Development of sustainable carbon nanotubes from fruit waste for iron adsorption

Abstract In this study, the mangosteen peel-derived carbon nanotubes (M-CNTs) were synthesized using a simple and environmentally friendly process. The green M-CNTs were oxidized with a concentrated mixture of phosphoric and sulfuric acids. Their physicochemical characterization revealed that both M-CNTs and oxidized M-CNTs exhibited well-developed surface morphologies, with surface areas of 408.5 m 2 /g and 223.3 m 2 /g, respectively, and points of zero charge at pH 9 and 7. Moreover, the FTIR characterization verified the enrichment of key functional groups, including hydroxyl and carboxyl. The adsorption capacity and the percentage of removal efficiency of the oxidized M-CNTs were determined using batch adsorption experiments, in which the impacts of adsorbent dosage, initial pH, iron concentration, and contact time were evaluated. The results demonstrated that oxidized M-CNTs exhibited an efficient iron adsorption capacity of 27.0 mg/g, and 92.1% iron removal at pH 8.5 (25 °C). The adsorption kinetics indicate that the iron adsorption followed a pseudo-second order with R 2 = 0.985, and a Fractional Power model with R 2 = 1, indicating the iron adsorption is chemisorption with a heterogeneous adsorption process. The thermodynamic properties showed that the process is exothermic and spontaneous, as evidenced by the negative ΔG values, which increased from − 2149.5 to − 1458.2 J/mol as the temperature increase from 25 to 75 °C. The findings confirm that mangosteen peel–derived CNTs constitute an effective green adsorbent, offering high iron removal efficiency from aqueous media and representing a promising alternative to conventional carbon-based materials.