Evaluation of bromine substitution factors of DBPs during chlorination and chloramination

Abstract

Bromine substitution factor (BSF) was used to quantify the effects of disinfectant dose, reaction time, pH, and temperature on the bromine substitution of disinfection byproducts (DBPs) during chlorination and chloramination. The BSF is defined as the ratio of the bromine incorporated into a given class of DBPs to the total concentration of chlorine and bromine in that class. Four classes of DBPs were evaluated: trihalomethanes (THMs), dihaloacetonitriles (DHANs), dihaloacetic acids (DHAAs) and trihaloacetic acids (THAAs). The results showed that the BSFs of the four classes of DBPs generally decreased with increasing reaction time and temperature during chlorination at neutral pH. The BSFs peaked at a low chlorine dose (1 mg/L) and decreased when the chlorine dose further increased. The BSFs of chlorination DBPs at neutral pH are in the order of DHAN > THM & DHAA > THAA. DHAAs formed by chloramines exhibited distinctly different bromine substitution patterns compared to chlorination DHAAs. Brominated DBP formation was generally less affected by the pH change compared to chlorinated DBP formation.

Introduction

Bromide ions are nearly ubiquitous in natural water sources. Chlorine can rapidly oxidize bromide ions in the water to form bromine during the drinking water chlorination process (Kumar and Margerum, 1987). Bromine and chlorine are active oxidants that react with natural organic matter (NOM) to produce halogenated disinfection byproducts (DBPs) (Rook, 1974, Stevens et al., 1989, Krasner et al., 1989, Reckhow et al., 1990, Cowman and Singer, 1996, Westerhoff et al., 2004, Hua and Reckhow, 2007). Bromide concentrations have a significant impact on the formation and speciation of DBPs. The formation of DBPs shifts to more brominated species as the bromide concentration increases (Symons et al., 1993, Pourmoghaddas et al., 1993, Heller-Grossman et al., 1993, Cowman and Singer, 1996, Wu and Chadik, 1998, Diehl et al., 2000, Hua et al., 2006). Because of their high molecular weights, brominated DBPs can cause difficulties for utilities to meet the regulatory limits of trihalomethanes (THMs) and haloacetic acids (HAAs). Moreover, brominated DBPs may present higher health risks than their chlorinated analogs based on some toxicological studies (Bull et al., 2001, Richardson, 2003). Therefore, it is important for utilities to control the formation of brominated DBPs to meet the requirements of the DBP rules and reduce the health risks of DBPs.

Many studies have been conducted to investigate the formation and speciation of THMs and HAAs. Researchers have developed indices to help quantify the bromine substitution during DBP formation. Bromine incorporation factor (BIF) was introduced by Gould et al. (1983) to investigate the bromine substitution degree of THMs. The BIF was defined as the ratio of the molar concentration of bromine incorporated into a given class of DBPs to the molar concentration of DBPs in that class. Equation (1) presents a generic method for calculating BIF.

$$\text{BIF}=\frac{\text{DBP}-\text{Br}}{\text{DBP}}$$

The BIF has been used by many researchers to evaluate bromine substitution of DBPs (Gould et al., 1983, Symons et al., 1993, Diehl et al., 2000, Chellam, 2000, Pope et al., 2006). It has been shown that higher bromide-to-chlorine ratios produce more bromine incorporation. The BIFs of THMs and HAAs can vary between 0 and 3, depending on the bromine substitution degree. The highest possible BIF is 1, 2, and 3, respectively, for mono-, di-, and trihalogenated DBPs. Because of this, the BIF may not be an ideal parameter to compare the bromination degrees among different classes of DBPs. Hua et al. (2006) introduced the bromine substitution factor (BSF) to investigate the bromine substitution of DBPs. The BSF is defined as the ratio of the molar concentration of bromine incorporated into a given class of DBP to the total molar concentration of chlorine and bromine in that class (Equation (2)). The BSF is the percentage of bromine in the total halogen of each class of DBP and it can vary from 0 to 1. Thus, the BSF can be used as an unbiased measure of bromine substitution among different DBP classes. Equation (3) uses THMs as an example to show the method for calculating the BSF. The BSF has been used to study the bromine substitution patterns of THMs, HAAs, total organic halogen and other DBPs in drinking waters (Hua et al., 2006, Hua and Reckhow, 2007, Chow et al., 2007, Engelage and Stringfellow, 2009).

$$\text{BSF}=\frac{\text{DBP}-\text{Br}}{\text{DBP}-\left(\text{Cl}+\text{Br}\right)}$$

$$\text{THM}-\text{BSF}=\frac{\sum _{n=1}^{3}n\times \left[{\text{CHCl}}_{(3-n)}{\text{Br}}_n\right]}{3\sum _{n=0}^3{\text{CHCl}}_{(3-n)}{\text{Br}}_n}=\frac{{\text{CHBrCl}}_2+2\times {\text{CHBr}}_2\text{Cl}+3\times {\text{CHBr}}_3}{3\times \left({\text{CHCl}}_3+{\text{CHBrCl}}_2+{\text{CHBr}}_2\text{Cl}+{\text{CHBr}}_3\right)}$$

Most studies on DBP speciation to date have focused on the effect of varying bromide concentrations on DBP formation (Symons et al., 1993, Pourmoghaddas et al., 1993, Heller-Grossman et al., 1993, Cowman and Singer, 1996, Hua et al., 2006). The DBP speciation is often evaluated as a function of Br⁻/Cl₂ or Br⁻/DOC ratios (Cowman and Singer, 1996, Chellam, 2000, Clark et al., 2001, Chow et al., 2007). Relatively less research has investigated the impact of specific disinfection conditions, such as reaction time, pH, and temperature on the DBP speciation. These disinfection conditions can affect the formation of the DBPs and they may also impact the competition between chlorine and bromine for the reactions with NOM. There is a lack of quantitative information in the literature on the effect of different disinfection conditions on DBP speciation. The primary objective of this study was to evaluate and quantify the effects of disinfectant dose, reaction time, pH, and temperature on the BSFs of DBPs formed by chlorine and chloramines. The BSF provides a direct measure of bromine content of the total halogen incorporated into each group of DBPs. Therefore; it can be used as an unbiased indicator to compare the bromination degrees of different classes of DBPs.

A natural water was treated with chlorine and chloramines at bench scale under controlled conditions. Four classes of DBPs were evaluated in this study and these included THMs, dihaloacetonitriles (DHANs), dihaloacetic acids (DHAAs) and trihaloacetic acids (THAAs). THMs, THAAs and DHAAs are three groups of DBPs that contain the regulated THM and HAA species. DHANs are important nitrogenous DBPs that are sometimes monitored by water utilities. Monohaloacetic acids were not evaluated due to their low concentrations in this study. DHAAs and THAAs are considered different classes of DBPs in this study because they respond differently to the variations of disinfection conditions (Obolensky and Singer, 2005, Hua and Reckhow, 2008). The investigation of the BSFs of these four groups of DBPs helps to improve our understanding of the formation and speciation of DBPs under different disinfection conditions.