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Environmental fate & pathways

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Abiotic degradation:

Bubnis (2009) reported the loss of chlorine dioxide in ballast water. This experiment clearly demonstrated that any chlorine dioxide residuals that might be present following the treatment of ballast water will quickly be consumed by dilution and demand. Chlorine dioxide and its transformation products (chlorite and chlorate) showed similar demand features when tested in diverse waters. The author observed an initial fast demand for chlorine dioxide along with a slower continuing loss of chlorite and chlorate ions.

Hydrolysis is a reaction in which a water molecule or hydroxide ion substitutes for another atom or group of atoms present in a chemical resulting in a structural change of that chemical. Potentially hydrolysable groups include alkyl halides, amides, carbamates, carboxylic acid esters and lactones, epoxides, phosphate esters, and sulfonic acid esters. The lack of a suitable leaving group renders compounds resistant to hydrolysis.

Results from Medir and Giralt (1982) demonstrated that aqueous solutions of chlorine dioxide are fairly stable at 25°C and pH 9 for an initial period of time before a fast decomposition takes place. The length of the initial stable period decreases with increasing chlorine dioxide concentration and in the presence of inert electrolytes. The reaction products are chlorate, chlorite, chloride and oxygen. Addition of sodium chloride reduces significantly the induction time, but slows down the second reaction and changes the product distribution to equal amounts of chlorite and chlorate.Therefore, this degradative process will contribute to their removal from the environment.

Two laboratory based experiments were carried out, one on abiotic chlorite degradation in 3 Swedish river waters with medium to low TOC values, and one using seawater. In the case of river water, a series of concentrations was prepared directly in river water with known total Organic Carbon values and degradation was measured over time. A strong correlation was found between TOC and half-life. At TOCs found in medium to low TOC, rivers' half-life was determined as minutes to hours but degradation rate depended upon chlorite concentration. Above 0.075 mg/L chlorite was more stable in river water. In the river water with the lowest TOC (8 mg/L), a half-life could only be calculated at the lowest concentration of 0.025 mg/L as at higher chlorite concentrations, insufficient oxidisable material was present for complete degradation to occur.

Bubnis (2009) for chlorite, demonstrated that dilution of the treated water with the source water showed demand for chlorine dioxide, chlorite and much slower loss of chlorate beyond what can be accounted for by dilution.

Other forms of abiotic transformations:

Phototransformation in air:

Standard tests for atmospheric oxidation half-lives are intended for single substances. Study from Cosson and Ernst (1994) showed that concentration of substance: 0.020 - 0.024 mol/L Products: Chlorine dioxide residual concentration vs. time. Quantum Yield (number of chlorine dioxide molecules divided by the number of photons adsorbed by the solution): 1.4 at 300 nm and 0.44 at 253.7 nm, both at 25°C.

Phototransformation in water and soil:

The direct photolysis of an organic molecule occurs when it absorbs sufficient light energy to result in a structural transformation. The absorption of light in the ultra violet (UV) -visible range, 110-750 nm, can result in the electronic excitation of an organic molecule. The stratospheric ozone layer prevents UV light of less than 290 nm from reaching the earth's surface. Therefore, only light at wavelengths between 290 and 750 nm can result in photochemical transformations in the environment.

Zika et al. (1984) demonstrated that ClO2 is readily decomposed by sunlight and fluorescent lights. This can lead to significant losses during water treatment. The characteristics of the water play an important role in the nature of products resulting from light-initiated reactions. The bromide ion was found to play a particularly important role in THM formation and in initiating light reactions that accelerated the decomposition of ClO2 in the dark.

Photodecomposition of chlorite in aqueous solution is rapid with a half-life of about 10 minutes depending on concentration. Oxygen, chloride and chlorate are produced as stable end-products with the formation of chlorine and chlorine dioxide as intermediates. pH (5<pH<9) had no effect on the productions of chlorine and chlorine dioxide or of chlorate and chloride.

Biodegradation

As demonstrated by different studies in this section, chlorine dioxide reacts with a number of inorganic and organic substances like iron, sulphuric compounds (organic as well as inorganic), phenolic compounds and humus acids. Surface waters, ground water, waste water etc. are unique in terms of their composition and therefore the combination of substances that can react with and degrade chlorine dioxide. The laboratory study has consequently to be seen as an example of how chlorine dioxide may decay in the aqueous environment.

Studies from Ottaviani et al. (2002) and Belluati (2007) demonstrated that Chlorine dioxide is completely degraded within 37 and 18 min respectively.

No decay of ClO2 could be detected using tap water during the evaluated time frame. The reason for the slow decay in tap water is the low amount of substances that can be oxidized. Still a low amount of ClO2 in the water leaving the water treatment plant is desired in order to prevent recontamination of the water and to avoid bio-fouling of the water pipes. The study from Van der Togt and van Ginkel (2005), on chlorate degradation, concluded that while chlorate is degraded to chloride, no chlorite is observed. Thus, chlorite is completely degraded within few minutes.

Degradation rates

Chlorine dioxide is completely degraded within seconds to minutes under the conditions of use. It is considered that no chlorine dioxide reaches the environment. Chlorine dioxide is entirely degraded to chloride and chlorate ions, via the transient intermediate of chlorite which has a really short half-life (few seconds).

Bioaccumulation

Based on the criteria for bioaccumulation stipulated in REACH Annex IX (9.3.2), low Log Kow and high water solubility and the expected rapid metabolism of chlorine dioxide and chlorite, it is considered that the potential for bioaccumulation is low. Therefore, the substance is considered non-bioaccumulable.