Photodegradation of the fungicide thiram in aqueous solutions. Kinetic studies and identification of the photodegradation products by HPLC-MS/MS.

In this study, the relevance of photodegradation processes on the persistence of the fungicide thiram in waters was investigated. The photodegradation of thiram in Milli-Q water and in aqueous solutions of humic and fulvic acids, as well as the photodegradation in spiked river water were studied. Both pure thiram and one of its commercial formulations were used to prepare the solutions which were irradiated in a solar light simulator. In general, thiram photodegradation follows pseudo-first order kinetics. The half-life time of thiram 2mgL(-1) in Milli-Q water was 28min. However, the degradation rate of thiram was significantly increased (p=0.02) by the inert components of the thiram commercial formulation as well as by commercial humic acids and by fulvic acids isolated from river water (p<0.004). Thus, the half-life time of thiram decreased to 24min in the presence of the inert formulation components, while, in the presence of both humic and fulvic acids (10mgL(-1)) it decreased to 22min. Furthermore, thiram photodegradation in natural river water showed that there is a significant enhancement of the degradation rate constant of thiram relatively to Milli-Q water, corresponding to a decrease of about 38% in its half-life time. This increase of the degradation rate in river water seems to be higher than that observed in the presence of FA, suggesting that beyond organic matter, other natural river components can increase the thiram photodegradation rate. These results allow us to conclude that photodegradation by solar radiation can be an important degradation pathway of thiram in natural waters. HPLC-MS/MS allowed to identify, for the first time, three products of the photodegradation of thiram in aqueous solution. Three compounds were identified and their structure was corroborated by the MS(n) spectra fragmentation profile. Pathways for the formation of the products from thiram photodegradation are proposed and discussed.