Bioadsorption of Manganese with Modified Orange Peel in Aqueous Solution: Box–Behnken Design Optimization and Adsorption Isotherm

Chemically demethoxylated and Ca-cross-linked orange-peel waste was engineered as a biosorbent for Mn(II) removal from water. A three-factor Box–Behnken design (biosorbent dose 3–10 g L−1, initial Mn2+ 100–300 mg L−1, contact time 3–8 h; pH 5.5 ± 0.1, 25 °C) required only 16 runs to locate the optimum (10 g L−1, 100 mg L−1, 8 h), at which the material removed 94.8% ± 0.3% manganese removal under the optimized conditions (10 g L−1, 100 mg L−1, 8 h, pH 5.5) of dissolved manganese and reached a Langmuir capacity of 29.7 mg g−1. Equilibrium data fitted the Freundlich (R2 = 0.968) and Sips (R2 = 0.969) models best, indicating a heterogeneous surface, whereas kinetic screening confirmed equilibrium within 6 h. FTIR and SEM–EDX verified abundant surface –COO−/–OH groups and showed Mn deposits that partially replaced residual Ca, supporting an ion-exchange component in the uptake mechanism. A preliminary cost analysis (<USD 10 kg−1) and > 90% regeneration efficiency over three cycles highlight the economic and environmental promise of this modified agro-waste for polishing Mn-laden effluents.