A QUANTITATIVE ESTIMATION OF THE COMPLEXATION OF SMALL ORGANIC-MOLECULES WITH SOLUBLE HUMIC ACIDS

Small organic molecules, such as the commonly used herbicides napropam ide [N,N-diethyl-2-(1-naphthyloxy) propionamide] and atrazine oro-4-et hylamino-6-isopropylamino-1,3,5-triazine), complex with soluble humic acids (HAs), producing a marked effect on apparent solubility, availab ility, and mobility. The overall objective of the present paper is to propose a procedure by which the extent of complexation of small organ ic molecules with stable humic acids can be quantitatively estimated. Experiments were carried out to investigate the effects of HA and herb icide concentrations, pH, and ionic composition and strength on the ex tent and nature of complexation using an equilibrium dialysis procedur e and C-14-labeled herbicides. The relative strength of complexes of n apropamide with a series of HAs from a wide range of sources, determin ed 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 grea ter for napropamide than for atrazine (K-c at pH 6.5, 0.83 and 0.12 mt mg(-1), respectively). As solubility of molecules In water increases, complex formation usually decreases. However, the complexation of nap ropamide and atrazine did not correlate inversely with their solubilit y. Environmental conditions, such as pH and ionic strength and composi tion, as well as the source of the HAs, had a strong effect on the com plexation of the investigated pesticides. The relationship between com plexation and the above factors suggests that it is their effect on co iling of the HA strands, which determines the extent of complexation.