Efficient adsorption of Cu(II) from wastewater via modified biochars from residues of Chinese herbal medicines Pueraria lobata and Leonurus japonicus and direct reuses for electrode materials.

Chinese herbal medicine residues represent a promising feedstock for biochar production, offering significant potential for resource recycling. In this study, residues of Chinese herbal medicines Pueraria lobata and Leonurus japonicus were initially prepared into raw biochars (PL-BC and LJ-BC), which were subsequently modified using KOH (KPL-BC and KLJ-BC) for Cu(II) removal from wastewater. The results demonstrated that adsorption behaviors of both KPL-BC and KLJ-BC were well described by the pseudo-first-order model. The isotherm analysis revealed that KPL-BC followed the Freundlich model, whereas KLJ-BC was better fitted by the Langmuir model. The calculated maximum adsorption capacities of KPL-BC and KLJ-BC were 172.0 mg/g and 163.0 mg/g, respectively, representing increases of 1.8-fold and 4.6-fold compared with PL-BC and LJ-BC. Additionally, after four consecutive adsorption-desorption cycles, KPL-BC and KLJ-BC retained adsorption capacities of 69.34% and 52.84%, respectively. KOH modification significantly improved the specific surface area, oxygen-containing functional groups and structural disorder of PL-BC and LJ-BC. Mechanistic analysis revealed that KPL-BC adsorption involved both physisorption and chemisorption, whereas KLJ-BC was dominated by chemisorption. Furthermore, Cu(II)-adsorbed biochars (KPL-BC/Cu and KLJ-BC/Cu) were directly employed as electrode materials for supercapacitors. The electrochemical testing revealed that KPL-BC/Cu and KLJ-BC/Cu electrodes achieved specific capacitances of 281.5 F/g and 289.5 F/g at a current density of 0.5 A/g. In summary, the results of this study provided an efficient and environmentally friendly option for the resource utilization of Chinese herbal medicine residues of P. lobata and L. japonicus and the removal of heavy metal in wastewater.