Synergistic carbon–silicate hierarchical biochar from waste activated sludge for high-performance flow-electrode capacitive deionization

This study investigates the efficiency of biochar derived from waste activated sludge (WAS) as an electrode material in flowing-electrode capacitive deionization (FCDI), demonstrating improved desalination performance compared to commercial activated carbon (AC). The WAS-derived biochar (B-800-KH-1), synthesized via 2-hour pyrolysis at 800 °C followed by KOH activation at a mass ratio of 1:1, exhibits a hierarchical micro/mesoporous architecture with an average pore diameter of 9.22 nm, a mesopore volume ratio of 76.57% and a disorder/graphite bands intensity ratio of 1.18. Moreover, the embedded silicate framework within the biochar synergistically interacts with the carbon phase, promoting ion pathways, and mitigating charge transfer resistance. Optimized operation of the FCDI system using the prepared B-800-KH-1 electrode achieved a maximum salt removal rate of 1.92 ± 0.23 μmol/cm 2 ·min and an average removal rate of 1.24 ± 0.07 μmol/cm 2 ·min, representing increments of approximately 56.1% and 7.8%, respectively, over commercial AC. Additionally, it demonstrated an energy-normalized salt removal of 8.35 ± 0.26 μmol/J at a 5.0 wt.% carbon content and an applied voltage of 1.2 V. These findings highlight that high desalination efficiency is not solely dictated by total carbon content or specific surface area; rather, the interplay of mesoporous carbon domains with the silicate network and hierarchical pore connectivity drives enhanced electrosorption. Utilizing of WAS as a biochar precursor not only provides an effective electrode material but also offers a sustainable solution for WAS management, paving the way for advanced and environmentally cost-effective conscious desalination technologies.