A green method to improve adsorption capacity of hydrochar by ball-milling: enhanced norfloxacin adsorption performance and mechanistic insight.

The application of hydrochar as a cost-effective solution has received much attention for the remediation of contaminated water. An economical and environmental approach to enhancing the physicochemical and adsorption performance of hydrochar is essential. In this study, the green technology of ball-milling was firstly employed to improve the adsorption capacity of hydrochar for the typical antibiotics norfloxacin. Aqueous batch adsorption experiment using both pristine and ball milled hydrochar derived from water hyacinth, prepared by hydrothermal carbonization at three temperatures (180, 200, 220 °C) was conducted. The results showed that ball-milling decreased the specific surface area of hydrochar, but still greatly enhanced their performance on the adsorption of norfloxacin. Surface functional groups, aromatization degree, and hydrophobicity of hydrochar were increased after ball-milling, as evidenced by measurements of Boehm titration, Raman spectra, and contact angle, respectively. With these changes, all the ball-milled water hyacinth hydrochar exhibited a better performance on the adsorption of norfloxacin than pristine hydrochar. Ball-milled 220 °C water hyacinth hydrochar showed the greatest norfloxacin adsorption (68.53 mg g⁻¹) compared to unmilled hydrochar (24.29 mg g⁻¹), and the enhancement was effective in a wide pH range (5–9) in aqueous solutions. The thermodynamics study indicated that the norfloxacin adsorption on ball-milled hydrochar was both physically spontaneous and exothermic. Combined physicochemical characterization of hydrochar and batch experiment results suggest that the enhanced adsorption capacity was owing to boosting H-bonds, π-π electron-donor–acceptor, and hydrophobic interaction. This study suggested that ball-milling can be served as a facile, green, and cost-effective method to obtain modified hydrochar for the removal of pollutants in water. Highlights: • Surface functional groups and hydrophobicity of hydrochar were increased after ball-milling. • Ball-milling was firstly used to modify hydrochar (BMHC) to remove antibiotics. • BMHC had a high adsorption capacity of 68.53 mg g^-1 for norfloxacin (NOR). • NOR adsorption on BMHC was enhanced by increased H-bonds, π-π EDA, and hydrophobic interaction. [ABSTRACT FROM AUTHOR]