Phosphate and myo-Inositol Hexakisphosphate Adsorption onto Hematite as Affected by Ca2+, Mg2+, and pH

myo-Inositol hexakisphosphate (IHP6) is typically the most abundant form of organic phosphorus (Po) in soils, and this species is highly reactive with Al and Fe (hydr)­oxides because of the six phosphoryl groups in its structure. In this study, the effects of pH (4.5–6.5) and cations (Ca2+ and Mg2+) were investigated using in situ attenuated total reflectance Fourier transform infrared (ATR–FTIR) spectroscopy and Mg K-edge X-ray absorption spectroscopy (XAS). IHP6 has more complex infrared (IR) spectra than inorganic phosphate, even though both share certain bands of absorbances. Ca and Mg influenced the IHP6 adsorption process on the hematite surface, but this effect was less evident for phosphate. The variation in pH promoted a shift of several bands in the adsorbed IR spectra. As for adsorption kinetics, IHP6 was sensitive to a pH change from 4.5 to 6.5, with a reduction in the adsorption rate at the highest pH. Phosphate, on the other hand, presented more rapid adsorption kinetics than IHP6, with equilibrium reached in ∼80 min. Two adsorption mechanisms were identified for IHP6 and phosphate kinetics, without the influence of the studied cations. However, differences in the shape of XAS spectra at the Mg K-edge revealed that there was a probable change in the atomic environment of Mg2+ caused by its association with phosphate. In other evaluated organic molecules, such as IHP6 and citrate, this effect was not observed as a result of the intense presence of MgCl2 in the linear combination fitting (LCF) analysis. In general, the results enhance the molecular-level understanding of oxyanion adsorption onto hematite surfaces that can predict IHP6 and phosphate behaviors in iron-oxide-rich soils.