Dual sodium/sulfur modified graphitic carbon nitride: structure-preserving doping and superior Pb(II)/Cd(II) uptake

Abstract

The discharge of toxic heavy metals such as lead and cadmium into water bodies represents a critical challenge due to their high toxicity, environmental persistence, and tendency for bioaccumulation. Herein, graphitic carbon nitride was prepared by thermal polycondensation of urea and subsequently modified through hydrothermal heteroatom doping using Na2S as a simultaneous sodium and sulfur precursor. Comprehensive characterization by XRD, FTIR, SEM, TG/DTG, and XPS confirmed effective Na/S incorporation while preserving the native layered architecture of gCN and enriching its surface chemistry. Doping shifted the point of zero charge from 6.8 to 7.4 and generated a more negatively charged and defect-rich surface, resulting in substantially improved adsorption of Pb(II) and Cd(II) compared with pristine gCN. Equilibrium data fitted the Langmuir isotherm, suggesting predominant monolayer uptake, with maximum capacities of 526-833 mg g- 1 for Pb(II) and 212-344 mg g- 1 for Cd(II) at 303-323 K. Adsorption followed pseudo-first-order kinetics and was thermodynamically spontaneous and endothermic. The enhanced performance is attributed to synergistic effects of Na and S sites that promote electrostatic attraction and inner-sphere metal coordination, increasing the density of accessible active centers.