Functional Group Specific Phytic Acid Adsorption at the Ferrihydrite–Water Interface

Phytic acid, as the dominant organic phosphorus species in soils, always preferentially adsorbs on Fe/Al minerals. In this study, adsorption mechanisms of phytic acid at the ferrihydrite-water interface were investigated as a function of pH using batch adsorption experiment, zeta potential measurements, in situ P K-edge X-ray absorption near edge structure spectroscopy, and solution ³¹P nuclear magnetic resonance spectroscopy. With increasing [phytic acid] from 0 to 50 μM, the isoelectric point of ferrihydrite decreased from ∼8.5 to <5. The phytic acid adsorption envelope showed that (i) adsorption decreased with increasing pH from 5 to 9 and (ii) adsorption was enhanced with increasing ionic strength. These macroscopic data are supported by the in situ spectroscopic data of inner-sphere complexation of phytic acid at the ferrihydrite-water interface. The results of solution ³¹P NMR showed the preferential affinities of P1,3 and P2 phosphate functional groups toward the adsorbent at pH 5 and 8.5, as evidenced by the changes in chemical shifts of the three major reactive phosphate groups. Furthermore, deconvoluted peaks of these functional groups showed that P1, P2, and P3 were the most active functional groups and followed by P5 and P4,6.