Adsorption performance of modified magnetic biochar for tetracycline removal from water

This study aimed to compare the properties and tetracycline (TC) adsorption performance of modified magnetic biochars derived from three types of biomass waste—coffee grounds, coconut shells, and bamboo—using dipotassium ferrate (K 2 FeO 4 ) as a modifying agent. The results demonstrated that the modified magnetic biochar prepared from coffee grounds at a pyrolysis temperature of 500 °C (Fe 6+ -CB) exhibited the highest adsorption capacity and magnetic separation efficiency. This superior performance could be attributed to the dual effects of K 2 FeO 4 modification, which enhanced the biochar's physicochemical properties through functional group enrichment and pore expansion via alkaline etching. The primary mechanisms for TC adsorption on Fe 6+ -CB included pore filling, hydrogen bonding, and complexation. The adsorption process was well described by the pseudo-second-order kinetic model, as well as the Langmuir and Freundlich isotherm models. Thermodynamic analysis indicated that the adsorption was spontaneous and exothermic. Moreover, Fe 6+ -CB maintained a TC removal efficiency of over 63% after five adsorption–desorption cycles, indicating moderate reusability with acceptable performance retention. This study provided a practical approach for selecting optimal biomass waste materials to prepare modified magnetic biochar for effective tetracycline removal from wastewater. • K 2 FeO 4 -modified magnetic biochars derived from three biomass wastes were synthesized for tetracycline adsorption comparison. • Fe 6+ -CB, a coffee grounds-based magnetic biochar pyrolyzed at 500 °C, showed favorable adsorption and magnetic separation. • Tetracycline adsorption involves pore filling, hydrogen bonding, and complexation, and follows pseudo-second-order kinetics. • The adsorption is spontaneous and exothermic, fitting well with both Langmuir and Freundlich isotherm models. • After five adsorption–desorption cycles, Fe 6+ -CB maintained ＞63% tetracycline removal efficiency with favorable reusability.