Improvement of zero valent iron nanoparticles by ultrasound-assisted synthesis, study of Cr(VI) removal and application for the treatment of metal surface processing wastewater

Abstract : Reductive zero-valent iron nanoparticles were prepared for the treatment of Cr(VI) solutions. The ferric ions reduction by borohydride was carried out through mechanical stirring or ultrasound irradiation either in an ultrasonic bath or in a 20 kHz cup-horn reactor. The prepared materials have been characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and N2 adsorption-desorption at 77 K. The microscopic observations showed iron nanoparticles with diameter in the range 40 nm-80 nm, arranged in chain aggregates regardless the variation of the preparation method. The ultrasonic synthesis allowed to obtain higher BET specific surface area (99 m2. g-1 for nanoparticles prepared in ultrasonic bath and 145 m2. g-1 for the ones prepared by probe sonication) than the ones of samples synthesized in silent conditions (22 m2. g-1). The Cr(VI) ions removal in pure water solution was found significant at acidic pH. The removal kinetics modeled by pseudo first order, are linearly proportional to the BET surface areas. They are rapid (< 30 min to attain equilibrium), and decrease as increasing the Cr(VI) concentration and as decreasing the iron nanoparticles dose. Cr(VI) ions were removed by reduction and co-precipitation of Cr(III) in the form of chromite at pH > 7. The iron nanoparticles were tested for the treatment of metal surface processing wastewater. For such treatment, they have proven to be efficient for removing Cr(VI) together with Ni(II) ions. The iron nanoparticles prepared by 20 kHz ultrasonic probe have been found more effective for Cr(VI) removal than any kind of iron nanoparticles conventionally prepared.