Acid-treated g-C3N4 with improved photocatalytic performance in the reduction of aqueous Cr(VI) under visible-light

• Acid-soaking treatment can improve the photocatalytic activity of g-C3N4.
• 5 mol/L HNO3 treatment can remove the Cr(III) deposited on the used g-C3N4.
• 5 mol/L HNO3-treated g-C3N4 exhibited higher photocatalytic efficiencies in reuses.

An alternative simple, economical and efficient acid-soaking method was proposed to improve the activity of g-C3N4 for photocatalytic reduction of aqueous Cr(VI) in this work. The improvement was achieved simply by soaking g-C3N4 (which was obtained by heating melamine in air at 550 °C for 4 h) in 5 mol/L HNO3 or HCl aqueous solution for 2 h (the HNO3 and HCl-treated samples were denoted as g-C3N4HNO3 and g-C3N4HCl, respectively). The compositions, structures, Brunner–Emmet–Teller (BET) specific surface areas, Zeta potentials and optical properties of g-C3N4, g-C3N4HNO3 and g-C3N4HCl were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, N2 adsorption/desorption isotherms, Zeta potential analyzer and UV–vis diffuse reflectance spectroscopy. The photocatalytic properties of g-C3N4, g-C3N4HNO3 and g-C3N4HCl were tested in the reduction of aqueous Cr(VI) under visible-light (λ > 420 nm) irradiation. It was observed that both g-C3N4HNO3 and g-C3N4HCl exhibited higher photocatalytic activity than g-C3N4. The higher photocatalytic activities of acid-treated samples may be attributed to their larger BET specific surface areas, positive surface charges and larger adsorption capacities for Cr(VI). Furthermore, the soaking treatment with 5 mol/L HNO3 aqueous solution can not only remove the Cr(III) species deposited on the surface of g-C3N4HNO3 after use in photocatalytic reduction of aqueous Cr(VI), but also enhance the photocatalytic efficiency of g-C3N4HNO3 in reuse.