If You Are Using Bromine in Your Pool or Spa, it Could be
Veritasium youtube channel: Why are 96,000,000 Black Balls on This Reservoir
There are many benefits in choosing bromine for use over chlorine for sanitation in an indoor swimming pool. Many aquatics venues go that route. After all, bromine is an effective sanitizer at higher ranges of pH, where chlorine is not. The combined form of bromine, monobromoamine (NH2Br), is an excellent sanitizer where the combined form of chlorine, monochloramine (NH₂Cl), is fairly weak and ineffective. Monobromoamine is not an irritant and does not have an odor, where monochloramine is foul smelling (that stereotypical “indoor pool smell”) and can cause discomfort to both the skin and eyes. Bromine is even a better algaecide than chlorine, but there may be a need to bring to light its darker side.
Sounds good so far, so what’s the issue?
When used in conjunction with a supplemental oxidizer such as Ozone or UV, sodium bromide has the potential to become problematic. When water containing bromide ions (Br-) is ozonated, hypobromite ions (OBr-) are formed and become hypobromous acid (HOBr) at the usual aquatic venue pH (7.2 – 7.8). Hypobromous acid, as hypochlorous acid (HOCl) is to Chlorine, is the killing form of Bromine. In the ozonation process, bromate will form due to oxidation of hypobromite ions by ozone. Bromate is hazardous when ingested and is regulated as a disinfection by-product in drinking water.
Disinfection By-Product: a secondary, often harmful, product that is created as a “side effect” of the synthesis of chemicals
Yeah, but what about UV?
UV may have actually received a bad rap regarding this DBP (Disinfection By-Product) when used with bromine. In the available research, there is no evidence that UV oxidation alone contributes to bromate (BrO3–). However, in UV/chlorination (or UV/persulfate) oxidation, bromate formation is recognized. Therefore, Sodium Bromide, when used with UV and some other form of oxidation does form some bromate. The question is does it form enough to pose a threat?
Ozone and UV are both powerful oxidizers and use of either can benefit water quality tremendously. However, caution should be exercised when matching a supplemental oxidizer to a primary disinfectant. When in doubt, consult your local Pool Professional.
Aren’t Bromine tablets mostly just chlorine anyway?
Bromine tablets are manufactured with a hefty amount of chlorine in the ingredients; the Cl (chloro) in the formula, 1-bromo-3-chloro-5,5-dimethylhydantoin (C₅H₆BrClN₂O₂), represents this. It is not in there by accident. We put it in there, because, under normal circumstances (minus additional oxidation) we need to have it. Bromide ions, by nature, are lazy. Without some type of oxidizer added as a kick in the pants, they do not do much of anything. The reaction between chlorine and bromine is actually a displacement reaction (oxidation-reduction); Chlorine displaces bromine from the sodium bromide. So, chlorine oxidation of sodium bromide is built right into the tablet!
Note: There are also 1,3-dibromo-5,5-dimethylhydantoin tablets and these produce only hypobromous acid.
Okay, but how is that a factor?
To simplify, chlorine oxidizes sodium bromide resulting in free bromine (hypobromous acid and hypobromite ions), then the UV splits the free bromine to create hydroxyl radicals that can then react with bromine to from bromate. However, the amount of hydroxyl radicals produced in this manner is minimal, as is the bromate formation. The formation of bromate requires an oxygen-based chemical such as ozone, (maybe) persulfate, or hydroxyl radicals. The latter can be formed from UV breakdown of chlorine and to a lesser extent from UV breakdown of bromine.
So, is Chlorine the problem?
Oxidation of sodium bromide through chlorine alone does not produce bromate, so using bromine tablets with chlorine as an oxidizer, provided that a supplemental system (especially Ozone and to a much smaller extent UV) is not in use, does not constitute a significant threat of BrO3– formation in an indoor application. Chlorine dioxide will also not result in the formation of bromate, but it will not produce hypobromous acid either.
Can we eliminate the threat?
No, not completely. However, if we wanted to minimize the formation of these carcinogens when UV is in use, we could change up the delivery and use a sodium bromide/bromine tablet combo instead. This can be done by simply establishing a “bromide bank” in the water, which would allow the chlorine (from the tablets) to oxidize sodium bromide ions to bromine quickly, preventing the chlorine from ever reaching the Ultraviolet system. Yes, bromine will still split, but it is much less sensitive to UV than Ozone.
With Ozone in place, the answer is not as simple.
Ozonation of bromine will form bromate without the presence of another oxidizer whether the delivery is bromine tablets or not. Therefore, with ozone, formation of this DBP is of a greater concern. We can take steps to minimize these by-products, but our efforts will likely be futile. Still, we do need to take into consideration that BrO3– is not known to be absorbed by the skin and is not volatile. It is taken in through accidental ingestion and people do not typically drink copious amounts of swimming pool water.
Dammit Jim! I’m a Pool Guy, not a Doctor… (nod to the Trekkies)
so, keep in mind; the method of bromate intake described above is as it has been explained to me.
Is there anything we can do?
Studies have shown that increasing/decreasing the levels of pH; increasing Total Alkalinity, dissolved organic carbon, or ammonia may be successful in limiting bromate formation. However, the World Health Organization reports that natural organic matter and high concentrations of carbonate/bicarbonate ions can mask the actual level. Historically, ion chromatography is the preferred method of testing water for the presence of bromate, though photometric options now exist.
What about UV rays from the Sun? 🌞
There are other applications where bromate formation from bromine is a concern and UV rays from sunlight (as in the 96,000,000 Black Balls on a Reservoir Video above) is one of them. The viral 96 million black plastic balls on the Los Angeles reservoir video explains that the real purpose of the balls was to reduce bromate formation witnessed in the drinking water as a direct result of interaction with UV rays from the sun. Electrolysis of sodium bromide, as a saltwater generator produces chlorine, has also been found to lead to the development of BrO3–.
Don’t have a Bromine Pool?
Still need to be careful. If you are using chlorine as your disinfectant, you’ll need to be cautious of other products that you add. Many algaecides marketed for the treatment of mustard/yellow algae (diatoms) utilizes sodium bromide as the active ingredient. Repeated addition of a sodium bromide based algae treatment in a pool treated with chlorine will result a bromine pool; this, combined with any of the above scenarios in this article, will contribute to bromate.
Similar Article: Available Chlorine Content vs. Active Strength… Huh? 🤷♀️
Richard Falk with an after hours chemistry assist
NATIONAL PRIMARY DRINKING WATER REGULATIONS, United States Environmental Protection Agency
Peng-Fei Chao, ROLE OF HYDROXYL RADICALS AND HYPOBROMOUS ACID REACTIONS ON BROMATE FORMATION DURING OZONATION, ARIZONA STATE UNIVERSITY, December 2002
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