Flow-electrode capacitive deionization using citric-acid-modified biochar for efficient treatment of acidic...

Flow-electrode capacitive deionization (FCDI) holds promise for treating complex electroplating wastewater, yet its performance is often limited by the electrode material. Herein, a citric-acid-modified watermelon-rind biochar was employed as a flow electrode to enhance the continuous purification of acidic wastewater containing chromium and zinc. The biochar, prepared via pyrolysis and hydrothermal modification, was evaluated under varying voltage, wastewater loading, reuse cycles and competitive removal in FCDI systems. At 2.8 V, the modified biochar achieved removals of 88.97 ± 0.66% for Cr and 98.31 ± 0.05% for Zn, with charge efficiencies of 4.7% and 91.3%, respectively. The system exhibited a total dissolved solids removal flux of 30.94 ± 0.24 g/(m²·h), while the effluent pH increased from 4.8 ± 0.15-6.8 ± 0.42, and ten-cycle reuse tests demonstrated stable Cr and Zn removal (>80%). Notably, the system maintained high Cr removal (>93%) even under severe Zn excess, yielding specific energy consumptions of 30.83 kWh/kg for Cr and 1.26 kWh/kg for Zn. DFT calculations revealed that citric-acid functionalization introduces oxygenated Lewis-base sites, converting weak π-surface physiochemical adsorption into multidentate chelation and hydrogen bonding. The improved performance arose from synergistic pathways, including Cr(VI) reduction and subsequent precipitation, double-layer electroadsorption, surface complexation of Cr(III), coordination adsorption of Zn(II) and hydrogen bonding. These results elucidate the functional role of citric acid and provide a practical electrode modification method that enables the effective treatment of highly toxic, acidic electroplating wastewater.