4–5, both enantiomers were degraded at similar rates (k(R) approximately k(S)), and
in aerobic soils with pH < 4 and in most anaerobic soils, the enantioselectivity was
reversed (k(R)
Liming and acidification of a “non-enantiomer-selective” soil prior to incubation
resulted in enantioselective degradation with k(R)>k(S) and k(R)
(k(R)+k(S))) of metalaxyl degradation in aerobic soils apparently correlated with soil
pH, no such correlation was found for metalaxyl acid. The examination of available
data for dichlorprop and mecoprop indicated similar correlations between soil pH
and ES as for metalaxyl.
A further study on the transformation of soil in metalaxyl is adding important
information on the enantiomer-selective profile in soil under different climate
conditions (Monkiedje et al. 2003). This investigation focused on the enantiomerselective metalaxyl degradation of the racemic mixture and determined the enantiomers of metalaxyl in typical soils from Germany and Cameroon. R-metalaxyl was
studied, as well as racemic (rac) metalaxyl in controlled incubation experiments.
The kinetics of the degradation or transformation was determined with RP-HPLC,
while the enantiomeric ratios were measured by HPLC with a chiral Whelk O1
column
(CSP:
based
on
1-(3,5-Dinitrobenzamido)-1,2,3,4,tetrahydrophenanthrene). The degradation followed first-order kinetics (R2 > or ¼
0.96). Higher metalaxyl acid metabolite concentrations were found in German soil
than in Cameroonian soil. The enantiomers of the fungicide had different degradation rates in both soils, with half-lives ranging from 17 to 38 days. All forms of
metalaxyl had lower transformation rates in the Cameroonian soil than in the
German soil. The transformation of the R-enantiomer was much faster than the Senantiomer in the German soil and slower than the S-enantiomer in the Cameroonian
soil suggesting that different microbial populations have different degradation
preferences.
Metalaxyl was also investigated in Mediterranean agricultural soils (Celis et al.
2013). For this investigation, soil incubation experiments with pure enantiomeric
compounds were performed under laboratory conditions. These experiments
revealed that the R-metalaxyl was transformed faster than the S-enantiomer in all
three soils, but the extent and enantioselectivity of metalaxyl transformation was
soil-dependent, occurring more slowly and with less enantiomer selectivity in the
fine-textured soil than in the coarse-textured soils. Therefore, it is assumed that
sorption and entrapment of metalaxyl enantiomers in the abundant small-sized pores
of soil 1 (i.e. pore radius < 100 nm) could have resulted in a fraction of the fungicide
of reduced bioavailability, and consequently, protected from enantioselective transformation. Metalaxyl leaching through soil columns was also enantiomer-selective;
the concentration of S-metalaxyl in all leachates collected was greater than that of Rmetalaxyl. Despite being non-enantioselective, sorption influenced the
enantioselectivity of metalaxyl leaching, as it determined the residence time of the
fungicide within the soil column, and consequently, the extent and enantioselectivity
of its transformation during leaching.
8.2 Transformation/Accumulation of Chiral Xenobiotics in Higher Organisms
209
in aerobic soils with pH < 4 and in most anaerobic soils, the enantioselectivity was
reversed (k(R)
Liming and acidification of a “non-enantiomer-selective” soil prior to incubation
resulted in enantioselective degradation with k(R)>k(S) and k(R)
(k(R)+k(S))) of metalaxyl degradation in aerobic soils apparently correlated with soil
pH, no such correlation was found for metalaxyl acid. The examination of available
data for dichlorprop and mecoprop indicated similar correlations between soil pH
and ES as for metalaxyl.
A further study on the transformation of soil in metalaxyl is adding important
information on the enantiomer-selective profile in soil under different climate
conditions (Monkiedje et al. 2003). This investigation focused on the enantiomerselective metalaxyl degradation of the racemic mixture and determined the enantiomers of metalaxyl in typical soils from Germany and Cameroon. R-metalaxyl was
studied, as well as racemic (rac) metalaxyl in controlled incubation experiments.
The kinetics of the degradation or transformation was determined with RP-HPLC,
while the enantiomeric ratios were measured by HPLC with a chiral Whelk O1
column
(CSP:
based
on
1-(3,5-Dinitrobenzamido)-1,2,3,4,tetrahydrophenanthrene). The degradation followed first-order kinetics (R2 > or ¼
0.96). Higher metalaxyl acid metabolite concentrations were found in German soil
than in Cameroonian soil. The enantiomers of the fungicide had different degradation rates in both soils, with half-lives ranging from 17 to 38 days. All forms of
metalaxyl had lower transformation rates in the Cameroonian soil than in the
German soil. The transformation of the R-enantiomer was much faster than the Senantiomer in the German soil and slower than the S-enantiomer in the Cameroonian
soil suggesting that different microbial populations have different degradation
preferences.
Metalaxyl was also investigated in Mediterranean agricultural soils (Celis et al.
2013). For this investigation, soil incubation experiments with pure enantiomeric
compounds were performed under laboratory conditions. These experiments
revealed that the R-metalaxyl was transformed faster than the S-enantiomer in all
three soils, but the extent and enantioselectivity of metalaxyl transformation was
soil-dependent, occurring more slowly and with less enantiomer selectivity in the
fine-textured soil than in the coarse-textured soils. Therefore, it is assumed that
sorption and entrapment of metalaxyl enantiomers in the abundant small-sized pores
of soil 1 (i.e. pore radius < 100 nm) could have resulted in a fraction of the fungicide
of reduced bioavailability, and consequently, protected from enantioselective transformation. Metalaxyl leaching through soil columns was also enantiomer-selective;
the concentration of S-metalaxyl in all leachates collected was greater than that of Rmetalaxyl. Despite being non-enantioselective, sorption influenced the
enantioselectivity of metalaxyl leaching, as it determined the residence time of the
fungicide within the soil column, and consequently, the extent and enantioselectivity
of its transformation during leaching.
8.2 Transformation/Accumulation of Chiral Xenobiotics in Higher Organisms
209