Ternary Model of the Speciation of I-/Br-/Cl-Trihalomethanes Formed in Chloraminated Surface Waters

This study examined the effects of the iodide concentration and pH on yields and speciation of the entire group of 10 species of iodine-, bromine-, and chlorine-containing trihalomethanes (THMs) formed at pH values from 6.5 to 8.5 in chloraminated surface waters in the presence of bromide and iodide. Pathways of iodine, bromine, and chlorine incorporation in the active sites in dissolved organic matter (DOM) were examined on the basis of a ternary halogenation/THM speciation model. The model assumed the occurrence of sequential three-step halogenation of the active site and competition of iodine, bromine, and chlorine species at each node of the halogenation sequence. A comparison of experimentally measured and modeled speciation coefficients and also iodine and bromine incorporation factors calculated for 10 THM species showed that the developed approach was sufficient to closely model the observed trends. Interpretation of preferred iodine incorporation pathways associated with the generation of THMs in all examined conditions showed that the susceptibility of the halogenated intermediates increases rapidly with the number of iodine atoms that have already been incorporated into the reaction site. In contrast, the incorporation of bromine and chlorine atoms in the intermediates involved in the generation of THMs makes them largely inactive in iodine incorporation reactions. The presented approach allows for a further understanding of the mechanisms of DOM/halogen interactions and prediction of the speciation of THMs formed at varying pH values, iodide concentrations, and other system conditions.