The divergent role of inorganic versus organic P during the Fe(II)-catalyzed transformation of ferrihydrite

In reduced soils, phosphorus (P) biogeochemistry is strongly governed by iron (Fe) oxides, particularly poorly crystalline ferrihydrite due to its large surface area. Ferrihydrite is metastable and naturally transforms into more stable phases such as lepidocrocite and goethite. Under reducing conditions, under reducing conditions, adsorption of Fe(II) on the Fe(III) oxyhydroxide surface catalyzes this transformation via iron atom exchange (IAE). Structural impurities, including retained anions and organic ligands, influence transformation kinetics and products. Stable ⁵⁷Fe isotope tracer experiments showed that neither P nor organic ligands prevent initial IAE, but P strongly suppresses lepidocrocite and goethite recrystallization, whereas organic ligands were less effective.

P speciation in natural environments is complex and includes inorganic (P_in) and organic (P_org) forms. P_org, especially inositol phosphates, often dominates total soil P and acts as a long‑term sink when retained by poorly crystalline Fe oxides. Different P compounds interact differently with ferrihydrite surfaces, thereby potentially influencing its transformation products in various ways. However, to the best of our knowledge, the influence of P speciation on Fe(II)-catalyzed ferrihydrite transformation remains unexplored.

In this study, ^NAFe ferrihydrite (⁵⁴Fe: ~5.84%; ⁵⁶Fe: ~91.76%; ⁵⁷Fe: ~2.12%; ⁵⁸Fe: ~0.28%) co‑precipitated with inorganic P (Pⁱⁿ‑Fh) and/or organic P (inositol phosphate; P_org‑Fh) suspensions were spiked with a ⁵⁷Fe(II) stock solution (⁵⁷Fe = 97.3%) to reach a Fe(II):Fe(III) ratio of 0.14 mol/mol. Suspensions were incubated for 4 weeks at pH 6.0 (40 mM MES buffer). Variations in aqueous P and Fe(II) concentrations, as well as the Fe isotopic composition of the solution, used to track atom exchange between solid ^NAFe(III) and aqueous ⁵⁷Fe(II), were measured by inductively coupled plasma mass spectrometry (QQQ‑ICP‑MS). The mineral composition of the solid phase was determined by X‑ray diffraction (XRD), Fe extended X‑ray absorption fine structure (EXAFS) spectroscopy, and ⁵⁷Fe Mössbauer spectroscopy.

The obtained results demonstrate that neither inorganic nor organic P fully prevented IAE; however, P_org‑Fh showed a faster and overall greater exchange compared to P_in‑Fh. After 4 weeks of incubation, P_org‑Fh resulted in nearly complete atom exchange, while only 60% of the atoms were exchanged in P_in‑Fh. This effect can be attributed to the pronounced decrease in ferrihydrite surface charge induced by P_org co‑precipitation, which may have promoted aqueous Fe(II), especially at pH < 7.

After 2 weeks of incubation, XRD showed that P_org‑Fh progressively started to transform into lepidocrocite, while no changes were detected for P_in‑Fh. Fe‑EXAFS showed that initial P_org‑Fh transformed into 40% lepidocrocite, and high‑resolution Mössbauer temperature profiles further confirmed the presence of <10% goethite fractions. P_in‑Fh was nearly unchanged in Fe‑EXAFS, while Mössbauer showed a slight increase in blocking temperature, likely associated with increased mineral structural ordering.

The release of P into solution was <1% of the P initially retained in the solid and was entirely attributed to P_in. No P_org release was detected. Overall, our results indicate that ferrihydrite is an effective sink for P during Fe(II)‑catalyzed transformation. P_org is much more strongly retained than P_in, which likely limits its availability for biological degradation processes and favors accumulation over time.