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???? from aqueous solutions using ultrasound-assisted aqueous solution ball milling. The Cr ??? removal rate were compared under the effect of ultrasound, ball milling, ultrasound-assisted ball milling for contaminated waters. Samples were characterized by X-ray diffraction, XPS, fluorescence measurements, atomic absorption and the diphenylcarbazide colorimetric method to explore Cr ??? removal mechanism by ultrasound-assisted aqueous solution ball milling. The decisive factors, including initial concentration of Cr ???, pH value, ball weight in per litre water, ultrasonic frequency, filling gases, and addition of reduction iron powder, were studied. The Cr ??? removal rates affected by different determining factors are discussed.1. Cr??? can be removed by ultrasound-assisted aqueous solution ball milling, aqueous solution ball milling, and ultrasonic processing iron ball, and the Cr??? reduction rates of three methods followed the order:ultrasound-assisted aqueous solution ball milling> ball milling> ultrasound treatment. The initial Cr??? concentration of 42 mg/L,83mg/L, and 166mg/L reached a value below 0.5 mg/L after the ultrasound-assisted milling of 2 min,10 min, and 120 min, respectively.2. The Cr ??? and total chromium concentrations in the sample of contaminated waters reduced by ultrasound-assisted aqueous solution ball milling were decreasing over time until they are undetectable. It means that Cr??? was reduced to Cr??? and settled down. The precipitate was detected by XPS, XRD. The results show that the reduction of Cr?VI? to Cr??? was complete in the precipitate, and Cr??? transformed into Cr?OH?3, FeCr2O4 could be removed from contaminated water, then the water can be reused. Fluorescence determination results show that H· and OH· free radicals were generated by three methods, and they participated in the reaction. Mechanism of removing Cr??? from contaminated waters by ultrasound-assisted aqueous solution ball milling is that Cr??? contacted with the zero-valent irons, trasformed into Cr??? precipitates of Cr?OH?3 and FeCr2O4 which were attached to the surface of iron balls with the participation of hydrogen radicals and hydrogen ions. Then the precipitates were peeled off from iron balls by ultrasonic cavitation shock and the impaction-abrasion behavior, and fresh surface was provided. The new surface of iron balls reacted with Cr??? until the Cr??? is reduced completely.3. By changing the initial pH value, the ball liquid ratio, ultrasonic frequency, adding different amounts of reduced iron powder, and filling different gas to discuss the effect for the Cr??? reduction rate. The batch data was analyzed using pseudo-first-order kinetic model to investigate the rate law of Cr??? removal for different factors. The results show that the optimal ball weight per litre water and ultrasonic frequency for ultrasound-assisted aqueous solution ball milling device are 1.7 kg/L and 20 KHz, respectively. It can be observed that the more reduced iron powder or the lower the pH value of the aqueous solution can lead to a faster reduction rate of Cr ???. The reduction rate of Cr??? with filling gas follows the order:Ar> air> N₂> O₂.