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 N2 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 °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. [Display omitted] •Kaolinites coating by ferrihydrite nanoparticles precipitation assisted by ultrasound for Cr(VI) adsorption.•Structural and textural characterization of ferrihydrite coated kaolinites.•Higher specific surface areas, more uniform deposits for the syntheses assisted by ultrasounds compared to silent conditions.•Ultrasound preparation increases Cr(VI) adsorption, as the ferrihydrite/kaolinite ratio increases and the particle size decreases.•Adsorption kinetics controlled by the diffusion through the ferrihydrite coating films.