Studies of the sorption of myo-inositol hexaphosphate, cadmium and humic acid to gibbsite and kaolinite

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Molecular Sciences, La Trobe University, Bendigo.

Organic phosphorus compounds account for up to 80 % of total phosphorus in soils, with myo-inositol hexaphosphate (IP6) an important source. Cycling of inositol phosphates represents a substantial gap in our understanding of the global phosphorus cycle. This thesis describes binary and ternary sorption systems involving IP6, cadmium (Cd(II)), humic acid (HA) and the soil minerals gibbsite and kaolinite. These systems were investigated using sorption experiments, solid-state 31P NMR measurements and surface complexation modelling. IP6 sorption to gibbsite decreased with increasing pH while with increasing temperature sorption increased at pH 6 and decreased at pH 10. 31P NMR information indicated sorption involved a combination of outer and inner-sphere complexation and surface precipitation. IP6 sorption to kaolinite generally decreased with increasing pH and the effect of temperature was relatively small. Using an Extended Constant Capacitance Model IP6 sorption was modelled on both minerals using a similar set of surface reactions. IP6 influenced Cd(II) sorption to both gibbsite and kaolinite. On gibbsite, Cd(II) sorption increased in the presence of IP6 with the increase more pronounced at higher concentrations. Only small differences were observed in 31P NMR spectra of binary and ternary systems. On kaolinite, IP6 increased Cd(II) sorption between pH 4-8 but supressed it at higher pH. Surface complexation modelling of both mineral systems suggested the formation of ternary surface complexes. For IP6 and HA systems, the presence of IP6 decreased HA sorption to both minerals. The influence of HA on IP6 sorption was more complex, with both the addition sequence and pH altering the changes found. Where HA was bound to the surface at lower pH before the addition of IP6, less IP6 was sorbed. At pH 6 and 10 there was relatively little desorption of IP6 from gibbsite. HA increased desorption somewhat at pH 6, but not at pH 10.