###### Abstract  
The removal of decabromodiphenyl ether (BDE 209), as a typical persistent organic pollutant (POP), is of worldwide concern. Mechanochemical (MC) processes are promising methods to degrade environmental pollutants, most of which use a single grinding reagent. The performance of MC processes with co-milling agents still needs to be further verified. In this study, an efficient MC treatment with combined utilization of lithium cobalt oxide (LiCoO\({}_{2}\)) and iron (Fe) as co-milling reagents for BDE 209 degradation was investigated. The synchronous action of LiCoO\({}_{2}\) and Fe with a LiCoO\({}_{2}\)/Fe/Br molar ratio of 1.5:1.67:1 and a ball-to-powder ratio of 100:1 led to almost thorough-paced abatement and debromination of BDE 209 within 180 min using a ball milling rotation speed of 600 rpm. The reduction in particle sizes and the destruction of crystal structure in mixture powders with the increase in milling time induced the enhanced degradation of BDE 209, as characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The X-ray photoelectron spectroscopy (XPS) characterization showed that the valence state of Co was converted from Co(III) to Co(II), and Fe(0) was changed to Fe(III) when treated with an MC process. This indicated that the reductive deformation of BDE 209 by Fe and the following oxidative degradation of debrominated products by LiCoO\({}_{2}\) were integrated in a concerted way. It proved the removal of BDE 209 via an MC treatment. The full breakage of C-Br and C-O bonds in BDE 209 was confirmed by Fourier transform-infrared spectrometry (FT-IR) spectra, and a possible abatement pathway was also proposed based on the identified intermediate products using gas chromatography-mass spectrometry (GC-MS). These obtained results indicated that a combination of LiCoO\({}_{2}\) and Fe as co-milling reagents is promising in the MC treatment of toxic halogenated pollutants like BDE 209.  
decabromodiphenyl ether; iron; lithium cobalt oxide; mechanochemical process; synergetic effect 2020 October such as air, water, soil, and sediment [9; 10; 11; 12; 13]. Moreover, due to its high lipophilicity, BDE 209 can be accumulated in biological organisms and eventually transferred to humans through the food chain. It has been reported that BDE 209 exposure might lead to neurotoxicity, endocrine disruption, and carcinogenicity [14; 15]. Thus, the elimination of BDE 209 has been currently capturing increased attention worldwide. Recently, many studies have focused on the degradation of BDE 209 via chemical oxidation techniques, such as incineration, Fenton reaction, or photocatalytic oxidation [16; 17; 18]. Each of these methods, however, has its limitations. Incineration generates secondary hazardous byproducts, such as polychlorinated dibenzo-\(p\)-dioxins, leading to secondary pollution [18]. As for advanced oxidation processes, namely the Fenton reaction and photocatalytic oxidation, they have been employed to decompose BDE 209 in liquid media [19]. However, its high hydrophobicity gives BDE 209 the tendency to enrich in solid matrices, which may significantly decrease the efficiency of the degradation treatment. Thus, it is urgent to develop new technologies to degrade BDE 209 in the solid phase. Recently, mechanochemical (MC) techniques based on grinding reactants in the solid phase have attracted particular attention as straightforward procedures for treating halogenated organic pollutions. In the MC procedure, solid-to-solid reactions are triggered by collisions with milling bodies in reaction devices [20]. Compared to the chemical oxidation approaches, the MC method holds an advantage due to its use of mild temperature and pressure, the absence of organic solvent, and the avoidance of emission of hazardous byproducts [21]. It has been proven that the grinding reactant is one of the key elements in the MC process, having significant impacts on the degradation efficiency of halogenated organics.