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Environmental fate & pathways

Additional information on environmental fate and behaviour

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Endpoint:
additional information on environmental fate and behaviour
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Peer-reviewed well documented publication

Data source

Reference
Reference Type:
publication
Title:
Ozone Decomposition in Water: Kinetic Study
Author:
Sotelo JL, Beltran FJ, Benitez FJ, and Beltran-Heredia J
Year:
1987
Bibliographic source:
Ind. Eng. Chem. Res. 26,39-43

Materials and methods

Test guideline
Qualifier:
no guideline followed
Principles of method if other than guideline:
Kinetic studies on ozone decomposition in water were performed over a range of temperatures from 10 to 40 °C and pH range from 2.5 to 9.The ozone decomposition chemical reaction was followed by determining the concentration of dissolved ozone by reaction with a buffered potassium iodide solution, measuring the triodide ions liberated spectrophotometrically at the wavelength of 352 nm.
GLP compliance:
no

Test material

Constituent 1
Chemical structure
Reference substance name:
Ozone
EC Number:
233-069-2
EC Name:
Ozone
Cas Number:
10028-15-6
Molecular formula:
O3
IUPAC Name:
trioxygen
Test material form:
gas
Specific details on test material used for the study:
- Test material: ozone gas
- An ozone-oxygen mixture was produced in an ozonator (SLO Constrema) which is able to operate at a maximum rate of 6 g of 03/h.
- Oxygen was taken directly from a commercial cylinder and dried with silica gel traps before entering the ozonator.
- Water was ozonated with an 02-03 stream, containing about 4% (v/v) ozone, at a flow rate of 40 L/h for 30 min.

Results and discussion

Any other information on results incl. tables

Effects of Agitation Speed, Temperature, and pH.
Though the process studied is only a homogeneous chemical reaction, agitation was provided to keep the temperature of the water uniform during the experiment. Agitation speed was varied between 100 and 700 rpm. At speeds below 300 rpm, the ozone conversion remained constant at a given reaction time. However, the decomposition rate was found to be higher when agitation speed was above 300 rpm, probably due to ozone desorption (Figure 1). Hence, the rest of the experiments were performed at 100 rpm.


Experiments were carried out while varying the temperature from 10 to 40 °C and pH from 2.5 to 9. When these variables were increased separately, a higher ozone conversion at a given reaction time was observed (see Figures 2 and 3). Decomposition was extremely fast in experiments performed at temperatures and pHs higher than 30 °C and 8, respectively.


Figures 1, 2 an 3 can be found under illustration.


Interpretation of Kinetic Data.
Experimental data and kinetic calculations based also on previous work from various other authors lead to the conclusion that the ozone decomposition in water involves two major contributions: direct ozone decomposition, and the overall hydroxide ion attack.

The global ozone decomposition rate was expressed as:
-rO3= 3.26 x 105(-4964/T)[O3] + 5.69 x 1018 x exp(-10130/T)[OH-]1/2[O3]3/2(mol/(L.min))

It was found that experimental and calculated values agreed ± 10% in 95% of cases. Figure 8 (see illustration) shows the comparison among the ozone half-lives obtained from this work and previous reports.
From figure 8 we can see that the calculated half-life in deionized and distilled water, at 20°C and pH 7, was calculated to be circa 2000 seconds (ca. 32 min).


Applicant's summary and conclusion

Conclusions:
Ozone is unstable in water. The half-life in deionized and distilled water, at 20°C and pH 7, was calculated to be circa 2000 seconds (ca. 32 min). In any real water (containing ions and other subtsances) the half-life will be much shorter.
Executive summary:

Kinetic studies on ozone decomposition in water were performed over a range of temperatures from 10 to 40 °C and pH range from 2.5 to 9. The ozone decomposition chemical reaction was followed by determining the concentration of dissolved ozone.
Ozone decomposition was observed to follow a two-term rate equation supported by a reaction mechanism. It was shown that at pHs lower than 7, whatever the temperature was, direct ozone decomposition and the initiation step involving the hydroxyl radicals are the main cause of ozone decomposition. At higher pHs, the importance of the peroxy radicals and of the hydroxide ion initiation step increases. Thus, at pHs around 9, the ozone decomposition rate depends on two major contributions: direct ozone decomposition, which leads to the formation of hydroxyl radicals, OH·, and hydroxide ion action, which produces not only peroxy radicals, HO2·, but also hydroxyl radicals.
Finally, the nature of the ionic species (carbonate, sulfate, phosphate, etc.) present in the water greatly influences ozone decomposition, inhibiting some of the reactions in the mechanism proposed.

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