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Environmental fate & pathways

Biodegradation in water: screening tests

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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:

Additional information

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

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