It has been discovered that several Brominated Flame Retardants (BFRs) have an adverse effect on the environment and human health, and are classified as Persistent Organic Pollutants (POPs). Mechanochemical destruction is considered to be a promising technology for the safe disposal of POPs as it is capable of achieving their complete carbonization by solvent-free high energy ball milling at room temperature, thus preventing the formation of unintentional dioxins during the destruction process.
With the financial support by the National High Technology Research and Development Program of China (2013AA06A305), stoichiometric quantities of Bi2O3 or La2O3 were employed as co-milling reagent (Bi/La-to-Br atomic ratio = 1) to destroy four BFRs (viz. decabromodiphenyl ether, decaBDE; hexabromocyclododecane, HBCDD; tetrabromobisphenol A, TBBPA; hexabromobenzene, HBB), which were selectively and completely transformed into their corresponding oxybromides (i.e. BiOBr and LaOBr). These products have extremely peculiar catalytic and optical properties, and hence they can be used for some actual and many more potential industrial applications. In this way, bromine is beneficially reused in the final product, while POPs carbon skeleton is safely destroyed to CO2 and amorphous carbon.
Mechanochemical Destruction of Brominated Pollutants
It is essential to ascertain carbonization of brominated pollutants to carbonaceous matter or their oxidation to CO2 during the mechanochemical destruction: This is considered to be a major issue because it assures that potentially toxic (brominated) organic by-products are not generated. Amorphous and graphitic carbon is generally found in the product of POPs mechanochemical destruction. On the other hand, oxidation to CO2 has been occasionally reported in literature. Some co-milling reagents induce its release in the milling jar headspace, while others (like various oxides) capture it to develop carbonates. In the latter case, CO2 percentages are close to 0.033%vol (i.e. the average amount in air), due to the thermodynamic equilibrium of carbonates with atmospheric CO2 (at room temperature). In this situation, it is not easy to discern whether the gas is really originated by the reaction, thus a reliable and precise quantification is necessary.
In the experiments conducted, a carbonization process was confirmed by Raman spectra, where two bands, called “G-band” (1540–1580 cm-1) and “D-band” (1330–1380 cm-1), were attributed to the presence of amorphous and graphitic carbon, respectively. Independently from the co-milling reagent, D- and G-band of TBBPA and HBB were sharper than those of HBCDD and DecaBDE, implying a higher carbonization degree.
Carbon Dioxide Quantification
Carbon dioxide quantification of the jar headspaces by HPR-20 QIC Real time Gas Analyzer highlighted major diversity, when the reactions were executed with Bi2O3 or La2O3 (Table 1). Bi2O3 was not carbonated by the CO2 produced during the destruction process. In particular, HBCDD and DecaBDE produced huge amounts of this gas, when compared to HBB and TBBPA. On the contrary, low percentages of carbon dioxide were observed in all samples after ball milling with La2O3. The formation of carbonates was corroborated by Fourier transform infrared analysis, establishing that the oxidation pathway also occurred with this reagent. In fact, the residual CO2 fractions (i.e. which did not react with La2O3) found with varied brominated pollutants in the jar headspace showed a similar trend to those obtained with Bi2O3, suggesting that the oxidation reaction extent was analogous with both oxides and that it mostly depended on the molecular structure of the brominated POP.
Carbon dioxide formation is capable of explaining why the reactivity of Bi2O3, which needed only 2 hour milling to attain >90% conversion, is higher than that of La2O3, which required at least 6 hours of milling in order to obtain almost complete conversion (Table 1). CO2 was not trapped into Bi2O3’s lattice and did not interfere with its reactivity. On the contrary, carbon dioxide was included by LaOBr and La2O3 to form La2(CO3)3 and La(CO3)Br, respectively. The presence of CO2 in lanthanum oxide lattice hampered its activation by high energy ball milling, thus bringing about a major reduction in the reaction rate.
Table 1. Atmosphere composition in the jar headspace after high
||Milling time (h)
Project summary by:
Associate Professor (with Tenure)
School of Environment, Tsinghua University
No.1 Qinghuayuan, Haidian District
Giovanni Cagnetta, Han Liu, Kunlun Zhang, Jun Huang, Bin Wang, Shubo Deng, Yujue Wang & Gang Yu (2016) “Mechanochemical conversion of brominated POPs into useful oxybromides: a greener approach” Scientific Reports, 6, Article number: 28394. doi:10.1038/srep28394
This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.
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