Sorptive interactions of pesticides in soils treated with modified humic acids

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Abstract
Summary: Different structural changes in peat humic acid (HA) were induced through treatments leading to chemical derivatization and/or the removal of functional groups. Eleven humic products were obtained after acetylation, amidation, ammonia fixation, methylation, nitration, oximation, sulphonation, selective degradations and ‘fixation’ of stearic acid. The sorption of 12 pesticides (alachlor, atrazine, carbofuran, 2,4‐D, 2,4‐DB, methyl parathion, metoxuron, monouron, prometryne, propanil, fenoprop and chloranil) on the above products was determined by ultraviolet derivatographic spectroscopy. In addition, pesticide sorption was evaluated on samples from neutral and calcic soils treated with the corresponding potassium humates. The most intense interactions occurred in samples from the neutral plot, whereas the greatest effect of the humic matter in enhancing pesticide sorption was found with the calcic soil. As expected, the sorption patterns in HA‐pesticide systems were poorly correlated with those in soil‐HA‐pesticide systems. In the former case, the sorption performance was influenced mainly by hydrophobic interactions, whereas in the latter it depended on the participation of multiple parameters suggesting that the molecular volume and the van der Waals energy of the pesticide molecule play a chief role. The HA characteristics most frequently correlated with pesticide sorption were those connected with the aromatic/aliphatic ratio and the particle size. In the calcic‐soil‐HA‐pesticide systems, the base saturation of the exchange complex correlated negatively with pesticide sorption.