Sewage sludge–coconut fiber co-pyrolysis biochar: Mechanisms underlying synergistic heavy metal stabilization and ciprofloxacin adsorption

The effects of co-pyrolyzed sewage sludge–coconut fiber biochar on heavy metal stabilization and antibiotic adsorption capacity have not yet been thoroughly investigated. The present study found that Zn and Cd levels were remarkably reduced in the acid-soluble and reducible fractions in sludge after co-pyrolysis with coconut fiber, whereas their levels increased significantly in the residual fraction, thereby reducing the ecological risk of biochar. Kinetics and Freundlich models of the adsorption of ciprofloxacin (CIP) onto biochar indicated chemisorption and multilayer processes. Negative enthalpy (ΔH0) and Gibbs free energy (ΔG0) values indicated that CIP adsorption was an exothermic and spontaneous reaction. The synergistic mechanisms of heavy metal stabilization and CIP adsorption of this biochar are caused by the formation of crystal structures, complexation of functional groups, electrostatic interactions, and pore filling. The sewage sludge–coconut fiber biochar reduces the risk posed by heavy metals and has demonstrated a higher capacity for adsorbing CIP from water.