Mechanism of Myo-inositol Hexakisphosphate Sorption on Amorphous Aluminum Hydroxide: Spectroscopic Evidence for Rapid Surface Precipitation

Inositol hexakisphosphates are the most abundant organic phosphates (OPs) in most soils and sediments. Adsorption, desorption, and precipitation reactions at environmental interfaces govern the reactivity, speciation, mobility, and bioavailability of inositol hexakisphosphates in terrestrial and aquatic environments. However, surface complexation and precipitation reactions of inositol hexakisphosphates on soil minerals have not been well understood. Here we investigate the surface complexation–precipitation process and mechanism of myo-inositol hexakisphosphate (IHP, phytate) on amorphous aluminum hydroxide (AAH) using macroscopic sorption experiments and multiple spectroscopic tools. The AAH (16.01 μmol m<sup>–2</sup>) exhibits much higher sorption density than boehmite (0.73 μmol m<sup>–2</sup>) and α-Al<sub>2</sub>O<sub>3</sub> (1.13 μmol m<sup>–2</sup>). Kinetics of IHP sorption and accompanying OH<sup>–</sup> release, as well as zeta potential measurements, indicate that IHP is initially adsorbed on AAH through inner-sphere complexation via ligand exchange, followed by AAH dissolution and ternary complex formation; last, the ternary complexes rapidly transform to surface precipitates and bulk phase analogous to aluminum phytate (Al-IHP). The pH level, reaction time, and initial IHP loading evidently affect the interaction of IHP on AAH. In situ ATR-FTIR and solid-state NMR spectra further demonstrate that IHP sorbs on AAH and transforms to surface precipitates analogous to Al-IHP, consistent with the results of XRD analysis. This study indicates that active metal oxides such as AAH strongly mediate the speciation and behavior of IHP via rapid surface complexation–precipitation reactions, thus controlling the mobility and bioavailability of inositol phosphates in the environment.