One-step preparation of activated pomelo peel biochar by using H₃PO₄ for removal of methylene blue: performance, isotherm, and kinetic studies

In this study, activated biochar was prepared using waste pomelo peel as a precursor using a one-step facial pyrolysis method with phosphoric acid in air. Different characterization methods indicated that the pyrolysis temperature significantly affected the morphology and surface properties of the prepared biochars. Under phosphoric acid activation, the biochar was successfully prepared at 700 °C (PBC-A7) with an area surface of 1192.18 m²/g. PBC-A7 exhibited a high adsorption capacity of methylene blue (MB) of 385.11 (mg/g). The adsorption isotherms of MB on PBC-A7 were effectively described using the Langmuir and Temkin models rather than the Freundlich model, which indicated that MB interacted differently with the biochar surface to form monolayers. In addition, the kinetics study demonstrated that MB adsorption on the biochar agreed well with the pseudo-second-order, Elovich, and intraparticle diffusion kinetics models. This result suggests that the adsorption mechanisms of MB on PBC-A7 include electrostatic interactions, pore filling, hydrogen bonding, and π−π interactions. This study reveals that biochar can be simply synthesized through the activation of phosphoric acid in air at high temperatures.