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

Abstract

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 (O_V) 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 O_V of Co1Zn2 and the low valence state Co(II) promote the adsorption of ${\text{HSO}}_5^{-}$${\text{HSO}}_5^{-}$ on the surface, which facilitates the OO bond-breaking interfacial reaction to produce $S{O}_5^{\bullet -}$$S{O}_5^{\bullet -}$ and $S{O}_4^{\bullet -}$$S{O}_4^{\bullet -}$. Quenching and reactive oxygen species (ROSs) trapping experiments showed that •OH, $S{O}_4^{\bullet -}$$S{O}_4^{\bullet -}$, $O_2^{\bullet -}$$O_2^{\bullet -}$, 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 •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.