Production of hydroxylated polybrominated diphenyl ethers (OH-PBDEs) from bromophenols in natural and engineered processes

The emerging environmental pollutant hydroxylated polybrominated diphenyl ethers (OH-PBDEs) have been of particular concern in recent years due to their ubiquitous distribution in environmental matrices and adverse health effects. Since there is no industrial-scale manufacturing of OH-PBDEs, their origins are inconclusive and not fully understood, thus triggering extensive discussions among the academic community. Recently, the polymerization of simple bromophenols (BrPs) has been identified as an important origin of OH-PBDEs, which are widely reported in various oxidation processes. This review comprehensively surveyed previous studies about OH-PBDEs formation from BrPs precursors in both natural (e.g., photochemical processes, naturally occurring manganese oxides, enzymes-mediated oxidation) and engineered processes (e.g., ozonation, chlorination, permanganate/ferrate associated oxidation, radicals based oxidation, nonradically activated persulfates). Generally, oxidative coupling reactions of bromophenoxyl radicals resulting from BrPs were recognized as the main mechanism for OH-PBDEs formation, which were discussed emphatically by combining theoretical calculations with the experimental evidence. Some discrepancies were found in interpreting the preferential coupling pathway of bromophenoxyl radicals according to different calculation methods. So, further efforts are needed to use advanced analytical methods for sensitive identification of OH-PBDEs to better evaluate the formation potentials of diverse OH-PBDEs from BrPs. Carbon materials associated oxidation technologies (e.g., carbon nanotube activated ferrate or persulfates) may be applied as a promising method for controlling OH-PBDEs during treatment of BrPs-containing waters. Overall, this review reveals a prevalent phenomenon of BrPs transformation to OH-PBDEs and meanwhile advances the mechanistic understanding. Moreover, the knowledge gaps are identified and some recommendations are proposed for future studies.

近年来，新兴环境污染物羟基化多溴二苯醚（hydroxylated polybrominated diphenyl ethers, OH-PBDEs）因其在各类环境介质中广泛分布且具有健康损害效应而受到广泛关注。由于羟基化多溴二苯醚并无工业化生产，其来源尚未明确且未被完全阐明，由此引发了学术界的广泛讨论。近期研究证实，简单溴酚（bromophenols, BrPs）的聚合反应是羟基化多溴二苯醚的重要来源之一，该类反应在各类氧化过程中已有大量报道。本综述全面梳理了关于溴酚前驱体在自然过程（如光化学过程、天然氧化锰体系、酶介导氧化）与人工处理过程（如臭氧化、氯化、高锰酸盐/高铁酸盐耦合氧化、基于自由基的氧化、非自由基活化过硫酸盐体系）中生成羟基化多溴二苯醚的既往研究。一般认为，由溴酚生成的溴苯氧自由基的氧化偶联反应是羟基化多溴二苯醚生成的主要机制，本文结合理论计算与实验证据对该机制进行了重点讨论。不同计算方法对溴苯氧自由基优先偶联路径的解释存在分歧，因此未来需采用先进分析方法实现羟基化多溴二苯醚的灵敏识别，以更准确评估各类溴酚前驱体生成羟基化多溴二苯醚的潜能。碳基材料耦合氧化技术（如碳纳米管活化高铁酸盐或过硫酸盐体系）有望应用于含溴酚水体的处理过程中，以实现对羟基化多溴二苯醚的有效控制。总体而言，本综述揭示了溴酚转化为羟基化多溴二苯醚的普遍现象，同时深化了对相关生成机制的认知。此外，本文还明确了当前研究存在的知识空白，并为未来研究方向提出了建议。