Photodegradation of 4,4′-dibrominated diphenyl ether in Triton X-100 micellar solution

Highlights

• The various pH lead to the different photodegradation rates.

• The hydroxybiphenyl and dibenzofuran was produced in the photodegradation of BDE-15.

• The formation mechanism of hydroxybiphenyl and dibenzofuran was induced.

• The degradation pathway of BDE-15 in TX-100 solution was discerned.

Abstract

This study has investigated the photochemical reactions of 4,4′-dibrominated diphenyl ether (BDE-15) in Triton X-100 (TX-100) solution by UV light. All photolysis experiments were performed in TX-100 solutions which were all above critical micelle concentration (CMC). BDE-15 photodegradation follows the pseudo-first-order kinetics under various conditions. The results showed that the degradation rates of BDE-15 increased with the increasing concentration of TX-100, and decreased when TX-100 was used in excess, because TX-100 can act as hydrogen donor, photosensitizer and light barrier. When the pH value was in the range of 1–11, the degradation rates increased with the increasing pH value due to the proton effect and free radical reaction. When pH reached 13, the corresponding degradation rate dropped significantly, which was attributed to the decreasing surface potential of micellar retarding the degradation process. BDE-15 was debrominated into 4-dibrominated diphenyl ether (BDE-3) and diphenyl ether (DE), subsequently. In addition, dibenzofuran (DF), ortho-hydroxydiphenyl and para-hydroxydiphenyl were identified as another group of photoproducts, indicating PBDEs can also undergo the photochemical rearrangement via CO bond cleavage and recombination of the radical fragments.

Introduction

Polybrominated diphenyl ethers (PBDEs) are an important group of brominated flame retardants (BFRs) which have been widely applied to the products used in daily life, such as plastics, textiles, building materials, and electronic appliances (Leonetti et al., 2016, Fromme et al., 2014). However, due to the way they are incorporated in the materials, PBDEs are easily released from these products. Therefore, they are frequently detected in water, soil, sediment and biota (Santos et al., 2016). They have been a major concern due to their persistence, bioaccumulation and even carcinogenicity. PBDEs are highly hydrophobic and therefore they can easily accumulate in biota through the food chain (De Wit, 2002, Darnerud, 2003). Studies have shown that PBDEs can potentially affect the endocrine system, liver, neuron development, immune system and reproduction system in humans and mammals (McDonald, 2002). In addition, attention should be paid on the derivatives generated during the natural attenuation of PBDEs because PBDEs generate not only lower brominated BDE products, but also methoxylated polybrominated diphenyl ethers (MeO-PBDEs), methoxylated polybrominated dibenzofurans (MeO-PBDFs) and hydroxylated polybrominated diphenyl ethers (HO-PBDEs), which will negatively affect the environmental and human health (Santos et al., 2016). Therefore, PBDEs have been a major concern for human health and one main target in environmental remediation.

Recently, high levels of PBDEs in soils and sediments near Guiyu and Taizhou (i.e., two electronic waste sites in China) have been reported (Zhao et al., 2016, Zeng et al., 2016a, Zeng et al., 2016b). Dismantling and recycling operations with primitive techniques have been carrying on for at least 30 years, and the processing capacities are now on the order of millions of tons of e-waste each year at these sites. In the combusted areas of plastic chips and cables, the highest concentration of BDEs in soil was 4667 ng/g dw, which were much higher than those from the control sites (2.0–6.2 ng/g dw) (Leung et al., 2007). Moreover, due to the hydrophobic properties of PBDEs, most of them can be absorbed onto natural solid matrices, such as soil, sediment, or clay minerals. Therefore, it is of necessity to find an effective and harmless way to remove PBDEs from the environment.

Surfactant enhanced remediation (SER) has been proposed as a promising technology for significantly increasing the remediation efficiency of aquifers contaminated with non-aqueous phase liquid (NAPL) or solid like polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) (Candida and West, 1992, Yang et al., 2015a, Yang et al., 2015b). Surfactant micelles can significantly enhance the apparent solubilities of hydrophobic organic contaminants (HOCs) and assist the remediation of contaminated soils and sediments. Studies have reported that surfactant-aided soil-washing are effective to remove organic pollutants from soil or sediment (Chu and So, 2001, Rosas et al., 2013, Chu and Kwan, 2003). Yang et al., (2015a) have suggested that SER is based primarily on two processes: (i) reduction of the NAPL–water interfacial tension and (ii) solubilization of HOCs by incorporating them into surfactant micelles. However, few studies have focused on the disposal of the extracts and the recycling of the washing agents.

Photodegradation is an important way to destruct the halogenated organic contaminants in surfactant solutions. The washing agent can be recycled after the removal of pollutants by photodegradation (Choy and Chu, 2001, Diehl et al., 2002). The surfactant micelles can act as both solubilizers and hydrogen donors which can improve the degradation rate of PBDEs. Additionally, surfactant can also facilitate the UV-induced free radical reactions owing to eliminating the quenching effect of aqueous dissolved oxygen (Shi et al., 1997; Chu, 1999). However, limited information was available on the photodegradation of PBDEs in surfactant solutions.

In recent years, 4,4′-dibromodiphenyl ether (BDE-15) has attracted considerable attention on its bioavailability, photooxidation kinetic, solubilization, and subacute oral toxicity (Zhou et al., 2011, Liu et al., 2011, Yang et al., 2016). Therefore, in this study, BDE-15 was used as the representative of low-brominated PBDE congeners. Triton X-100 (TX-100) were selected in our study due to its low CMC and strong capacity to solubilize BDE-15. This study aimed to investigate the effects of the substrates on photodegradation of BDE-15. Moreover, the effect of pH value was also studied to explain the differences in various pH levels. Finally, the comprehensive photodegradation pathway of BDE-15 in TX-100 was discussed.