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 neu tral pH, no disappearance of the herbicide was detected either in the enric hment cultures or in the growth media of the axenic microbial cultures. Dur ing the growth of some of the microbial strains, however, the pH of the med ium dropped below 6, resulting in the hydrolysis of primisulfuron. The rate of primisulfuron hydrolysis was clearly pH dependent; primisulfuron was mo re persistent in neutral or weakly basic solutions than in acidic solutions . After hydrolysis of the herbicide, four products were observed. These wer e identified as methyl 2-(aminosulfonyl)benzoate, 2-amino-4,6-(difluorometh oxy)pyrimidine,2-N-[[[[[4,6-bis(difluoromethoxy)-2-pyrimidinyl]amino]carbon yl] amino]sulfonyl]benzoic acid, and 2-(aminosulfonyl)benzoic acid. After h ydrolysis, it was found that the fungus Phanerochaete chrysosporium mineral ized 27 and 24% of C-14-phenyl- and C-14-pyrimidine-labeled products, respe ctively, after 24 days of incubation. Similarly, Trametes versicolor minera lized 13 and 11% of C-14-phenyl- and C-14-pyrimidine-labeled hydrolysis pro ducts, 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-(difluoro methoxy)pyrimidin-2-ylurea]. When C-14-phenyl-labeled primisulfuron was exp osed 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 lab eled at the C-14-pyrimidine position. Mineralization of C-14-pyrimidine-lab eled products of photolyzed primisulfuron by P. chrysosporium was similar t o 25% after 24 days. These results clearly indicate that hydrolysis and pho tolysis of primisulfuron facilitated microbial degradation.