Hierarchical nickel cobalt oxide spinel microspheres catalyze mineralization of humic substances during wet air oxidation at atmospheric pressure

Humic substance (HS) is a key pollutant in water supply or wastewater treatment. In this work, hollow microspheres constituted of NiCo2O4 (NCO) spinel were synthesized and used as the catalyst of catalytic wet air oxidation, aiming to remove HS at atmospheric pressure and low temperature. The results show that NCO had excellent hierarchical architectures with large surface area (66.88 m(2)/g), solid-state redox couples, numerous surface oxygen species, and low reduction temperature. When NCO was used to treat HS solution at 90 degrees C, total organic carbon concentration decreased by 93.4% after 24 h. NCO itself had the ability to mineralize the HS adsorbed on the surface, and dissolved oxygen can fill oxygen vacancies and recover NCO to its original state. Hydroxyl radicals also assisted the HS degradation. These findings provide some insight into the HS degradation process and promise an inexpensive and easy way to clean HS-rich water or wastewater.