Sustainable Mn²⁺ Removal Using Chayote-Derived Activated Carbons: Porosity, Surface Chemistry, and Reusability
The post-harvest collected leaves, stems, and vines from chayote (Sechium edule), an abundantly cultivated yet largely discarded agro-waste rich in cellulose and lignin, were transformed into activated carbon through KOH, HNO₃, and FeCl₃ treatment. The KOH route yielded a highly microporous structure (BET surface area 1103 m²/g⁻¹; pore volume 0.601 cm³/g), whereas HNO₃ enhanced oxygenated functional groups and FeCl₃ left behind iron species. In batch studies, the materials removed up to 95% of Mn²⁺ ions under favourable conditions. The adsorption process was better explained by pseudo second order kinetic model (R² = 0.979–0.984) and Langmuir isotherm, with maximum uptake capacities of (qmax - 37.69, 35.58, and 33.43 mg/g) for HAC, KAC, and FAC, respectively. Among them, KAC showed the best reusability, retaining about two-thirds of its efficiency after five cycles. Further field trials of manganese-contaminated groundwater from Dimapur, Nagaland, were found to be lowered below the WHO safety limit (0.1 mg L⁻¹). These findings reveal how chayote, often considered as agro-waste, can be repurposed into a low-cost, renewable, and reusable adsorbent for sustainable heavy-metal clean-up from water.