Efficiency and mechanism of adsorption for imidacloprid removal from water by Fe-Mg co-modified water hyacinth-based biochar: Batch adsorption, fixed-bed adsorption, and DFT calculation

In the study, iron (Fe, III) and magnesium (Mg, II) co-modified water hyacinth-based biochar (Fe/Mg-WHBC) to adsorb imidacloprid from water was prepared by using water hyacinth as raw material, and Fe and Mg salts as modifiers. The results showed that the adsorption capacity of Fe/Mg-WHBC (114 mg/g) was greatly improved, 15.34 times that of the unmodified water hyacinth biochar (WHBC, 7.41 mg/g). The imidacloprid adsorption of Fe/Mg-WHBC follows the pseudo-secondary-kinetics and Sips adsorption isotherm. The mechanisms of Fe/ Mg-WHBC adsorption for imidacloprid included surface complexation, hydrogen bonding, pi-pi interaction, electrostatic interaction, and pore-filling. DFT calculation indicates that surface complexation between the metal oxides on Fe/Mg-WHBC and imidacloprid has been the major adsorption mechanism. The Fe/Mg-WHBC had good adsorption efficiency for a variety of pollutants, environmental safety, stability, and reusability. Moreover, the adsorption efficiency pattern of Fe/Mg-WHBC for different target pollutants was verified by the DFT calculation on the binding energy of biochar to the pollutants. The bench scale fixed-bed adsorption experiments indicated that the BJP model could better describe the dynamic adsorption behavior of imidacloprid in the biochar fixed bed. The penetration time was only reduced by 27.6% after five adsorption-regeneration cycles of the fixed-bed. Overall, this study demonstrated that the Fe/Mg-WHBC adsorption to remove imidacloprid from water could be potentially applicable in water treatment engineering.