Mechanisms of Cu(II) Adsorption onto Biochars Derived from Fallen and Non-Fallen Maple Leaves

The ability of biochars derived from fallen (F-BC) and non-fallen (NF-BC) maple leaves to adsorb Cu2+ ions from aqueous solutions was examined. Biochars were produced at pyrolysis temperatures of 350, 550, and 750 °C. Higher pyrolysis temperatures resulted in enhanced specific surface areas and promoted CaCO3 crystallization while limiting MgCO3 formation. The Cu2+ adsorption capacity depended on the biochar type and pyrolysis conditions. Although the Cu2+ adsorption efficiency of NF-BCs decreased with increasing pyrolysis temperature, F-BC550 exhibited a higher Cu2+ adsorption capacity than F-BC750. Additionally, the Cu2+ adsorption performance of both NF-BC350 and F-BC550 improved with increasing solution pH. Cu2+ adsorption onto NF-BC350 and F-BC550 followed the two-compartment first-order (involving fast and slow adsorption compartments) and Langmuir (meaning homogeneous monolayer adsorption) models, respectively. The maximum Cu2+ adsorption capacity of F-BC550 (147.3 mg Cu/g BC) was 7.8-fold higher than that of NF-BC350 (18.8 mg Cu/g BC), as determined by isotherm studies. The enhanced adsorption performance of F-BC550 may be attributable to physical adsorption facilitated by increased surface area and multiple mechanisms, including cationic attraction via functional groups, ion exchange (Ca and Mg), and van der Waals interaction facilitated by increased surface area. These findings suggest that F-BC550, derived from waste biomass, exhibits superior Cu2+ adsorption performance compared to NF-BCs, making it a promising adsorbent for wastewater treatment applications.