On the interaction mechanisms of atrazine and hydroxyatrazine with humic substances

Atrazine (6-chloro-N-2-ethyl-N-4-isopropyl-1,3,5-triazine-2,4-diamine) is r etained against leaching losses in soils principally by sorption to organic matter, but the mechanism of sorption has been a matter of controversy. Co nflicting evidence exists for proton transfer, electron transfer, and hydro phobic interactions between atrazine and soil humus, but no data are conclu sive. In this paper we add to the database by investigating the role of (i) hydroxyatrazine (6-hydroxy-N-2-ethyl-N-4-isopropyl-1,3,5-triazine-2,4-diam ine) and (ii) hydrophobicity in the sorption of atrazine by Brazilian soil humic substances. We demonstrate, apparently for the first time, that hydro xyatrazine readily forms electron-transfer complexes with humic substances. These complexes probably are the cause of the well-known. strong adsorptio n by humic acids and they may be the undetected cause of apparent electron- transfer complexes between soil organic matter and atrazine, whose transfor mation to the hydroxy form is facile. We also present evidence that support s the important contribution of hydrophobic interactions to the pH-dependen t sorption of atrazine by humic substances.