Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) is retained against leaching losses in soils principally by sorption to organic matter, but the mechanism of sorption has been a matter of controversy. Conflicting evidence exists for proton transfer, electron transfer, and hydrophobic interactions between atrazine and soil humus, but no data are conclusive. In this paper we add to the database by investigating the role of (i) hydroxyatrazine (6-hydroxy-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and (ii) hydrophobicity in the sorption of atrazine by Brazilian soil humic substances. We demonstrate, apparently for the first time, that hydroxyatrazine readily forms electron-transfer complexes with humic substances. These complexes probably are the cause of the well-known strong adsorption by humic acids and they may be the undetected cause of apparent electron-transfer complexes between soil organic matter and atrazine, whose transformation to the hydroxy form is facile. We also present evidence that supports the important contribution of hydrophobic interactions to the pH-dependent sorption of atrazine by humic substances.

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10 2.1 .2 Quantidade de herbicidas utilizado e o impacto no Meio Ambiente 2.1.3 Matéria Orgânica do Solo (MOS) 12 19 2.1.4 Substâncias Húmicas (S.H. 's) e sua importância na interação com Pesticidas 21 2.1.5 Formas de adsorção de herbicidas por Substâncias Húmicas

CAPiTULO I INTRODU~O CAPiTULO II MECANISMOS DE DEGRADA~O E ADSOR~O . DE HERBICIDAS NO SC)LO 04 2.1 Mecanismos de adsoroao ao solo 2.2 Degradaoao quimica e fotodegradaoao 07 CAPiTULO III TEORIA DE RESSC)NAN"CIAPARAMAGNETICA . ELETRONICA 3.1 0 efeito Zeeman eletronico e 0 termo Hamiltoniano correspondente REFE~NCIAS BIBLIOCRAFICAS