Thermal catalytic degradation of α-HBCD, β-HBCD and γ-HBCD over Fe₃O₄ micro/nanomaterial: Kinetic behavior, product analysis and mechanism hypothesis

Highlights

• The new POPs, α, β, γ-HBCD were highly catalytic degraded over Fe₃O₄ at 200 °C.

• Comprehensive products were obtained by various products detection methods.

• A high-throughput non-target product detection method was applied.

• The average carbon oxidation state was introduced to study the types of reactions.

• β-HBCD was slightly stable than γ-HBCD and both of them readily convert into α-HBCD.

Abstract

The new persistent organic pollutant (POP), 1,2,5,6,9,10-hexabromocyclododecane (HBCD), has been widely detected in various environmental media and proved to be biotoxic. However, the research on catalytic degradation of HBCD is in its infancy. Herein, we examined the degradation of α-HBCD, β-HBCD and γ-HBCD, over Fe₃O₄ micro/nanomaterial at 200 °C. The pseudo-first-order kinetic rate constants were in the range of 0.04–0.15 min⁻¹, with half-life values of 5–19 min. γ-HBCD is slightly less stable than β-HBCD, but both of them readily convert into α-HBCD, as consistent with the Gibbs free energies of isomers themselves. The four products containing pentabromocyclododecene, two isomers of tetrabromocyclododecene and 1,5,9-cyclododecatriene were detected by conventional GC–MS. Interestingly, a high-throughput non-target product detection were performed by ESI-FT-ICR-MS, where up to 59 types of intermediate products were determined. It is tentatively proposed that different types of bromine-removed products (C₁₂H₁₇Br₅, C₁₂H₁₈Br₄, C₁₂H₁₈, C₁₂H₁₉Br₅, C₁₂H₂₄ and C₁₂H₁₉Br₅O) and cyclododecane ring-opened products (C₁₂H₁₉Br₇, C₁₂H₂₀Br₆O and C₁₂H₂₀Br₆) form via elimination reaction, nucleophilic substitution, hydrodebromination and addition reaction. Besides, most of the products that were detected contained oxygen. The average carbon oxidation state ($\overline{{\mathit{OS}}_{\mathrm{c}}}$) of the products indicate that the oxidation reaction is the dominant reaction type. Deep oxidation products, such as small molecular organic acids (formic, acetic, propionic, and butyric acids) and gas-phase oxidation products (CO₂ and CO) were further detected by ion chromatography and GC-FID, respectively. This study might provide an alternative technique for the low-cost treatment of HBCD waste.

Introduction

1,2,5,6,9,10-Hexabromocyclododecane (HBCD) is the brominated cyclic alkane that is used primarily as additive flame retardant in polystyrene and textile products for fire protection (Marvin et al., 2011). In May 2013, HBCD was listed in Annex A as the new persistent organic pollutant (POP) by the Stockholm Convention, and with a global ban on the use of HBCD to begin in 2016 (UNEP, 2013). However, the use of HBCD in expanded and extruded polystyrene in the building industry has been exempted from this ban until 2021 (UNEP, 2013). HBCD can be transmitted to the atmosphere, soil, water, animals (Jo et al., 2017; Peck et al., 2008; Zhu et al., 2012), plants (Zhu et al., 2016), human skin (Pawar et al., 2016) and even breast milk (Fujii et al., 2018). As established, the high toxicity of HBCD to organisms can cause neuroendocrine disorder and developmental toxicity (UNEP, 2013). Therefore, the safe and efficient removal of HBCD in the environment is essential.

The removal methods of HBCD reported to date，include material adsorption (Li et al., 2018), thermal degradation (Barontini et al., 2001; Larsen and Ecker, 1988), biodegradation (Davis et al., 2005), photocatalysis (Gao et al., 2011), and ultrasonic degradation (Ye et al., 2014). However, research on the catalytic degradation of HBCD is still in its infancy. A few studies on reduction catalytic degradation of HBCD showed that bromine atoms in HBCD could be effectively removed. However, it is unknown that whether these large alkanes generated after debromination could be effectively degraded to small molecules (Li et al., 2016a; Li et al., 2017a; Tso and Shih, 2014). Studies have shown that the surface of metal oxide catalyst is rich in oxygen species and affords good catalytic oxidation performance owing to the defects of low coordination and the good activity of lattice oxygen (O²⁻) (Bielański and Haber, 1979; Hetrick et al., 2011). In addition to their potential in removing halogens from organic compounds, metal oxides can degrade organic compounds into small molecules through catalytic oxidation. Amiridis et al. (Hetrick et al., 2011) observed the generation of CO and CO₂ during the degradation of dichlorobenzene over V₂O₅/TiO₂. Of the metal oxides studied to date, the non-toxic and low cost micro/nano Fe₃O₄ with a three-dimensional hierarchical structure has attracted much attention in environmental remediation-related fields (Zhong et al., 2006). It is composed of nanosized building blocks, while the particle size is in the micrometer range. When compared with a conventional Fe₃O₄ power, the high catalytic activity could be achieved since the composite structure is made up of interconnected nanoparticles with the relatively large specific surface area. Furthermore, because of the micrometer scale particle size, separation and recycling are easier than for general nanoparticles (Jia et al., 2011b). The former studies have shown that the aromatic POPs such as decabromodiphenyl ether (Li et al., 2016b), hexachlorobenzene (Jia et al., 2011a) and octachloronaphthalene (Su et al., 2014) could be efficiently degraded by Fe₃O₄ micro/nanomaterial. Although HBCD also contains halogens, as the nonaromatic compound, the involved degradation mechanism over Fe₃O₄ is likely to differ.

Elucidating the degradation mechanism is highly dependent on a comprehensive analysis of the degradation products. However, the detection of degradation products has mainly been conducted by conventional gas chromatography (GC) (Liu et al., 2015; Sustar et al., 1992) or combined with low resolution mass spectrometry (MS) (Barbosa et al., 2018; Lu et al., 2017; Murugan and Vasudevan, 2018), typically affording the identification of few products. With the development of high-resolution mass spectrometry, high-throughput and non-target compound analysis has been realized. Such a method has been widely used in environmental research (Gago-Ferrero et al., 2015; Peter et al., 2018; Yu et al., 2018), food regulation (Gomez Ramos et al., 2019) and metabolomics research (Salihovic et al., 2019; Villette et al., 2018). Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) affords the highest mass resolution available to date, thereby providing precise molecular masses and element ratios, enabling the analysis of complex mixtures on a molecular level (Colati et al., 2013). Moreover, FT-ICR-MS as a high-throughput and non-target product detection method can detect more compounds (Gago-Ferrero et al., 2015; Gomez Ramos et al., 2019; Peter et al., 2018; Salihovic et al., 2019; Villette et al., 2018; Yu et al., 2018). However, it is rarely used in the identification of degradation products.

Considering the six stereogenic centers of HBCD at positions 1, 2, 5, 6, 9, and 10, theoretically, 16 stereoisomers can be deduced (Heeb et al., 2005). Commercially available HBCD consists mainly of α-HBCD, β-HBCD, and γ-HBCD, with fractional compositions of 8, 15, and 75%, respectively. The structural dissimilarities of the three stereoisomers of HBCD lead to differences in their properties (Li et al., 2017a). Therefore, studying the difference in the catalytic degradation of the three diastereomers over Fe₃O₄ micro/nanomaterial and the involved mechanism are of interest.

Herein, α-HBCD, β-HBCD, and γ-HBCD were degraded over Fe₃O₄ micro/nanomaterial at 200 °C. The degradation kinetics and interconversion of the three diastereomers were studied. To better understand the degradation mechanism, the degradation products were identified by various techniques, including conventional GC–MS, electrospray ionization (ESI) FT-ICR-MS, ion chromatography (IC) and gas chromatography- flame ionization detector (GC-FID). A variety of product detection methods, especially the high-throughput and high-resolution ESI-FT-ICR-MS, contribute a comprehensive degradation product. The average carbon oxidation state ($\overline{{\mathit{OS}}_{\mathrm{c}}}$) typically increases upon oxidation and can be used to monitor the progress of the reaction. Therefore, based on the abundant degradation products and the determination of $\overline{{\mathit{OS}}_{\mathrm{c}}}$, the degradation mechanism was finally proposed.