Nitric-acid-activated biochars from agricultural wastes for methylene blue removal: characterization and pseudo-second-order adsorption kinetics

Abstract

The discharge of synthetic dyes such as methylene blue (MB) into water bodies poses a serious environmental and health concern, driving the search for low-cost, sustainable adsorbents. In this work, five biochars were synthesized from agricultural biomass wastes pineapple leaves (PL), pineapple peels (PP), sunflower seed hulls (SH), macadamia nut shells (MS) and flamboyant pods (FP) through nitric-acid activation followed by pyrolysis at 500 °C, and were evaluated for MB removal from aqueous solution. The materials were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and N₂ adsorption–desorption analysis. XRD confirmed an amorphous, turbostratic carbon structure, while FT-IR revealed abundant oxygen- and nitrogen-containing functional groups generated by acid activation. SEM showed precursor-dependent morphologies, with FP exhibiting an open, interconnected macroporous network. N₂ isotherms were of type IV, and the BET surface areas increased in the order SH < PL < PP < MS < FP. Batch experiments showed that MB removal reached equilibrium within 120 min, with FP reaching 92%. FP, combining the highest surface area, largest pore volume and richest surface functionality, was identified as the optimum adsorbent. Under the identical, low-severity activation conditions used here the FP biochar reaches a Langmuir monolayer capacity of 2.43 mg.g^-1. The value of the present work therefore lies in the systematic comparison of five agricultural residues under a common protocol, and in identifying flamboyant pods as the most promising precursor for further activation, rather than in the absolute capacity of the material as prepared.