Ethylene oxide is used widely to sterilize soils in order to study the abiotic degradation of pesticides. The drawbacks of ethylene oxide sterilization are increase in pH and increase in the organic carbon content of soils. In order to understand the mechanism of these modifications of soil characteristics, we investigated the effect of ethylene oxide on the chemical structure of soil humic acids in sterile soils by FT-IR spectrophotometry and CP-MAS13C NMR. The FT-IR spectra of sterilized humic acids showed two absorptions in the out of phase -C-C-O- stretching region of primary alcohols (1075-1045 cm-1) which are absent in the spectra of original humic acids. These absorptions are very close to those of ethylene glycol and ethanol, suggesting the presence of a -CH2-CH2-OH group in the sterilized material, formed by reaction of ethylene oxide with the acidic hydrogen atoms of humic acids. This reaction is confirmed by several other features of the IR spectra of sterilized humic acids; for example, the original broad band of the OH stretching (3300 cm-1) is shifted to a higher wavenumber (about 3350 cm-1). Furthermore, an increase of the aliphatic bands (2850-2925 cm-1) is observed, and the original absorption of the carbonyl groups is shifted from 1715 cm-1 to 1725 cm-1, which can be caused by the esterification of a part of the carboxylic groups of the humic acids. The NMR spectrum of a sterilized humic acid exhibits new peaks at 64 ppm (-CH2-OH group), 74 ppm (ether group), and 168 ppm (ester group), which confirm that the hydroxyalkylation of humic acids has occurred during sterilization. This reaction is likely to be responsible for increasing the pH and organic carbon content of sterilized soils. In order to evaluate the influence of the sterilization of soil on the interaction with pesticides, studies of the adsorption of the herbicide acifluorfen have been performed on sterile and unsterile humic acids and soils. Sterilization decreases adsorption on humic acids and on soils characterized by a high organic carbon content.
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