Ludwig (1991) also found that in the case of rac-α-HCH, rac-β-PCCH and rac-γPCCH, the mixed culture of marine micro-organisms was able to transform the two
enantiomers of these pollutants, although this was achieved at different rates. As a
consequence, a shift of the enantiomeric ratios was observed. DCPP is being
transformed also enantioselectivity, however, in contrast to the results with rac-αHCH and the HCH metabolites, the marine microbial community exclusively transforms the R-enantiomer, while the S-enantiomer appears to be persistent and
unchanged.
Instead of applying enrichment cultures of marine micro-organisms, Buser and
Müller performed anaerobic microbial transformation experiments with sludge from
the anaerobic stabiliser of the communal sewage treatment plant Zürich-Glatt,
Switzerland, for the investigation of α-HCH degradation (Buser and Müller 1995).
This plant is considered representative for today’s sewage treatment technology in
Switzerland and Central Europe. They studied the transformation of the four most
common hexachlorocyclohexane isomers, the chiral α-HCH and the prochiral γHCH, δ-HCH and β-HCH, in sewage sludge under anaerobic conditions. Approximately 250 g of sewage sludge in a 300 mL clear glass serum bottle was fortified
with 100 μL of an ethyl ethanoate solution containing 400–500 μg of α-HCH. The
bottles were tightly capped and incubated on a horizontal shaker at 298 K (25
C) for
up to 14 days in the dark. Samples were taken at different time intervals, extracted
with n-hexane and analysed using enantioselective cGC/MS. The significant transformation was observed for γ- and α-HCH with half-lives between 20.4 and
99 h. High enantioselectivity in the transformation of the α-HCH was indicated by
significantly different rates for the (+)-enantiomer (20.2 Â 10
À3 h
À1 ) and the (À)enantiomer (7.26 Â 10
À3 h
À1 ) resulting in an apparent enrichment of (À)-α-HCH in
the digested samples (Buser and Müller 1995). Buser and Müller, however, also tried
to investigate the transformation products that were formed during their transformation experiment. In preceding model experiments, they synthesised β-PCCH by
dehydrohalogenation, thus confirming previous conclusions earlier drawn (Ludwig
1991; Hühnerfuss et al. 1993). But though Buser and Müller could thus reliably
verify their experimental approach, they were unable to detect β-PCCH in any of the
sewage sludge samples spiked with α-HCH and, consequently, assumed that the
metabolite, initially formed, was further degraded at rates that are significantly faster
than the rate of their formation. A series of recently published experiments focussed
on the microbial transformation mechanisms. A specific focus was laid upon the
predisposition of the microorganisms able to transform HCH isomers (CamachoPerez et al. 2012; Lovecka et al. 2015). A summary of the new findings on
enantiomer-selective transformation processes in aqueous environments can be
found in a recent review (Ribeiro et al. 2017).
An experimental approach that moved somewhere along the borderline between
unspecific microbial and targeted enzymatic transformation of environmental pollutants was earlier reported by Garrison et al. (1997; Lewis et al. 1999). They
investigated the chiral compound o,p
0 -DDT, which comprises 12–20% of
technical-grade DDT. This isomer is presumed to be a human endocrine disruptor
because of its strong estrogenic activity in mammals (Zhou et al. 2014a, b; Pestana
116
8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants
enantiomers of these pollutants, although this was achieved at different rates. As a
consequence, a shift of the enantiomeric ratios was observed. DCPP is being
transformed also enantioselectivity, however, in contrast to the results with rac-αHCH and the HCH metabolites, the marine microbial community exclusively transforms the R-enantiomer, while the S-enantiomer appears to be persistent and
unchanged.
Instead of applying enrichment cultures of marine micro-organisms, Buser and
Müller performed anaerobic microbial transformation experiments with sludge from
the anaerobic stabiliser of the communal sewage treatment plant Zürich-Glatt,
Switzerland, for the investigation of α-HCH degradation (Buser and Müller 1995).
This plant is considered representative for today’s sewage treatment technology in
Switzerland and Central Europe. They studied the transformation of the four most
common hexachlorocyclohexane isomers, the chiral α-HCH and the prochiral γHCH, δ-HCH and β-HCH, in sewage sludge under anaerobic conditions. Approximately 250 g of sewage sludge in a 300 mL clear glass serum bottle was fortified
with 100 μL of an ethyl ethanoate solution containing 400–500 μg of α-HCH. The
bottles were tightly capped and incubated on a horizontal shaker at 298 K (25
C) for
up to 14 days in the dark. Samples were taken at different time intervals, extracted
with n-hexane and analysed using enantioselective cGC/MS. The significant transformation was observed for γ- and α-HCH with half-lives between 20.4 and
99 h. High enantioselectivity in the transformation of the α-HCH was indicated by
significantly different rates for the (+)-enantiomer (20.2 Â 10
À3 h
À1 ) and the (À)enantiomer (7.26 Â 10
À3 h
À1 ) resulting in an apparent enrichment of (À)-α-HCH in
the digested samples (Buser and Müller 1995). Buser and Müller, however, also tried
to investigate the transformation products that were formed during their transformation experiment. In preceding model experiments, they synthesised β-PCCH by
dehydrohalogenation, thus confirming previous conclusions earlier drawn (Ludwig
1991; Hühnerfuss et al. 1993). But though Buser and Müller could thus reliably
verify their experimental approach, they were unable to detect β-PCCH in any of the
sewage sludge samples spiked with α-HCH and, consequently, assumed that the
metabolite, initially formed, was further degraded at rates that are significantly faster
than the rate of their formation. A series of recently published experiments focussed
on the microbial transformation mechanisms. A specific focus was laid upon the
predisposition of the microorganisms able to transform HCH isomers (CamachoPerez et al. 2012; Lovecka et al. 2015). A summary of the new findings on
enantiomer-selective transformation processes in aqueous environments can be
found in a recent review (Ribeiro et al. 2017).
An experimental approach that moved somewhere along the borderline between
unspecific microbial and targeted enzymatic transformation of environmental pollutants was earlier reported by Garrison et al. (1997; Lewis et al. 1999). They
investigated the chiral compound o,p
0 -DDT, which comprises 12–20% of
technical-grade DDT. This isomer is presumed to be a human endocrine disruptor
because of its strong estrogenic activity in mammals (Zhou et al. 2014a, b; Pestana
116
8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants