Study on the Catalytic Mechanism of MAl₂O₄:Cr Catalysts (M = Mg, Co, Zn) under the Synergistic Effect of Ultrasound and Simulated Solar Irradiation on the Degradation of Oil and Gas Field Wastewater

A simple sol-gel method has been utilized to synthesize MAl₂O₄:Cr catalysts (M = Mg, Co, Zn). The spinel-type MAl_1.9Cr_0.1O₄ was formed after Cr ions were introduced into MAl₂O₄, as confirmed by various characterization methods. The particle size of MAl₂O₄:Cr catalyst increases with the increasing of the M ion radius, and CoAl₂O₄:Cr catalyst has the highest uniformity with an average particle size of 60 nm. The CoAl₂O₄:Cr catalyst is effective in degrading oil and gas field wastewater through the combination of ultrasonic and simulated solar irradiation and has a high visible light absorption coefficient. The CoAl₂O₄:Cr catalyst has high cyclic stability, and its optimal catalyst content is 0.5 g/L. Capture experiment and possible mechanism analysis confirm that the electrons, holes, hydroxyl radicals and superoxide radicals are the main active species in CoAl₂O₄:Cr catalyst for the degradation of oil and gas field wastewater.