Nitrogen‐Doped Graphitic Carbon Nitride With Defect‐Rich Structure for Efficient Pb (II) and Cd (II) Removal From Aqueous Solutions

The presence of toxic heavy metals such as Pb (II) and Cd (II) in water poses severe risks to ecosystems and human health. In this study, nitrogen‐doped graphitic carbon nitride (N@g‐C 3 N 4 ) was synthesized via an ammonia‐assisted hydrothermal route to enhance its adsorption performance toward Pb (II) and Cd (II) ions. Structural and surface analyses confirmed that nitrogen heteroatom doping generated abundant defect sites and nitrogen‐containing functional groups, significantly improving surface reactivity compared to pristine g‐C 3 N 4 . Batch adsorption experiments revealed that N@g‐C 3 N 4 exhibits markedly enhanced adsorption capacity, strongly dependent on solution pH, contact time, and temperature. The adsorption isotherms were well described by the Langmuir model, indicating monolayer adsorption on homogeneous active sites. Kinetic data followed the pseudo‐first‐order model, suggesting a surface‐controlled adsorption process. Thermodynamic analysis demonstrated that the adsorption of both Pb (II) and Cd (II) is spontaneous and endothermic. The enhanced performance is attributed to synergistic electrostatic attraction, surface complexation, and coordination interactions between metal ions and nitrogen‐rich functional groups. This work highlights nitrogen heteroatom engineering as an effective strategy to develop high‐performance g‐C 3 N 4 –based adsorbents for wastewater treatment.