title: Removal of azobenzene from water by kaolinite.
abstract: The use of natural kaolinite clay to remove azobenzene from aqueous solutions under different pHs, ionic strengths, initial solid mass used, and initial solution concentrations was investigated. Batch kinetic experiments showed that the adsorption of azobenzene onto kaolinite followed a pseudo-second-order kinetics with an initial rate of 7.2 mg/g-h and a rate constant of 0.19 g/mg-h. The equilibrium azobenzene adsorption on kaolinite was well described by the Langmuir and Freundlich isotherms with an adsorption capacity of 11 mg/g, or 60 mmol/kg, corresponding to a monolayer adsorption on the surface of kaolinite. Adsorption increased with decreases in solution pH and increases in solution ionic strength. The enthalpy change of adsorption was -38 kJ/mol, suggesting that both physical and chemical adsorption was responsible for the retention of azobenzene on kaolinite. The high affinity of azobenzene for siloxane and gibbsite surfaces was attributed to the attractive Coulombic and van der Waals' forces between the surface and the planar structure of the organic molecule.
authors.affiliation: The Faculty of Earth Sciences, China University of Geosciences, Wuhan, Hubei 430074, China.
authors.forename: Xiaoling
authors.initials: X
authors.lastname: Zhang
authors.forename: Hanlie
authors.initials: H
authors.lastname: Hong
authors.forename: Zhaohui
authors.initials: Z
authors.lastname: Li
authors.forename: Junfang
authors.initials: J
authors.lastname: Guan
authors.forename: Laura
authors.initials: L
authors.lastname: Schulz
mesh_terms: D000327:Adsorption; D000465:Algorithms; D001391:Azo Compounds / Q000302:isolation & purification*; D006863:Hydrogen-Ion Concentration; D007616:Kaolin / Q000737:chemistry*; D007700:Kinetics; D008855:Microscopy, Electron, Scanning; D013056:Spectrophotometry, Ultraviolet; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature; D014874:Water Pollutants, Chemical / Q000302:isolation & purification*; D014961:X-Ray Diffraction
chemical_list: D001391:Azo Compounds; D014874:Water Pollutants, Chemical; D007616:Kaolin; C009850:azobenzene