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Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Additional information

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. 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 chloratehas 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 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)