Diguanidinium carbonate

Biodegradation in water and sediment: simulation tests

Description of key information

Inherent, ultimate biodegradation

Key value for chemical safety assessment

Half-life in freshwater:

33 d

at the temperature of:

25 °C

Additional information

The relevant part of the submission item Guanidine carbonate is the Guanidine (CAS 113-00-8) as discussed in the section on Environmental fate and pathways above. It is present as the Guanidinium kation (see section Dissociation constant). The carbonate, which is fully mineralized, is negligible for the environmental assessment and disregarded in the following discussion.

For assessment data from other guanidine salts than the submission item are used (read-across), which is justified as follows. Guanidine carbonate (target chemical) and guanidine mononitrate or monohydrochloride (source chemical) dissociate in aqueous media to yield the Guanidinium kation and the respective anions. Therefore it is reasonable to discuss the effects of the ions separately. The environmental fate of the Guanidinium kation will be independent from accompanying inorganic anions, which cannot be degraded in the environment. Accordingly any data regarding dissolute guanidine salts of whatever inorganic anion may be used for read across.

Guanidine was primary degraded when incubated in stream water samples obtained near 2 nitroguanidine (CAS 556-88-7) pilot plant facilities following long lag periods. Complete guanidine dissipation in the water samples ranged from 20 days with a lag period of 11 days to 68 days with a lag period of 52 days (HSDB 2008, Mitchell 1987 Chemosphere). As the Guanidinium kation was followed analytically primary Degradation (Dissipation) times 50 % can be assumed to be ca. 15.5 to 60 days. Considering the chemical structure of the guanidinium kation primary degradation is likely to indicate complete degradation. This is experimentally evidenced by additional experiments of Mitchell (1987 Chemosphere), in which carbon-14 radiolabelled guanidinium kations were followed.

In the first experiment Mitchell (1987 Chemosphere) investigated the dissipation of the guanidinium kation in natural surface waters from different sources and seasons in a sufficiently documented publication, which meets generally accepted scientific standards and is considered reliable with restrictions (Klimisch 2). After addition of the 11 mg guanidinium/L as 20 mg guanidine mononitrate (CAS 506-93-4)/L and 0.2 % potassium phosphate buffer, pH 7, the waters were incubated with agitation at 25 °C in the dark. The initial bacterial titer was determined. Aliquots were withdrawn throughout the course of the experiments and assayed for the test item spectrofluorometrically. Autoclaved controls were employed to conclude on biotic degradation. The same experiments were conducted with addition of a readily accessible carbon source, which was added at 500 mg/L. Two such controls were performed, one with glucose and the other with arginine as carbon source. Beginning of disappearance of the guanidine kation was recorded after lag times from 11 to 52 days and was completed after 9 to 68 days. Considering that the primary degradation may be approximately linear, the DT50 can be approximated by adding the lag time and half the duration of the dissipation. This leads to 50 % dissipation times (DT50) between ca. 15.5 and 60 days, whereof the lower values are from probably adapted inocula as the sampling sites are assumed having received nitroguanidine (CAS 556-88-7) waste-water discharges (Hansen Creek and Kill Creek, Desoto, KA, U.S.A). During incubation with readily accessible carbon/energy sources, glucose or arginine (which bears a guanidine group) at 500 mg/L, the guanidinium kation was fully degraded within less than 3 or 2 days, respectively, in samples from all sites. This indicates that microorganisms capable of degrading guanidinium in the presence of the added carbon source are widely distributed.

In the second experiment taken from the same publication the concentration dependence of guanidinium biodegradation was investigated, in a sufficiently documented publication, which meets generally accepted scientific standards and is considered reliable with restrictions (Klimisch 2). The author incubated buffered natural surface water samples with 5 different guanidinium kation levels (added as 64 % guanidinium chloride) and 1 µCi carbon-14 guanidinium. The final guanidinium kation concentration in the 500 mL test volume were 0.0005, 0.01, 0.1, 1.0 and 10 mg/L. Controls consisted of water samples autoclaved prior to the addition of the chemicals. Surface water samples were obtained from two sites in the vicinity of Frederick, MD, U.S.A. (Carroll Creek and Monocacy river) considered out of the vicinity of nitroguanidine (CAS 556-88-7) releases. The percentages of guanidinium carbon converted to carbon dioxide were determined from levels of trapped radioactivity in each sample, less abiotic background, and the measured amount of total carbon-14 guanidinium added. Cumulative percentages of guanidinium converted to carbon dioxide were then scaled for plotting by multiplying them by the original guanidinium concentration for the same flask. Levels of carbon dioxide production after 1 and 10 days were less than 1 and 5 % respectively for all of the concentrations tested. The mineralisation rate did not exceed the initial rates in the two lower concentrations, while at the three highest concentrations of 0.1, 1.0 and 10 mg/L, inflections in carbon dioxide production occurred with continued incubation such that by the end of the experiment 77, 80 and 85 %, respectively, of the guanidinium carbon was converted to carbon dioxide. These levels were reached after 93, 75 and 75 days, respectively. Similar extensive mineralisation did not occur at or below 0.01 mg/L and developed more slowly at 0.1 mg/L than at the two higher concentrations. The Monocacy river mineralization patterns for were reproducible in surface water from a second site (Carroll Creek). From the graphically given results of the publication the degradation times for 50 % of the initial concentrations can be estimated to be ca. 70, 41 and 30 days at 0.1, 1.0 and 10 mg/L. Thus, the development of the microbial populations capable of enhanced guanidinium mineralization is related to the concentration of the kation. At higher concentrations, a biodegrading population could well develop that would ultimately effect the persistence of the kation. In conclusion the data evidence mineralisation of the guanidine-carbon to carbon dioxide in non-adapted surface water samples. The guanidinium kation can be expected to be biodegraded in environmental freshwaters with DT50 of ca. 70 to 30 days decreasing with increasing concentrations.

In summary the DT50 values of the submission item guanidine carbonate were reported in a range from 15.5 to 70 days, decreasing with concentration, adaptation of the microorganism community and the presence of carbon/energy sources, which were shown to reduce DT50 to values as low as 2 to 3 days, even in non-adapted microorganisms. Considering the omnipresence of guanidinium in the environment and the generally present additional carbon/energy sources for microorganisms, the DT50 for assessment is set to the geometric mean of the extrema 15.5 and 70 days and assessed to be DT50 = 33 days at 25 °C.

• HSDB Hazardous Substances Databank (2008). GUANIDINE, CASRN: 113-00-8, Number: 7603. Last Revision Date: 2008-08 -26, Last Review Date: Reviewed by SRP 5/8/2008, Update History: Complete Update on 2008 -08-26, 38 fields added/edited/deleted, Created 2007-12-13