Understanding the formation and fate of halonitrophenols during the water disinfection

Disinfection is crucial in water treatment. The use of disinfectants has significantly decreased waterborne diseases. However, recent research reveals that this process can produce toxic disinfection byproducts (DBPs) from reactions between disinfectants and dissolved organic matter in source water. Many DBPs in drinking water are regulated by countries and organizations; but the known DBPs still cannot fully account for the adverse health effects caused by consuming chlorinated drinking water.

Given the huge number of unknown byproducts, research of DBPs should be focused on investigating the emerging unknown class of DBPs – aromatic DBPs, due to their higher toxicity and potential as major toxicity drivers. Moreover, aromatic DBPs contain phenyl or heterocyclic structures, which increase their ability to permeate the skin – hence possibly absorbed during daily activities such as swimming and bathing. Aromatic DBPs also have high thermal stability, which means that the concentration of some halogenated aromatic DBPs may increase after boiling. Otherwise, boiling is thought to be an effective way to reduce the total organic halogens in chlor(am)inated water. Halonitrophenols (HNPs) are an important group of emerging aromatic DBPs that warrants urgent attention. HNPs exhibit considerable toxicity and potential endocrine disrupting properties. They have already been discovered in several tap waters in China, as well as in estuarine and swimming pool water around the world. However, studies of their formation are still very limited. To evaluate the impact of these emerging byproducts on drinking water safety in Hong Kong, we need to know: (a) their occurrence in our local drinking water; (b) the relevance of phenolic groups present in micropollutants and natural organic matters as precursors of HNPs; (c) the formation mechanism; and (d) their transformation and toxicity changes during disinfection.

To answer these questions, the present work will firstly, conduct a comprehensive occurrence study of finished drinking water from water treatment works and complimentary tap water in different regions of Hong Kong. The formation mechanism of HNPs, using 2,6-dichloro- nitrophenol as the model compound, under chloramination will be studied with selected precursors. Different parameters used in chloramination will be evaluated for their impact on HNPs formation. The transformation of 2,6-dichloro-nitrophenol will then be studied to determine its fate and impact in a simulated drinking water system, with structural elucidation and toxicity evaluation of the transformation products.

The chemical data produced in this study will provide details of the fundamental mechanisms of HNPs formation through representative phenolic compounds, whose structures are commonly present in micropollutants and natural organic matter as precursors. The success of this work will accomplish two important goals. First, it will, establish a database to track HNPs occurrence in Hong Kong water. Second, it will provide fundamental understanding of HNPs formation in drinking water, their further transformation in drinking water system and toxicity changes. This will be of global value, as we all seek to protect our most vital resource, water.