Abiotic reduction of dinitroaniline herbicides

The importance of abiotic reductive transformations as a sink for four dinitroaniline herbicides (trifluralin, pendimethalin, nitralin, and isopropalin) has been evaluated. Using reductants representative of abiotic reductants found in natural systems, the results of this study indicate that nitro groups present on the dinitroaniline herbicides can be reduced by surface-bound Fe(II) species in goethite suspensions or by hydroquinone moieties such as (mercapto)juglone in a hydrogen sulfide solution. Aqueous iron species are also effective at pH values above 7.0. The reaction in aqueous Fe(II) and in Fe(II)/goethite systems is strongly pH dependent, with rates increasing with increasing pH. Montmorillonite clay, however, is not effective in mediating the reduction of dinitroaniline herbicides in the presence of Fe(II). Because the selected dinitroaniline herbicides have a mixture of electron withdrawing and electron donating groups, linear free energy relationships were developed for the H₂S/(mercapto)juglone and Fe(II)/goethite systems. Anilines resulting from reduction of the nitro group as well as cyclization products (benzimidazoles) were observed in the degradation of trifluralin. Only one aniline product was observed for pendimethalin.