The microscopic insights into the adsorption of Cu2+, Pb2+ and Zn2+ onto g-C3N4 surfaces by a combined spectroscopic characterization and DFT theoretical calculations

Abstract

The graphitic phase carbon nitride (g-C3N4) is increasingly employed to remove heavy metals from wastewater, but the intrinsic imobilization mechanism still needs to be explored. Herein, the g-C3N4 before and after adsorption for Cu2+, Pb2+ and Zn2+ was firstly characterized using high resolution transmission electron microscope, energy dispersive X-ray, fourier transform infrared, and X-ray diffraction. Then two types of C3N4 structures were calculated, i.e. the triazine (C3N4a) and tri-s-triazine units (C3N4b) using the DFT theoretical calculation. The results indicated that the g-C3N4 materials possessed a well-developed 2D lamellar sturcture, basic tris-triazine units, and abundent C-N, C˭N, –NH–, and –NH2 groups. After successfully adsorption for Cu2+, Pb2+ and Zn2+, the surface functional groups, typical tris-triazine units, and the amino functions of g-C3N4 were still well retained. The Cu2+, Pb2+ and Zn2+ ions combined with C3N4a and C3N4b, trapped in the hole surrounded by triazine molecules, and produced both three-cordinated [C3N4a-Cu/Pb/Zn]2+ and six-coordination [C3N4b-Cu/Pb/Zn]2+ structures. Interestingly, the heavy metals could be trapped within the C3N4b plane but be fixed above the C3N4a plane, and a higher sorpiton energy and coordination number were found for the structure of [C3N4b-Cu/Pb/Zn]2+ compared to [C3N4a-Cu/Pb/Zn]2+. In total, this study provides a new insight into the adsorption mechanism of g-C3N4 for heavy metal ions in water.