An Improved Crutch.

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Analysis of halogen-specific TOX revisited: Method improvement and application

A method was optimised and evaluated for the analysis of total organic halogen (TOX) in drinking water samples. It involved adsorption of organic halogen onto activated carbon, followed by combustion of the activated carbon and adsorbed material, absorption of the resulting hydrogen halide gases in an absorbing solution, and analysis of halide ions in the solution using an on-line ion chromatograph. Careful optimisation and validation of the method resulted in significant improvements compared to previously reported methods. Method detection limits were 5 µg L−1 for TOCl (as Cl−), 2 µg L−1 for TOBr (as Br−), and 2 µg L−1 for TOI (as I−). Interferences with TOI measurement occurred when iodide or iodate was present in the sample at concentrations at or above 100 µg L−1 and 500 µg L−1, respectively. In general, excellent method recoveries were determined for a wide range of model compounds. The method was used to investigate the formation of halogen-specific TOX through a water treatment plant and in laboratory-scale disinfection experiments. Up to 70% of bromide in the water was converted to TOBr following disinfection at the plant. In the disinfection experiments, TOI was preferentially formed in chloraminated samples, and trihalomethanes only constituted a small fraction (≤20%) of TOX, highlighting the significant proportion of halogenated organic DBPs that are not measured regularly. This is the first report of a comprehensive assessment of the key parameters influencing the efficiency and reliability of the analysis of halogen-specific TOX in drinking water with demonstration of its applications

Influence of bromide on iodate and iodo-trihalomethane formation during chlorination of iodide-containing waters

The kinetics of iodate formation during chlorination of iodide-containing waters is a key factor in the formation of iodoorganic compounds. In contrast to bromate, iodate is considered to be non-toxic. A strategy to reduce the formation of potentially toxic iodoorganic compounds could be to ensure the rapid conversion of iodide to iodate. The observed kinetics of oxidation of iodide by chlorine cannot explain the conversion of iodide to iodate typically observed in water treatment. It has been demonstrated in this study that the formation of bromine by oxidation of bromide during chlorination enhances the oxidation of iodide to iodate. The kinetics of oxidation of iodide by bromine were determined to be relatively fast. Oxidation of iodide by bromine was found to depend on the pH, where a maximum of the reaction rate occurred at pH 9.6, which is the mean of the two pKa values of the main species involved in the limiting reaction (HOBr and HOI). The rate was controlled by the reaction of HOBr + IO- under typical drinking water conditions. A kinetic model was formulated which allows demonstration of catalysis of the oxidation reaction of iodide by bromide. Experiments with various natural waters collected in Switzerland and in Western Australia were performed. These waters were diluted or spiked to achieve different levels of dissolved organic carbon (DOC) and different Br-/I- ratios. The rate and efficiency of iodate formation was found to depend on the water quality, mainly the concentration of bromide and the concentration and the type of DOC. Among the disinfection processes, chloramination usually leads to the highest formation of Iodinated disinfection by-products (I-DBPs). In contrast to ozone or chlorine, the formation of iodate does not occur with monochloramine. Prechlorination followed by addition of ammonia is a potential process to mitigate the formation of I-DBPs during chloramination. The formation of iodinated trihalomethanes (I-THMs) was studied during treatment involving prechlorination followed by addition of ammonia. The formation of iodo-THMs and especially iodoform was significantly reduced by increasing the contact time of chlorination and increasing the Br-/I--ratio. In addition, a relatively low formation of brominated and chlorinated THMs was obtained by this process, with a formation significantly lower than the one obtained during the chlorination process. The optimum prechlorination time for minimal I-THM formation depended strongly on the Br- level; it was determined to be at 60% iodide conversion to iodate. The prechlorination method of chloramination thus shows promise as a method to mitigate I-DBP formation, by promoting iodate formation. In addition, the concentration of bromide is among the relevant parameters which should be taken into consideration in the evaluation of the risk of iodinated DBP formation

Toxicity evaluation of synthetic waters based on Br-Cl-I-THMs formation during the chlorine/ammonia process

Monochloramine (NH2Cl) is commonly used as an alternative to chlorine for disinfection because it is less reactive with the organic matrix, therefore forms less regulated DBPs and leads to a more stable residual. However, emerging DBPs such as I-DBPs which are more cytotoxic and genotoxic than the corresponding regulated Cl-Br-DBPs may be produced during chloramination. To mitigate the formation of I-DBPs, a common option is the application of the chlorine/ammonia process. The water is allowed to be in contact with chlorine to oxidise iodide to iodate, therefore mitigating the formation of I-DBPs. Then ammonia is added to form NH2Cl to control the formation of regulated DBPs. To better understand the mechanisms involved in the mitigation of I-DBPs during the chlorine/ammonia process, synthetic waters spiked with iodide and bromide at typical drinking water concentrations and different DOM extracts from the IHSS were subject to 3 disinfection scenarios: NH2Cl alone, pre-chlorination at different contact times followed by ammonia addition and HOCl alone. A theoretical toxicity evaluation was carried out based on the THMs formation and their relative toxicity equivalents to discriminate the 3 disinfection strategies studied. Results showed that the pre-chlorination time, the bromide concentration and the type and concentration of DOM are important parameters that control the formation of I-THMs and iodate. Regarding the cytotoxicity, the chlorine/ammonia process is not always favourable. For highly reactive DOM, the decrease in toxicity induced by the conversion of iodide to iodate during chlorination was compensated by the toxicity of regulated THMs.Conversely, for DOM with lower reactivity, the application of chlorine clearly reduced the formation of I-THMs while the toxicity of regulated THMs remained relatively low. In this case, because of the I-THMs mitigation, the application of chlorine led to a lower relative toxicity (based on THMs), as compared to the application of NH2Cl