Nanofiltration combined with membrane capacitive deionization for efficient classification and recovery salts from simulated coal chemical industrial wastewater

Achieving water reuse and salt resources recycling in high-salt wastewater from the coal chemical industry is crucial to the sustainable development. However, the current widely used RO (reverse osmosis)-NF (nanofiltration)-crystallization process requires significant energy consumption in the step of heat-driven crystallization. Here, RO-NF-membrane capacitive deionization (MCDI) system was designed for classify and recover salt from high-salt wastewater of coal chemical industry. Sludge-derived biochar with high specific surface area and hierarchical structure was used as the electrode for MCDI. The MCDI cell can stabilize desalination of source water containing NaCl up to 300 mM, i.e., the total dissolved solids (TDS) of 18,000 mg·L⁻¹, which can be applied to the desalination treatment after RO-NF salt separation. Based on the MCDI, a two-channel architecture was designed, in which a concentration channel was constructed between the electrode and the anion exchange membrane, while the desalination channel remains between the anion and cation exchange membrane. Continuous operation of desalination and concentration was achieved by exchanging the inlet and outlet of the concentration and desalination channels during the charging and discharging phases. This work demonstrates that the RO-NF-MCDI system could be a feasible method for classification and recovery salts from coal chemical industrial wastewater.