Fate of atrazine and alachlor in redox-treated ferruginous smectite

The oxidation state of structural iron (Fe) in clay minerals exerts a large influence on clay surface chemistry and may affect the adsorption and degr adation of pesticides in the environment. This effect, however. has been li ttle investigated. In the present study, herbicides atrazine and alachlor w ere reacted with ferruginous smectite (sample SWa-1) in its oxidized, reduc ed (either chemically or bacterially), and reduced-reoxidized states. In so me experiments the herbicide was labeled with C-14. Gas chromatography/mass spectrometry (GC-MS) was also used to detect alachlor degradation products . Compared to oxidized clays, reduction by both chemical and microbial trea tments decreased the concentration of both herbicides in the surrounding so lution. Reoxidized clay exhibited behavior similar to the oxidized clay. Hy drolysis-dechlorination of atrazine occurred in the presence of chemically reduced SWa-1, and GC-MS analysis of alachlor revealed at least 14 degradat ion products after treatment with reduced clay and only two with the oxidiz ed clay. Interaction of atrazine and alachlor with the clay may be through a H bond with the waters of hydration surrounding interlayer cations, the e xtent of which should increase with increasing acidity; but under reduced c onditions, the validity of this model is unclear. Reduction of structural F e may affect pH-dependent phenomena in two ways: The increased surface char ge density increases the number of hydrated interlayer cations, thereby enh ancing surface acidity, and increased electron density at basal surface oxy gens increases their Bronsted basicity. Atrazine could, therefore, adsorb a nd/or degrade through either acid or alkaline hydrolysis pathways. Increase d reduction potential of the reduced clay surfaces may also promote degrada tion.