Sustainable Hydrochar from Orange Peel and Bagasse: A Wet Pyrolysis Approach for Efficient Fe2+ and Mn2+ Removal from Water Using a Factorial Design

Water pollution is a global concern, especially due to iron and manganese, which, at high concentrations, affect water quality by altering taste, odor, and color. This work explores the sustainable synthesis of hydrochar from orange peel and bagasse using hydrothermal carbonization (HTC) and a 23 factorial design to optimize Fe2+ and Mn2+ removal for water treatment polishing. HTC was performed by varying (1) temperature (100–200 °C), (2) residence time (8–14 h), and (3) activation agent (H3PO4 or NaOH), with a central point at 150 °C for 11 h without activation. Characterization was performed using FTIR, TGA, SEM, nitrogen adsorption (BET) for surface area determination, elemental analysis, Brønsted acidity measurements, and zeta potential analysis. The hydrochar synthesized at 100 °C for 14 h with NaOH (HC6) showed the best Fe2+ and Mn2+ removal performance. The equilibrium time was 400 min, with pseudo-first-order kinetics best fitting the Fe2+ adsorption data, while pseudo-second-order kinetics provided the best fit for Mn2+ adsorption. The adsorption process was best described by the Freundlich and Langmuir isotherms, with maximum adsorption capacities (qmax) of 21.44 and 33.67 mg g−1 for Fe2+ and Mn2+, respectively. It can be concluded that HTC-derived hydrochars offer a sustainable and efficient solution for Fe2+ and Mn2+ removal. This strategy presents a potentially valuable approach for sustainable water treatment, offering advantages for industrial application by operating at lower temperatures and eliminating the need for biomass drying, thereby reducing energy consumption and environmental impact.