Registration Dossier

Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Additional information

Bromine is unstable in water hydrolysing rapidly. The rate constant for bromine hydrolysis is in the range of 97sec-1 (Becker RH et al., 2001, Beckwith RC et al 1996). Under relevant exposure conditions pseudo first order kinetics can be assumed and the half life corresponds to 0.007 s indiacting an instant reaction.

The identity of the degradation products of bromine have been outlined in Lewis S et al (1994). In aqueous solutions made with pure water, bromine hydrolyses to hypobromous acid (HOBr) (free bromine) which further dissociates to give hypobromite ions (OHBr-) and hydrogen ions (H+) with the degree of dissociation being highly pH dependent. An overview of the dissociation of hypobromous acid over various pH values are given below:

Effect of pH on Hypobromous acid :

pH

Bromine

6.0

5 % Br2

95 % HOBr

7.0

99 % HOBr

1 % OBr-

8.0

90 % HOBr

10 % OBr-

9.0

50 % HOBr

50 % OBr-

As under the exposure conditions covered in the registration dossier concentrations of bromine are low, water is in excess and the hydronium ion concentration is buffered to physiological pH values, the equilibrium is shifted to the hydrolysis products. The above reaction is pseudo first order. With the rate constant k1 of 97 s-1 a half-live of ln2 / 97 = 0.007 s can be calculated which confirms the rapid hydrolysis of the substance.

In natural waters, bromine or HOBr respectively reacts with ammonia and organic nitrogenous materials to form bromamines. Bromine will react more quickly with other organic materials in water, to form other chemicals analagous to chlorinated products (e.g. bromoform, bromophenols, bromoaldehydes, bromoacetonitriles, bromoacetic acids, bromoketones, bromohydrins). In seawater, the likely predominant species will be inorganic bromamines (especially dibromamine) because of the relatively high ammonia concentrations. As a result, because of the limited stability of inorganic bromamines, it is likely that the total bromine residual will decay fairly rapidly to bromide. In river waters, with typically low ammonia concentrations, and relatively high organic carbon contents, it is likely that higher concentrations of inorganic and organic bromamines would result. In neither case, is there likely to be any free bromine remaining after a short contact period. This could be difficult to confirm since standard analytical procedures are incapable of distinguishing free and combined bromamines (i.e. incapable of distinguishing hypobromous acid and the inorganic bromamines. If any free bromine should persist in an open environment, it would be destroyed by photolysis yielding bromide, and possibly bromate