Enantioselective determination of two persistent chlorobornane congeners in sediment from a toxaphene‐treated yukon lake

Abstract

The composition of toxaphene residuesand the enantiomeric ratios of the major congeners were studied in the top 10 slices (1935–1992) of a sediment core collected from Hanson Lake in the Yukon Territories (Canada). This lake was treated with toxaphene as a picsicide in July 1963. Electron‐capture negative ion mass spectrometry and electron‐capture detection were applied in combination with enantioselective gas chromatography using modified cyclodextrin columns. Of the 10 toxaphene peaks identified in each of the sediment slices, two congeners, 2‐exo,3‐endo,6‐exo,8,9,10‐hexachlorobornane (B6‐923 or Hx‐Sed) and 2‐endo,3‐exo, 5‐endo,6‐exo, 8,9,10‐heptachlorobornane (B7‐1001 or Hp‐Sed), were the most abundant. Each of the three chiral stationary phases used in this study was successful at resolving the enantiomers in at least of one of the two congeners. Among them was permethylated β‐cyclodextrin (Chirasil‐Dex), which, up until now, has never been used for enantiomer separation of compounds of technical toxaphene. B6‐923 and another less abundant, as of yet unknown, hexachlorobornane were racemic in all sediment core slices. For B7‐1001, the latter eluting enantiomer was always more predominant. Furthermore, the enantiomeric ratio was found to consistently decrease with time from approximately 0.8 in the early slices (∼1950) to 0.7 in the slice representing 1992, implying that the first eluted B7‐1001 enantiomer is less persistent than the second. B6‐923 and B7‐1001 are most likely metabolites of the more highly chlorinated congeners. A structural evaluation provided evidence that reductive dechlorination at carbons with geminal chlorine atoms is the primary microbial degradation pathway in sediments from Hanson Lake.

Chirality as an Environmental Forensics Tool

“…The first measurements of chiral PCBs in sediments found racemic residues of PCBs 95, 132, and 149 in sediments of the river Elsenz in southern Germany [71]. Similar results were found for PCBs 91,95,136,149,174,176,and 183 in Environment Canada Certified Reference Material EC-5, consisting of sediments from the mouth of the Humber River in Toronto [105]. Thus, no evidence for stereoselective PCB microbial degradation was observed at these sites.…”

“…Several chlorobornane enantiomers were quantified in a 1992 sediment core from Hanson Lake, Yukon, Canada, treated with toxaphene as a piscicide in 1963 [149]. Two hexachlorinated congeners, B6-923 and an unidentified congener, were racemic throughout the core.…”

“…However, another dominant heptachlorinated congener, B7-1001, had nonracemic EFs ranging from 0.44 (Equation (4.4) on Chirasil-Dex) in the deepest sediments deposited around 1950 to 0.41 in the most recent 1992 surficial sediments. While both B6-923 and B7-1001 are congeners likely formed by reductive dechlorination of more chlorinated bornane congeners [149], the lack of higher chlorinated congeners and the reduced number of technical toxaphene congeners in the core suggested enantioselective degradation of B7-1001 after it was formed. Only one enantiomer of B6-923 was produced by laboratory reductive dechlorination of a heptachlorobornane in an anaerobic soil [150], suggesting that microbial consortia responsible in Hanson Lake were not those present in the laboratory study.…”

“…Microbially mediated biotransformation of PCBs has occurred in these sediments by both anaerobic reductive dechlorination [155] and aerobic oxygenase and dioxygenase activity [153]. A subsequent, more extensive survey found nonracemic amounts of PCBs 91,95,132,136,149,174,176,and 183 in river sediments throughout the US [160], while a recent survey of sediments from Toronto-area lakes and ponds found PCB 95 EFs as low as 0.409 [148]. Patterns in EFs among congeners were consistent with known reductive dechlorination patterns by microbial consortia [155] in the Hudson River.…”

Chiral xenobiotics bioaccumulations and environmental health prospectives

“…Vetter et al (1998) identified enantiomers of technical toxaphene in Canadian lake sediment. The same authors (Vetter et al 1999) also detected chloroborane congeners in the sediment of toxaphenetreated Yukon Lake. The sediment samples were collected for more than 60 years (from 1935 to 1992).…”