Extraction pH Controls Assessed Biotoxicity of Chlorination Disinfection Byproducts from Amphoteric Precursors

Effect-based toxicity assessments are crucial for evaluating the risks of disinfection byproducts (DBPs), particularly unknown species, generated during drinking water chlorination. However, the accuracy of this approach is highly dependent on unbiased sample extraction. Conventional methods often employ single, low-pH extraction, which may fail to recover pH-sensitive amphoteric DBPs derived from amphoteric precursors (e.g., nitrogenous compounds). This study investigated how extraction pH affects the measured biotoxicity of DBPs formed from three model precursors: biopterin (Bip), cytosine (Cyt), and tryptophan (Trp). Under excess chlorine conditions, all three precursors degraded rapidly. The formation of aliphatic DBPs followed the order Trp > Cyt > Bip, and the maximum toxicity of the non-volatile extracts, assessed via a Vibrio fischeri bioassay, followed the reverse order: Bip > Trp > Cyt. This toxicity profile was significantly influenced by extraction pH, with maximum toxicity observed for Bip at around pH 4.0, under weakly acidic conditions for Trp, and under neutral to alkaline conditions for Cyt. For all precursors, the total organic carbon concentration remained constant throughout chlorination, indicating negligible mineralization and the predominant formation of non-aliphatic, likely heteroaromatic, products. These findings demonstrate that conventional extractions at a single low pH can lead to the incomplete recovery of toxic DBPs from amphoteric precursors. Therefore, pH-optimized extraction protocols are necessary for a more accurate risk assessment of chlorinated drinking water.