Development of nanoporous activated carbon from waste red onion peels for small-scale and large-scale effective wastewater treatment: two-stage reactor modeling and optimization.

Industrial growth has intensified water pollution from chromium (Cr) and methylene blue (MB), posing serious health hazards. This study synthesized nanoporous activated carbon (NPAC) from waste red onion peels through chemical and physical activation, thereby enhancing surface functionality and porosity for pollutant removal. Characterization using FTIR, XRD, BET, FE-SEM, EDX, TEM, and XPS confirmed the presence of diverse functional groups, nano-sized pores, and a surface area of 338.85 m2/g. NPAC-WROP exhibited high adsorption capacities of 171.98 mg/g for Cr(VI) and 177.53 mg/g for MB at 298 K. This study also developed single-stage and two-stage adsorption batch reactor models, providing a theoretical framework for optimizing NPAC-WROP dose in adsorption processes for efficient removal of pollutants such as MB dye and Cr(VI) ions from wastewater. The theoretical results showed that 266.63 and 197.53 g of NPAC-WROP were required to treat a 100 mg/L pollutant concentration of Cr(VI) or MB dye, respectively, achieving 95% conversion at a volumetric flow rate of 100 dm³ in the single-stage system, whereas these requirements were reduced to 132.70 and 106.23 g, respectively, in the two-stage system.