Carbon Nitride Gels: Synthesis, Modification, and Water Decontamination Applications

Graphitic carbon nitride (g-C3N4)-based materials hold significant promise for environmental remediation, particularly water purification, owing to their unique electronic structure, metal-free composition, and robust chemical stability. However, powdered g-C3N4 faces challenges such as particle aggregation, poor recyclability, and limited exposure of active sites. Structuring g-C3N4 into hydrogels or aerogels—three-dimensional porous networks offering high surface area, rapid mass transport, and tunable porosity—represents a transformative solution. This review comprehensively examines recent advances in g-C3N4-based gels, covering synthesis strategies such as crosslinking (physical/chemical), in situ polymerization, and the sol–gel and template method. Modification approaches including chemical composition and structural engineering are systematically categorized to elucidate their roles in optimizing catalytic activity, stability, and multifunctionality. Special emphasis is placed on environmental applications, including the removal of emerging contaminants and heavy metal ions, as well as solar-driven interfacial evaporation for desalination. Throughout, the critical interplay between gel structure/composition and performance is evaluated to establish design principles for next-generation materials. Finally, this review identifies current challenges regarding scalable synthesis, long-term stability, in-depth mechanistic understanding, and performance in complex real wastewater matrices. This work aims to provide valuable insights and guidance for advancing g-C3N4-based hydrogel and aerogel technologies in environmental applications.