Iron dependent degradation of an isothiazolone biocide (5-chloro-2-methyl-4-isothiazolin-3-one)

An isothiazolone biocide, 5-chloro-2-methyl-4-isothiazolin-3-one (CMI), was degraded in the presence of iron. According to the Fe-dependent degradation of CMI, stoichiometric production of chloride was observed. Copper and stainless steel did not enhance the physico-chemical degradation of CMI, whilst phosphate inhibited the Fe-dependent degradation. Neither aerobic nor anaerobic conditions influenced the Fe-dependent CMI degradation. Furthermore, FeO(OH)-powder and Fe3O4-powder did not lead to the physico-chemical degradation of CMI. Rapid disappearance of CMI was observed in an operating cooling water plant. CMI added to the cooling tower declined from 1.4 mg l(-1) to < 0.1 mg l(-1) in 2 d. This finding is important in optimising the use of CMI and combating resistance if encountered.