Langmuir and Freundlich Isotherms and Pseudo-Order Kinetics of Xylene Adsorption on Succinic Acid-Modified Hardwood Sawdust

In this study, batch sorption of xylene on chemically modified hardwood sawdust (0.1, 0.2, 0.3, 0.4, and 0.5 M) was conducted. Pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models were derived from time-dependent uptake data, and isotherms (Langmuir and Freundlich) were also obtained. The linearized Langmuir regression produced an apparent capacity of Qm ≈ 2.5 × 104 mg∙g−1. However, it yielded a negative physical affinity constant (KL Qm should be interpreted as an overall (apparent) sorption capacity rather than a monolayer adsorption capacity. A strong regression was observed in the Freundlich plots. However, the sign of the fitted slope (1/n) was non-physical, suggesting that equilibrium was not achieved uniformly across concentrations or that non-ideal mechanisms (e.g., matrix partitioning/capillary retention in a non-single-phase system) contributed to uptake. Kinetically, PSO provided a better description of the uptake curve than PFO under the studied conditions. The enhanced performance of succinic-acid-modified sawdust is attributed to increased accessible sorption domains and strengthened non-specific interactions (hydrophobic and π–π interactions) with aromatic xylene. It was concluded that the modified sawdust is a potential, low-cost lignocellulosic sorbent for aromatic hydrocarbons. It is also necessary to consider the need for equilibrium validation and model selection based on physically meaningful parameters.