Impact of iron–organic matter complexes on aqueous phosphate concentrations

• Stable Fe(III)–organic matter (OM) complexes prevent interactions with phosphate.
• A mechanistic and thermodynamic model describing the ternary P(V)–Fe(III)–OM system is proposed.
• The model predicts that increased OM concentrations lead to increased aqueous phosphate concentrations.
• Balance between formation of Fe(III)–OM complexes and Fe(III) (hydr)oxides plays a decisive role for phosphate sequestration.

The close linkage between iron (Fe) and phosphorus (P) suggests that changes in Fe speciation may have a strong effect on the bioavailability of P. At the same time Fe speciation in natural oxic environments is known to be affected by the presence of organic matter (OM), pH and total Fe concentrations, thus these parameters should also influence the Fe–P interactions. The main objective of the present work was to study how OM affected the distribution of P(V) in the presence of Fe(III) and to address the questions if and by what mechanism(s) OM influenced the concentration of aqueous phosphate. This was accomplished by investigating the ternary P(V)–Fe(III)–OM system over a wide range of chemical conditions; [Fe]tot = 5000–50,000 μg g− 1, Fe/P = 0.5–2.0 at pH 2.9–7. Iron speciation was probed via Fe K-edge X-ray absorption spectroscopy, P speciation and concentrations were analyzed via infrared spectroscopy, and chemical equilibrium modeling was conducted to simulate the distribution of chemical species of the system. The collective results showed that the dominating species were Fe(III)–OM complexes and ferric phosphate (FePO4(s)). At low concentrations, the Fe(III)–OM complexes suppressed the formation of FePO4(s), which resulted in elevated aqueous phosphate concentrations. At high concentrations, FePO4(s) was formed and co-existed with Fe(III)–OM complexes; ternary P(V)–Fe(III)–OM complexes were not detected under any experimental condition. The collective spectroscopic and equilibrium modeling results offer a mechanistic and thermodynamic consistent explanation to why OM contributes to elevated concentrations of soluble P and thereby to increased bioavailability of P in soils and waters.