Ferrihydrite Transformation Impacted by Coprecipitation of Phytic Acid

Phytic acid is a common phosphate monoester that is present in soils due to the deposition of plant-derived materials. Thus far, its interaction with dissolved Fe and Fe minerals has not been as extensively investigated as phosphate, although it is expected be highly reactive due to its multiple phosphate functional groups. In this study, the effects of phytic acid on the formation of iron oxyhydroxide was investigated at near neutral pH as a function of the phytic acid/Fe ratio (0.05-0.5) and aging time using zeta potential measurements, X-ray diffraction, Fe K-edge X-ray absorption spectroscopy, and scanning electron transmission spectroscopy. It was found that an iron(III) phytate-like precipitate was formed when the phytic acid/Fe ratio was as low as 0.05. On increasing the ratio to 0.5, the quantity of iron(III) phytate-like precipitate increased to ∼60% in the ferrihydrite background. Interestingly, 10 month aging at 22 °C or hydrothermal treatment at 70 °C for 60 h did not transform the background ferrihydrite into goethite or hematite, suggesting the adsorbed phytic acid played an important role in inhibiting the transformation of ferrihydrite. The adsorption and incorporation of phytic acid into the Fe(III)O₆ polymers should be useful in understanding the complex phosphorus, iron, and hard acid chemistry in a terrestrial environment.