Spectroscopic examination of effects of iodide on the chloramination of natural organic matter

•Iodide consumption can be quantified by the changes of absorbance of NOM at 225 nm.•Iodide consumption demonstrates the presence of two kinetics reactive site in NOM.•The decrease of NOM emission intensity is linear with the increase of S325–350 values.•Iodide consumption is well correlated with the decrease of NOM emission intensity.•There are two distinct types of iodine incorporation into NOM in chloramination.

This study examined reactions of NOM with chloramine in the presence of varying iodide concentrations using in situ methods of absorbance and fluorescence. The data showed that the consumption of iodide during chloramination could be quantified by the differential absorbance of NOM at 225 nm. Examination of the kinetics of iodide consumption demonstrated the presence of two different reaction stages corresponding to the engagement of fast and slow reactive site in NOM. Interactions between the fast site and chloramine in the presence of iodide did not show specific effects of the incorporation of iodine compared to that of chlorine but the engagement of the slow site caused changes of NOM fluorophores and chromophores specific to iodine. Comparison of changes of the slope of log-processed NOM absorbance spectra (S325–350) and decreases of NOM fluorescence intensity showed the presence of heavy atom effects whose extent was proportional to the incorporation of iodine in the organic substrate. The data also indicated that the engagement of the slow site was accompanied by the formation of intermediate species whose nature remains to be elucidated.