Application of acidic treated peanut shell biochar in methyl orange removal from aqueous medium: elucidating isotherms, kinetics and proposed mechanism

Abstract

The urgent need for sustainable water treatment solutions has driven global scientific efforts toward developing novel materials. Biochar, a low-cost material produced from biomass waste, represents a promising and versatile adsorbent class. In this study, acid-treated peanut shell biochar (PS-BC) was synthesized and evaluated for the removal of the methyl orange (MO) dye from aqueous solution. Characterization via Fourier Transform Infrared Spectroscopy (FT-IR) confirmed the presence of carbonaceous groups, such as C=O, C-O, C=C and derived phosphoric groups, such as P=O. Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS) revealed a mesoporous and macroporous structure, while X-ray Diffraction (XRD) indicated a predominantly amorphous material containing amorphous SiO₂ phases. The adsorption was followed by monitoring 464 nm band of MO in UV-Vis spectroscopy, and the adsorbent achieved a maximum removal of above 89% of MO within a 60-minute experiment. The adsorption kinetics were analysed in pseudo-first and pseudo-second models, which the adsorption was described better by the pseudo-first-order model. Also, adsorption isotherms (Langmuir, Freundlich and Temkin) were studied, and the equilibrium data closely fit the Langmuir isotherm model, pointing to monolayer adsorption onto a homogeneous surface. The FT-IR analysis of post-adsorbed PS-BC confirmed bands associated to MO, such as N=N, SO₃^- and change in aromatic C=C, indicating the possible adsorption pathways. These results confirm the successful application of the acid-treated PS-BC as an efficient and eco-friendly adsorbent for organic pollutant removal from water.