Competitive adsorption of Cu and Zn in water by pyrolysis biochar from willow leaves and its adsorption mechanism: A combination of experiments, 2D-COS and DFT calculations

Copper and zinc are prevalent in industrial effluents, and their excessive discharge aggravates heavy metal pollution in aquatic systems, posing serious risks to ecosystems and human health. In this study, biochar derived from willow leaves (LBC), a scarcely explored precursor, was applied for the efficient removal of Cu and Zn from complex wastewater. Adsorption experiments, structural characterization, 2D-COS, and DFT clarified the competitive adsorption mechanism and quantified contributions of each mechanism. LBC exhibits monolayer chemisorption, with maximum capacities of 174.76 mg·g -1 for Cu and 129.65 mg·g -1 for Zn in single-metal systems, maintaining high efficiency at pH 3-6. In binary systems, competitive adsorption favors Cu, with slightly higher capacity (105.13 mg·g -1 ) than Zn (104.46 mg·g -1 ), attributed to Cu’s higher electronegativity and smaller hydration radius. DFT calculations revealed lower adsorption energy (-1.07 eV vs. -0.49 eV) and greater electron cloud overlap, confirming LBC’s preference for Cu. 2D-COS analysis indicates that the priority order of functional groups interacting with metal ions on LBC is C-H > C=O > C=C > C-C. Various characterization and experimental analyses show that mineral precipitation is an important adsorption mechanism for Cu and Zn adsorption by LBC, accounting for 45.1% and 38.0% of the total contribution rate, respectively. In conclusion, LBC effectively adsorbs Cu and Zn, providing a theoretical basis for its functional modification to enhance performance. • In binary-metal systems (Cu and Zn) with LBC exhibiting a higher affinity for Cu. • Biochar LBC maintains a high adsorption efficiency in the pH range of 3 ~ 6. • The role of functional groups on LBC was revealed by 2D-COS. • DFT calculations show LBC has higher affinity for Cu 2+ (-1.07 eV) than Zn 2+ (-0.49 eV).