The effect of pyrolysis temperature on the structure and uranium separation characteristics of kaolin

Kaolin (KL) is regarded as an available adsorbent for U-containing wastewater treatment owing to its rich resources, low price, high chemical stability and excellent ion-exchange ability. While, the U removal performances of KL are constrained by its structure and poor active sites. Thermal activation is an efficient way to improve the U removal capacity of KL through expanding contact aera and providing more available adsorption sites. While, there were few researches about elaborating the relationship between KL structural properties, adsorption performances and pyrolysis temperature. In this study, the structure of KL at different pyrolysis temperatures relationship with its U separation characteristics was investigated at first. According to the results of characterizations, pyrolysis temperature obviously affected the crystal structure, phase composition and microstructure of KL. By comparison, KL pyrolyzed at 500 °C (KL-500) possessed rougher surface and stripped layer structure, thus providing more adsorptive sites to combine with uranyl. The experimental results indicate that KL-500 showed higher U separation rate (90.4 %) and larger U adsorption capacity (530.8 mg/g). Meanwhile, the maximum U removal capacity of KL-500 was larger than most of reported clay-based materials, illustrating the potential of practical application. Besides, KL-500 also performed favorable recoverability with the U adsorption rate of 80.2 % at the fifth cycle, meaning that KL was expected for treating natural wastewater. The fitting results of isotherm and kinetics models verified that the removal process of U on KL-500 was mainly single layer chemical adsorption. The surface charge of KL was negative due to deprotonation at pH 5.0, thus causing strong electrostatic attraction between adsorbents and uranyl. Combining the results of XPS spectra, the immobilization of U on KL-500 was mainly realized through electrostatic interaction and complexation, which was a combination process of physical and chemical adsorption, with chemical adsorption dominating.