Chlorine dioxide

Hydrolysis

Description of key information

One key study on chlorine dioxide stability is available (Medir and Giralt, 1982). This study (KS2) shows 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.

Additional information

A total of 3 studies are available to assess the stability of chlorine dioxide:

The key study (Medir and Giralt, 1982)(KS2) shows that aqueous solutions of chlorine dioxide (CAS 10049-04-4) 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.

 A second study used a chemical model to understand the disappearance of disinfection residuals (chlorine dioxide, chlorite and chlorate) in ballast water treated with up to 5 mg/L chlorine dioxide is based on analytical data gathered from numerous studies of treated waters from various locations world-wide.
Consider that a defined volume of water previously treated with chlorine dioxide and having a residual concentration of about 1 mg/L chlorine dioxide (e.g., treated ballast) is diluted with an equal volume untreated water. If no additional demand is contained in the dilution water one would expect to see a 50% reduction in concentration due to the 1:1 dilution … or a concentration of 0.5 mg/L. Chlorine dioxide, chlorite and chlorate show similar demand features when tested in diverse waters… NIOZ (The Netherlands); NJ (East Coast USA). Both waters show an initial fast demand for chlorine dioxide along with a slower continuing loss of chlorite and chlorate ions. Dilution of the treated water with the source water shows demand for chlorine dioxide, chlorite and chlorate beyond what can be accounted for by dilution. This dilution-demand act to further remove any residuals that might be present at the time of ballast release. The initial rate of chlorine dioxide loss appears to be relatively constant however increases are observed for more industrial source waters compared to more pristine source waters. The initial rate of chlorite ion loss appears to be similar to NIOZ and NJ source waters regardless of temperature.

Last results are collected from a report on the comparison of ecotoxicological properties of chlorine dioxide with chlorine/hypochlorite (Thomas, 2010). A half-life of approximately 8h was observed at the lowest concentration of sodium chlorite (0.025 mg/L) with natural river water (Molndalsan, TOC 8 mg/l). Longer half-lives were noted for higher concentrations. Results with another sample of water show a total recovery of sodium chlorite within 4 and 7 hours depending on the concentration tested.

According to the TNsG on Data Requirements, and REACH Guidance, a study of hydrolysis as a function of pH and identification of breakdown products is required. Chlorine dioxide is not expected to hydrolyse under abiotic test conditions in ultrapure water. Transformation of chlorine dioxide to sodium chloride will occur in use and in the environment via the transient intermediates of chlorite and chlorate. The rate being dependent on the concentration of organic matter and/or of certain metal ions in the water column. The performance of a Guideline study such as OECD 111, without modifications of the study design to use natural water or artificially increase organic matter or suspended solids, will not provide any information that is relevant to the behaviour of chlorine dioxide in the environment, and is, therefore, scientifically unjustified. In recent work (Thomas et al., 2008) performed as part of the calibration for the daphnid chronic test on sodium chlorite, the chlorite ion was found to degrade abiotically in the mineral medium in the presence of algae. As part of the preliminary work documented in the report, analysis was performed on sodium chlorite under semi-static conditions and the test substance was found to be unstable in the medium at concentrations lower than a nominal 45 µg/l.