Competitive Adsorption of Pb2+ from Aqueous Solutions by Multi-Source Lignocellulose-Derived Hydrothermal Humic Acid

This study explores the conversion of lignocellulosic biomass from softwood, hardwood, and grasses into humic acid via a mild hydrothermal process and its application in Pb2+ adsorption. The investigation focused on adsorption isotherms, kinetics, thermodynamics, and the intraparticle diffusion model to evaluate the adsorption performance of humic acids from different sources. The results indicate that the humic acid of broad-leaved wood (Eucalyptus-HA) possesses the optimal adsorption capacity and removal efficiency of Pb2+. When the initial concentration of Pb2+ is 100 mg/L, the adsorption capacity and removal efficiency of Eucalyptus-HA reach 49.75 mg/g and 25.57%, respectively, which are far higher than the adsorption capacity (26.82 mg/g) and removal efficiency (13.71%) of commercial humic acid (Commercial-HA). The pore structure of humic acid plays a critical role in its Pb2+ adsorption capacity. High Pb2+ concentrations and a low pH negatively impact adsorption efficiency, and instability in the humic acid pore structure affects reproducibility. Adsorption isotherm fitting showed that Pb2+ adsorption conforms most closely to the Langmuir model. While commercial humic acid exhibited faster adsorption rates, its capacity was constrained by thermodynamic limitations and lower specific surface areas. The intraparticle diffusion model revealed that Pb2+ diffusion proceeded more efficiently in hydrothermal humic acids than in commercial ones due to lower diffusion resistance. This study highlights the potential of feedstock source regulation to enhance humic acid’s heavy metal adsorption capabilities, expanding its application across various fields.