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A Quantitative Estimation of the Complexation of Small Organic Molecules with Soluble Humic Acids

USDA-ARS and Dep. of Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN 55108;

U. Mingelgrin

ARO, Volcani Center, P.O. Box 6, Bet Dagan, Israel;

R. Liu

Dep. of Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN 55108;

H. Zhang

Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK;

M. H. B. Hayes

The School of Chemistry, Univ. of Birmingham, Edgbaston, England.

^* Corresponding author (eclapp{at}soils.umn.edu).

ABSTRACT

Small organic molecules, such as the commonly used herbicides napropamide [N,N-diethyl-2-(1-naphthyloxy) propionamide] and atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine), complex with soluble humic acids (HAs), producing a marked effect on apparent solubility, availability, and mobility. The overall objective of the present paper is to propose a procedure by which the extent of complexation of small organic molecules with stable humic acids can be quantitatively estimated. Experiments were carried out to investigate the effects of HA and herbicide concentrations, pH, and ionic composition and strength on the extent and nature of complexation using an equilibrium dialysis procedure and ¹⁴C-labeled herbicides. The relative strength of complexes of napropamide with a series of HAs from a wide range of sources, determined at a concentration of 2.0 mg HA mL^–1, was in the order of soil > peat > water. Complexation of the herbicides with the soil HA was greater for napropamide than for atrazine (K_c at pH 6.5, 0.83 and 0.12 mL mg^–1, respectively). As solubility of molecules in water increases, complex formation usually decreases. However, the complexation of napropamide and atrazine did not correlate inversely with their solubility. Environmental conditions, such as pH and ionic strength and composition, as well as the source of the HAs, had a strong effect on the complexation of the investigated pesticides. The relationship between complexation and the above factors suggests that it is their effect on coiling of the HA strands, which determines the extent of complexation.

Paper no. 22,432 of the Journal Series, Minnesota Agric. Exp. Stn.

Received for publication July 11, 1996.

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