Mineralogical confirmation of a near-P:Fe = 1:2 limiting stoichiometric ratio in colloidal P-bearing ferrihydrite-like hydrous ferric oxide

We present a chemical and mineralogical explanation, derived from powder X-ray diffraction and Mössbauer spectroscopy measurements of synthetic samples, of the P:Fe = 1:2 limiting ratio of P incorporation (as PO₄) that was previously observed in natural aquatic oxic iron precipitates. The ⁵⁷Fe Mössbauer hyperfine parameters are interpreted with the help of state-of-the-art ab initio electronic structure calculations. We find that there is a strong tendency for solid solution P-Fe mixing in the P-bearing hydrous ferric oxide (P-HFO) aqueous coprecipitate system, interpreted as occurring between the P-free (ferrihydrite) end-member and an inferred P:Fe = 1:2 end-member beyond which P is not incorporated in the structure of the P-HFO solid. Up to and somewhat beyond the limiting end-member P:Fe ratio, all available P is scavenged by the coprecipitation reaction, suggesting strong P-Fe complexation in the precipitation-precursor dissolved species. The P-HFO solids are more stable (i.e., have stronger chemical bonds) than the P-free ferrihydrite end-member. We show that in coprecipitation the P specifically incorporates within the nanoparticle structure rather than complexing to the nanoparticle surface. Our results are relevant to the question of the mechanisms of coupling between the Fe and P cycles in natural aqueous environments and highlight a strong affinity between Fe and P in aqueous environments.