Modification of acid-activated kaolinite with TiO2 and its use for the removal of azo dyes

Acid-activated kaolinite (AAK), prepared by the calcination and acid activation of coal-bearing kaolinite, was modified with TiO2 to improve its ability to adsorb and hence remove azo dyes. X-ray diffraction analysis, N2 adsorption–desorption analysis, and high-resolution transmission electron microscopy were used to characterize the synthesized samples and confirm the formation of anatase TiO2 nanoparticles on the surface of the AAK bulk. The obtained AAK/TiO2 nanocomposites retained the porous structure of AAK and exhibited a Brunauer–Emmett–Teller surface area as high as 358.6 m2 g− 1. The dye-adsorption abilities of the synthesized samples were investigated using anionic dyes such as Congo red (CR), Direct Fast Scarlet 4BS (4BS), and weak acid dark blue 5R (5R), which are common pollutants. The dye-removal efficiency increased from 25 to 95%, 11 to 90%, and 13 to 97% for CR, 4BS, and 5R, respectively, under the same adsorption conditions, when the TiO2 content in the nanocomposite was increased from 0 to 30 mass%, indicating the anionic-dye-removal ability of AAK improved significantly after modification with TiO2. The dye-adsorption kinetics fit well the pseudo-second-order rate equation, while the adsorption isotherm data could best be described by the Langmuir adsorption model.