Influence des ions chlorure sur la determination quantitative de la tribromamine en milieu aqueux par spectrophotometrie d'absorption electronique

Abstract

Use of seawater in the condenser cooling systems of plants such as those of electric power stations poses serious problems because of marine biofouling in the cooling circuits. Chlorination at the condenser inlet has proved to be the most reliable and effective method to prevent the growth of common biofoulers as molluscs; in counterpart this process leads to the discharge of toxic chlorinated and/or brominated effluents around the outlet. When chlorine or hypochlorite is added to seawater a great number of reactions occur; in addition to organic matter, bromide and ammonium ions must be taken into account not only because of their reactivity towards chlorine but also because of their usual relatively high concentration levels: NH + 4+Br −+ ClO HClO − →(NHBr 2, NBr 3, NHBrCl, NBr 2Cl−HBrO, BrO− etc. . In this study we have taken an interest in the stability of the tribrominated species (NBr 3) which occurs, mixed with dibromochloramine (NBr 2Cl), when the chlorine to ammonium ratio is high (Cl 2/NH + 4 > 3) as it is ussually the case. Our work is about the effect of salinity, a specific seawater parameter, upon the formation and decomposition steps. To prevent other side-reactions, especially with organic compounds, all experiments have been carried out in artificial seawater, the biocide reagent used being bromine. The main analytical technique has been the electronic absorption spectrophotometry. From the results which are detailed in the study and illustrated in Figs 1–5, two important features appear: 1. (i) The quantitative determination of tribromamine in chlorinated seawater by using uncorrected values of the spectrophotometric parameters measured in fresh water leads to wrong results: the higher the chloride concentration is the higher the absorption spectra are modified. Chloride interferences can only be neglected when the halide concentration is lower than 0.1 M which is not the case of most seawaters. 2. (ii) Chloride ions promote hydrolysis of tribromamine as bromide ions do. To investigate quantitatively this reaction we have chosen the kinetic model described in the literature: − d(NBr 3) d t =k 1 (NBr 3) 2(HO −) (HOBr) +k 2 (NBr 3) 2 As shown by the comparison of the apparent rate constants k i ́ measured in the case of SO 2− 4 and Cl − ions k′ 1 μ SO 2− 4 0.1 ≃k′ 1 μ SO 2− 4 0.5 <k′ 1 μ Cl− 0.1 ≪k′ 1 μ Cl− 0.5 the nature of the ionic species seems to have a more important effect on the tribromamine decomposition than the ionic strength.