Enhanced Degradation of Decabromodiphenyl Ether via Synergetic Assisted Mechanochemical Process with Lithium Cobalt Oxide and Iron

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 (LiCoO₂) and iron (Fe) as co-milling reagents for BDE 209 degradation was investigated. The synchronous action of LiCoO₂ and Fe with a LiCoO₂/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 debromination of BDE 209 by Fe and the following oxidative degradation of debrominated products by LiCoO₂ 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 LiCoO₂ and Fe as co-milling reagents is promising in the MC treatment of toxic halogenated pollutants like BDE 209.

1. Introduction

Polybrominated diphenyl ethers (PBDEs) have been widely used as brominated flame retardants in manufactured products, such as plastics, construction materials, textiles, and electronic equipment [1,2,3,4]. Decabromodiphenyl ether (BDE 209) is one of the commercial PBDEs found worldwide [5,6]. However, BDE 209 is one of the typical persistent organic pollutants (POPs), with characteristics of long transportation range and environment persistence, due to its indestructible chemical bonds making it difficult to be decomposed [7,8]. Previous studies have consistently found BDE 209 in different parts of the environment, 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. In general, the common co-milling reagents employed for organic decomposition can be divided into two main groups, reductive agents such as zero-valent metals (e.g., Fe, Al) [22,23] and oxidative substances like persulfate and manganese dioxide [24,25]. During the MC processes, zero-valent irons (ZVIs) are utilized as electron donors to accomplish the reductive dehalogenation of the target chemicals [26]. For oxidative degradation of halogenated pollution, the MC process is mainly activated by losing electrons to oxidants. Manganese dioxide is usually used as the electron acceptor of the redox couples Mn(IV)→Mn(II) or Mn(IV)→Mn(III) [27]. The above-mentioned MC methods are all conducted using a single milling reagent. Recently, the combination of reductive metals and oxides has been tested to improve the destruction efficiency of halogenated contaminants in MC treatment [28,29,30]. Zhang et al. researched the synergistic action of Bi₂O₃ and Fe on the MC process for BDE 209 degradation [31]. The results showed that the destruction efficiency using Fe and Bi₂O₃ is significantly higher than those using only Fe or Bi₂O₃ under comparable operational conditions.

Lithium cobalt oxides (LiCoO₂), the cathode material of lithium-ion batteries (LIBs), have a certain oxidation capacity, as cobalt can be reduced from Co(III) to Co(II). Saeki et al. proposed an MC method for extracting the valuable metals Li and Co from LiCoO₂ with polyvinyl chloride (PVC) as a co-grinding reagent [32]. It is interesting to note that inorganic chlorides were detected in the MC products as about 90% of the chlorine in PVC had been transformed. This might suggest that LiCoO₂ could be applied to degrade halogenated organics in MC treatment. ZVI, as a cheaper but strong electron donor, has an excellent reductive dehalogenation efficiency in both liquid solvents and MC processes. In our previous study [33], reductive iron (Fe) powder could significantly promote the extraction efficiency of Li and Co from waste lithium cobalt oxide batteries via the MC process, and the valence state of Co was converted from Co(III) to Co(II) after an MC reductive process with Fe. Therefore, we anticipated that the combination of LiCoO₂ and Fe could improve the destruction of halogenated organics via the MC process.

Hence, the objective of the present study was to investigate the degradation behavior of BDE 209 with the assistance of LiCoO₂ and Fe by implementing the MC method. The MC parameters including the molar ratio of LiCoO₂/Fe, rotational speed, and ball-to-powder ratio were examined. The physical chemistry changes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) meter, X-ray photoelectron spectroscopy (XPS), and Fourier transform-infrared spectrometry (FT-IR). Ultimately, the intermediate products of BDE 209 were detected by GC-MS to illustrate the details of the mechanism of the BDE 209 degradation. As such, it will offer new insights with regard to both the resource utilization of the cathode material LiCoO₂ from the increasing number of spent LIBs and the elimination of POPs.

2. Experimental

2.1. Materials and Reagents

Decabromodiphenyl ether (BED 209, 99%) was obtained from Alfa Aesar. Lithium cobalt oxide (LiCoO₂, 99.8%) was purchased from Aladdin Co., Ltd., Shanghai, China. Reductive iron powder (Fe, >98%) was supplied by Sinopharm Chemical Reagent Co., Ltd., Beijing, China. Acetonitrile and tetrahydrofuran (THF) of chromatographic grade were from Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China. All the other chemical reagents were analytical grade and used without further purification. Milli-Q ultrapure water (18.2 MΩ·cm) was used for all the experiments.

2.2. MC Treatments

MC experiments were carried out using a planetary ball mill (QM-QX04, Nanjing University Instrument Corporation, Nanjing, China) at 25 °C and standard atmosphere conditions. In each trial, 0.46 g of BDE 209 powder was blended with LiCoO₂ and Fe in a mortar in specific ratios. The mixture was then transferred into stainless-steel pots, and steel balls with a diameter of 6 and 10 mm were added. To determine the optimal experimental conditions, the ratio of n_LiCoO2:n_Fe was varied from 0.36 to 1.08, as the amount of LiCoO₂ increased gradually, and the n_Fe:n_Br was fixed at 1.67:1, maintaining the amount of BDE 209 at 0.48 mM. The balls-to-powder mass ratio ranged from 50:1 to 200:1. The rotational speed was adjusted from 300 to 700 rpm, with automatic rotation direction changes every 15 min. Control experiments involved milling BDE 209 with Fe only, LiCoO₂ only, or without additives. Most batch experiments were duplicated, with an error of less than 3%.

2.3. 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 × 10⁴ rpm for 5 min, the supernatant was filtered with a 0.22 μm 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 × 250 mm, 5 μm). 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-5HT (30 m × 0.25 mm × 0.25 μm) capillary column was employed with helium gas serving as the carrier at a flow rate of 1 mL/min. Injection of 1 μL of the sample occurred in splitless mode, while the injector temperature was maintained at 250 °C. The temperature program followed the following steps: 100 °C held for 2 min, increased to 200 °C at 40 °C/min, elevated to 330 °C at 20 °C/min, then held at 330 °C for 2 min. The ion source temperature was set at 230 °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 μm membrane. This solution was utilized for monitoring bromide ion concentration with a Dionex ICS-2100 ion chromatography equipped, with a Dionex IonPac AS18-4 μM anion exchange column (4 × 150 mm). The debromination efficiency of BDE 209 could be evaluated by Equation (1) [34]:

$$\mathrm{Debromination}=\frac{C_t}{C_0}\times 100\%$$

(1)

where C_t is the content of inorganic Br⁻ at ball milling time t; C₀ is the initial content of total bromine in BDE 209.

FT-IR measurement (VERTEX 70, Bruker, Saarbrücken, 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_α X-ray source. XRD analysis (D8 Advance, Bruker, Saarbrücken, Germany) was conducted to determine the phases and crystal structure of the milling mixtures using Cu K_α radiation at a scan speed of 8° (2θ) per minute ranging from 2θ = 10° to 70°. 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]:

$$\eta =(1-\frac{\sum I_{\mathrm{Activated}}}{\sum I_{\mathrm{Raw}}})\times 100\%$$

(2)

where η is the degree of disorder (%), ΣI_Raw is intensity of each crystal plane diffraction peak for raw samples, and ΣI_Activated is the intensity of each activated crystal plane diffraction peak for activated samples.

3. Results and Discussion

3.1. Performance of BDE 209 Degradation

To investigate the impact of co-milling agents, BDE 209 was decomposed via the MC process employing LiCoO₂ Fe powder separately, a mixture of LiCoO₂ and Fe, and a control without additives (Figure 1). The results revealed that the degradation rate of BDE 209 was only 13.5% within 300 min without a grinding reagent. When LiCoO₂ or Fe was added individually, there was a modest increase, with removal rates of 34.6% and 59.3% for BDE 209 within 300 min, respectively. However, the simultaneous addition of Fe and LiCoO₂ significantly enhanced the decomposition of BDE 209, achieving a remarkable removal rate of 99.8% after 180 min. This indicates a synergistic effect in the degradation of BDE209 in the MC treatment when Fe and LiCoO₂ are used in combination. Therefore, subsequent experiments were performed to explore the influence of milling conditions on the decomposition and debromination of BDE 209, with both Fe and LiCoO₂ participating in the MC process within a milling time of 240 min.

3.2. Effect of Milling Conditions on BDE 209 Degradation and Debromination

The impact of milling conditions is illustrated in Figure 2 through quantification of the decomposition and debromination efficiency of BDE 209. Further details are provided in the Supplementary Materials (Figures S1–S3). As seen in Figure 2a, the effect of the grinding reagent molar ratio between LiCoO₂ and Fe (n_LiCoO2/n_Fe) was investigated, and BDE 209 exhibited a faster degradation with a higher n_LiCoO2/n_Fe ratio after 240 min co-milling. When n_LiCoO2/n_Fe increased from 0.36 to 0.9, the reduction of BDE 209 was rapidly and significantly escalated from 23.5% to 99.9%, while the debromination rate was varied from 21.0% to 92.0%. However, with n_LiCoO2/n_Fe reaching 1.08, the removal of BDE 209 remained almost constant, while the debromination decreased to 79.0%. In the low n_LiCoO2/n_Fe range, the higher LiCoO₂ load enhanced the oxidation of BDE 209. Conversely, in the high n_LiCoO2/n_Fe range, the excessive LiCoO₂ oxidized the Br⁻ yielded in the MC process to Br₂, leading to a low Br⁻ concentration being detected, and consequently the debromination rate being decreased [36].

Figure 2b presents the influence of the rotational speed on the abatement and debromination of BDE 209 within 240 min. As the rotational speed varied from 300 to 600 rpm, both the removal and debromination of BDE 209 increased, reaching a maximum of 99.9% and 92.0% at 600 rpm, correspondingly. These results suggest that a higher ball milling speed could lead to a better degradation and debromination of BDE 209. Previous studies confirmed that the impact energy is a key factor in the MC treatment of pollutants, (see Equation (3)) [37,38]. The faster rotation speed increases the collision velocity and collision number per unit time, leading to a higher impact energy of the MC process [38]. Hence, the C-C and C-Br bonds in BDE 209 are broken more easily with higher impact energy. When the rotation speed increased to 700 rpm, the degradation and debromination rates remained stable. This indicates that an excessively high rotation speed is unnecessary in this combined system, thus saving energy consumption:

$$E={\displaystyle \sum _{\mathrm{j}=1}^{\mathrm{n}}\frac{m_{\mathrm{b}}}{2m_{\mathrm{p}}}{v}_{\mathrm{j}}^2}$$

(3)

where E is the impact energy of milling balls, v_j is the relative speed of the impact between the ball and the tank wall or the balls, m_b is the mass of milling balls, n is the collision number per unit time, and m_p is the mass of sample powders charged into the mill pots.

Figure 2c depicts the effect of the ball-to-powder ratio (m_b/m_p) on the removal and the debromination of BDE 209 during MC treatment. The results indicate a positive correlation between the degradation rate and the yield of Br⁻ with m_b/m_p. As m_b/m_p was set at 50, the removal of BDE 209 was 52.1%, and the debromination was 48.9%; as m_b/m_p increased to 100, 150, and 200, the removal rate reached nearly 100%, and the debromination was 93.1% when m_b/m_p was at 200. The swift removal of BDE 209 was attributed to the large number of balls at high m_b/m_p, increasing the collision frequency per unit time, and consequently, the amount of impact energy transferred to the solid particles becoming higher (Equation (3)) [39]. It is important to note that the higher the m_b/m_p ratio, the higher is the energy consumption [37,38,39]. Hence, the m_b/m_p value of 100 was selected as the optimal experimental parameter, considering operability and cost of practical application.

3.3. Characterization Analysis and Mechanism

3.3.1. SEM Analysis

To elucidate the detailed reaction mechanism, physical chemical property changes of the sample powders were examined. Figure 3 shows the morphology of activated samples at different ball-milling times as observed by SEM. It can be seen that the surfaces of the particles became significantly rougher with the extended milling time. These outcomes imply that substantial energy was generated during the MC process and transferred to the samples, resulting in the samples being damaged and inducing the degradation and debromination of BDE 209.

3.3.2. XRD Analysis

XRD patterns of milled samples before and after MC treatment are presented in Figure 4. The peaks corresponding to the crystal planes 003, 101, and 104 represent the layer structure of LiCoO₂. It is obvious that the intensity of these peaks weakened gradually with prolonged ball milling time, indicating the destruction of the crystal structure of LiCoO₂. Accordingly, the structure of Fe reflected by the diffraction peak of 110 crystal plane, which disappeared after milling, and the new peaks of 211 crystal plane corresponding to Fe₂O₃ were observed, showing the oxidation of Fe(0) to Fe(III). Fe(0) acted as a reducing reagent to reduce BDE 209 during the MC process. The grain size of 003 and 110 peaks decreased from about 1000 nm to 453 and 243 nm after 240 min grinding, respectively, which also indicated the decrease of LiCoO₂ and Fe (Figure S4). Furthermore, the FWHM of the two main crystal planes, 003 and 110, broadened with the increase of MC reaction time (Figure S5). Results suggest that the crystal structure of LiCoO₂ and Fe was gradually converted to an amorphous state within the process. The disorder degree (Figure S5), representing the degree of amorphousness, became higher with MC treatment time from 0 to 240 min, very consistent with the results of FWHM.

3.3.3. XPS Analysis

XPS analysis was conducted to measure the chemical state of elements in the milled samples. The XPS spectra of Br 3d, C 1s, Co 2p, O 1s, Fe 2p before and after 180 min MC treatment are presented in Figure 5a. In non-activated samples, two types of bromine, Br 3d_5/2 and Br 3d_3/2 with binding energies 70.8 and 71.9 eV, were attributed to covalently bonded bromine atoms in BDE 209. After 180 min grinding, two new peaks appeared at 68.8 and 69.5 eV, assigned to Br⁻ (Figure 5b), suggesting that the organic bromine from BDE 209 was released and transformed to Br⁻ [31]. The C 1s peaks at 286.4 eV and 288.8 eV, attributed to C-Br of BDE 209, disappeared after milling (Figure 5c). The peak at 284.8 eV assigned to C-C decreased significantly, and a new peak at 291.1 eV was observed. These results reveal that the organic structure of BDE 209 was decomposed and transformed into graphitic and amorphous carbon [36]. For Co 2p (Figure 5d), the binding energies of Co 2p_3/2 and Co 2p_1/2 at 779.8 and 794.8 eV, respectively, belong to Co(III), consistent with Co(III) in non-activated LiCoO₂.The binding energies of Co 2p_3/2 and Co 2p_1/2 representing Co(II) appear at 781.3 and 796.3 eV, respectively [40], indicating that the valence state of Co was changed from Co(III) to Co(II). The XPS profile of O 1s (Figure 5e) for the sample before reaction at the peak of 529.4 eV can be fitted with the lattice oxygen in LiCoO₂ [41]. Afterwards, this peak disappeared, and the relative intensity of the peak at 532.9 eV, attributed to oxygen vacancies, significantly increased due to the MC process. This indicates that the active oxygen in LiCoO₂ can transfer electrons to the aromatic structure thus inducing the reduction of BDE 209. Fe 2p spectra are shown in Figure 5f, with binding energies of Fe 2p_2/3 and Fe 2p_1/2 measured at 710.8 eV and 724 eV, characteristic of Fe(III) [42]. This confirmed that Fe(0) was converted to Fe(III) and acted as the reducing agent during the treatment. From these results, it can be concluded that the reductive debromination of BDE 209 by Fe, and the subsequent oxidative abatement of debrominated products by LiCoO₂ were integrated in a concerted procedure, and as a result, leading to the elimination of BDE 209.

3.3.4. FT-IR Analysis

The samples, both before and after MC treatment for 180 min, were prepared for FT-IR analysis. The spectra in Figure 6 offer valuable insights into the structural and functional properties of the milled samples. In non-milled samples, the peaks at 1499 and 1321 cm⁻¹ represented the chain vibrations of the aromatic ring, while the peaks at 962, 763, and 695 cm⁻¹ were attributed to the stretching vibration of the C-Br bond. Additionally, the peak at 1210 cm⁻¹ denoted the asymmetrical stretching vibration of the C-O-C bond in BDE 209 [43]. Interestingly, all the abovementioned peaks disappeared in the 180 min ball-milled sample, suggesting that BDE 209 was degraded completely, and the bromine was removed from the organic structure. These observations align with the findings from the degradation and debromination experiments.

3.3.5. Degradation Pathway of BDE 209

The degradation intermediates of BDE 209 during the MC process at different ball milling times were identified by GC-MS, as detailed in Figures S6–S8. It is evident that the peak intensity of BDE 209 weakened, and dibromodiphenyl ether, tribromodiphenyl ether, and 3,5-dibromophenol were observed after 60 min of milling. Furthermore, the peaks of BDE 209 or low brominated aromatics disappeared after 180 min of milling, while aliphatic products such as 2-bromo-3-methyl-hexane-1,5-diol, 3-methyl-1,5-hexanediol, and 3-methyl-1-hexanol were identified (Additional mass spectral data are provided in Figures S9–S15). From all the identified intermediates, we concluded that the decomposition of BDE 209 involved a series of consecutive reactions, namely the C-Br and C-O-C bonds of BDE 209 were first broken, and low brominated aromatic products were produced during the initial period of the MC process. Subsequently, the aromatic structures underwent ring opening to generate aliphatic products; the degradation pathways of BDE 209 are proposed in Figure 7.

4. Conclusions

The present work first reports the degradation of BDE 209 by using MC treatment with LiCoO₂ and Fe as co-grinding agents. The favorable interaction between LiCoO₂ and Fe was found to be dependent upon the LiCoO₂ to Fe molar ratio, rotation speed, and the ball-to-powder ratio. Under optimum conditions (n_LiCoO2/n_Fe/n_Br = 1.5:1.67:1, m_b/m_p = 100:1, rotation speed = 600 rpm), a 180-min ball milling in LiCoO₂ 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 LiCoO₂-only system, respectively. SEM, XRD, XPS, FT-IR, and GC-MS analyses confirmed that Fe acted as the reducing agent 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 LiCoO₂ 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 LiCoO₂ 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 LiCoO₂ 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.