Bromine can be used for the disinfection of swimming pool and cooling tower water. It is not used for the disinfection of drinking water.
When was bromine discovered?
What are the characteristics of bromine?
Where can bromine be found?
Figure 1: bromine exists mostly as bromide salts in the sea
How can bromine be produced?
NaBr (s) + H2SO4 (l) → HBr (g) + NaHSO4 (s)
Another method is the electrolysis of bromide solutions. On the positive electrode bromine is formed:
2Br- + Cl2 → 2Cl- + Br2
What are the applications of bromine?
Can bromine be used for the disinfection of swimming pools?
Br2 + 2H2O « HOBr + H3O+ + Br-
This equilibrium strongly depends on the pH value. At the pH value that is usually found in swimming pools, bromine is mainly present as hypobromous acid (HOBr). Bromine has to be used combined with an oxidizing agent (for example chlorine or ozone).
Br2 + 2H2O « HOBr + H3O+ + Br-
Table 1: influence of the pH on the formation of hypobromous acid.
A bromine-containing stick was developed in 1958, because of the risks of using fluid bromine. This stick exists of bromine-chlorine-dimethylhydantoin (Dihalo, DMH). Both chlorine and bromine are attached to a nitrogen atom, which acts as an organic support. Applied to water, DHM is hydrolyzed and forms hypochlorous acid. Some HOCl is formed as well. The hypochlorite ion reacts with bromides to form hypobromous acid. Bromine-chlorine-dimethylhydantoine (BCDMH) is an organic substance; after disinfection and oxidation free bromine remains. When BCDMH is dissolved in water, hypobromous acid and hypochlorous acid are released. Those substances react with bromides (Br–), causing additional hypobromous acid to be produced. This is why bromine can be used both as a disinfectant and as an oxidizing agent. The concentration of BCDMH in water should not reach 200 mg/L or higher, otherwise the equilibrium between the residual disinfectant and the organic matter is disturbed. An advantage of BCDMH is that it is harmless when it is stored. It is easy to apply. Occasionally, the pH value has to be adjusted.
BCDMH is provided as tablets or cartridges. It has a long shelf life and it dissolves very slowly. Another system that can be used is dissolving bromine salt (sodium bromide) in water and activating it by the addition of an oxidator (hypochlorite or ozone). At first, salt is added to the water. Second, the oxidator is added to activate the bromine and hypobromous acid is formed.
Figure 2: different forms of bromine at various pH values and various concentrations of ammonia.
The most important bromide substances used as a biocide are sodium bromide and BCDMH.
Can bromine be used for the disinfection of drinking water?
Bromine can be used for the disinfection of cooling tower water. Hypobromous acid is slightly less effective than hypochlorous acid in killing microorganisms. The pH value of the cooling water determines which form of bromine is present. When the pH value is below 8,7, more hypobromous acid (HOBr) is formed. This is more effective than hypobromite ions, which will be more abundant above pH 8,7. This is why bromine is a better disinfectant for alkalic cooling tower water than chlorine. At pH 7,6 and higher, mainly hypochlorite ions are formed. These are less effective than hypochlorous acid. Bromine reacts with ammonia to form bromamines. In contrast to chloramines, bromamines are unstable and will dissociate into hypobromous acid. Most microorganisms in cooling towers can be treated with bromine, as long as there is enough bromine present.
Figure 3: dissociation of hypobromous acid and hypochlorous acid at various pH values
Bromine is very reactive. To maintain an adequate disinfection, the amount of bromine that is added must be high. Bromine aggressively reacts with metals and it is a corrosive material.
Bromine concentrations around 0.5 mg/L in swimming pools cause eye and mucous membrane irritation and can lead to odor nuisance.
What are the environmental effects of bromine use?
When bromine is used to disinfect water, bromamines and hypobromous acid react with organic matter in the water to form brominated disinfection byproducts. These can be harmful to human health.