Biodegradation and Product Identification of [¹⁴C]Hexabromocyclododecane in Wastewater Sludge and Freshwater Aquatic Sediment

Abstract

In a previous study the biodegradation of hexabromocyclododecane (HBCD) was reported to occur under realistic environmental concentrations in soils and freshwater aquatic sediments with biotransformation half-lives ranging from approximately 2 days to 2 months. In this study we extend our knowledge as to the environmental behavior of HBCD with respect to the fate of the three major diastereomers of HBCD (alpha, beta, and gamma) as well as to the identification of major intermediate metabolites formed during degradation. Substantial biological transformation of the alpha-, beta-, and gamma-[14C]HBCD diastereomers was observed in wastewater (i.e., digester) sludge and in freshwater aquatic sediment microcosms prepared under aerobic and anaerobic conditions. Concomitant with the loss of [14C]HBCD in these matrixes there was a concurrent production of three [14C]products. Using a combination of high performance liquid chromatography atmospheric pressure photoionization mass spectrometry and gas chromatography electron impact ionization mass spectrometry these metabolites were identified as tetrabromocyclododecene, dibromocyclododecadiene, and cyclododecatriene. We propose that HBCD is sequentially debrominated via dihaloelimination where at each step there is the loss of two bromines from vicinal carbons with the subsequent formation of a double bond between the adjacent carbon atoms. These results demonstrate that microorganisms naturally occurring in aquatic sediments and anaerobic digester sludge mediate complete debromination of HBCD.

Reductive transformation of hexabromocyclododecane (HBCD) by FeS

“…Yet, our results on additional transformation products indicated that only a fraction of the HBCD in the reaction mixtures was debrominated completely by the synthetic FeS into CDT within the time frame of this study. On the one hand, two tetrabromocyclododecene (TBCDe) isomers and five DBCDi isomers were identified as less brominated BCD intermediate products (see Text A4 and Table A6 for more details on the identification) by comparing their mass spectra with those of the corresponding transformation products reported in the literature (Barontini et al, 2001;Davis et al, 2006). Moreover, most of these intermediate products were still observed at the end of the experiment (48 h, Fig.…”

“…During each step, two Br atoms could be removed from two vicinal carbons and a double bond was subsequently formed between the two adjacent carbon atoms. Such a pathway was believed to be also operative in the microbially mediated debromination of HBCD (Davis et al, 2006), whilst part of this pathway has been proposed recently for the reductive debromination of HBCD by nanoscale ZVI (Tso and Shih, 2014). The scenario that abiotic and biotic reductive debromination of HBCD may share some common reaction steps deserves further investigation, since it likely has important implications for the remediation of anoxic sediments contaminated by HBCD.…”

“…No definitive evidence is currently available on the ability of abiotic reductants to transform HBCD in reaction mixtures into the completely debrominated product 1,5,9-cyclododecatriene (CDT), although it was reported that HBCD could be transformed biotically into CDT (Davis et al, 2006). To our knowledge, only two attempts have been made in recent years to identify the products of HBCD transformation by abiotic reductants in reaction mixtures.…”

“…Growing evidence indicates that TBBPA and HBCD can be biodegraded via reductive dehalogenation in anoxic sediments (Davis et al, 2005(Davis et al, , 2006Ronen and Abeliovich, 2000;Voordeckers et al, 2002). For examples, Voordeckers et al (2002) observed microbial dehalogenation of TBBPA to bisphenol A in anoxic sediments, and Davis et al (2006) reported sequential debromination of HBCD by microbes in anoxic sediments via a series of dihaloelimination steps. However, several studies showed that biodegradation of decaBDE could be very slow (La Guardia et al, 2007;Tokarz et al, 2008;Zhu et al, 2014).…”

Levels and distribution of hexabromocyclododecane (HBCD) in environmental samples near manufacturing facilities in Laizhou Bay area, East China

“…Tissue-specific accumulation of a-and g-HBCD was also reported in the study of bib and whiting 30 and juvenile rainbow trout, 32 in which a preferential accumulation of a-HBCD was found in the liver. In addition, the apparent concentrations in these tissues could also be affected by biotransformation, metabolism, biodegradation and more, which were previously discussed in the studies on cytochrome P450 mediated biotransformation in marine mammals, 33 metabolites and biodegradation of HBCD in rats 34,35 and bioisomerization of HBCD isomers in fish. 36 Compared with the distribution in other environmental matrices, the elevated ratio of a-HBCD in the aquatic species could probably be affected by factors such as physical-chemical properties (aqueous solubility and half life in the environment) of the three isomers and isomer-specific bioavailability in the organisms.…”