Clapp, C.E., USDA-ARS, Dep. of Soil, Univ. of Minnesota, St. Paul, MN 55108, United States
Mingelgrin, U., ARO, Volcani Center, P.O. Box 6, Bet Dagan, Israel
Liu, R., Dep. of Soil, Univ. of Minnesota, St. Paul, MN 55108, United States
Zhang, H., Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK, United States
Hayes, M.H.B., School of Chemistry, Univ. of Birmingham, Edgbaston, United Kingdom

Small organic molecules, such as the commonly used herbicides napropamide [N,N-diethyl-2-(1-naphthyloxy) propionamide] and atrazine (2- chloro-4-ethylamine-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(e) at pH 504, 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 effects on coiling of the HA strands, which determines the extent of complexation.