Surface complexation modeling of inositol hexaphosphate sorption onto gibbsite.

Abstract The sorption of Inositol hexaphosphate (IP 6 ) onto gibbsite was investigated using a combination of adsorption experiments, 31 P solid-state MAS NMR spectroscopy, and surface complexation modeling. Adsorption experiments conducted at four temperatures showed that IP 6 sorption decreased with increasing pH. At pH 6, IP 6 sorption increased with increasing temperature, while at pH 10 sorption decreased as the temperature was raised. 31 P MAS NMR measurements at pH 3, 6, 9 and 11 produced spectra with broad resonance lines that could be de-convoluted with up to five resonances (+5, 0, −6, −13 and −21 ppm). The chemical shifts suggest the sorption process involves a combination of both outer- and inner-sphere complexation and surface precipitation. Relative intensities of the observed resonances indicate that outer-sphere complexation is important in the sorption process at higher pH, while inner-sphere complexation and surface precipitation are dominant at lower pH. Using the adsorption and 31 P MAS NMR data, IP 6 sorption to gibbsite was modeled with an extended constant capacitance model (ECCM). The adsorption reactions that best described the sorption of IP 6 to gibbsite included two inner-sphere surface complexes and one outer-sphere complex: AlOH + IP 6 12 - + 5 H + ↔ Al ( IP 6 H 4 ) 7 - + H 2 O 3 AlOH + IP 6 12 - + 6 H + ↔ Al 3 ( IP 6 H 3 ) 6 - + 3 H 2 O 2 AlOH + IP 6 12 - + 4 H + ↔ ( AlOH 2 ) 2 2 + ( IP 6 H 2 ) 10 - The inner-sphere complex involving three surface sites may be considered to be equivalent to a surface precipitate. Thermodynamic parameters were obtained from equilibrium constants derived from surface complexation modeling. Enthalpies for the formation of inner-sphere surface complexes were endothermic, while the enthalpy for the outer-sphere complex was exothermic. The entropies for the proposed sorption reactions were large and positive suggesting that changes in solvation of species play a major role in driving the sorption process.