Hydrolysis and photolysis of brominated flame retardants and some traditional persistent organic pollutants : Reactivity as a tool in fate assessement of chemicals

Abstract: An organic compound that remains in the environment for a long time is regarded as a persistent organic pollutant (POP). Still, persistency is not a well defined property with a firm definition. Instead it is a conceptual property of chemicals for which DDT and PCB are typical representatives. No organic chemicals in the environment are inert and are thus subjected to chemical transformation reactions in air, water, soil sediments and in biota. Environmental pollutants are distributed in the environment based on their physicochemical properties and their reactivity. While the former properties have been thoroughly studied and used in modelling of the fate of chemicals in the environment, the latter property – reactivity – has been investigated poorly. The far-reaching objective with the present thesis is to introduce the reactivity of chemicals into the concept of persistency. More specifically the aim is to develop methodologies for defining chemical reactivity by determination of disappearance reaction constants for hydrolysis and photolysis of actual or potential environmental contaminants. Other transformation reactions in the environment such as oxidations reductions and radical reactions are not included at this stage. In this thesis, hydrolysis include both substitution and elimination reactions. The developmental work is performed largely by comparing the reactivity of brominated flame retardants but also include several traditional contaminants for comparative reasons. A new method for determination of the rate of hydrolysis of chemicals is developed using methoxide in methanol and DMF, which is done by using hexachlorobenzene (HCB) as the test chemical. The method is applied to several other chemicals, particularly polybrominated diphenyl ethers (PBDEs) but also traditional POPs. Chemicals such as DDT,TBBPA 2,3-dibromopropyl ether react very fast and the perbrominated diphenyl ether (BDE-209) is shown to undergo nucleophilic aromatic substitution faster than HCB. Compounds that can undergo elimination are rapidly degraded while the rates of substitution reactions of aromatic chemicals depend on the degree of halogenation. A new method is used for studying photolytic reactions based on irradiation of UV light at environmentally relevant wavelengths and with the study compounds dissolved in methanol/water (4:1). Disappearance constants and quantum yields are determined for several traditional and potential environmental contaminants. Halogenated phenols are rapidly transformed under these conditions. Polybrominated aromatics are indicated to react faster than the corresponding chlorinated compounds. The two methods for studying reactivity of chemicals have the potential of being useful to predict the persistency of chemicals in the environment. Such predictions must however also include the compound’s susceptibility to oxidation, reduction and radical reactions in all major environmental compartments and knowledge of their physicochemical characteristics.