title: Surface complexation modeling of inositol hexaphosphate sorption onto gibbsite.
abstract: The sorption of Inositol hexaphosphate (IP6) 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 IP6 sorption decreased with increasing pH. At pH 6, IP6 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 -21ppm). 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, IP6 sorption to gibbsite was modeled with an extended constant capacitance model (ECCM). The adsorption reactions that best described the sorption of IP6 to gibbsite included two inner-sphere surface complexes and one outer-sphere complex: ≡AlOH + IP₆¹²⁻ + 5H⁺ ↔ ≡Al(IP₆H₄)⁷⁻ + H₂O, ≡3AlOH + IP₆¹²⁻ + 6H⁺ ↔ ≡Al₃(IP₆H₃)⁶⁻ + 3H₂O, ≡2AlOH + IP₆¹²⁻ + 4H⁺ ↔ (≡AlOH₂)₂²⁺(IP₆H₂)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.
authors.affiliation: La Trobe University, P.O. Box 199, Bendigo, VIC 3552, Australia.
authors.forename: Maika
authors.initials: M
authors.lastname: Ruyter-Hooley
authors.affiliation: Chemistry of Interfaces, Luleå University of Technology, S-971 87 Luleå, Sweden.
authors.forename: Anna-Carin
authors.initials: AC
authors.lastname: Larsson
authors.affiliation: La Trobe University, P.O. Box 199, Bendigo, VIC 3552, Australia.
authors.forename: Bruce B
authors.initials: BB
authors.lastname: Johnson
authors.affiliation: Chemistry of Interfaces, Luleå University of Technology, S-971 87 Luleå, Sweden; Department of Physics, Warwick University, CV7 4AL Coventry, UK.
authors.forename: Oleg N
authors.initials: ON
authors.lastname: Antzutkin
authors.affiliation: La Trobe University, P.O. Box 199, Bendigo, VIC 3552, Australia. Electronic address: m.angove@latrobe.edu.au.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Angove
mesh_terms: D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D010833:Phytic Acid / Q000737:chemistry*; D013499:Surface Properties; D013816:Thermodynamics
chemical_list: D010833:Phytic Acid
keywords: (31)P MAS NMR; Adsorption; ECCM; IP(6); Organic; Phosphorous