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@PHDTHESIS{Luks:841217,
author = {Luks, Ann-Katrin},
othercontributors = {Schäffer, Andreas and Hollert, Henner},
title = {{N}on-extractable residues of pendimethalin in soil},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2022-01565},
pages = {1 Online-Ressource : Illustrationen, Diagramme},
year = {2021},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2022; Dissertation, RWTH Aachen University, 2021},
abstract = {Two main scenarios were investigated regarding the
formation of non-extractable residues and their
characteristics of the herbicide pendimethalin in soil.
First, the incubation in soil alone was examined and second,
the incubation in soil amended with 20 $weight-\%$ compost.
Incubation of pendimethalin in a LUFA 2.2 soil (loamy fine
sand, USDA) at an application rate of 5 mg kg-1 soil led to
a mineralization of 26.2 $\%$ AR and a formation of
non-extractable residues of 32.0 $\%$ after 400 days (end of
study), hence moderate amounts of non-extractable residues
were found. The amount of non-extractable residues increased
over time and 14CO2-development was observed over the whole
time. 14C-pendimethalin was degraded to several metabolites
of which one could be identified as M455H001
(2-methyl-3,5-dinitro-4-(pentan-3-ylamino) benzoic acid)
amounting to 5 $\%$ AR after 400 days of incubation.
Amendment with compost (incubation time until 217 days) to
the soil led to a slightly higher formation of
non-extractable residues (38.2 $\%$ AR, 217 days) and
reduced the mineralization almost completely (1.7 $\%$ AR
after 217 days of incubation). Also in this study,
metabolite M455H001 was identified but detected in minor
amounts (2.4 $\%$ AR after 217 days, highest amount
detected). A tenfold application of pendimethalin to soil
led to a delayed formation of non-extractable residues and
the degradation of pendimethalin occurred at a lower rate,
too, but this might have been due to the incorrect water
content which was not adjusted the whole time. Humic matter
fractionation revealed that in soil alone, the radioactivity
was almost evenly distributed among the fractions of fulvic
acids, humic acids and humins at all sampling dates
investigated. When soil was amended with compost, the major
part of the radioactivity in the residues was found in the
fraction of humins. The purified fulvic acid fraction was
analyzed with size-exclusion chromatography. The major part
of the injected radioactivity of the fulvic acid fraction
eluted rather early, together with the fulvic acids
themselves. This amount increased with time. It amounted to
92 $\%$ of the radioactivity in the fulvic acids fraction or
9.4 $\%$ of applied radioactivity after 400 days of
incubation. A minor portion of the radioactivity eluted at a
later time point from the SEC column. This was assigned to
radioactivity not associated firmly with fulvic acids. This
portion amounted to 0.4 $\%$ AR (400 days of incubation).
Comparing with the retention times of the parent
pendimethalin and the metabolite M455H001, this portion of
radioactivity was tentatively assigned to the metabolite
M455H001 whereas no pendimethalin could be identified. When
soil was amended with compost, the amount of radioactivity
eluting in the range of radioactivity not associated firmly
with fulvic acids was higher. A silylation reaction was
conducted on extracted soil to evaluate whether the
non-extractable residues were covalently bound to or rather
sequestered in pores of the soil matrix. From soil alone
(study B), ~8 - 20 $\%$ (or ~1.5 - 3.6 $\%AR)$ of the
non-extractable residues could be released. Considering the
amount released by chloroform treatment of the soil, the
amount released by silylation alone would have been even
lower. The amount of non-extractable radioactivity released
by silylation decreased with increasing incubation time,
hence suggesting a firmer association with the soil matrix.
It is concluded that the major portion of the bound residues
is indeed bound covalently to the soil matrix and only a
minor portion was present rather in a sequestered form. The
analysis of the obtained extracts additionally revealed that
only part of the released (thus sequestered) radioactivity
was present as parent pendimethalin. HPLC of a sample
suggested the presence of metabolite M455H001 and
radioactive residues associated with soil organic matter. In
contrast to incubation in soil alone, silylation could
release more radioactivity of the non-extractable residues
formed in compost amended soil. However, higher amounts of
radioactivity released might not be linked to silylation
itself but to the initial treatment with CHCl3 prior to the
silylation process. These leads to the suggestion that the
methanolic extraction method suitable for extraction of
pendimethalin and its metabolites from soil alone was not
suitable for exhaustive extraction of pendimethalin and
metabolites from soil with compost addition. The higher
amount of organic matter resulted probably in the higher
amount of pendimethalin (equivalents) being sorbed to the
soil matrix due to the higher amount of possible adsorption
locations in compost amended soil. In an additional
experiment, the remobilization experiment assessed the
potential of non-extractable residues to be released in the
future. Small quantities were biodegraded, indicated by
minor portions degraded completely to 14CO2 and minor
amounts becoming extractable after incubation in fresh soil.
It was concluded that only minor amounts could become
bioavailable with time, provided that the organic matter
matrix is thoroughly altered by degradation or oxidation.
All in all, with the results of this investigation, the
non-extractable residues formed by pendimethalin in this
soil are considered to be mainly covalently bound to the
soil matrix. Compost amendment enhanced the formation of
non-extractable residues and thus, the herbicide was less
bioavailable and was degraded to a lesser degree than in
soil alone. The methods used in the present dissertation are
a tool to investigate the formation and characteristics of
xenobiotic non-extractable residues and to distinguish those
being sequestered (type I) and covalently bound (type II).},
cin = {162710 / 160000},
ddc = {570},
cid = {$I:(DE-82)162710_20140620$ / $I:(DE-82)160000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2022-01565},
url = {https://publications.rwth-aachen.de/record/841217},
}
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