Azimsulfuron Interaction with Iron(III) Soil Colloids
J. Agric. Food Chem., Vol. 52, No. 26, 2004 8085
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mode, giving rise to a six-membered chelated ring, explains
the peculiar IR features of the AZS-iron oxide complex. In
fact, metal coordination to the sulfonamide group shifts the SO2
stretches, particularly the asymmetric one, to lower wavenum-
bers, owing to the electron-withdrawing effect of metal coor-
dination (12, 24).
It was not possible to perform a FT-IR study of the AZS-
iron humate complex because the intense absorption bands due
to the humic moiety mask those due to the herbicide.
Effect of Oxalic Acid on Iron-Humate Sorption. To obtain
spectroscopic information about the binding mode of AZS to
iron-humate, a further sorption experiment was performed. An
iron-humate sample (20 mg) was added to a solution containing
equimolar amounts of AZS and oxalic acid (12.7 µM), which
is a strong ligand for Fe(III) ions (25). After equilibration (20
h), the suspension (pH 4.8) was tested for AZS. The presence
of oxalic acid decreased drastically the amount of adsorbed AZS
(∼75%), indicating that oxalic acid competes with AZS for the
binding sites on iron-humate. This feature may be ascribed to
a greater tendency of oxalic acid compared with AZS to form
complexes with ferric ions. Therefore, it is plausible that also
the interaction between AZS and iron-humate can occur via a
metal coordination, even if actually we cannot assign the AZS
donor groups coordinated to Fe ions.
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of two quinolinecarboxylic acid herbicides on homoionic mont-
morillonites. Clays Clay Min. 2003, 51, 143-149.
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C.; Bottero, J. Y. Iron speciation in natural organic matter
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Substances in the Ecosystems and in EnVironmental Protection;
Drozd, J., Gonet, S. S., Senesi, N., Weber, J., Eds.; PTSH:
Wroclaw, Poland, 1997; p 671.
Conclusions. The sorption mechanisms of AZS on Fe3+-rich
soil colloids depend on the type of colloid. According to FT-
IR spectra, the sorption of AZS on iron-clay involves the
protonation of one of the two basic nitrogen atoms of the
pyrimidine ring of the herbicide due to the acidic water
surrounding Fe3+-saturating ions. On the other hand, the
formation of a six-membered chelated complex, involving
carbonyl and sulfonyl oxygen atoms, promotes AZS adsorption
on iron oxide.
The different sorption mechanisms affect the degradation rate
of AZS. The hydrolysis is much faster when the herbicide
interacts with colloids by formation of coordination bonds, as
in the case of iron oxide. The complexation of iron by oxygen
atoms of the sulfonylurea bridge weakens the sulfonamidic C-N
bond and facilitates its hydrolytic cleavage from water. Most
probably, a similar mechanism acts also in the case of iron-
humate.
In the presence of iron-clay, the lower pH of the suspension
affords the protonation of the pyrimidine ring and draws electron
density away from the sulfonylurea bridge. This electronic effect
is less important than a chelation process in weakening the C-N
bond and slows the AZS degradation.
(20) Jouany, C.; Chassin, P. Wetting properties of Fe and Ca humates.
Sci. Total EnViron. 1987, 62, 267-270.
(21) Pusino, A.; Petretto, S.; Gessa, C. Sorption of primisulfuron on
soil, and inorganic and organic soil colloids. Eur. J. Soil Sci.
2004, 55, 175-182.
(22) Cook, D. Vibrational spectra of pyridinium salts. Can. J. Chem.
In conclusion, the results indicate that the distribution of Fe3+
ions among organic and inorganic colloids can affect differently
the sorption and degradation of AZS herbicide.
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(N1-pyrimidin-2-yl-sulfanilamido)2‚H2O. J. Inorg. Biochem.
1993, 49, 201-207.
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Received for review July 29, 2004. Revised manuscript received October
11, 2004. Accepted October 12, 2004. Financial support was provided
by the Italian Ministry of University and Research (PRIN Project
ex-40%).
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