Biotic and abiotic degradation of S-metolachlor and its commercial product Mercantor Gold® in aqueous media

S-metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl] acetamide) is a selective chloroacetanilide herbicide intensively used for annual grassy weeds, corn, soybeans, peanuts, and other crops. In recent years more effective and risk-reduced S enantiomer has replaced the existing rac-metolachlor in the worldwide market since 1976. Mercantor Gold® is a mixture of S-metolachlor (86%) with mixture of aromatic hydrocarbons (14%). Once the compound is released into the environment it undergoes both biotic and non-biotic processes. As large amounts of pesticide are introduced to the environment, it is of great importance to investigate the biodegradation and photodegradation in the natural environment.
The present study investigates the environmental pathways of S-Metolachlor and its commercial product Mercantor Gold® by Syngenta. Two tests of different bacterial density from OECD series were applied to test the biodegradation: the Closed Bottle Test (CBT, OECD 301D) and Manometric Respiratory Test (MRT, OECD 301 F). Photodegradation in aqueous media under simulated solar irradiation was performed using a UV/VIS xenon lamp (TXE 150, UV consulting Peschl, Mainz), which emits spectra similar to natural sun light (300-800nm). Samples after 8h photodegradation were administered the CBT and MRT biodegradation tests. The primary elimination of parent compound was monitored at 220 nm by high performance liquid chromatography (HPLC-UV). The formed transformation products (TP’s) were analyzed by liquid chromatography coupled with multiple stage ion trap mass-spectrometry (LC-Ms).
S-metolachlor was efficiently photo-degraded under laboratory conditions. Analyses by HPLC-UV showed a 69% elimination of pure compound and a 74% elimination of commercial product, both dissolved in water. Abundant photoproducts were detected by LC-Ms. No biodegradation has been observed for two compounds in the CBT and MRT tests as well as in the samples after photo-treatment. However, analyses through the MRT test revealed new TP’s in the samples after photodegradation.
This study obtained results which show that S-metolachlor as well as its commercial product Mercantor Gold® efficiently undergo abiotic primary elimination in aqueous media, however their photo-TP’s are not biodegradable in performed tests and therefore might be persistent in the environment or exhibit further transformations.