Kinetics and Mechanism of Methylene Blue Adsorption by a TiO2/Diatomite Nanocomposite and Its Components

A study was made of the kinetics of the adsorption of the methylene blue dye from an aqueous solution on the photocatalyst DDT (nanosized titanium dioxide in the anatase phase, deposited on diatomite) and its components: diatomite D and anatase TiO2. The effect of the initial concentration and pH of the methylene blue solution on the rate of the adsorption process was investigated. The kinetic data of adsorption were processed using two simplified kinetic models, one of the pseudo-first-order and the other of pseudo-second-order. To investigate the adsorption mechanism, a model of intraparticle diffusion kinetics was employed. The adsorption kinetics of methylene blue on the surfaces of D, TiO2, and DDT was found to be best described by the pseudo-second-order model. It was shown that the adsorption of methylene blue on the D and DDT adsorbents is a multistep process involving adsorption on the external surface and inside particles, with the limiting step being a chemical reaction. For the adsorption on TiO2, the limiting step is the external diffusion.