Arsenic Removal using De-oiled Mentha Biomass Biochar: Adsorption Kinetics and the Role of Iron Modification

Arsenic contamination in water threatens millions of people globally, demanding the development of sustainable and effective remediation strategies. This work investigates the removal of As(V) using Fe-modified biochar derived from de-oiled Mentha waste (MMBC). The study aimed to synthesize MMBC and evaluate its adsorption capacity; secondly, to investigate adsorption kinetics; and lastly, to elucidate the governing mechanisms behind the process. Proximate analysis revealed MMBC's stability (low moisture: 6.48%, high fixed carbon: 54.7%) and high adsorption potential (low volatile matter: 9.77%). Characterization techniques (SEM-EDX, XRD, FTIR, TGA) confirmed a desirable porous structure, favourable chemical composition, and surface functionality critical for As(V) adsorption. Importantly, MMBC exhibited a significantly larger surface area providing more active sites (378.08 m² g⁻¹) as compared to the unmodified biochar. Concerning operational conditions, the optimal As(V) removal was achieved at pH 7.5 with a 25 mg/50 mL MMBC dosage in a 24 h contact time. Further, kinetic modelling indicated a pseudo-second-order mechanism, suggesting chemisorption as the dominant process. However, isotherm studies revealed favourable multilayer adsorption, with the Freundlich model best describing the data. The combined effects of MMBC's porosity, functional groups, and Fe modification facilitated both physical and chemical adsorption mechanisms. These findings highlight MMBC's potential as a promising biochar-based adsorbent for efficient As(V) removal from contaminated water for sustainable remediation.