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Isothermal titration calorimetry as a useful tool to examine adsorption mechanisms of phosphate on gibbsite at various solution conditions
Author(s) -
Hong Zhineng,
Yan Jing,
Jiang Jun,
Li Jiuyu,
Xu Renkou
Publication year - 2020
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.1002/saj2.20101
Subject(s) - gibbsite , adsorption , isothermal titration calorimetry , chemistry , endothermic process , enthalpy , titration , inorganic chemistry , exothermic reaction , calorimetry , fourier transform infrared spectroscopy , isothermal process , exothermic process , thermodynamics , kaolinite , chemical engineering , mineralogy , organic chemistry , physics , engineering
Despite extensive prior work with batch and spectroscopic methods, little information is available about adsorption thermodynamics of phosphate (P) on Al (oxyhydr)oxides. Here we directly quantified for the first time the adsorption heat of P on gibbsite at varying pH (3.0–9.0) and ionic strength (IS, 1–100 mM) with isothermal titration calorimetry (ITC). The obtained thermograms showed a strong effect of pH but a negligible influence of IS on the adsorption thermodynamics. Coupled with results from other methods including batch experiment, in‐situ attenuated total reflectance Fourier‐transform infrared spectroscopy, and nuclear magnetic resonance, the ITC data revealed contrasting mechanisms for P adsorption at pH 9.0 and 3.0. At pH 9.0, P adsorption was probably initiated by a reaction of P with Al(OH) 4 − (originated from the solubilization of gibbsite) with high equilibrium constant ( k ) and small exothermic enthalpy change (Δ H ), followed by a ligand exchange between P and the > Al–O − (Al hydroxyl groups on gibbsite surface) with low k and relatively large exothermic Δ H . In contrast, P adsorption at pH 3.0 likely underwent a fast ligand exchange between P and the > Al‐OH 2 + on gibbsite surface with high k and exothermic Δ H and then a slow surface precipitation of P with Al 3+ due to gibbsite solubilization with low k and endothermic Δ H . This study provides complementary information to the prior knowledge on P adsorption on (hydr)oxides and demonstrates how to use ITC in combination with other methods for mechanistic examination of ions adsorption on the surfaces of soils and sediments.