Quantifying the Mineralization of ¹³C-Labeled Cations and Anions Reveals Differences in Microbial Biodegradation of Herbicidal Ionic Liquids between Water and Soil

TLDR

Characterization of herbicidal ionic liquids (HILs) biodegradability using activated sludge alone is insufficient because cations and anions may degrade differently in aquatic versus terrestrial systems. The study aimed to test the hypothesis that HIL cations and anions exhibit distinct biodegradation potentials in different environmental compartments. Laboratory microcosm experiments were performed with C‑13‑labeled cations and anions of benzyldodecyldimethylammonium 4‑chloro‑2‑methylphenoxyacetate in soil with herbicide exposure history and in mineral water supplemented with activated sludge microorganisms. The cation mineralized in water while the anion was fully utilized in soil, demonstrating that biodegradation depends on the test system and that compartment‑specific C‑13‑labeled studies are required; aqueous‑system kinetic parameters should not be used to predict terrestrial degradation rates.

Abstract

Characterization of the biodegradability of herbicidal ionic liquids (HILs) using the industry standard activated sludge methodology is thought to be insufficient to fully understand the biodegradation of HILs in the environment because cations and anions of the HILs may have different potential for biodegradation in aquatic and terrestrial systems. To test this hypothesis, we conducted laboratory microcosm experiments using C-13-labeled cations and anions forming benzyldodecyldimethylammonium 4-chloro-2-methylphenoxyacetate ([C-12-BA][MCPA]) and evaluated their biodegradation potential in soil with a known history of herbicide exposure, and in water (mineral medium) augmented with microorganisms present in activated sludge. The biodegradability of the cation and anion was found to depend on the test system (water or soil). The cation was mineralized in water, whereas the anion was fully utilized in soil. These results suggest that performing biodegradation tests using differently C-13-labeled species of the target HIL and mimicking various environmental compartments (e.g., soil, activated sludge) is needed to provide a better understanding of the fate of HILs in the environment. They also indicate that biodegradation kinetic parameters of HILs derived from experiments performed in aqueous systems should not be used to estimate biodegradation rates in terrestrial environments.