Phosphate and Myo-inositol hexakisphosphate adsorption onto hematite as affected by Ca2+, Mg2+ and pH

Authors: FIRMANO, RUAN - FEDERAL UNIVERSITY OF SAO PAULO; PEAK, DEREK - UNIVERSITY OF SASKATCHEWAN; Schmidt, Michael - Mike; ALLEONI, LUIS - FEDERAL UNIVERSITY OF SAO PAULO

Submitted to: ACS Earth and Space Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2023
Publication Date: 11/9/2023
Citation: Firmano, R.F., Peak, D., Schmidt, M.P., Alleoni, L.R. 2023. Phosphate and Myo-inositol hexakisphosphate adsorption onto hematite as affected by Ca2+, Mg2+ and pH. ACS Earth and Space Chemistry. 7, 2420-2429. https://doi.org/10.1021/acsearthspacechem.3c00192.
DOI: https://doi.org/10.1021/acsearthspacechem.3c00192

Interpretive Summary: Soil acidity is a major problem in agricultural soils worldwide, reducing crop production. Adding lime (CaCO3 and MgCO3) to soils is a common strategy for managing soil acidity. Added Ca2+ and Mg2+ have benefits in addition to raising soil pH, including an increase in soil organic carbon (SOC) content through enhancing bonding to mineral surfaces by formation of SOC-Ca/Mg2+-mineral ternary complexes. Inorganic and organic P forms are known to bond particularly strong to Fe-oxide minerals, such as hematite, but their role in forming ternary complexes with Ca/Mg2+ and, therefore, impact on P-availability are not well established. The objective of this study was to better understand the bonding of a prevalent organic and inorganic P molecule (Myo-inositol hexakisphosphate (IHP6) and phosphate) with and without added Ca2+ and Mg2+ to better understand the role of liming on availability of soil P. Results show Ca2+ and Mg2+ addition had a greater impact on IHP6 adsorption on hematite relative to inorganic phosphate, suggesting ternary complexation may play a larger role for organic P forms. As a summary, our results suggest that inorganic forms of P would be little affected by the type of limes added (calcitic or dolomitic) in highly weathered soils of humid tropical regions, in which crystalline Fe-oxides like hematite prevail.

Technical Abstract: 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 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 the 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 influence of the studied cations . However, differences in the shape of XAS spectra at the Mg K-edge revealed that there was probably a 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 due to the intense presence of MgCl2 in the Linear Combination Fitting (LCF) analysis. In general, the results enhance molecular-level understanding of oxyanions adsorption onto hematite surfaces that can predict IHP6 and phosphate behaviors in iron oxide-rich soils.