Acid activated orange peel waste adsorbent for the elimination of phenol with insights into isotherm, kinetics, and thermodynamics

This study explored the potential of acid-treated orange peel (ATOP) and raw orange peel (ROP) as sustainable, low-cost biosorbents for the elimination of phenol from aqueous solutions. Acid modification was employed to enhance the textural and surface chemistry properties of orange peel–derived biomass. Comprehensive characterization using FTIR, SEM, BET, and XRD confirmed successful surface functionalization, improved porosity, and structural modification after adsorption. The findings showed that ATOP demonstrated superior performance relative to ROP, achieving a maximum removal efficiency of 85.25% at 45 °C and reaching equilibrium within 150 min. The maximum adsorption capacities obtained from the Langmuir model were 80.32 mg g−1 for ROP and 133.13 mg g−1 for ATOP. Equilibrium data indicated Langmuir behavior for ROP and Freundlich behavior for ATOP, reflecting increased surface heterogeneity induced by activation. Kinetic assessment showed that phenol removal on ROP followed the pseudo-first-order model, while ATOP was best explained by the pseudo-second-order model, signifying a shift toward stronger surface interactions after acid modification. Thermodynamic parameters established the process to be spontaneous and endothermic, with ΔH° values of 14.5 kJ mol−1 (ROP) and 45.32 kJ mol−1 (ATOP). Overall, the results demonstrate that acid-treated orange peel biomass is an efficient and environmentally benign adsorbent for phenol remediation in wastewater systems.