What inorganic disinfection products are in drinking water?
Drinking water disinfection removes, inactivates, or kills human pathogens and microbes that are present in raw water. This treatment process produces disinfection byproducts (DBPs). Inorganic disinfection byproducts include chlorite, chlorate, and bromate, which are also referred to as oxyhalides or DBP anions. In comparison, organic disinfection byproducts include total trihalomethanes (TTHMs) and haloacetic acids (HAAs).
How are inorganic disinfection byproducts produced?
Water treatment facilities use different water disinfectants, which in turn, produces different DBPs. For example:
- Chlorination produces trihalomethanes, haloacetic acids, and chlorate
- Chlorine oxide treatment produces chlorite and chlorate
- Chloramine treatment produces chlorite
- Ozonation generates bromate by reacting with naturally occurring bromide and organic matter in water
Among the inorganic DBPs, both bromate and chlorite are regulated by the U. S. EPA under the 1998 Disinfectants/Disinfection Byproducts (D/DBP) Stage 1 Rule, with bromate having a maximum contaminant level (MCL) of 10µg/L and chlorite having an MCL of 1mg/L. Although a regulatory determination has not been made for chlorate, it was monitored and reviewed between 2013 and 2015 during the most recent unregulated contaminant monitoring rule 3 (UCMR 3).
Analysis of inorganic disinfection byproducts
EPA Method 300.0 is the foundation method for monitoring inorganic anions in drinking water using ion chromatography.
- Part A of the method is used to monitor common anions (fluoride, chloride, bromide, nitrite, nitrate, sulfate, phosphate)
- Part B of the method is used to monitor oxyhalides, including chlorite, chlorate, and bromate
The 1998 bromate standard in place today could not be met using EPA Method 300.0, which has a detection limit for bromate at 20µg/L. This led to the promulgation of EPA Method 300.1. The updated method lowers the detection limit for bromate to 1.4µg/L, which is sufficient for measuring bromate concentrations at or below the current bromate standard. The 2003 Stage 2 Rule of the D/DBP did not change the bromate and chlorite standards, but approved additional more sensitive bromate monitoring methods using post-column derivatization, such as EPA Methods 317.0 and 326.0, which are not discussed here.
Analytical columns using hydroxide eluent or carbonate eluent can be used for oxyhalide analysis. However, hydroxide columns provide better sensitivity than carbonate columns, even newer carbonate columns and background reduction actions are taken. Regardless of which eluent is used, availability of reagent-free ion chromatography (RFIC) systems speeds up the process for analysis and reduces errors potentially caused by manual eluent preparation.
Using hydroxide columns and eluents to separate inorganic DBPs and bromide
Determination of inorganic anions has been traditionally performed using carbonate eluent columns. Hydroxide-selective columns, despite providing better sensitivity than carbonate eluent columns, were not widely adapted due to the lack of appropriate column selectivity and easy contamination during the preparation process for hydroxide eluent. Automated eluent generation and, therefore, reagent-free ion chromatography (RFIC) by just adding water has totally changed the views of using hydroxide eluents to determine inorganic anions in a variety of environmental samples.
In some matrices, the total ionic content may lead to column overloading, resulting in failure to detect trace levels of bromate in a high background of chloride or other interfering anions. These high concentrations of interfering anions may lower the recovery of bromate. High capacity hydroxide columns can be successfully used for trace levels of oxyhalide monitoring in drinking water. The use of high capacity Dionex IonPac AS19 columns drastically improves linearity, MDLs, precision, and resolution between bromate and chloride compared to the Dionex IonPac AS9-HC column, which was used in the original EPA Method 300.1. The high capacity of the Dionex IonPac AS19-HC column tolerates concentrations of chloride up to 150 ppm without lowering the bromate recovery. The small particles in the Dionex IonPac AS19-4µm column using hydroxide eluents allow faster separation of inorganic anions without compromising data quality.
Using carbonate columns and eluents to separate inorganic DBPs and bromide
Traditionally, carbonate eluent is used for anion separations because carbonate eluent is easy to prepare without contamination concerns and delivers results that meet the analytical requirement for regulatory standards of inorganic DBPs as in EPA Method 300.1 (Part B). However, the detection sensitivity using carbonate eluent columns is much lower compared to hydroxide eluent columns as described above due to the higher background and therefore lower signal-to-noise ratio. For instance, the background conductivity after suppression for hydroxide eluent using the Dionex IonPac AS19 column is less than 1 µS while the background conductivity after suppression for carbonate eluent using the Dionex IonPac AS23 column is about 19 µS.
To overcome the high background, a carbonate removal device (CRD) can be used after suppression to decrease the detection limit. Bromate can be detected at less than 5µg/L level using the Dionex IonPac AS23 column with the carbonate removal device. The detection limit can be further reduced to less than 1µg/L when the Dionex IonPac AS23-4µm column with smaller resin particles is used with both carbonate suppressor and carbonate removal device. These solutions allow carbonate eluent users to achieve better sensitivity if they are unwilling to change to hydroxide eluent columns.