Green closed-loop preparation-recovery-regeneration of Co-Zn oxide nanoblocks for PMS activation using novel deep eutectic solvent-microfluidic injection method

The cobalt-based oxide-activated PMS process for organic wastewater treatment has received widespread attention. However, the recovery and regeneration of catalysts are still restricted by many limitations. Herein, a novel green and close-loop recyclable deep eutectic solvents (DESs)-microfluidic injection method (choline chlorine-oxalic acid system) for the preparation of cobalt-zinc oxide nanoblocks with high oxygen vacancy (OV) content is proposed. The catalyst Co1Zn2 prepared with the optimal elemental molar ratio achieved a 99.9 % degradation rate of RhB in 20 min. Its reaction kinetic constant k is 40.9 times that of Co3O4. Moreover, the catalyst can be completely green-regenerated after several cycles. DFT calculations indicate that the OV of Co1Zn2 and the low valence state Co(II) promote the adsorption of HSO 5 on the surface, which facilitates the O-O bond-breaking interfacial reaction to produce SO center dot 5 and SO center dot 4. Quenching and reactive oxygen species (ROSs) trapping experiments showed that center dot OH, SO center dot 4 , O center dot 2 , and 1O2 all contributed to the degradation of RhB, with 1O2 dominating the reaction. The RhB degradation mechanism is investigated experimentally and combined with DFT calculations. The results indicate that the thermodynamically spontaneous barrier-free reactions of center dot OH at the C6 and C11 sites on the A and C rings are the predominant initial reaction pathways. Finally, RhB degradation pathways and intermediate product toxicity were analyzed. The work provides a closed-loop green cycle strategy from preparation, utilization to regeneration of metal oxide for PMS activation.