Graphene oxide modified magnetic sludge biochar as a green adsorbent for environmental concentration level of pesticide removal

In addition to the advantage for resource utilization of municipal sludge which is an important solution to reduce the secondary pollution (e.g. landfill and incineration) to the environment, biochar derived from municipal sludge displayed a potential adsorption capacity for emerging contaminants, particularly with appropriate modification. This study synthesized a green adsorbent (Graphene oxide modified magnetic sludge biochar, GO/CoFe₂O₄-SBC), which showed the superiority of low cost, effective adsorption and separation for the environmental concentration level of pesticide (Imidacloprid, IMI) removal from water. The results suggested that higher dose and temperature would facilitate the contaminant removal. The optimum removal rate of IMI obtained at pH=6 was due to the weakest electrostatic repulsion. Pseudo-second kinetic, Freundlich and Temkin isotherm models fitted the experimental data, which indicated that both physisorption and chemisorption were involved in the adsorption process. The maximum adsorption capacity of sludge biochar (SBC), magnetic sludge biochar (CoFe₂O₄-SBC) and GO/CoFe₂O₄-SBC were 3.11×10³, 5.99×10³ and 8.92×10³ μg g^-1, respectively. Physicochemical characteristics, kinetics, isotherms and thermodynamics analysis suggested that the better adsorption performance for GO/CoFe₂O₄-SBC was attributed to pore filling, π-π electron donor-acceptor and oxygen-containing functional groups (e.g. C=O, C-O, -OH, Fe-O and Co-O) interaction. IMI adsorption was a spontaneous endothermic process and GO/CoFe₂O₄-SBC exhibited the greater spontaneous. Ethanol extraction increased the regeneration of adsorbents and maintained more than 90% of adsorption capacity in the recycles (1-5 cycles) of the fresh. The leaching concentrations of Fe and Co of adsorbent (GO/CoFe₂O₄-SBC) were below drinking water standard (0.5 mg L^-1) in the range of pH 3 to 12. In conclusion, this work not only provided a promising biochar (e.g. GO/CoFe2O4-SBC) with superior removal capacity for pesticides in water at low concentrations, but also offered a sustainable and cost-effective way for the resource utilization of municipal sludge.