From Strong Fluoride Binding to Reversible Electrodesorption: S, N-Regulated La-MOF-Derived Carbon Electrodes for Capacitive Deionization Defluoridation

La-MOFs exhibit strong affinity toward anions such as F− and phosphate. However, conventional La-MOFs show limited regeneration performance when used as CDI electrodes, posing a major challenge for practical applications. In this study, a high-performance sulfur and nitrogen co-doped La-BDC-140-derived carbon electrode (La-CNS3) was fabricated via a coupled carbonization and doping strategy. The optimized La-CNS3 electrode possessed abundant defects, a mesoporous structure, favorable hydrophilicity, and rapid charge-transfer capability, which collectively enhanced fluoride electrosorption. At 1.4 V, La-CNS3 achieved a fluoride removal capacity of 31.86 mg·g−1 for 10 mg·L−1 F− solution and up to 195 mg·g−1 at an initial F− concentration of 100 mg·L−1. More importantly, partial fluoride desorption was realized solely under reverse voltage, and the electrode maintained favorable defluoridation performance over 50 adsorption–desorption cycles. In actual groundwater treatment, the effluent fluoride concentration decreased to below 1.0 mg·L−1 after 120 min. XPS analysis and DFT calculations revealed that fluoride removal was mainly governed by La-F coordination, surface hydroxyl/water ligand exchange, and interfacial charge redistribution. The La2O2S/g-C3N4 structure provided a favorable balance between fluoride adsorption strength and desorption reversibility. This work offers a promising strategy for designing efficient, selective, and electrically regenerable rare-earth-based CDI electrodes for fluoride-contaminated water treatment.