Valorisation of Orange Peel into Biochar Using Pyrolysis for Phenolic Contaminant Removal from Water: Experimental and Quantum Chemical Insights

This study investigates orange peel valorisation through KOH pre-treatment and high-temperature pyrolysis (800 °C) to develop a highly porous activated char for the efficient removal of phenolic compounds, specifically 2,4-dinitrophenol (DNP) and aminophenol (AP), from water. The main objective of the study is to synthesise high-surface area activated char from orange peel and investigate its performance for the adsorption of DNP and AP from water. The synthesised adsorbent exhibited a Brunauer–Emmett–Teller (BET) specific surface area of 965 m2/g, contributing to its excellent phenol adsorption efficiency. Batch adsorption experiments were performed, and a maximum removal efficiency of 99% and 92% was observed at pH 4 and 7 with initial concentration 50 mg/L, contact time 60 min, and adsorbent dosage 0.6 g/L, for DNP and AP, respectively. The adsorption process was described by the Langmuir isotherm model (R2 = 0.99), indicating monolayer adsorption and followed pseudo-second-order kinetics, achieving a maximum adsorption capacity of 366 mg/g for DNP and 341 mg/g for AP. Furthermore, DFT analysis revealed that DNP possesses a lower HOMO-LUMO energy gap (−0.54 eV), favouring a stronger adsorption interaction, whereas AP exhibited a relatively higher energy gap (−0.27 eV), corresponding to its comparatively lower adsorption capacity. Overall, the findings demonstrates that a single step chemical-thermal conversion of orange peel into biochar-based adsorbent offers a sustainable pathway for the removal of phenolic compounds from water.