Effect of organic carbon chemistry on sorption of atrazine and metsulfuron-methyl as determined by 13C-NMR and IR spectroscopy

Soil organic matter (SOM) content is themajor soil component affecting pesticide sorption. However,recent studies have highlighted the fact that it is notthe total carbon content of the organic matter, but itschemical structure which have a profound effect on thepesticide’s sorption. In the present study, sorption ofatrazine and metsulfuron-methyl herbicides was studiedin four SOM fractions viz. commercial humic acid,commercial lignin, as well as humic acid and huminextracted from a compost. Sorption data was fitted to theFreundlich adsorption equation. In general, theFreundlich slope (1/n) values for both the herbicideswere <1. Except for atrazine sorption on commercialhumic acid, metsulfuron-methyl was more sorbed. Desorptionresults suggested that atrazine was moredesorbed than metsulfuron-methyl. Lignin, whichshowed least sorption of both the herbicides, showedminimum desorption. Sorption of atrazine was bestpositively correlated with the alkyl carbon (adjustedR2 = 0.748) and carbonyl carbon (adjusted R2 = 0.498)but, their effect was statistically nonsignificant(P = 0.05). Metsulfuron-methyl sorption showed bestpositive correlation with carbonyl carbon (adjustedR2 = 0.960; P = 0.05) content. Sorption of both theherbicides showed negative correlation with O/N-alkylcarbon. Correlation of herbicide’s sorption with alkyland carbonyl carbon content of SOM fractions suggestedtheir contribution towards herbicide sorption.But, sorption of metsulfuron-methyl, relatively morepolar than atrazine, was mainly governed by the polargroups in SOM. IR spectra showed that H-bonds andcharge-transfer bonds between SOM fraction and herbicidesprobably operated as mechanisms of adsorption