Granulated active carbon (GAC) is commonly used as a chemical barrier for the removal of organic micropollutants (OMPs) in drinking water treatment plants (DWTPs). However, little is known about the impact of dissolved organic carbon (DOC) and its long-term performance with regard to OMP removal efficiency. This study examined the performance of two GAC types (Norit 830W and Filtrasorb 400) in the removal of OMPs and DOC from natural lake water, in pilot-scale and full-scale tests run for almost one year. Potential early warning indicators of the exhaustion of GAC sorption capacity were also evaluated. The seven OMPs investigated (carbamazepine, lamotrigine, cetirizine, fexofenadine, oxazepam, fluconazole and N,N-diethyl-meta-toluamide (DEET)) all showed decreasing removal efficiencies after ~20,000 bed volumes (BV) in the pilot-scale Norit 830W and Filtrasorb 400 columns. However, columns with an 18-min empty bed contact time (EBCT) showed better performances than columns with 6-min EBCT. DEET was the OMP adsorbed most weakly. We found that DOC concentrations, methylene blue sorption kinetics, UV and fluorescence did not sufficiently explain the OMP breakthrough in the GAC columns. We concluded that carbamazepine, lamotrigine and fexofenadine can be used as indicators of decreasing GAC adsorption performance, due to their later breakthrough. Based on the results, UV and fluorescence removal could be used for the early detection of declining DOC removal, and online solid-phase extraction (SPE)–liquid chromatography–tandem mass spectrometry (SPE-LC-MS/MS) could be used for the early detection of OMPs in drinking water.

Wavelength-dependent Φ_λ of uridine, atrazine, PNA, sulfamethoxazole, and diclofenac were determined, which can be used to characterize and model the photodegradation of chemicals of interest.

Predicting the environmental fate of anthropogenic chemicals remains a top priority for scientists and regulators; however, these efforts are hindered by the complexity of environmental systems. For example, in aquatic photodegradation, multiple photochemically produced reactive intermediates (PPRI) are present simultaneously, such as hydroxyl radicals (˙OH), singlet oxygen (¹O₂), and triplet excited states of chromophoric dissolved organic matter (³CDOM*). This makes it difficult to isolate contributions of individual PPRI to overall photodegradation as well as to measure bimolecular reaction rate constants with target contaminants (k_PPRI), which could subsequently be used to predict degradation rates under variable environmental conditions and in engineered water treatment systems. As an alternative approach, simplified model systems can be used to isolate reactions with each PPRI. Yet, a systematic comparison of the results obtained in different model systems has not been conducted. In this study, at least two model systems were used to quantify k_PPRI between each PPRI (i.e., ˙OH, ¹O₂, and three ³CDOM* proxies) and each of the 28 pesticides evaluated. Results were consistent for most pesticides across the set of model systems used to evaluate a given PPRI. However, significant discrepancies were observed in some cases. For some pesticides, reactions with ˙OH appeared faster than the diffusion-controlled limit, suggesting additional reactions with unidentified PPRI were occurring. In ¹O₂ model systems, unexpected reactions occurred between some pesticides and the triplet excited states of the model sensitizer. Lastly, there was not a consistent trend between the calculated k_PPRI and the photochemical properties of the three ³CDOM* proxies evaluated, as suggested in previous studies. Overall, the results from this study showed that model systems are a powerful tool for investigating indirect photodegradation reactions and should be adopted in formal evaluations of the environmental fate of anthropogenic chemicals. Key considerations and recommendations to ensure accurate and reliable use of model systems are provided and areas benefiting from further investigation are identified.

Predicting pesticide photodegradation rates in natural waters based on laboratory data and identifying the role of individual degradation pathways.