Removal of hexavalent chromium from contaminated waters by ultrasound-assisted aqueous solution ball milling

Batch mode experiments were conducted to study the removal of hexavalent chromium (Cr(VI)) from aqueous solutions using ultrasound-assisted aqueous solution ball milling. The results show that the reduction rate of Cr(VI) by ultrasound-assisted aqueous solution ball milling was significantly faster than that by ball milling or ultrasound treatment alone, and an initial Cr(VI) concentration of 166 mg/L could be decreased to 0.35 mg/L at 120 min. The decisive factors, including initial concentration of Cr(VI), pH value, ultrasonic frequency and filling gas, were studied. It was found that the optimal ultrasonic frequency for ultrasound-assisted aqueous solution ball milling device was 20 kHz, and the rate of Cr(VI) reduction as a function of filling gas followed the order: Ar > air > N₂ > O₂. Samples were characterized by X-ray diffraction, fluorescence measurements, atomic absorption and the diphenylcarbazide colorimetric method. The Cr(VI) transformed into a precipitate that could be removed from the contaminated water, after which the water could be reused.