Robust High-Entropy Mg(Al0.2Cr0.2Co0.2Fe0.2Ni0.2)2O4 Oxides for Peroxymonosulfate Activation toward Oxytetracycline Degradation: Structure Identification, Performance, Mechanism, and Toxicity

A Mg(Al_0.2Cr_0.2Co_0.2Fe_0.2Ni_0.2)₂O₄ high-entropy oxide (HEO) was fabricated using a polyacrylamide gel method and applied in the photoactivated peroxymonosulfate (PMS) system for the efficient degradation of oxytetracycline (OTC). Various structural characterizations such as neutron powder diffraction have been used to confirm that the nano Mg(Al_0.2Cr_0.2Co_0.2Fe_0.2Ni_0.2)₂O₄ high-entropy oxide is obtained at 800 °C. Meanwhile, the effect of calcination temperature on the optical properties and photocatalytic activities of high-entropy oxides was explored. Results show that the Mg(Al_0.2Cr_0.2Co_0.2Fe_0.2Ni_0.2)₂O₄ xerogel sintered at 800 °C (HEO-800) showed the best catalytic activity, and the removal rate of the degradation of the OTC (75 mg/L) within 1 h was 75.9%. By exploring the influence of different reaction conditions on photocatalytic activity, the optimal reaction conditions were finally determined; namely, the initial drug concentration, catalyst content, PMS concentration, and pH value of the reaction solution were 75 mg/L, 0.75 g/L, 1 g/L, and 7.57, respectively. Through experiments such as cycling experiments and characterization of charge states and functional groups before and after photocatalysis, it was found that the HEO-800 catalyst exhibited high cycling stability and structural stability. The capture experiment and electron spin resonance (ESR)/electron paramagnetic resonance (EPR) test findings show that the hole and superoxide radical have a significant influence on the degradation process of OTC. Based on the band structure, DFT calculation, capture experiment, ESR/EPR test, and Mott-Schottky curve, the possible photocatalytic mechanism of the HEO-800/PMS/Vis system was proposed, and the degradation process of OTC after the activation of PMS molecules by transition metals was confirmed. This work offers both a method for synthesizing highly efficient and stable high-entropy oxide photocatalysts and a mechanism for effective activation of PMS.