## 4 Conclusions  
The present work first reports the degradation of BDE 209 by using MC treatment with LiCoO\({}_{2}\) and Fe as co-grinding agents. The favorable interaction between LiCoO\({}_{2}\) and Fe was found to be dependent upon the LiCoO\({}_{2}\) to Fe molar ratio, rotation speed, and the ball-to-powder ratio. Under optimum conditions (\(n_{\rm LiCoO2}/n_{\rm Fe}/n_{\rm Br}\) = 1.5:1.67:1, \(n_{\rm b}/m_{\rm p}\) = 100:1, rotation speed = 600 rpm), a 180-min ball milling in LiCoO\({}_{2}\) and Fe combined system resulted in a degradation efficiency of 99.8% for BDE 209. This efficiency was approximately 3.5 and 4.2 times higher than that in the Fe-only and LiCoO\({}_{2}\)-only system, respectively. SEM, XRD, XPS, FT-IR, and GC-MS analyses confirmed that Fe acted as the reducing agent  
Figure 6: FT-IR spectra of samples before and after ball milling for 180 min. Conditions: \(n_{\rm LiCoO2}/n_{\rm Fe}/n_{\rm Br}\) = 1.5:1.67:1, \(n_{\rm b}/m_{\rm p}\) = 100:1, rotation speed = 600 rpm. Figure 7: The possible pathways for BDE309 degradation by MC treatment with combined LiCoO\({}_{2}\) and Fe as co-milling reagents. to initiate the debromination of BDE 209 during the MC process. The ball milling process activated the lattice oxygen of LiCoO\({}_{2}\), facilitating the oxidation of the debromination products of BDE 209. Namely, during the MC process, Co(III) in LiCoO\({}_{2}\) was reduced to Co(II), Fe(0) was oxidized to Fe(III), and the organic bromine in BDE 209 was converted to dissolvable inorganic Br\({}^{-}\). The BDE 209 was firstly reduced to low brominated aromatic intermediates, and then decomposed to aliphatic products through ring opening triggered by the mechanical force and LiCoO\({}_{2}\) oxidation. These findings highlight that the MC method with co-milling reactants is a potential technology for removing BDE 209 in solid matrices. The strategy of combined utilization of LiCoO\({}_{2}\) and Fe as co-grinding reagents in the MC treatment proposes an alternative route for the degradation of toxic halogenated contaminants together with implementation of ball milling. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app132312924/s1](https://www.mdpi.com/article/10.3390/app132312924/s1)]([https://www.mdpi.com/article/10.3390/app132312924/s1](https://www.mdpi.com/article/10.3390/app132312924/s1)), Figure S1: The effect of grinding reagent molar ratio between LiCoO\({}_{2}\) and Fe (nLiCoO\({}_{2}\)/nFe) on the degradation of BDE209 in MC treatment. Figure S2: The effect of rotation speed on the degradation of BDE209 in MC treatment. Figure S3: The effect of ball-to-powders mass ratio (\(m_{\text{b}}/m_{\text{p}}\)) on the degradation of BDE209 in MC treatment. Figure S4: Grain size changes of different crystal planes with varying milling times. Figure S5: FWHM and disorder degree changes of different crystal planes with varying milling times. Figure S6: GC chromatogram of BDE209 before MC treatment. Figure S7: GC chromatogram of BDE209 after 60 min ball milling time. Figure S8: GC chromatogram of BDE209 after 180 min ball milling time. Figure S9: GC-MS spectrum of decabromodiphenyl ether. Figure S10: GC-MS spectrum of dibromodiphenyl ether (P1). Figure S11: GC-MS spectrum of tribromodiphenyl ether (P2). Figure S12: GC-MS spectrum of 3,5-dibromophenol (P5). Figure S13: GC-MS spectrum of 2-bromo-3-methyl-hexane-1,5-diol (P6). Figure S14: GC-MS spectrum of 3-methyl-1,5-hexanediol (P7). Figure S15: GC-MS spectrum of 3-methyl-1-hexanol (P8). Writing--original draft, X.L. ; Writing--review, editing, and funding acquisition, J.G., Y.S. ([PERSON]), and X.L. ; Validation, Q.Z. and C.F. ; Visualization, Y.T. and X.Z. ; Methodology and investigation, X.L. and Y.S. ([PERSON]); Data curation: G.Y. ; Supervision and funding acquisition, D.H. and Y.G. The manuscript was written with the contributions of all authors. All authors have read and agreed to the published version of the manuscript. Not applicable.