A circular economy approach for valorization of Okra stalk waste via its hydrothermal carbonization into sorbents for wastewater treatment and byproduct fuels for energy recovery

• Transforming underutilized unmodified okra stalks into a high-performing hydrochar. • Removing pharmaceutical contaminants efficiently by the hydrochar in real wastewater. • Reusing the hydrochar via regeneration using green solvents. • Utilizing the tar byproducts of hydrochar synthesis for energy recovery. • Applying life cycle assessment to the circular economy of okra agricultural wastes. This study investigates the sustainable utilization of okra stalk waste by converting it into hydrochar through hydrothermal carbonization (HTC) for the removal of ciprofloxacin (CPX), levofloxacin (LV), and methylene blue (MB) from water. The raw okra stalk waste showed minimal removal efficiency (0% for CPX, 11% for LV, and 50% for MB), which motivated its transformation into hydrochar at 180°C for 16 h. Characterization using FTIR, XRD, SEM, TGA, BET, and zeta potential analyses confirmed moderate improvements in surface properties, with the BET surface area increasing from 1.33 to 7.04 m²/g and the pore volume rising from 0.0059 to 0.039 cm³/g. The environmental impact of the hydrochar synthesis process was evaluated through life cycle assessment studies, and the cost effectiveness was assessed by estimating the material and energy costs. Batch adsorption experiments achieved a maximum CPX removal of 70% and a maximum adsorption capacity of 13.74 mg/g at pH 6, 3 g/L dosage, and 180 min contact time. FTIR analysis confirmed the involvement of hydrogen bonding and electrostatic interactions, while BET results indicated pore-filling as a major mechanism. In real wastewater samples, the removal efficiency decreased from 70% to 50%. Competitive adsorption also occurred in binary systems. Regeneration tests using methanol, and alternative green solvents of hibiscus and cinnamon maintained about 80% efficiency after four cycles, proving good reusability. The tar byproduct showed a calorific value of 8542 kJ/kg, demonstrating an energy recovery potential. Overall, okra-derived hydrochar offers a green, cost-effective, and efficient solution for pharmaceutical wastewater treatment, aligning with circular economy principles.