency under soil conditions derive from their low average stability and very large size, which cause chelate and metal precipitation in soil 10, 11. A number of studies show that metal complexation by humic systems involves the formation of large molecular aggregates via intermolecular metal bridges 12. This suggests that only a small fraction of metal, that involved in metal bridges, is complexed with high stability, whereas a major metal fraction is associated with molecular aggregation through interactions with low stability 13.…”

Incorporation of humic-derived active molecules into compound NPK granulated fertilizers: main technical difficulties and potential solutions

“…In fact, several studies reported that humic-metal complexes involving many diverse cations were able to complex significant amounts of phosphate in aqueous solutions with pH values ranked between 4 and 8 [40][41][42]. These results also showed that the stability of humic-metalphosphate complexes did not significantly vary with pH [42].…”

“…Further studies demonstrated that these types of humic-metalphosphate complexes can be obtained in the laboratory [40][41][42][43]. These studies also showed that these complexes were stable and soluble in soil solutions with pH values ranked between 4 and 8 [41,42]. Other studies showed that these humic-metal-phosphate complexes prevented phosphate soil fixation and provided available P to different plant species [46].…”

Theoretical chemical characterization of phosphate‐metal–humic complexes and relationships with their effects on both phosphorus soil fixation and phosphorus availability for plants

“…As far as the formation of stable and soluble PMHA is concerned, recent studies have described their viability, stability, and solubility in solution, as well as the electronic conditions that favour their formation . However, no information exists about both their dynamics in soil and plant availability in relation to their physicochemical properties.…”