Environmental photodegradation of emerging contaminants: A re-examination of the importance of triplet-sensitised processes, based on the use of 4-carboxybenzophenone as proxy for the chromophoric dissolved organic matter

The photoreactions sensitised by the excited triplet states of chromophoric dissolved organic matter (3CDOM*) are very important in the photochemical attenuation of emerging contaminants in natural waters. Until quite recently, anthraquinone-2-sulphonate (AQ2S) was the only available CDOM proxy molecule to estimate the contaminant reaction kinetics with 3CDOM*, under steady-state irradiation conditions. Unfortunately, the AQ2S triplet state (3AQ2S*) is considerably more reactive than average 3CDOM*. We have recently developed an alternative protocol based on 4-carboxybenzophenone (CBBP), the triplet state of which (3CBBP*) is less reactive compared to 3AQ2S*. Here we show that in the case of ibuprofen (IBP), paracetamol (APAP) and clofibric acid (CLO), the reaction rate constants with 3CBBP* are more reasonable as 3CDOM* reactivity estimates than those obtained by using AQ2S. In contrast, similar rate constants are measured for the reaction of atrazine (ATZ) with either 3AQ2S* or 3CBBP*. Moreover, the reactivity of ATZ with both 3AQ2S* and 3CBBP* is very similar to that with 3CDOM*, available through a literature estimate. The possibility to validate the ATZ-3CBBP* reactivity estimate against the 3CDOM* data, and to accurately predict the reported IBP and CLO field lifetime, support the suitability of CBBP as CDOM proxy. The replacement of AQ2S with CBBP as proxy molecule does not reverse the qualitative prediction, according to which 3CDOM* would be the main process involved in the photodegradation of the studied contaminants in waters with high dissolved organic carbon (DOC). However, the CBBP-based data prompt for an important reconsideration of the estimated lifetimes at high DOC.