A simple dual-salt catalysis method to synthesize lignin-derived biochar towards efficient Na+ removal via FCDI

The large-scale discharge of Na + -rich wastewater has intensified the global freshwater crisis. Flow-electrode capacitive deionization (FCDI) offers a promising electrosorption technology for desalination. Lignin is the representative by-product of the papermaking industry, and lignin-derived biochar has been intensively investigated as an active material for flow-electrode material. In this study, we proposed a facile dual-salt (KCl and LiCl) catalysis method during hydrothermal carbonization to prepare lignin-derived biochar (Li-K-LCN). The characterization results showed that the dual-salt catalysts exerted combined effect by physical structure optimization and surface chemical modification based on the cleavage of low bond energy moieties (β-O-4 ether bond and methoxyl group) in the lignin structure. When employed in a flow-electrode capacitive deionization (FCDI) system for Na + removal, Li-K-LCN electrode achieved a stable removal efficiency of over 83.3%, an electrosorption capacity of 26.03 mg g −1 , and low energy consumption of 0.015 J mg −1 . Toxicity tests indicated a preliminary favorable improvement in the environmental safety of the treated effluent. This work presented a simple and efficient dual-salt catalysis method to synthesize lignin-derived biochar towards competitive Na + removal via FCDI, achieving the goal of a low-cost, high-value-added by-product lignin utilization.