## 2 Experimental
### Analysis  
To determinate the degradation of BDE 209, 0.1 g of milled mixture was collected and subjected to extraction with THF (50 mL) through ultrasonic treatment for 20 min. After centrifugation at 1.0 \(\times\) 10\({}^{4}\) rpm for 5 min, the supernatant was filtered with a 0.22 \(\upmu\)m membrane. The extracted liquid underwent analysis for BDE 209 concentration using high-performance liquid chromatography (HPLC, LC-20A, Shimadzu, Kyoto, Japan) equipped with an SPD-20A UV detector at 230 nm with an Athena C18 column (4.6 mm \(\times\) 250 mm, 5 \(\upmu\)m). The mobile phase consisted of 95% acetonitrile and 5% water, with a flow rate of 1.0 mL/min.  
The extracted solution underwent additional analysis employing a gas chromatograph-mass spectrometer (GC-MS, Agilent 7890B-5977B, Santa Clara, CA, USA) to identify the intermediate products of BDE 209. A DB-5 HT (30 m \(\times\) 0.25 mm \(\times\) 0.25 \(\upmu\)m) capillary column was employed with helium gas serving as the carrier at a flow rate of 1 mL/min. Injection of 1 \(\upmu\)L of the sample occurred in splitless mode, while the injector temperature was maintained at 250 \({}^{\circ}\)C. The temperature program followed the following steps: 100 \({}^{\circ}\)C held for 2 min, increased to 200 \({}^{\circ}\)C at 40 \({}^{\circ}\)C/min, elevated to 330 \({}^{\circ}\)C at 20 \({}^{\circ}\)C/min, then held at 330 \({}^{\circ}\)C for 2 min. The ion source temperature was set at 230 \({}^{\circ}\)C, with an electron energy of 70 eV and a scan mass (\(m/z\)) range from 20 to 800. Analysis of the products was referenced against the NIST 08 mass spectrometry library.  
An additional 0.1 g of milled mixture was extracted by 50 mL ultrapure water under ultrasonic concussion for 20 min, followed by centrifugation. The supernatant was then filtered with 0.22 \(\upmu\)m membrane. This solution was utilized for monitoring bromide ion concentration with a Dionex ICS-2100 ion chromatography equipped, with a Dionex IonPacAS18-4 uM anion exchange column (4 \(\times\) 150 mm). The debromination efficiency of BDE 209 could be evaluated by Equation (1) [34]:  
\[\text{Debromination}=\frac{C_{t}}{C_{0}}\times 100\% \tag{1}\]  
where \(C_{t}\) is the content of inorganic Br\({}^{-}\) at ball milling time \(t\); \(C_{0}\) is the initial content of total bromine in BDE 209.  
FT-IR measurement (VERTEX 70, Bruker, Saarbrucken, Germany) was implemented to detect the chemical structure changes of BDE 209 during the MC process based on the standard KBr method. The microscopic appearance was recorded using SEM (JSM 6400, JEOL, Tokyo, Japan). The element valence was characterized by XPS (Kratos AXIS Ultra DLD, Shimadzu, Kyoto, Japan) with a monochromatic Al K\({}_{\alpha}\) X-ray source. XRD analysis (D8 Advance, Bruker, Saarbrucken, Germany) was conducted to determine the phases and crystal structure of the milling mixtures using Cu K\({}_{\alpha}\) radiation at a scan speed of 8\({}^{\circ}\) (20) per minute ranging from 20 = 10\({}^{\circ}\) to 70\({}^{\circ}\). The full width at half maximum (FWHM) of peaks, crystal size, and degree of disorder were calculated using Jade 6.0, and the degree of disorder in the MC process was obtained from Equation (2) [35]:  
\[\eta=(1-\frac{\sum I_{\text{Activated}}}{\sum I_{\text{Raw}}})\times 100\% \tag{2}\]  
where \(\eta\) is the degree of disorder (%), \(\Sigma I_{\text{Raw}}\) is intensity of each crystal plane diffraction peak for raw samples, and \(\Sigma I_{\text{Activated}}\) is the intensity of each activated crystal plane diffraction peak for activated samples.