Dissolved Organic Matter Interaction with Napropamide and Four Other Nonionic Pesticides

Equilibrium dialysis was used to measure the extent of interaction between dissolved organic matter (DOM) and five nonionic pesticides. The association-dissociation of DOM with the relatively polar, nonionic pesticide napropamide [2-(a-naphthoxy)-N,N-diethylpropionamide], was emphasized in this study. A negative linear correlation was found between the log of the solubility and the log of the association coefficient of these five pesticides with dissolved humic acid derived from peat (peat-DHA). However, napropamide, which is more water soluble and therefore considered less hydrophobic than lindane (γ-1,2,3,4,5,6-hexachlorocyclohexane) and prometryn [2,4-bis(isopropylamino-6-methyl-thio-s-triazine] had a greater extent of association with peat-DHA, suggesting that hydrophobicity of nonionic pesticides is not the only factor governing their interaction with DOM. When comparing DOM from several sources, dissolved humic acid had a higher affinity than dissolved fulvic acid for napropamide, DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane], and lindane. Association of napropamide with peat-DHA increased with increasing pH above pH 6.4, decreasing ionic strength, and decreasing charge on cations added to the system. Dissociation studies showed that the interaction between napropamide and peat-DHA was not fully reversible. The results suggest that the source of the DOM, properties of the solution environment, and reversibility of the interaction should be taken into account when evaluating the interaction between DOM and nonionic pesticides. In addition, dissolved humic and fulvic acid released from two soils showed a different affinity for napropamide compared to bulk soil organic matter when the ability of the whole soil to retain napropamide was expressed on the fraction organic-carbon-content basis. Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside.