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Environmental fate & pathways

Biodegradation in water: screening tests

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L’Haridon (2003)
CG van Ginkel, CM Plugge and CA Stroo (1995) Reduction of chlorate with various energy substrates and inocula under anaerobic conditions. Chemosphere 31 4075-4066
Bryan, E.H. & Rohlich, G.A. (1954) Biological reduction of sodium chlorate as applied to measurement of BOD. Sew.Wastes 26 11 1315-1324
HA Painter and EF King (1983) Reg Toxicol Pharmacol 3 144-151

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Ready biodegradability tests

Biotic conversions of sodium chlorate, an inorganic substance should not be assessed in standard OECD TG 301 tests for ready biodegradability, and OECD TG 302 tests for inherent biodegradability because these tests only detect biodegradation of organic compounds under aerobic conditions. 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 byand 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 biodegrable.  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.