Inverse 13C and 15N Isotope Fractionation During Photodegradation of Bromoxynil in Aqueous Solution

Nadav Knossow, Environmental Hydrology & Microbiology, Ben-gurion University, Midreshet Ben-Gurion, Israel

Bromoxynil is a brominated nitrile herbicide used mainly in field corn, wheat and barley. It has been classified as a possible human carcinogen and was also shown to be an endocrine disrupter impairing reproduction in birds, fish and aquatic invertebrates. In the environment, Bromoxynil is subjected to two distinct modes of degradation: abiotic degradation - mainly photodegradation, which takes place at the soil/plant surface, and microbial degradation (both aerobic and anaerobic) which takes place primarily in the subsurface. The different modes of degradation may potentially be differentiated using stable isotope tools; raising the motivation to study the isotope fractionation along these processes.

Bromoxynil in an aqueous solution was photodegraded in the laboratory, under direct UV light (254 nm), and analyzed for changes in its carbon and nitrogen stable isotope ratio.  Thin Layer Chromatography (TLC) was used as a first purification step followed by Elemental Analysis - Isotope Ratio Mass Spectrometry (EA-IRMS). Carbon and nitrogen bulk enrichment factors (ε_bulk) were calculated to be 5.23±0.55‰ and 0.57±0.13‰ respectively, denoting an inverse isotope fractionation in both elements.

It may be hypothesized that microbial degradation processes would be accompanied by normal isotope effect, thus enabling to distinguish between the biotic and abiotic modes of degradation. Complementary biotic degradation experiments are currently under process.