A universal model to predict yields of THMs and HAAs based on UV-Visible absorption spectra

Differential absorption spectroscopy (DAS) quantifies changes in the UV-Visible absorbance of dissolved organic matter (DOM) caused by reactions of its chromophores. As a result of its precision and sensitvity, DAS serves as a powerful tool for characterizing the formation of disinfection by-products (DBPs) in generated in DOM chlorination reactions. However, the nonlinear relationship between the intensity of DAS and DBP concentrations as well as the need to develop site-specific fitting parameters limit its practical applications. This study investigated the physico-chemical nature of DAS of chlorinated DOM through experimental measurements and theoretical calculations. Results of this study provide molecular-level evidence that electrophilic substitution reactions involving DOM reactive sites result in the emergence of DAS feaures ascribed to the "fast" chromophores. The ring opening in the cyclic enones-like structures which can be present either in the original DOM or are generated as intermediates in its chlorination, leads to the emergence of DAS features associated with the "slow" chromophores and high yields of DBPs. The kinetic study of chlorination of real waters reveals a strong linear relationship (R² > 0.91) between ln([DBP]) and the long-wavelength (λ > 325 nm) parameter of the DAS, notably (ln(-DA₃₅₀)). This relationship varies among different water sources due to the differences in the heterogeneity of Band A₃ whose maximum is near 350 nm. Band A₃ is one of the Gaussian bands that comprise the overall UV-Visible spectrum of DOM. A new function (f(-DA₃₅₀)) is proposed in this study to quantify DBP formation. This function, which is determined by the Band A₃'s area, allows establishing a universal linear relationship between f(-DA₃₅₀) and ln([THMs]), as well as f(-DA₃₅₀) and ln([HAAs]), across various water sources. The findings of this study will stimulate further development of spectroscopy-based DBP monitoring technology for monitoring and optimization of water disinfection processes.