The effect of humic acid on the sorption and desorption of myo‐inositol hexaphosphate to gibbsite and kaolinite

Summary The effect of humic acid (HA) on myo-inositol hexaphosphate (IP6) sorption and desorption to gibbsite and kaolinite was investigated with measurements of macroscopic sorption. Gibbsite tended to have a larger sorption capacity for both HA and IP6. The behaviour in the ternary systems, where both HA and IP6 were present, was complex. Both pH and the addition sequence of HA and IP6 were found to be important in affecting IP6 and HA sorption to both minerals; with more noticeable effects on the gibbsite surface than on kaolinite. When HA was added first, IP6 sorption decreased when the pH was less than 9, whereas HA sorption decreased over the whole pH range. The addition of HA to mineral-bound IP6 had little effect on IP6 sorption, but HA sorption decreased. The observed decreases in HA and IP6 sorption most probably result from a reduction in the availability of sites at the surface and electrostatic repulsion between the negatively charged HA and IP6 ions at the surface. Desorption experiments indicate that IP6 was bound strongly to the gibbsite surface, especially at pH 6 where little IP6 was released into solution, even in the presence of HA. Highlights Aim to understand how the sorption of inositol hexaphosphate (IP6) and humic acid (HA) are affected when both are present in soil. Both the nature of the mineral surface and order of addition of adsorbates affect the amount of each sorbed. The presence of HA on the mineral surface decreases the amount of IP6 that sorbs at lower pH values, but it cannot displace IP6 from the surface. The presence of sorbed HA is likely to affect the plant-available IP6 in soil systems.