The impact of bromide/iodide concentration and ratio on iodinated trihalomethane formation and speciation

The objective of this study was to evaluate the formation and speciation of iodinated trihalomethanes (I-THMs) from preformed chloramination of waters containing bromide (Br⁻) and iodide (I⁻) at a Br⁻/I⁻ weight ratio of 10:1. The factors investigated were pH, iodide to dissolved organic carbon (I⁻/DOC) ratio, and NOM characteristics, specifically SUVA₂₅₄. A Br⁻/I⁻ ratio of 1:2 was also evaluated to determine the importance of Br⁻ and I⁻ concentrations and ratio on I-THM formation and speciation. Regulated triholamethanes (THMs) were measured alongside I-THMs for a more complete understanding of trihalomethane formation. The results showed that, in general, both I-THM and THM formation increased with decreased pH. Greater formation at lower pH was likely attributed to monochloramine decomposition and the formation of additional oxidants and substituting agents, most notably chlorine. For pH ≥ 7.5, I-THM yield increased with increasing I⁻/DOC ratio and decreasing specific ultraviolet absorbance (SUVA₂₅₄) of the water. The Br⁻/I⁻, Br⁻/DOC and I⁻/DOC ratios were important factors for I-THM and THM speciation. At pH 6, dichloroiodomethane (CHCl₂I) and bromochloroiodomethane (CHBrClI) were the dominant species at the common bromide and iodide levels. For pH ≥ 7.5 and for elevated bromide and iodide levels, iodoform (CHI₃) was always the dominant specie regardless of the Br⁻/I⁻ ratio. The results demonstrated that it is important to examine I-THM formation and speciation at typical Br⁻/I⁻ ratios (∼10) of natural waters, which have often been overlooked in previous investigations, in order to obtain practical and relevant results.