Sodium chlorate

Endpoint summary

Description of key 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.

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.In this study, NaClO3 is not readily bidoegradable.

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. 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 andthat 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.

 

 

Table Summary of degradation rate constants in various (eco)systems

 

(Eco)system	Method	Result	validity and comments
Surface water (fresh)	TGD default value	Half-live 15 days	Growth-linked biodegradation is not expected
Surface water (fresh)	Calculated with nitrate-nitrogen uptake rates for phytoplankton	half lives ranging from 0.2 day to 18 days	Literature search was not comprehensive
Surface water with sediment	OECD TG	DT50 8 and 29 days in overlying aerobic water column	Valid test
Sediment	OECD TG	DT50 values ranging from <1 day in sediment to 29 days in water phase	Valid test
Seawater	Calculated with nitrate-nitrogen uptake rates	half lives ranging from 5.2 days to 60 days	Literature search was not comprehensive
Soils	TGD default value	Half-live 30 days	valid
Soils	OECD TG 307	DT50 ranging from 39 to 58 days	Validity restricted due to very high initial chlorate concentrations
Degradation in sewage treatment plants	TGD default value	1 h-1  (67 % removal)	Only restricted to low chlorate influent concentrations
Degradation in plants with anaerobic zones	Assessments in laboratory-scale reactors	>90%	Valid (shown by many research groups)