Adsorption-Desorption of Myo-Inositol Hexakisphosphate on Hematite

Abstract

Adsorption, desorption, and precipitation reactions at environmental interfaces impact the bioavailability, mobility, and fate of organic phosphates in terrestrial and aquatic environments. Myo-inositol hexakisphosphate (IHP) is the most abundant organic phosphate in soils. The adsorption/desorption characteristics of IHP on/from hematite were studied using batch adsorption and desorption experiments, Zeta (ζ) potential measurement, and in situ attenuated total reflectance Fourier transform infrared spectroscopy. The experiments were also conducted with phosphate (Pi) for comparison. Adsorption of both IHP and Pi has an initial rapid uptake followed by a slow adsorption process. The adsorbed amounts of IHP and Pi on hematite were 0.67 and 1.78 μmol m at pH 5, respectively, decreasing with increasing pH. At all pH levels between 3 and 10, the surface of IHP adsorbed hematite was highly negative charged, leading to increasing IHP adsorption with ion strength. The desorption of IHP/Pi decreased sharply with the increasing desorption cycle and decreasing preadsorption amount. The desorption curves of IHP/Pi by 0.02 mol L KCl or H2O can be well fitted by an exponential equation, whereas those for citrate followed a linear equation. Readsorption occurred in the later stage of Pi desorption by H2O, which was not observed in case of IHP probably because of the greater steric hindrance of IHP with the larger molecular size. In combination with adsorption-desorption characteristics, ζ potential measurements, and attenuated total reflectance Fourier transform infrared spectroscopy analysis, it is suggested that each adsorbed IHP molecule binds on hematite in the form of inner-sphere surface complexes via two phosphate groups.