Degradation of primisulfuron by a combination of chemical and microbiological processes

Microbial degradation of the herbicide primisulfuron was investigated using enrichment cultures from contaminated soils and 20 axenic cultures. At neutral pH, no disappearance of the herbicide was detected either in the enrichment cultures or in the growth media of the axenic microbial cultures. During the growth of some of the microbial strains, however, the pH of the medium dropped below 6, resulting in the hydrolysis of primisulfuron. The rate of primisulfuron hydrolysis was clearly pH dependent; primisulfuron was more persistent in neutral or weakly basic solutions than in acidic solutions. After hydrolysis of the herbicide, four products were observed. These were identified as methyl 2-(aminosulfonyl)benzoate, 2-amino-4,6-(difluoromethoxy)pyrimidine,2-N-[[[[[4,6-bis(difluoromethoxy)-2-pyrimidinyl]amino]carbonyl] amino]sulfonyl]benzoic acid, and 2-(aminosulfonyl)benzoic acid. After hydrolysis, it was found that the fungus Phanerochaete chrysosporium mineralized 27 and 24% of C-14-phenyl- and C-14-pyrimidine-labeled products, respectively, after 24 days of incubation. Similarly, Trametes versicolor mineralized 13 and 11% of C-14-phenyl- and C-14-pyrimidine-labeled hydrolysis products, respectively. In addition, primisulfuron in a hydrolytically stable solution, at pH 7.0, was rapidly decomposed after ultraviolet irradiation, and two photolysis products were isolated [methylbenzoate and 4,6-(difluoromethoxy)pyrimidin-2-ylurea]. When C-14-phenyl-labeled primisulfuron was exposed to photolysis for 24 h, 32% of the initial radioactivity was recovered as (CO2)-C-14, whereas no (CO2)-C-14 was detected if the herbicide was labeled at the C-14-pyrimidine position. Mineralization of C-14-pyrimidine-labeled products of photolyzed primisulfuron by P. chrysosporium was similar to 25% after 24 days. These results clearly indicate that hydrolysis and photolysis of primisulfuron facilitated microbial degradation.