Development of novel composite materials based on kaolinitic clay modified with ZnO for the elimination of azo dyes by adsorption in water

In this work, ZnO nanoparticles, synthesized by the sol-gel process, are immobilized on the external surface of raw kaolinite particles and kaolinite activated by different treatments: heat treatment at 600, 700 and 800 °C; treatment in a dimethyl sulfoxide (DMSO) medium; hot acid treatment (HCl, 6M) under reflux conditions or heat treatment at 800 °C followed by acid treatment. Characterization confirmed the successful immobilization of the nanocrystalline ZnO particles in the hexagonal structure of the different clay matrices. Measurement of the zeta potential showed a sudden inversion of the nature of the surface charge of certain composite materials obtained, through zeta potential values ranging from −31 mV before doping with ZnO to +36 mV after doping. The raw kaolinite and certain composites obtained were tested in batch mode for the adsorption in aqueous solution of three anionic azo textile dyes: a monozoic (Mordant Red 19, MR19), diazoic (Direct Blue 53, DB53) and a triazoic (Direct Green 1, DG1) dye. Compared to raw kaolinite, a linear and rapid increase in the quantity of dye adsorbed is observed during the first 5 min with retention rates around 95% for the best composite materials. The adsorption efficiency strongly depends on the zeta potential of the material: the higher the latter is towards positive values, the better the adsorption capacities of the samples towards these anionic textile dyes.