Adsorption of Phenol from Aqueous Solution using Activated Char Synthesized by One-Step and Two-Step KOH Activation and Pyrolysis of Orange Peel

Abstract The present study synthesized activated char from orange peel through one-step (KAC-1) and two-step (KAC-2) methods involving the use of KOH activation and pyrolysis at 800 ℃. The one-step method combined both chemical and thermal activation in a single process which reduced the complexity involved with the traditional approach where initial carbonisation step is followed by chemical activation before another carbonisation. Adsorption process parameters, such as phenol concentration (50–250 ppm), adsorbent dosage (50–250 mg), contact time (5–50 min), and pH (2–12) were optimized for both activated chars using Box Behnken design (BBD) response surface methodology design. The BBD statistical tool showed that the quadratic model describing the adsorption efficiency was significant with an R 2 ≥ 0.98. The maximum adsorption efficiency and capacity were 95% and 360 mg/g for KAC-1 and 99% and 467 mg/g for KAC-2 activated char. The SEM image showed a porous morphology with a BET-specific surface area of 956 m 2 /g for KAC-1 and 1404 m 2 /g for KAC-2. Quantum chemical DFT analysis revealed that KAC-2 lowered the HOMO–LUMO energy gap (ΔE) for the KAC-2 phenol complex and enhances the adsorption capacity with a relative free energy of 60.05 kJ/mol. The regeneration study showed that both KAC-1 and KAC-2 can be used up to five cycles maintaining a removal efficiency of > 85%. Hydrogen bonding and π-π interaction between phenol and adsorbent surface were the main adsorption mechanisms identified. The adsorption isotherms for KAC-1 followed Langmuir (monolayer) while it was Freundlich (multilayer) for KAC-2. Overall, the finding demonstrates that orange peel can be an efficient and recyclable adsorbent percussor material for the removal of phenol from water. Graphical Abstract