Study Of Adsorption Of Azobenzene Onto Kaolinite Surfaces

Azo dyes are the most common components of organic pigments.The issues of how to solve the azo dye wastewater caused widely concerned.Kaolinite is a kind of clay minerals and it is widely applied for adsorbing toxicant.This paper aims to study the interaction between azobenzene and kaolinite surfaces.Adsorption of organic molecule azobenzene onto kaolinite surfaces was calculated using ab-initio quantum chemistry method in Gaussian 03 program.The results show that adsorption of azobenzene on the siloxane surface of kaolinite has the lowest energy,and therefore,the siloxane surface is the most possible position for adsorbing azobenzene.Adsorption of azobenzene at different surface positions of kaolinite suggests that the stability of the reaction models and the possibility of electron transfer decrease as the following order:the siloxane surface,side(010) plane,the surface ofgibbsite layer.Azobenzene adsorption at other positions of kaolinite surface the adsorption reaction will probably not occur according to the higher energies and lower electron transfer of the reaction models.The adsorption of azobenzene onto kaolinite:surfaces were studied by different conditions:pHs, initial solution concentrations,kaolinite dosages,crystallinity,granularity,ionic strengths.Batch adsorption kinetic experiments revealed that the adsorption of azobenzene onto kaolinite involved fast and slow processes.It was found that the adsorption in the azobenzene/kaolinite system follow pseudo-second-order kinetics.The azobenzene adsorpion on kaolinite is well described by the Langmuir isotherm.The adsorption capacity reached as high as 11 mg/g.The enthalpy change of (ΔH) is -35 kJ/mol,indicating retention of azobenzene on kaolinite is mainly via physical and chemical adsorption.The high affinity of azobenzene for siloxane surface and gibbsite surface was attributed to the attractive Coulombic and van der Waals' forces between the surface and the planar structure of the organic ligand.The Azobenzene adsorbed increased as the kaolinite dosages increasd and the pH value decreased.Low crystallinity and small size is propitious to the adsorption.It is showed a positive correlation between Na~+ concentration and the adsorption.The studies showed that the main factors influenced the adsorption results were as follows:solution concentration,pH,temperature. The best conditions for kaolinite to adsorbing azobenzene adsorption were:normal temperature, the concentration of the solution is not more than 30mg/L,pH value is between 4 to 6.XRD and FTIR analyses show high affinity of azobenzene for siloxane and gibbsite surfaces, which was attributed to the attractive Coulombic and van der Waals' forces between the surface and the planar structure of organic ligand without any intercalation.