Nitrogen and sulfur co-doped cyanobacteria-derived biochar for efficient capacitive removal of Pb2+ from wastewater

Capacitive deionization (CDI) has emerged as a promising and energy-efficient technique for the removal of heavy metal ions such as lead (Pb²⁺) from aqueous solutions. In this study, a novel nitrogen and sulfur co-doped biochar (NSBAC) was developed from cyanobacteria‑nitrogen-fixing algae known for their ecological hazards. The NSBAC electrode exhibited excellent capacitive performance with a specific capacitance of 270.69 F/g at a current density of 0.5 A/g. In the CDI system, NSBAC achieved an outstanding Pb²⁺ adsorption capacity of 30.42 mg/g (1.2 V, 100 mg/L) and maintained 93.1 % of its initial capacity after ten adsorption-desorption cycles. Furthermore, density functional theory (DFT) calculations revealed that N, S-co-doping introduces more defects and edge-N sites compared to single N-doping, significantly enhancing the Pb²⁺ adsorption capability of NSBAC. This study elucidates the synergistic effects of dual heteroatom doping and provides new insights for CDI-based Pb²⁺ removal technologies.