Constructing Advanced Oxidation Field in Ultrasonic toward Efficient Recovery of Palladium from Spent Catalysts

The palladium content of spent catalysts is significantly higher than that found in most natural ores, making spent catalysts important raw materials for palladium recovery. However, the palladium in the waste catalyst is encapsulated by insoluble Al2O3, and the oxidation potential of palladium is high, which makes the palladium recovery efficiency of conventional methods low. To solve this problem, this paper proposes ultrasound-enhanced persulfate oxidation to recycle palladium from waste catalysts. The leaching percentage of palladium under an ultrasound-enhanced persulfate and hydrochloric acid system is up to 97%. Kinetic studies showed that the activation energy of the reaction under ultrasonic conditions was reduced by 32.41 kJ/mol compared with the conventional conditions. Mineralogical analyses of the leaching slag are made to examine the mechanism of the ultrasound-enhanced leaching of palladium from persulfate. Measurements of the contact angle and surface energy of the solids were shown to indicate that the introduction of the ultrasonic external field accelerates the reaction rate by increasing the hydrophilicity of the solids. Electron paramagnetic resonance was introduced to probe the effect of ultrasound on the highly oxidized free radicals produced by persulfate. This work demonstrates that ultrasound-enhanced persulfate is an efficient method for recycling palladium from waste catalysts.