Low-temperature degradation of humic acid via titanium zirconium oxide@copper single-atom activating oxygen: Mechanism and pathways

Humic acid (HA), a refractory organic pollutant, has always been a key obstackle in wastewater treatment (such as biochemical landfill leachate). And catalytic air oxidation is one of the most promising methods to degrade HA efficiently and economically. In this study, a novel high-loading Cu single-atom catalyst (TiZrO4@CuSA) was facilely synthesized using isovolume vacuum impregnation and co-calcination method, and then used to accelerate the oxidation of HA at low temperatures. The TiZrO4@5%CuSA catalytic system showed a superior performance, and 91 % of HA was degraded and 90 % of organic carbon was removed from HA solution within 10 min at 90 °C. Aberration-corrected electron microscopy, electron paramagnetic resonance spectroscopy, and density functional theory calculations were applied to reveal the catalytic mechanism of TiZrO 4@5%CuSA. The active center of Cu single-atom co-catalyst with Cu − O coordination sites can quickly transfer electrons to O2 in the water to form O2.−, exhibiting high activation efficiency. Gas chromatography (GC), liquid chromatograph-mass spectrometer (LC-MS), liquid chromatography-organic carbon detection (LC-OCD), and Fourier-transform infrared spectroscopy (FT-IR) spectroscopy provided the results of intermediate products, including 3-phenoxy-benzaldehy, phenylglyoxal, ethanol, acetic acid, butyric acid, and valeric acid, and thus a catalytic degradation pathway of HA was speculated. Based on the new catalyst, a new process can be conceived for efficient and economic removal of HA from HA-containing wastewater.