Environmental fate of S-metolachlor in its pure form and as a part of commercial product - Mercantor Gold®: biodegradation and sorption onto sediment

S-metolachlor, an herbicide, is an active compound in many commercially available products. One such product is Mercantor Gold® which is a combination of S-metolachlor with a mixture of organic solvents. The aim of the following study was to investigate whether this mixture has an influence on environmental fate of S-metolachlor namely in its biodegradation and elimination by sorption onto sediment.
Biodegradation and sorption studies were performed using the screening biodegradation Water sediment test (WST). WST is mostly based on OECD Guideline No. 308. However the test was further modified by creating an artificial matrix to which all components were standardised based on their corresponding existing OECD Guidelines. Concentration of compound and mixture was 40 mg/L of the theoretical oxygen demand (ThOD). The S-metolachlor concentration was 19 mg/L (pure) and 17 mg/L (Mercantor Gold®). Additionally analytical assessment of S metolachlor removal from water phase by high performance liquid chromatography coupled with UV-detector was performed.
Biodegradation level of S-metolachlor was 8%±19% (n=6) and Mercantor Gold® was 27%±12% (n=3). Neither Mercantor Gold® nor S-metolachlor was toxic to the test bacteria (biodegradation in toxicity controls in both cases was above 30%). At day 28 of the WST, the elimination of S-metolachlor from the water phase was 60% in test series as a pure substance with a corresponding 62% in the abiotic control and 53% in test series as a component of Mercantor Gold® with a corresponding 45% in the abiotic control. S-metolachlor pure and in Mercantor Gold® showed similar sorption rates.
In conclusion this study showed that additives in the Mercantor Gold® have not influenced the biodegradation of the compound S-metolachlor. Higher biodegradation level of Mercantor Gold® is attributed to biodegradation of some constituents of the organic solvent mixture. S-metolachlor removal from water phase in the WST also proved that the organic solvent mixture had no influence on sorption. Therefore, WST provides a promising tool for the assessment of the fate of pesticides in water sediment systems.