Adsorption of reactive blue (RB) 21 dye in the aqueous solution has been carried out on TiO2 alone and in combination with rare earth ions [La3+, Ce4+, Pr3+ and Gd3+] in the presence and absence of ultrasound. The formation of adsorbent (TiO2) from tetra n-butyl orthotitanate and its characterisation has been done through X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), Raman spectroscopy and N2 adsorption techniques. Complete decolourisation was achieved in 5 min in the presence of US + TiO2 + Ce. The effects of initial concentration of dye, adsorbent dose and contact time on the decolourisation of dye have been examined under different experimental conditions. The removal of dye in the presence of ultrasound was (88–99%) compared to conventional stirring (62–69%).
Adsorption behaviour has been analysed using Langmuir, Freundlich, Dubinin–Radushkevich and Temkin isotherm models. RE–TiO2 combination had higher adsorption equilibrium constant (Kc) and better adsorption capacity (qmax) than TiO2 alone, both in the absence and presence of ultrasound, indicating the formation of Dye-RE complex before sorption. This is confirmed from the shift in λmax from 626 to 616 cm−1. The kinetic data fit well with the pseudo-second order kinetic model, and the adsorption process was spontaneous in the presence of rare earths. The mechanism of adsorption has been explained with the help of Weber–Morris intraparticle diffusion and Boyd kinetic models. A batch adsorber has been proposed for different TiO2 dose to effluent volume ratios using the Langmuir equation.