Ag-modified g-CN with enhanced activity for the photocatalytic reduction of hexavalent chromium in the presence of EDTA under ultraviolet irradiation.

The photocatalytic reduction of Cr to Cr in an aqueous solution, using 3 wt% Ag/g-CN in the presence of ethylenediaminetetraacetic acid (EDTA), has been investigated here. The photocatalytic reduction of Cr with pure g-CN was very low. The addition of Ag and EDTA can significantly improve the photocatalytic reduction of Cr using g-CN. In the presence of EDTA, the efficiency with Ag/g-CN was better than those with Au/g-CN and Cu/g-CN. With EDTA, the reduction rate constant increased from 0.0005 for pure g-CN to 0.12 min for 3 wt% Ag/g-CN. By increasing the concentration of EDTA from 0 to 500 mg L, the reduction efficiency of Cr increased extremely, and the rate constant raised from 0.008 to 0.12 min. The optimal EDTA concentration was 500 mg L for the photocatalyst Ag/g-CN. The Ag-EDTA complex may be reduced to metallic silver by the conduction band electrons of g-CN. The electron-hole recombination was significantly suppressed by the electron trapping of Ag. EDTA may act in by the formation of Cr-complex and the separation of Cr from the g-CN surface and by the valence band hole scavenger of g-CN. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy (DRS) and photoluminescence spectra (PL) were used to characterize g-CN and Ag/g-CN nanoparticles. A possible mechanism for photocatalytic Cr reduction has also been demonstrated.