Kinetics and adsorption behaviour of benzoate and phthalate at the α-alumina–water interface: Influence of functionality

A systematic study on the influence of functionality on adsorption of benzoate and phthalate at α-alumina–water interface was investigated. Kinetics of adsorption of benzoate and phthalate on α-alumina surfaces were performed at constant ionic strength, I = 5 × 10−4 mol dm−3 and pH 5 at 25, 30 and 40 °C. Kinetics results show that the rate of adsorption of phthalate is ≈2 times more than that of benzoate on α-alumina surfaces at 25 °C. The adsorption of benzoate and phthalate on α-alumina surfaces were carried out over a wide range of concentration of adsorbate at fixed ionic strength, I = 5 × 10−4 mol dm−3 and pH 5–10. The adsorption isotherms for both the systems were found to be Langmuir in nature. The maximum adsorption density of phthalate is 1.1–5.1 times more than that of benzoate on the same adsorbent under similar condition. This difference is attributed to the presence of adjacent COOH group in phthalate. The activation energy for both the systems was calculated using Arrhenius equation. The other thermodynamics parameters like Gibbs free energy, enthalpy and entropy were also calculated. The solubility of α-alumina in presence of benzoate and phthalate was found to be depended on the pH of the medium and the concentration of sodium chloride. The surface complexation of benzoate and phthalate on α-alumina surfaces were investigated using the Fourier transform infrared (FTIR) spectroscopy. Benzoate forms outer-sphere complexes with α-alumina surfaces at pH 5 and 6 depending on the shifting of the asymmetric and symmetric bands. Whereas phthalate forms both outer- and inner-sphere surface complexes with α-alumina surfaces.