Organic wastewater treatment by a single-atom catalyst and electrolytically produced H2O2

Abstract

The presence of organic contaminants in wastewater poses considerable risks to the health of both humans and ecosystems. Although advanced oxidation processes that rely on highly reactive radicals to destroy organic contaminants are appealing treatment options, substantial energy and chemical inputs limit their practical applications. Here we demonstrate that Cu single atoms incorporated in graphitic carbon nitride can catalytically activate H₂O₂ to generate hydroxyl radicals at pH 7.0 without energy input, and show robust stability within a filtration device. We further design an electrolysis reactor for the on-site generation of H₂O₂ from air, water and renewable energy. Coupling the single-atom catalytic filter and the H₂O₂ electrolytic generator in tandem delivers a wastewater treatment system. These findings provide a promising path toward reducing the energy and chemical demands of advanced oxidation processes, as well as enabling their implementation in remote areas and isolated communities.

Fig. 1 ∣. Schematic drawing of our wastewater treatment system.

The system includes the H₂O₂ electrolyzer, the Fenton filter and the Fe₃O₄-carbon filter.

Fig. 2 ∣. Characterizations of Cu-C₃N₄.

a, Structural illustration of Cu-C₃N₄. b, Aberration-corrected HR-TEM image of Cu-C₃N₄ showing the absence of crystalline structure. Darker area in the top-left corner is due to the lacey carbon of TEM grid. Scale bar, 20 nm. c, Aberration-corrected HAADF-STEM image of Cu-C₃N₄. Circles indicate Cu single atoms. Scale bar, 2 nm. d, Normalized k²-weighted Fourier transform of the EXAFS spectra of Cu-C₃N₄ and other reference materials in radial distance. e, XPS Cu 2p spectrum and Cu LMM Auger spectrum (inset) of Cu-C₃N₄. The black lines show the raw data, and the colored lines correspond to the deconvoluted components. f, FTIR spectra of Cu-C₃N₄ and undoped C₃N₄.

Fig. 3 ∣. Catalytic activity and degradation product.

a, Degradation of 10 μM RhB with the presence of 1 g L⁻¹ H₂O₂ and different catalysts. Reaction conditions: 10 mL aqueous solution, 1 g L⁻¹ catalyst (if present), pH = 7 (pH is adjusted to 7 at the beginning of reaction without buffer control). b, LC-MS chromatograms of the reaction solution at different degradation time intervals. The reaction conditions are the same as those for panel a, with Cu-C₃N₄ as the catalyst. c, TOC removal during the degradation of 50 μM RhB in the presence of 1 g L⁻¹ H₂O₂ and 1 g L⁻¹ Cu-C₃N₄. Reaction conditions: 100 mL aqueous solution, pH = 7.

Fig. 4 ∣. Fenton filter.

a, Photo of a proof-of-concept Fenton filter. Cross-sectional area, 1 cm². Length, 5 cm. Scale bar, 5 cm. b, SEM image of the filter medium. Scale bar, 100 μm. Inset, magnified SEM image showing the Cu-C₃N₄ catalyst coated on the surface of a carbon fiber. Scale bar, 5 μm. c, Dye removal and Cu concentration in effluent as functions of filtration time. Flow rate, 10 mL h⁻¹. Contact time, 30 min. H₂O₂ dosage, 1 g L⁻¹. Pollutant, 10 ppm RhB for the first 100 h, changed to 10 ppm MB for the second 100 h.

Fig. 5 ∣. Electrodes and electrolyte of H₂O₂ electrolyzer.

a, Schematic drawing of the GDE for 2e-ORR. O₂ diffuses in the gas phase through the porous PE to the gas-catalyst-electrolyte interface, and then gets reduced by the electrons transported from the carbon paper. b, Cross-sectional SEM image of the GDE, with dashed lines denoting the boundaries between each component. The top surface is visible because the sample is slightly tilted. Scale bar, 50 μm. c, Magnified SEM image of the porous PE showing interconnected micron-sized pores, which provide the gas diffusion pathway and prevent flooding. Scale bar, 2 μm. d, Comparison of the Tafel plots of a GDE supplied with pure O₂, a GDE supplied with atmospheric air, and a carbon paper electrode immersed in electrolyte. All the electrodes were loaded with the same amount of catalyst (0.5 mg cm⁻²). Electrolyte, 0.1 M Na₂SO₄. e, LSV curves of the IrO₂ anode before and after 200 cycles of continuous CV scanning in 0.1 M Na₂SO₄. Inset, photo of the IrO₂ anode. Scale bar, 5 mm. f, Cost estimate for producing 10 g L⁻¹ (1 wt.%) H₂O₂ solution using different electrolytes (0.1 M), based on the condition that both electrodes are operated at 20 mA cm⁻². The market price shown here is a 50th of the market price for 50 wt.% H₂O₂.

Fig. 6 ∣. Reactor design and performance of H₂O₂ electrolyzer.

a, Schematic drawing of the H₂O₂ electrolyzer. b, Front-view photo (the same view direction as the schematic). Scale bar, 1 cm. c, Side-view photo (from the OER chamber side). Scale bar, 1 cm. d, Operation of the electrolyzer by controlling the working current and the electrolyte flow rate. The air flow was kept at 10 mL min⁻¹. A 10 g L⁻¹ H₂O₂ solution was produced at a total cost of $4.66 m⁻³, when the working current and the electrolyte flow rate were 100 mA and 5 mL h⁻¹, respectively.