Trifluralin Degradation under Microbiologically Induced Nitrate and Fe(III) Reducing Conditions

Trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] ranks among the most commonly used herbicides in the United States. The compound persists under most environmental conditions, yet it is rapidly transformed under certain anaerobic conditions. In this study, the fate of trifluralin in anoxic environments and the contribution of Fe(II) to its anaerobic degradation have been investigated. Trifluralin was rapidly degraded under anaerobic conditions in a range of soils representing typical agricultural usage in the Midwest. The presence of nitrate or oxygen suppressed trifluralin degradation. Degradation rate increased under iron-reducing conditions, and the addition of trifluralin appeared to promote reoxidation of extractable Fe²⁺. Transformation of trifluralin under iron-reducing conditions apparently involved the soil solid phase and was not limited by bioavailability. In a soil-free aqueous system, no reaction of trifluralin with dissolved Fe²⁺ was detected in the presence or absence of kaolinite clay under anoxic conditions. Reduced but not oxidized or reoxidized forms of purified ferruginous smectite (sample Swa-1) catalyzed rapid transformation (72% of applied in 30 h) of trifluralin to polar products with a concomitant reoxidation of structural Fe in the clay. Results indicate that, as for other nitroaromatics, trifluralin is subject to reaction with Fe(II) associated with the minerals in anoxic environments.