Asymmetric Flow-Electrode Capacitive Deionization for Copper-Containing Electroplating Wastewater Treatment Using Modified Sugarcane Bagasse Biochar

Copper-containing electroplating wastewater is strongly acidic and highly saline, making efficient treatment challenging. Oxalic-acid-modified sugarcane bagasse biochar was prepared as an anode-side flow electrode for asymmetric flow-electrode capacitive deionization (AFCDI). Sugarcane bagasse biochar modified with 1.25 M oxalic acid (1.25OA-SC) was selected for its favorable overall balance of structural, electrochemical, and treatment characteristics. At 2.8 V, AFCDI achieved 98.12 ± 0.36% Cu2+ removal and lowered conductivity from 22.6 to 2.35 ± 0.66 mS/cm. This performance was associated with the combined effects of accessible porosity, oxygen-containing surface functionality, and electrochemical transport properties. Compared with conventional FCDI, AFCDI facilitated Cu2+ migration and cathodic deposit formation while achieving higher charge efficiency and lower electrical energy consumption. At 62.5 mL/g, Cu2+ removal remained above 98%, with an average TDS removal rate of 298.91 ± 7.19 g/(m2·h). During 10 consecutive AFCDI cycles using recovered 1.25OA-SC, Cu2+ removal remained above 98% and SO42− removal close to 80%, indicating short-term system-level performance retention. Cathodic deposits consisted mainly of Cu0 and Cu2O, whereas SO42− migrated through the anion-exchange membrane and was stored mainly by electric-double-layer charge compensation. These results demonstrate a biochar-based AFCDI strategy for coupled copper removal, recovery, and desalination of acidic sulfate-rich wastewater.