Insight to the mechanism of tetracycline removal by ball-milled nanocomposite CeO2/Fe3O4/Biochar: Overlooked degradation behavior

A novel nanocomposite CeO2/Fe3O4/biochar (BC) was prepared using a straightforward, solvent-free mechanical ball milling method. Comparatively to the material prepared by chemical co-precipitation (77.14 %) and raw BC (33.64 %), CeO2/Fe3O4/BC prepared by ball milling achieved better tetracycline (TC) removal (94.35 %) under the optimal condition (TC concentration 10 mg·L-1, catalyst dose 0.5 g·L-1 and pH 9.01). Ball milling not only reduced the particle size, but also increased the surface area of composite, which was favorable to the adsorption of TC on CeO2/Fe3O4/BC. Meanwhile, the number of oxygen-containing functional groups in as-prepared nanocomposite increased after ball milling, which could serve as the bridge of electron transfer to enhance TC degradation. Dissolved oxygen (DO) was activated by CeO2/Fe3O4/BC to produce reactive oxygen species (ROS) for the TC degradation. X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) characterization indicated that the enhanced oxygen activation should be attributed to the presence of Ce4+/Ce3+ and Fe3+/Fe2+ redox pairs and persistent free radicals (PFRs) in CeO2/Fe3O4/BC. Desorption experiments suggested that the adsorption and degradation induced by ball-milled CeO2/Fe3O4/BC were accounted for about 79.8 % and 20.2 % of total TC removal, respectively. This study revealed that the organic pollutants removal by BC-based nanocomposite is not alone contribution of the adsorption and the degradation behavior during this process should be concerned.