Ferrihydrite coating on kaolinite assisted by ultrasound for improving Cr (VI) adsorption in aqueous solutions

Kaolinite was coated with ferrihydrite nanoparticles at various ferrihydrite/kaolinite ratios by Fe3+ precipitation under either magnetic stirring or ultrasound irradiation. The prepared materials were characterized by TGA, SEM-EDX, XRD, optical microscopy, Raman spectroscopy and by N-2 adsorption-desorption measurements (77 K). Cr(VI) removal was studied in batch experiments. The influence of pH on Cr(VI) adsorption, Cr(VI) adsorption kinetics and isotherms were studied at pH 3 and 5 (25 degrees C). The content and composition of ferrihydrite precipitates on kaolinite were determined by TGA. The ultrasound-assisted synthesis route slightly increased the specific surface area of the prepared materials and allowed the formation of a homogeneous coating of ferrihydrite nanoparticles on the kaolinite platelet surface (observed by SEM and Raman). Cr(VI) removal by the coated ferrihydrite kaolinites is maximal at acidic pH (pH <= 3). The Cr(VI) removal kinetics at pH 3 and 5 are well described by a pseudo-second-order and diffusion model. The diffusion-controlled Cr(VI) adsorption kinetics are promoted by the ferrihydrite coating films formed under ultrasound irradiation. The Langmuir-type Cr(VI) adsorption isotherms at pH 3 on coated kaolinites prepared by magnetic stirring exhibit adsorption uptake increasing from 6.17 mg.g(-1) to 14.40 mg.g(-1) as the ferrihydrite content increases from 12% to 34%. At the latter ferrihydrite content, the adsorption capacity of the ultrasonically coated material reaches 20.41 mg.g(-1). The higher Cr(VI) adsorption efficiency of ultrasound-prepared ferrihydrite deposits compared to samples prepared in silent conditions is attributed to their smaller nanoparticle size and the consequent higher amount of hydroxyl surface groups.