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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Guanidine chloride is inherently biodegradable. This was shown in a non guideline, read-across study with Guanidine nitrate (Mitchell 1987). Test conditions especially test substance concentration and bacteria concentration are comparable to guideline OECD301, inoculum is expected to be adapted.

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable, fulfilling specific criteria

Additional information

Guanidine chloride is inherently biodegradable.The relevant study is a read-across study, made with Guanidinium nitrate. Read-across is justified as both salts are completely dissociated and for both salts the Guanidinium ion is the component to determine velocity of biodegradation (see detailed justification below).

Mitchell (1987) followed the biodegradation of Guanidinium nitrate in different surface water samples by chemical analysis. No guideline was followed but the test conditions were similar to OECD 301 with the exception that no inocculum but the bacteria naturally occurring in river water were used. The inoculum is expected to be adapted as it is originated downstream an ammunition factory. For this reason, the substance is classified as inherently instead of readily biodegradable.

The test substance concentration (20mg/l) and bacteria concentration (105CFU/ml) were comparable to OECD Guideline 301. Without the addition of a carbon source, only little decrease of Guanidinium could be observed for the first 11 days of incubation. After this time period a degradation rate of approximately 0.07mg/L/hour of Guanidinium was detected and after 20 days Guanidinium was not detectable anymore. The parallel test of a sterilised batch gave no degradation of Guanidinium. In the presence of 500mg/L glucose, after the first day of incubation a significant decrease of Guanidinium could be detected and after 2 to 3 days the level decreased under the detection limit. Degradation half-life is calculated to be 5 days.

According to this reliable study Guanidinium nitrate is inherently biodegradable.

The ready biodegradation of Guanidine hydrochloride was investigated in a study conducted according to OECD Guideline 301 C (Ready Biodegradability: Modified MITI Test) over a period of 33 days and using activated sludge as inoculum (Alzchem 1990). The biodegradation rate was determined by measurement of oxygen consumption. Inoculum blank and control with the reference substance sodium acetate were performed. The study is regarded as reliable without restrictions and satisfies the guideline requirements for ready biodegradation.

The test item proved to be not biodegradable under the test conditions.

The functional control reached the pass level >60% after 14 d. The test item does not influence the degradation of Sodium acetate at 30mg/l (toxic control).

The biodegradability of Guanidine chloride is supported by positive biodegradation study results in simulation tests in water (Mitchell in Chemosphere (1987)) and soil (Mitchel in Bull. Environ. Contam. Toxicol. (1987).).

As is usual in ecotoxicology for this endpoint, the positve biodegradation screening test with river water superimposes the negative result according to AlzChem (1990). This proposition is supported by the Reach guideline R.7B, May 2008 (version 1.1), R. Laboratory data on degradation/biodegradation, BIODEGRADATION, READY BIODEGRADABILITY (page 176): "Realising that ready biodegradability tests may sometime fail because of the stringent test conditions, in general, and the differences among the individual tests in terms of their stringency, consistent positive test results from test(s) should generally supersede negative test results."

Justification for read-across:

Guanidine hydrochloride and guanidine nitrate dissociate in aqueous media to yield the guanidine ion and the respective anion. Therefore it is reasonable to discuss the effects of the ions separately. The chloride ion is a naturally occurring essential ion in human beings with well-known metabolism and mechanisms of action as described in standard textbooks on pharmacology and physiology. As well it is found as salt in the Earth´s crust and is dissolved in seawater. Effects of guanidine hydrochloride are expected to be based primarily on the guanidine ion. The physiological processing of the guanidine ion is expected to be independent of the individual source. Therefore read-across from guanidine hydrochloride for effects of guanidine dissociated from guanidine nitrate is considered valid. This strategy is supported by a quite similar toxicological profile of both substances, as shown in acute toxicity, irritation, sensitization and genotoxic studies.

A more detailed justification is attached and outlined in CSR chapter 1.1.2 as well.