What it is:
• An inorganic ion, bromate (Br03–) is a member of the oxyhalide group of chemicals that includes hypochlorite (OCl–), perchlorate (ClO4–) and iodate (IO3–) and others. It contains bromine in its highest oxidation state +5, so it is an oxidizing agent, especially under acidic conditions. It is found in some treated drinking waters at parts per billion (ppb) levels.
• This ion is tasteless at low concentrations, colorless and has very low volatility.
• Bromate can be present in drinking water from two principal sources: As a residue in hypochlorite solutions produced by electrolysis of sodium chloride salt that contains some bromide and as a disinfection byproduct (DBP) that forms when bromide ion in water reacts with ozone (O3): Br– + O3 → BrO3–. Bromate can also form from hypochlorite oxidation of bromide in bright sunlight, e.g. in uncovered finished drinking water reservoirs.
• Bromate is generally not formed in chlorinated drinking water from oxidation of bromide because the hypobromite (OBr–) that is initially formed is rapidly reactive with organic carbon that is present to form brominated disinfection byproducts like bromoform.
• In ozonation of water, the rate and extent of bromate formation depends upon the concentration of ozone used, the bromide ion concentration, pH and contact time. The reaction rate increases with increasing pH and levels off at about pH 8.8.
• Salts of bromate include sodium bromate (NaBr03) and potassium bromate (KBr03), both white crystalline substances that readily dissolve in water. Both are used in industrial dyeing processes and hair treatments and were used as dough conditioners. In the later case, the bromate is decomposed during the baking process, but it has been replaced by other chemicals such as ascorbic acid.
• Ingestion of very large amounts of bromate (as in acute poisoning from suicide attempts) causes nausea, diarrhea, vomiting and abdominal pain, as well as effects on the kidneys and nervous system and hearing loss.
• It is an animal carcinogen from high dose testing, causing DNA damage and, therefore, a potential human carcinogen under appropriate dose conditions. However, recent studies have shown that bromate is rapidly decomposed in stomach acid and in blood. The kidney cancers found uniquely in male rat testing are not relevant to human risk. According to Joseph Cotruvo and Associates LLC and others, bromate is unlikely to be genotoxic (cause DNA damage and genetic mutations) under conditions as in drinking water and, therefore, likely not a human cancer risk in drinking water.
• The Environmental Protection Agency (EPA) has set the maximum contaminant level (MCL) for bromate in public water systems at 10 ppb and the World Health Organization (WHO) guideline is also 10 ppb. These were based upon conservative assumptions about human cancer risks (approximately 1/10,000 per lifetime) at low doses as well as the analytical method capabilities at the time. Analytical methods can now achieve quantitation limits below 1 ppb.
• Bromate is included in the current EPA review of disinfectant byproduct regulations so the MCL will be reconsidered in the next four or five years. Even if the original basis for the regulation is shown to be incorrect, there is a provision in the Safe Drinking Water Act, euphemistically called “antibacksliding,” which would make it difficult for EPA to raise the MCL.
• Bromate is difficult to remove from water so it is best controlled by preventing its presence in the source. For example, use of chlorine gas rather than hypochlorite, use of purified hypochlorite with low bromate content and not ozonating water that contains bromide are three possibilities. Some removal treatments include anion exchange and ammonia impregnated granular activated carbon (GAC). In bottled water production, precise control of the ozonation process is strongly recommended. Bromide concentration relative to ozone dose is a good predictor of ozone formation. Bromide can be removed prior to ozonation, but that is also expensive.
• Lowering pH to less than 8, adding ammonia or controlling ozone reaction time and the ozone/dissolved organic carbon ratio have been recommended, although these methods can have disadvantages as well.
Dr. Joseph Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the U.S. EPA Drinking Water Standards Division.