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The rapid progress of modern industry not only brings convenience to people's lives, but also brings negative effects. Industrial development produces a large amount of waste metal, which brings harm to the environment and human health. Carbon nitride (g-C3N4) was successfully prepared using the thermal-polymerization method and petal-like g-C3N4 (CA-g-C3N4) was impregnated with citric acid (CA). Compared with g-C3N4, CA-g-C3N4 showed extremely high photocatalytic activity because the petal-like g-C3N4 (CA-g-C3N4) had a larger specific surface area, which increased the active sites on the surface of the photocatalyst and improved the photocatalytic activity. After citric acid treatment, the removal of hexavalent chromium (Cr(VI)) by g-C3N4 increased from 48% to 93%. The photocatalytic materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) and UV-vis diffuse reflectance spectra (UV-vis). In summary, this study confirmed that citric acid can improve the photocatalytic activity of g-C3N4 by increasing its specific surface area and the active site of the photocatalytic material so as to achieve the purpose of removing hexavalent chromium from water. [ABSTRACT FROM AUTHOR]