Interaction of imidazolinone herbicides with soil humic acids. Experimental results and molecular modeling

Adsorption and desorption isotherms of the herbicides imazapyr, imazethapyr and imazaquin on a soil humic acid have been performed at pH 2.8 and 4.0 ( below and above the pKa of the herbicides). At both pH, adsorption increase d according to the lipophilic character of the molecules (imazapyr < imazet hapyr much less than imazaquin). The extent of adsorption was higher at pH 2.8 than at pH 4.0 because of the partial ionization of the carboxylic grou ps of both herbicides and humic acids at increasing pH. Desorption of imaza pyr and imazethapyr was nearly complete at pH 4 and higher than 60% at pH 2 .8 while desorption of imazaquin was 45 and 8% at pH 4 and 2.8, respectivel y. No differences between adsorption isotherms at 10 degreesC and 25 degree sC were observed a pH 4.0 indicating that adsorption involved very weak bon ds while at pH 2.8, adsorption was higher at 10 degreesC than at 25 degrees C indicating an exothermic process. The isosteric enthalpy of adsorption of each herbicide was low (about -1 kJoule mole(-1)) suggesting that low ener getic bonds were involved. Adsorption on different humic acids has indicate d that for each herbicide, the extent of adsorption expressed as K-d was co rrelated with the amount of carboxylic and aromatic groups of humic acids s uggesting that hydrogen bonding and/or charge-transfer complexes formation could take place. Molecular modeling and geometry optimization of humic aci d and soil organic matter (SOM) herbicide complexes were also performed. Th e results obtained with this throretical approach gave a consistent chemica l interpretation of the experimental results. To the best of our knowledge this is the first report to contribute to a better understanding of site-sp ecific bonding of herbicides in SOM complexes by nanochemical modeling and distinct energy descriptors.