Removal of toxic cadmium (II) from zinc sulfate solution with zinc powder enhanced by ultrasound: Kinetics and mechanism

Abstract

Currently, removing cadmium (II) from zinc sulfate electrolytes using zinc powder presents several problems, such as the agglomeration and encapsulation of the zinc powder, which leads to increased consumption of the zinc powder and poor removal performance. To overcome this limitation, ultrasound is used to assist the replacement of the zinc powder to remove cadmium (II) from zinc sulfate electrolytes. Compared with conventional replacements, the ultrasonic-assisted replacements reduce the reaction time from 120 to 40 min, the reaction temperature from 60 to 55 ℃, and the required amount of zinc powder from 5 to 3 g/L. Under the same process conditions, the replacement rate of cadmium (II) is increased from 68.9 to 98.7 %. According to kinetic studies, ultrasound reduces the required activation energy for replacement from 34.77 to 16.83 kJ/mol, which significantly reduces the energy barrier of the cadmium precipitation. In addition, ultrasound can remove the reaction layer between the zinc powder and cadmium (II), thus accelerating the chemical reaction rate. The kinetic behavior changes when the initial cadmium (II) concentration is in the range of 800–1200 mg/L in conventional experiments, and 1600–2000 mg/L in ultrasound experiments. The characterization of the purification slags confirms that ultrasound can refine particles, improve the replacement rate of cadmium (II), and significantly reduce the consumption of the zinc powder. The ultrasound-enhanced purifying technology of zinc sulfate electrolytes with zinc powder is of great significance for zinc electrowinning and comprehensive recovery of cadmium.