Enhancement of the Cr(VI) adsorption and photocatalytic reduction activity of g-C3N4 by hydrothermal treatment in HNO3 aqueous solution

• Hydrothermal treatment in HNO3 aqueous solution is adopted to modify g-C3N4.
• Treated g-C3N4 exhibits enhanced adsorption and photocatalytic reduction of Cr(VI).
• Hydrothermal treatment conditions affect the structure and performance of g-C3N4.
• Reasons for improved performance of treated g-C3N4 are discussed.
• Different photocatalytic Cr(VI) reduction mechanisms over g-C3N4 and treated g-C3N4.

An effective and environmental-friendly method was adopted to enhance the Cr(VI) adsorption and photocatalytic reduction activity of g-C3N4, which was prepared by thermal condensation of melamine. The enhancement was realized by hydrothermal treatment of g-C3N4 in 1.9–7.6 mol/L HNO3 aqueous solutions at 80–120 °C for 3–12 h. The compositions, structures and physiochemical properties of the hydrothermally treated g-C3N4 were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, N2 adsorption/desorption isotherms, Zeta potential analysis, UV–vis diffuse reflectance spectroscopy and photocurrent measurement, and their dark adsorption and visible-light (wavelength >420 nm)-driven photocatalytic reduction of Cr(VI) in aqueous solution were tested. It was found that the hydrothermally treated g-C3N4 exhibited significantly enhanced Cr(VI) adsorption and photocatalytic reduction activity than g-C3N4. Moreover, larger concentration of HNO3 aqueous solution, higher hydrothermal temperature or longer treatment time resulted in higher Cr(VI) adsorption and photocatalytic reduction activity of the hydrothermally treated g-C3N4. The reasons accounting for the enhanced Cr(VI) adsorption and photocatalytic reduction activity of the hydrothermally treated g-C3N4 were discussed, and the different mechanisms for the photocatalytic reduction of Cr(VI) over g-C3N4 and the hydrothermally treated g-C3N4 were also proposed.

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