Understanding the behaviour of UV absorbance of natural waters upon chlorination using model compounds

Abstract

Differential UV absorbance (ΔA) is a promising indicator that could allow operators and utility managers to routinely monitor and manage disinfection by-product (DBP) concentrations without the limitations of regulatory sampling and analyses. So far, empirical relationships between DBP formation and ΔA at 272 nm have been proposed, but these relationships are known to be specific to the waters being analyzed. The objective of this study is qualitative rather than quantitative; it is to identify features of the spectra of simple compounds that are distinguishable in the spectra of natural water and relevant to predict DBP formation empirically. In order to meet this objective, compounds that could model the different components of natural organic matter (NOM) were chlorinated, and their UV spectra were collected at various reaction times, along with samples for DBP analyses. The same procedure was conducted on natural waters that were pretreated using coagulation and filtration. Results show that both diketone and phenolic moieties could be responsible for the initial decrease in absorbance observed around 270 nm, while the continuous gradual decrease that follows is attributable to phenolic structures. This gradual decrease is most notable around 250 nm, which means that ΔA at this wavelength might be more closely related to DBP formation than the usual ΔA at 272 nm. Other compounds such as carboxylic acids and amino acids do not seem to contribute significantly to trihalomethanes, haloacetic acids or ΔA in natural waters, although amino acids probably make up a significant proportion of the haloacetonitrile precursors.