Adsorption of Lead on Sulfur-Doped Graphitic Carbon Nitride Nanosheets: Experimental and Theoretical Calculation Study

Two-dimensional (2D) layered materials for environmental remediation require abundant functional groups and high affinity for target pollutions. In this work, a heteroatom sulfur-doped graphitic carbon nitride (S-3.9%-g-C3N4) material had been fabricated by pyrolysis for S- and N-rich supra-molecular polymer precursor. A hierarchically porous structure with multimodal pore size distribution can facilitate Pb(II) ions' diffusion from wastewater to the S-3.9%-g-C3N4 surface. Batch adsorption experiments revealed that the resulting S-doped conjugated system of g-C3N4 exhibited enhanced adsorption capacity (52.63 mg g(-1)) as compared with that of individual g-C3N4 (31.25 mg g(-1)) at pH = 4.5. Accordingly, the higher adsorption ability for S-3.9%-g-C3N4 was on account of its binding sites (i.e., soft S ligand) for coordination with Pb(II) than g-C3N4. Significantly, thermodynamic parameters demonstrated that the adsorption of Pb(II) on S-3.9-g-C3N4 was a spontaneous and endothermic process, and the kinetic experiments suggested that the chemisorption governed the adsorption process. Additionally, all geometries of (S-g-C3N4) Pb(II) systems were thoughtfully constructed to explore the optimized structure, and these results were combined with the results of Density Functional Theory calculations to show that the enrichment of Pb(II) on S-3.9%-g-C3N4 was energetically favored in C3N4-S-N-3-the monosubstituted system with high E-ad value (4.75 eV), in which the chemical binding sites of S-Pb and N-Pb were further evidenced by XPS analysis. The presented results not only demonstrated a facile and environmentally friendly strategy to synthesize porous S-3.9%-g-C3N4 nanosheets with preferable Pb(II) adsorption ability, but also revealed the underlying adsorption mechanism for Pb(II) by experiments and theory simulation.