Interfacial mechanism of the synergy of biochar adsorption and catalytic ozone micro-nano-bubbles for the removal of 2,4-dichlorophenoxyacetic acid in water

• Biochar gained from NaOH-pretreated straw shows high performance for 2,4-D removal. • Synergistic behavior benefits the enhancement of the effective utilization of ozone. • Surface OH is the main active site to adsorb and decompose ozone molecules. • The degradation pathways of 2,4-D were elucidated. • The toxicity of the 2,4-D solution was reduced significantly. A high-efficiency catalytic ozonation system consisting of NaOH-pretreated rice straw biochar (OHBC) and ozone micro-nano-bubbles (O 3 -MNBs) was developed to remove 2,4-dichlorophenoxyacetic acid (2,4-D) in water, and the interfacial mechanism was further studied in detail. The removal rate of 2,4-D was improved by 47.0% through the synergy of adsorption and catalytic ozonation in the O 3 -MNBs/OHBC system (89.0%) compared with the O 3 -MNBs alone system (42.0%), and the ozone utilization rate was increased from 0.43 mg/mg to 0.66 mg/mg. High performance for 2,4-D removal from water with diverse matrix factors could also be obtained. The many hydroxyl groups ( OH) (1.25 mmol/g) on the surface of the OHBC were more powerful than the carboxyl and lactone groups ( COOH and COOR) at catalyzing ozone to produce OH according to XPS and Boehm titration results. The theoretical calculations based on density functional theory (DFT) showed that ozone molecules were more readily adsorbed on the surface OH groups due to the relatively high negative adsorption energy (–0.31 eV), and they further extended the two O O bond length from 1.277 Å to 1.303 Å and 1.311 Å, thus promoting its decomposition. The degradation mechanism of 2,4-D included dechlorination, electrophilic substitution, and OH substitution, and the degraded water showed relatively low toxicity.