Bromate

Guideline

The interim maximum acceptable concentration (IMAC) for bromate in drinking water is 0.01 mg/L (10 µg/L).

Identity, Use and Sources in the Environment

The bromate ion (BrO₃¯) exists in a number of salts, the most common of which are potassium bromate and sodium bromate. Potassium bromate is soluble in water (7.5 g/100 mL) at 25°C and is highly stable in water at room temperature. Bromate does not volatilize and adsorbs only slightly to soil or sediment. Because it is a strong oxidant, it reacts with organic matter, which ultimately leads to the formation of bromide ion.

Potassium bromate is used primarily as a maturing agent in flour and as a dough conditioner in bread making. In Japan, it was formerly used in fish paste products.¹ It may be used in the production of cheese and beer. Potassium bromate and sodium bromate are also components of neutralizing solutions in home permanent wave kits.² Although bromate is unlikely to be formed during water chlorination, evidence has been found in U.S. and British studies that water treatment grade sodium hypo-chlorite solutions may contain bromate as a contaminant. Data collected suggest that bromate concentrations range from <2 to 51 mg/L in the United States.³ In the United Kingdom, ranges from 50 to 1150 mg/L were noted.⁴ Other researchers have found bromate concentrations much greater than 10 µg/L in sodium hypochlorite solutions.⁵ Since chlorination activity of the solution decreases with time, it may be necessary to use larger quantities of the sodium hypochlorite solution in order to obtain the required level of disinfection. As a result, bromate levels could be high as a result of bromate's stability during long-term storage (as occurs in smaller municipalities).

Bromate is not a natural component of water but may be formed during the disinfection of drinking water using ozone⁶ or a combination of ozone and hydrogen peroxide.⁷ The concentration of bromide in raw water is a major factor in the formation of bromate. The bromine in well waters is primarily inorganic. The major natural sources of bromide in groundwater are saltwater intrusion and bromide dissolution from sedimentary rocks.⁸ Sewage and industrial effluent as well as road and agricultural runoff may also contribute to elevated bromide levels in surface waters.⁸

Exposure

For most Canadians, exposure to bromate is unlikely to be significant, because relatively few Canadian treatment plants (except in Quebec) use ozone for disinfection. This situation may change as water utilities seek alternatives to chlorination, which may lead to the formation of other toxic disinfection by-products (DBPs).

Bromides must be present in the water for bromate formation to occur. Bromide in water causes a catalytic disintegration of ozone and forms hypobromite (OBr¯) as an intermediate product. Hypobromite is predominantly present at higher pH values (pH > 8); at lower pH values, increasingly more hypobromous acid (HOBr) is formed. Hypobromite reacts further with an overdose of ozone to form bromate. Hypobromous acid does not react further with ozone; therefore, at low pH, no bromate is formed. In the presence of organic matter, HOBr does lead to the formation of brominated organic compounds, such as bromoform, mono- and dibromoacetic acid, dibromoacetonitrile, bromopicrin and especially cyanogen bromide.^6,9No monitoring data on bromides in Canadian raw water or drinking water were found. Krasner et al.¹⁰measured bromide concentrations in the incoming water (after final disinfection but prior to distribution) of 35 drinking water companies in the United States. Quarterly median values ranged between 0.07 and 0.1 mg/L (overall range from <0.01 to 3.00 mg/L). In general, a good correlation existed between bromide concentration and chloride concentration, with the concentration of bromide approximately 0.0031 times the concentration of chloride.¹⁰ In another study, bromide levels ranged from 0.33 to 0.48 mg/L in raw water from northern California where there was seawater infiltration and from 0.03 to 0.07 mg/L in Colorado River water.¹¹

There is little information available on concentrations of bromate in drinking water following chlorination, but bromate does not appear to be formed by this process, which favours the formation of brominated organics. Ozonation of water containing bromide concentrations of $0.18-0.37 mg/L resulted in the formation of bromate at concentrations of $5 µg/L (the detection limit) in pilot plant research in the United States.^9,11 Bromate was measured at concentrations of 8-180 µg/L, depending on temperature, pH, ozone and peroxone dose, ammonia-nitrogen concentration and bromide concentrations in raw water (0.3-1.4 mg/L).

Results from a small survey, in the summer of 1996, of 12 Quebec drinking water systems that use ozone indicated that bromate levels increased significantly in distributed water compared with raw water at seven of 12 sites. Bromate levels for distributed water ranged from 0.55 to 4.42 µg/L, with an average value of 1.71 µg/L. In many instances, bromate levels reached their maximum at the treatment plant and decreased in the distribution system.¹² In a follow-up survey in the summer of 1997 at these same sites, significantly increased bromate levels in distributed water compared with raw water were again seen. Bromate levels in distributed water ranged from 0.73 to 8.00 µg/L, with an average value of 3.17 µg/L. With the exception of two sites, bromate levels were found to be highest in the distribution system.¹² Another limited survey of the 12 municipal water supplies using ozonation in Quebec was conducted in the winter of 1998. This study found that bromate concentrations generally increased from raw to treated to distributed water, although concentrations in distributed water were occasionally the same as or lower than those in treated water. Concentrations of bromate ranged from <0.20 to 1.79 µg/L in raw water, from 0.43 to 5.98 µg/L in treated water and from 0.42 to 6.80 µg/L in distributed water.¹³ Spatial and temporal variations have been found to affect levels of chlorinated disinfection byproducts (CDBPs) and may account for the differences in results of these three surveys.¹⁴ A limited survey of bromate concentrations in ozonated treated waters in the United Kingdom detected concentrations of bromate of 10-20 µg/L at two of four sites sampled, whereas bromate concentrations ranging from 3 to 28 µg/L were found in final waters from treatment works using commercial sodium hypochlorite as a disinfectant.¹⁵ Bottled water was also tested in two limited surveys carried out in 1995 and 1996. In 1995, 27 types of bottled water were tested, with bromate concentrations ranging from below detection (0.3 µg/L) to 19.7 µg/L. The 1996 testing of eight bottled water samples that used ozone as a disinfectant showed bromate concentrations ranging from 2.0 to 33.0 µg/L.¹⁶

In 1996, a bottled water survey of 18 different brands of bottled spring water, all bottled in Canada, was carried out.¹² Eleven of these samples were ozonated, and the results showed that most samples had bromate levels that were much higher than in non-ozonated samples. The average bromate concentration in the non-ozonated bottled waters was 3.72 µg/L, with a range of <0.20-12.90 µg/L, whereas the average concentration for the ozonated bottled waters was 18.14 µg/L, with a range of 4.28-37.30 µg/L. Although this procedure was used to determine bromate in bottled water, this type of water is regulated under the Food and Drugs Act and thus is not subject to Canadian drinking water guidelines.

Another bottled water survey was carried out in 1998 by the Food Directorate of Health Canada. In this survey, bromate levels of 206 bottled waters were tested, with bromate concentrations ranging from below detection (0.5 ppb) to 144 ppb. An overall average of 6.88 ppb was calculated. There was no apparent correlation between bromate levels and bromide or use of ozone.¹⁷ A small amount of potassium bromate is added to flour during the preparation of bread; however, it breaks down to bromide during baking.² The recommended maximal potassium bromate doses are 30 and 50 mg/kg flour for Japan¹ and the United States, respectively.¹⁸

Analytical Methods and Treatment Technology

Bromate has seldom been monitored in drinking water until recently. In most cases, only bromide concentrations are measured. The National Testing Laboratories network performs bromate analysis via EPA Method 300.0,¹⁹ the only EPA-approved method for bromate analysis in drinking water to date. The current detection level of 5 µg/L is difficult to attain; there are not many accredited laboratories in the United States, and very few are capable of reaching this low a level. Samples with high chloride levels must often be diluted, thus increasing the detection level to 10 µg/L for this method.

Analytical methods with increasingly lower limits of detection are being developed for the measurement of bromate in drinking water. Ion chromatography is a potentially useful detection method; however, it is subject to significant interferences from chlorides, sulphates and bicarbonate/carbon dioxide. The practical quantitation limit (PQL) achievable by most certified laboratories is 5 µg/L; however, it can be as high as 20 µg/L, as any significant level of interference requires sample dilution, thus doubling or tripling the PQL. As well, the method has not been properly validated through round-robin testing. No performance evaluation sample testing has been done; thus, the method's scientific validity is considered unreliable at this time. Although it shows promise, this method is not widely used, is complex and requires very experienced chromatographers. In addition, it is not sufficiently available to all parties concerned with maintaining regulatory compliance.

Ion chromatography can isolate a large number of anions using specific ion exchange columns. A number of inorganic DBPs, such as chlorate, chlorite and bromate, can be analysed within one sample cycle of 25 minutes by this method.^20,21 The detection limit depends on the presence of major components, especially the concentration of chloride present. Pfaff and Brockhoff²⁰ tested the recovery of bromate from Cincin-nati tap water and deionized water spiked with bromate at different concentrations. The method detection limit for bromate was 0.01 mg/L in tap water and 0.02 mg/L in deionized water. This detection limit was subsequently lowered to 0.005 mg/L.²² Selective anion concentration has been demonstrated to be quite an effective technique for analysing bromate at lower levels. This new application of ion chromatography performs multi-column separation with automated peak fractionation of high-volume injections.²³ The method detection limit was reported as 0.18 µg/L and 0.25 µg/L for deionized water and river water, respectively. However, this technique is complex and may not be amenable to utilities with inexperienced ion chromatographers.²⁴ Another application of selective ion chromatography performed by multi-column separation of pre-treated samples also appears to be a promising technique.²⁵ Surface water samples were analysed with and without pre-treatment to determine the effect on the chromatographic resolution of the bromate peak. Pre-treatment of the sample prior to ion chromatography was shown to minimize interference from chloride and sulphate ions as well as reduce the carbonate level. The method detection limit was reported as 0.2 µg/L for surface water. This method is preferred to the Sorrell and Hautman²⁴ method owing to its success in minimizing or eliminating interference by other DBPs; however, it, too, is a complex technique requiring experienced chromatographers.

A simple concentration technique for the analysis of bromate at low levels in drinking water has been reported by Sorrell and Hautman.²⁴ A rotary evaporator is used to remove the excess water from the sample (sample volume reduced from 750-1000 mL to 10 mL); for a 1000-mL sample, the concentration of bromate has been increased by a factor of 100. The method detection limit is 0.1 µg/L. The average recovery of bromate was 96 ± 4% and 94 ± 2% from deionized water samples fortified with bromate at 2 µg/L and 5 µg/L, respectively, and 94 ± 17% from raw surface water fortified with bromate at 4 µg/L. This technique, however, has not been incorporated in the approved ion chromatography technique currently used by many laboratories. The method requires proper validation through round-robin testing and performance evaluation sample testing.

There are no practical methods currently available to remove bromate from water. Advanced treatment processes that have been suggested as warranting further evaluation include ion exchange and membrane filtration.²¹ Bromate in ozonated drinking water supplies is best controlled by limiting its formation, which is influenced by the bromide concentration ($0.18 mg/L),²² the source and concentration of organic precursors, pH, temperature, alkalinity and ozone dose.²⁶ For example, reductions in bromate formation can be achieved by lowering the pH to less than 8, adding ammonia or controlling the ozone reaction time and the ozone/dissolved organic carbon ratio.^21,27,28 These and other measures have both advantages and disadvantages; a low pH, while reducing bromate formation, increases the formation of bromoform and other brominated organic byproducts, in addition to being undesirable from the point of view of corrosion control; addition of ammonia results in the conversion of HOBr to monobromamine, which in turn may be oxidized to nitrate.²⁸ Because of the large number of factors that influence bromate production, it will be necessary to optimize treatment by balancing the advantages and disadvantages of various measures on an individual basis for each water supply.