Preparation of S-doped g-C3N4 with C vacancies using the desulfurized waste liquid extracting salt and its application for NOx removal

[Display omitted] •“Waste for waste control” to efficiently removal NOx.•The amino group inhibits the intermediate accumulation of NO2.•C vacancy defect forms electron capture center and causes O2 activation.•The sample has high S doping content presented favorable visible light absorption.•The carrier recombination rate was reduced by molten salt modification. In this study, graphitized carbon nitride photocatalysts were successfully prepared by using the desulfurized waste liquid extracting salt from coking plants. The NOx removal efficiency of g-C3N4 synthesized using ammonium thiocyanate (NH4SCN), ammonium thiosulfate ((NH4)2S2O3) and ammonium sulfate ((NH4)2SO4) with different mass ratios were evaluated. The results showed that due to the addition of (NH4)2S2O3 and (NH4)2SO4 a large amount of sulfur-containing gas was released during pyrolysis, which significantly increased the specific surface area and pore volume of the photocatalysts. The release of sulfur-containing gas also resulted in the incorporation of sulfur, and due to this doping process the band gap of the photocatalyst was widened to 2.94 eV, thus enhancing its visible light response. C vacancies were formed in the hepazine ring, which increased the electron density around the N–(C)3 bridge bond and formed an electron trapping center, thus inhibiting the recombination of charge carriers. The photocatalysts had increased O2 adsorption capacity and increased number of amino groups on their surface. This facilitated the rapid generation and consumption of reactive oxygen species, inhibited the accumulation of NO2 on the surface, reduced the production of NO2, thus improving the NOx removal efficiency of the photocatalysts. The principle of using “waste for waste control” was achieved, and a new method for nitrogen oxide removal was provided.