Comparison of Effects of Bromination and Iodination on the Spectroscopic Properties of Natural Organic Matter

Toxicological studies of disinfection by-products (DBPs) suggest that brominated and iodinated DBPs may be more carcinogenic than their chlorinated analogues. This makes it important to examine the pathways of formation of brominated and iodinated DBPs on a microscopic level. Examination the engagement of natural organic matter (NOM) that interacts with dissimilar halogen species can be done by the in situ methods of differential absorbance and fluorescence. spectroscopy. These measurements were carried out using three different NOM sources. In selected experiments, gas chromatography was used to determine concentrations of trihalomethanes and haloacetic acids formed in the studied systems at varying bromide concentration and reaction times. The data showed that chlorine and bromine species engage similar reactive sits in NOM but bromine incorporation takes place much faster than that of chlorine. The incorporation of bromine in NOM was observed to be associated with largely similar changes of the absorbance spectra of NOM but changes of NOM fluorescence were more specific and exhibited an increasingly prominent decrease of emission intensity. This was hypothesized to be caused by the heavy atom effect on NOM fluorophores. Several phenomena specific to NOM iodination were observed as well. NOM reactions with iodine species formed via the oxidation of iodide by chloramine were accompanied by the emergence of a prominent peak at around 225 nm; this peak reflected the consumption of iodine species. Changes of differential absorbance of NOM in wavelengths above 300 nm also indicated the presence of weak but indicative bands that were absent in the cases of chlorination or bromination. Fluorescence measurements indicated that, similarly to the observations made for NOM bromination, the incorporation of iodine caused the intensity of NOM fluorescence to decrease, most likely due to the heavy atom quenching effect. The decrease of NOM fluorescence immediately after the beginning of NOM halogenation in the presence of bromide or iodide appears to be correlated with increases of the fraction of bromine or iodine incorporated into the reactive sits of NOM. This observation was confirmed by the attendant changes of DBP speciation and concentration. The observed effects can have high importance for further exploration of NOM halogenation pathways and kinetics but more research is necessary to examine these effects in more detail.