Selective Capacitive Removal of Pb²⁺ from Wastewater over Biochar Electrodes by Zinc Regulation

Biochar materials have shown great potential for broad catalytic application. However, using these materials in the capacitive deionization technology (CDI) system for heavy metal removal still faces a significant challenge due to their low specific capacity and removal capability. Here, a comprehensive regulation on the interfacial/bulk electrochemistry of biochar by Zn doping is reported, which suggests a high renewable capacity (20 mg g^-1) and outstanding selective capacitive removal ability (SCR) of Pb²⁺ from leachate. The SCR efficiency of Pb²⁺ is as high as 99% compared to K⁺ (8%), Na⁺ (13%), and Cd²⁺ (37%). This work proves that the doped Zn on the biochar can combine with OH^- generated by water splitting to form M─OH bonds, which is beneficial for improving the specific capacity. Significantly, the relationship between double-layer capacitance and pseudo-capacitance can also be optimized by regulating the content of Zn, leading to different removal abilities of heavy metals. Therefore, this work offers insights into charge-storage kinetics, which provide valuable guidelines for designing and optimizing the biochar electrode for broader environmental applications.