Low-Cost Activated Carbon from Cassava Rhizome Waste via Hydrothermal Method for Methylene Blue and Community Wastewater Treatment: Isotherm and Kinetic Studies

Current pollution of natural water sources led to the utilization of agricultural waste, such as cassava rhizome, for conversion into activated carbon in order to reduce waste, promote a circular economy, and enhance the absorption of pollutants, particularly dyes, in wastewater treatment. In this study, the activated carbon was prepared through a hydrothermal process and potassium hydroxide (KOH) activation at 1:1, 1:2, and 1:3 ratios. The resulting activated carbon exhibited increased porosity, smaller pore sizes, and a higher surface area compared to cassava rhizome biochar, especially at a KOH 1:2 ratio. The surface area reached 42.68 m²/g. In the methylene blue adsorption test, the activated carbon showed better adsorption efficiency than cassava rhizome biochar, achieving a maximum removal percentage of 98.35%. Moreover, the results were consistent with the Langmuir adsorption model, with maximum adsorption capacities of 16.08, 81.97, 100, and 96.15 mg/g for biochar and activated carbon prepared with KOH at the respective ratios of 1:1, 1:2, and 1:3, respectively. The kinetic adsorption study aligned with pseudo-second-order kinetic modeling, suggesting that chemisorption may contribute to the adsorption process. In addition, when activated carbon and cassava rhizome biochar are used for natural wastewater treatment, the oxidation-reduction potential (ORP) and dissolved oxygen (DO) are enhanced, which suggests activated carbon and cassava rhizome biochar could effectively be used in real-world applications. HIGHLIGHTS Preparation of Activated Carbon from cassava rhizome, an agricultural waste for minimization of waste and effective treatment of wastewater. The hydrothermal activation method with a KOH ratio of 1:2 is most effective; the surface area of 68 m²/g was 8.82 times higher than that of cassava rhizome biochar. The activated carbon with a KOH ratio of 1:2 is capable of adsorbing 35% of methylene blue at initial concentration is 30 mg/L. Adsorption followed the Langmuir adsorption model with Qmax = 100 mg/g and KL= 33 L/mg and pseudo-second-order kinetic model with k2 = 0.0006 g/mg·min, R² = 0.9961, and h = 2.7 mg/g·min. Both cassava rhizome biochar and activated carbon show potential for natural wastewater treatment. GRAPHICAL ABSTRACT