Abstract
In this article, we demonstrate the major advantages of performing online kinetic studies of the formation of disinfection byproducts (DBPs) during chlorination using membrane inlet mass spectrometry (MIMS) supplemented with simultaneous UV-vis absorbance. MIMS is a well-known technique for online monitoring of small volatile organic compounds, but it cannot measure polar and/or nonvolatile organic compounds. This is problematic, since chemical DBP precursors, such as pesticides and pharmaceuticals, are generally not detectable by MIMS and must be analyzed offline using typical GC/MS or LC/MS. Supplementation of MIMS with simultaneous UV-vis absorbance makes it possible to monitor many of these non-MIMS detectable precursors. In addition, in many cases, UV-vis absorbance can be used to monitor the active chlorination agent, hypochlorite, during reactions. This is important for understanding mass balances in the chlorination process, since significant amounts of oxidant are lost to oxidation of reactive constituents in natural waters other than DBP precursors. Additionally, the UV absorbance spectra recorded simultaneously with MIMS monitoring can verify the identification of short-lived transient intermediates. This article will demonstrate the benefits of supplementing MIMS monitoring of DBP formation with simultaneous UV/vis absorbance for kinetic chlorination studies of the common water contaminants acetaminophen (a pain reliever), caffeine (a lifestyle compound), and difenoconazole (an herbicide). Concretely, UV/vis demonstrated that acetaminophen and its aromatic degradation product 1,4-benzoquinone were completely transformed into nonaromatic DBPs within 30 min, caffeine was only reduced by 30% after 24 h, and difenoconazole was unaffected after 24 h. In all three cases, the oxidant hypochlorite could be monitored during the reactions, and it was possible to separate the used hypochlorite into that used to oxidize the precursor itself and that used to oxidize oxidizable natural constituents present in the tap water. Finally, UV/vis supplemented the MIMS identification of the intermediate degradation product, 1,4-benzoquinone, from acetaminophen, and halophenols from phenol.