Sodium chlorate

Biodegradation in water: screening tests

Link to relevant study record(s)

Reference

Endpoint:

biodegradation in water: ready biodegradability

Type of information:

experimental study

Adequacy of study:

key study

Study period:

07-01-2002 - 21-03-2003

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Qualifier:

according to guideline

Guideline:

EU Method C.4-C (Determination of the "Ready" Biodegradability - Carbon Dioxide Evolution Test)

Qualifier:

according to guideline

Guideline:

OECD Guideline 301 B (Ready Biodegradability: CO2 Evolution Test)

Principles of method if other than guideline:

Sodium chlorate did not contain carbon. The biotic degradation of the test item was therefore measured by chemical analysis.

GLP compliance:

yes

Oxygen conditions:

aerobic

Inoculum or test system:

activated sludge, domestic, non-adapted

Details on inoculum:

- Type of sludge: sewage sludge
- Source: sewage treatment plant treating predominantly domestic wastewater, Evreux, France
- Sampling site: aeration tank
- Preparation of inoculum: The inoculum was prepared by initially decanting and sieving sewage sludge. The sludge was then centrifuged for
5 minutes, the supernatant was rejected and the pellet was redispersed in the mineral medium.
- Pretreatment: aerated for 6 days
- Initial concentration: 15.0 mg/L (dry weight) in all test vessels

Duration of test (contact time):

28 d

Initial conc.:

63.8 mg/L

Based on:

test mat.

Parameter followed for biodegradation estimation:

test mat. analysis

Reference substance:

other: sodium acetate

Test performance:

TEST CONDITIONS:
- Composition of medium: reconstituted water was prepared using deionized water which contained no more than 5% of the organic carbon content
introduced by addition of the test or reference item and analytical grade reagents.
- Test temperature: 20 to 24°C
- pH value: 7.33 to 7.81
- Aeration of dilution water: yes
- Concentration of suspended solids: 15 mg/L
INTERMEDIATES / DEGRADATION PRODUCTS: Sodium chlorite and sodium chloride
TEST SYSTEM:
- Culturing apparatus: flasks
- Number of culture flasks per concentration: Six flasks were used for the test: Two flasks containing the inoculum (inoculum blanks), two flasks
containing the test item at 63.8 mg/L (50.0 mg/L expressed as CHLORATE ION) and inoculum (test solutions), one flask containing the reference
item (sodium acetate) at 10.0 mg/L of TOC and inoculum (procedure control), one flask containing the test item at 63.8 mg/L, the reference item
(sodium acetate) at 10.0 mg/L of TOC) and inoculum (toxicity control).
- Initial test substance concentration: 63.8 mg/L (50.0 mg/L expressed as chlorate ion)
- Method of preparation of test solution: The test item was prepared by dissolving 957 mg of test substance in 1000 mL of mineral medium.
The stock solution was agitated during 5 minutes. The pH of this solution was 7.26 after agitation. The quantity of stock solution added per flask was 200 mL (i.e. 191.4 mg or 63.8 mg/L of test substance, corresponding to 150 mg).
- Analytical parameter: CO2 production and concentration of sodium chlorate, -chlorite and -chloride in the test flasks over time.

Parameter:

% degradation (CO2 evolution)

Value:

1

Sampling time:

28 d

Details on results:

- Degradation % after time:
Sodium chlorate: 1% after 28 days
Sodium acetate: 95% after 28 days

Validity criteria fulfilled:

yes

Interpretation of results:

under test conditions no biodegradation observed

Conclusions:

At the end of the test, the biotic degradation of SODIUM CHLORATE was 1%.
Under our experimental conditions, the test item SODIUM CHLORATE is therefore non readily
biodegradable in the 28-day modified Sturm test.

Executive summary:

All validity criteria were respected:

• biodegradation values of test item replicates (based on chemical measurements) deviated by less than 20% at the end of the test,

• biodegradation in the reference test (based on CO2 release) was at least 60% within 14 days,

• the blank value (a measurement of CO2 evolved uniquely from the breakdown of organic matter in the inoculum, mineral medium, etc...) was ≤ 70 mg of CO2 evolved per liter of suspension at the end of the test (average of the two controls),

• biodegradation in the toxicity control (based on CO2 release coming from the reference item degradation) was at least 25% within 14 days. Test item biotic degradation Based on chemical measurements, biotic degradation of the test item SODIUM CHLORATE was 1% at the end of the test (flask No. 1: 0%; flask No. 2: 2%). All concentrations of SODIUM CHLORITE measured in the inoculum blanks, the test solutions and the toxicity control were lower than the limit of quantification (0.5 mg/L) while SODIUM CHLORIDE was stable (around 180 mg/L, already present in the mineral medium) in these solutions during the test. Furthermore, the quantity of carbon dioxide evolved from the inoculum blanks and the test solutions was similar over the test period. This result also demonstrated that the test item had any toxic effect on the inoculum under our experimental conditions.

Conclusion

Under our experimental conditions, the test item SODIUM CHLORATE is non readily biodegradable in the 28-day modified Sturm test.

Description of key information

OECD Guideline 301B, GLP, key study, validity 2 (L'Haridon, 2003):

1% after 28 days

The substance is not readily biodegradable.

Key value for chemical safety assessment

Biodegradation in water:

not biodegradable

Type of water:

freshwater

Additional information

Sodium chlorate is an inorganic substance so readily biodegradability could be waived based on Annex VII (9.2.1.1.)

Nevertheless, studies under aerobic and anaerobic conditions are available to assess the potental biodegradability of sodium chlorate.

 

Ready biodegradability tests

To assess the ready and inherently biodegradability potential of the registered substance, only studies under aerobic conditions have to be taken into an account. Among all studies available, only study from L'Haridon (2003) is considered as key study because it has been performed under aerobic condition.

The attempt of L’Haridon (2003) to detect biodegradation of sodium chlorate in the Sturm test (OECD TG 301 B) using a specific analysis of chlorate was therefore unsuccessful. Degradation of sodium chlorate in the Sturm test was though to be possible by L’Haridon (2003) because of the existence of anaerobic niches within the sludge particles used as inoculum. These anaerobic niches do occur in properly operated biological wastewater treatment plants (high activated sludge concentrations and low oxygen levels of~2 mg/L) but not in an OECD TG 301 tests (low level of activated sludge and oxygen levels of >>9 mg/L). Moreover, the amount of biodegradable reducing agents in a standard OECD TG 301 test is limiting also preventing chlorate reduction.

“Ready” biodegradability of sodium chlorate transformation can be shown easily using the methodology of the Closed Bottle test (OECD TG 301 D) with one major modification (van Ginkel et al, 1995). The test was modified by adding excess amounts of reducing agents such as fatty acids, amino acids, carbohydrates. A minor part of the reducing agent was oxidized with the molecular oxygen present in the bottles thereby creating anaerobic conditions. The tests were inoculated with low concentrations of activated sludge, soil, digested sludge or dilutions of river and ditch water in line with the OECD TG 301. Complete removal of chlorate was achieved with in 28 days with all inocula tested and most reducing agents.

The ease with which chlorate reduction occurs naturally is also demonstrated by Bryan and Rohlich (1954). Bryan and Rohlich (1954) used chlorate reduction as a measure for the Biological Oxygen Demand (BOD) showing that chlorate is rapidly reduced by microorganisms using organic compounds as carbon and energy source present in sewage.

A valid ready biodegradability test result is not available for sodium chlorate because chlorate is an electron acceptor like molecular oxygen. Nevertheless all aspects important for achieving a ready biodegradability test result i.e. ultimate (complete) biodegradation, rate of biodegradation and number and occurrence of competent micro-organisms present in “unacclimated” ecosystems and biological treatment plants have been investigated (see above). Ready biodegradability tests only detect growth-linked biodegradation. Microorganisms are capable of growth on sodium chlorate in the presence of reducing agents under anaerobic conditions. The biodegradation pathway proves that chlorate is reduced completely to chloride.The biodegradation kinetics of chlorate have been determined with mixed and pure cultures. The maximum growth rates of chlorate reducing microorganisms range from 0.04 to 0.56h^-1, which is comparable or much higher than growth rates of nitrifying bacteria. Ammonium is oxidized readily in OECD TG 301 tests due to these nitrifying bacteria. Painter and King (1983) used a model based on the Monod equation to interpret the biodegradation curves in ready biodegradability tests. According to this model, growth rates of competent micro-organisms of 0.01 h^-1or higher do result in a ready biodegradation of the test substance. Reduction of chlorate has been detected in terrestrial ecosystems, fresh water, marine environment, compost, and aquifers. These findings demonstrate the wide distribution of chlorate-reducing micro-organisms and that sodium chlorate is rapidly biodegradable. Tests only deviating from OECD TG 301 TG methodology with respect to the absence of oxygen do indicate sodium chlorate is rapidly biodegradable.

 

Chlorate, a naturally occurring substance

Up to recently, perchlorate and chlorate were thought to be primarily antropogenic. Recent evidence makes a strong case for more widespread natural occurrence of perchlorate, outside of the long-established occurrence in caliches of the Atacama Desert in. Improved sensitivity of perchlorate detection techniques shows widespread existence of ppb levels of perchlorate. Not all perchlorate detected could be traced to anthropogenic sources. Natural perchlorate in soils is rare but occurs in other arid environments at levels up to 0.6 weight %.In the southern high plains groundwater, perchlorate is better correlated with iodate, known to be of atmospheric origin, compared to any other species(Dasgupta et al, 2005).

Natural perchlorate may be formed from chloride aerosol by electrical discharge and by exposing aqueous chloride to high concentrations of ozone (Bao and Gu, 2004; Bohlke et al 2005).Information regarding the perchlorate formation process is however, still largely unknown.Perchloric acid is the stable end product of the atmospheric chemistry because of its resistance to photolysis (Simonaitis and Heicklen, 1975) and occurs in aerosols in stratosphere of the earth at 0.5 to 5 parts per trillion (Murphy and Thomson, 2000). Perchlorate was also detected in rain and snow samples. This strongly suggests that some perchlorate is formed in the atmosphere and a natural perchlorate background of atmospheric origin should exist. In soils and surface waters perchlorate is reduced via chlorate. Chlorate is therefore part of natural chloro-oxy acid cycle . The existence of a chloro-oxy acid cycle does explain the enormous potential for chlorate reduction in the environment.

 

Bao HM and Gu BH. 2004. Natural perchlorate has auniqueoxygenisotopesignature. Environmental Science and Technology; vol 38; no 19; 5073-5077

BohlkeJK et al. 2005. Perchlorateisotopeforensics. Analytical chemistry 77, 7838-7842

Dasgupta PK, et al. 2005. The origin of naturallyoccuringperchlorate; the role of atmospheric processes. Environ Sci Technol 39; 1569-1575

Murphy DM and Thomson DS. 2000. Halogens ions and NO+ in the mass spectra of aerosols in the upper troposphere and lower stratosphere. Geophysical research letters, vol 27, no 19, 3217-3220

SimonaitisR andHeickleJ. 1975. Perchloric acid: possible sink for stratospheric chlorine. Planet Space Sci; 23; 1567 -1569