Evaluation of highly active nanoscale zero-valent iron coupled with ultrasound for chromium(VI) removal

An integrated technology consisting of highly active nanoscale zero-valent iron (nZVI) and ultrasound (US) was proposed for chromium(VI) removal. The parameters affecting Cr(VI) removal, such as the ultrasonic frequency, power, reaction temperature, initial pH and Cr(VI) concentration, were investigated. Based on the investigation and the characterization of the nZVI particles, the removal mechanism of Cr(VI) in US-nZVI system is proposed. Ultrasound not only contributed to an increase on available surface area, but also activated the surface of nZVI particles to induce many new reactive sites for the chemical reaction, which greatly enhanced the rate and efficiency of Cr(VI) reduction. Moreover, ultrasound could also remove the products that covered the surface of the nZVI particles, thus helping them to retain adequate reactive sites for Cr(VI) removal. The apparent kinetics for Cr(VI) removal by this new system could be well expressed by two-parameter pseudo-first-order model, and the observed rate constant (kobs) under ultrasound was much higher than that under shaking. The activation energy (Ea) of the process was calculated to be 21.53 kJ mol−1. The results indicated that the value of Ea was effectively reduced by introducing ultrasound into the nZVI/Cr(VI) reaction system, which was a benefit for Cr(VI) removal.