Atrazine Complexation by Soil Humic Acids

Atrazine [6‐chloro‐ N ‐ethyl‐ N′ ‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine] complexation pathways with soil humic acid remain a controversial issue, in part because of the absence of direct spectroscopic information. Recent optical and magnetic resonance spectral data indicate that proton‐transfer and, to a lesser extent, hydrogen bonding are as important as electron‐transfer in the mechanisms of reaction between atrazine and humic acid, whereas other s ‐triazines appear to react more strongly by electron transfer mechanisms. Two representative temperate‐zone humic acids were selected to probe the issue of complexation mechanisms more decisively. They were reacted at pH < 7 with 140 mmol m −3 atrazine solution under exclusion of light. Infrared and electron spin resonance spectra of the resulting products gave evidence for proton‐transfer and, under certain conditions, electron‐transfer reactions. The spectroscopic data, and those from other published studies, were analyzed in terms of the molecular properties of humic acids and s ‐triazines. The analysis suggested a general guiding principle to characterize the complexation mechanisms of s ‐triazines with soil humic acids. Proton‐transfer is favored (and electron‐transfer is disfavored) for humic acids of high acidic functional group content and for s ‐triazines of low basicity. Electron‐transfer mechanisms are favored for humic acids of low acidic functional group content and for s ‐triazines of high basicity. This principle illustrates the importance of specific molecular structure, as opposed to class structure, to understanding reactivity between the s ‐triazines and humic acids.