Solution NMR investigation of phytic acid adsorption mechanisms at the calcite-water interface

As one of the most abundant organic phosphorus (P) species in soils, phytic acid could serve as a mineralizable P reservoir in soils and sediments. It has been widely acknowledged that the adsorption of phytic acid to soil minerals retards P mineralization in soils. However, the adsorption mechanisms of phytic acid in the minerals are not clearly understood. Using solution P-31 NMR and H-1 -P-31 2D NMR, the adsorption mechanism of phytic acid was investigated at the calcite-water interface at pH 6 and 8. Maximum phytic acid adsorption reached 3.07 mmol/g, 2.60 mmol/g, 2.39 mmol/g at pH 6, 8, and 9.5, respectively. The presence of outer-sphere surface complex was evident by a lack of significant change in zeta-potential of phytic acid reacted calcite. Solution NMR analysis showed a fast exchange process between adsorbed and unreacted phytic acid at the mineral surface on an NMR time scale, also indicating the outer-sphere complexation mechanism at both pH values. Interestingly, a more active role of P5 and P4,6 in binding with calcite surface was observed at pH 6. Adsorbed phytic acid on the calcite surface should be labile and is not limiting P mineralization in the terrestrial environment.