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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Fast mineralisation (ultimate biodegradation) in soils,  DT50 from 2.5 to 17 days reported, DT90 from 49 to 70 days

Key value for chemical safety assessment

Half-life in soil:
11.8 d
at the temperature of:
21.1 °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.

Key study

The biodegradability of Guanidines among them the test item Guanidine carbonate (CAS 593-85-1) has been investigated by Lees & Quasel (1946) in a soil nitrification assay. No GLP or guideline were available at the time of the experiments but their publication is sufficiently documented and meets generally accepted scientific standards. The conclusive results are considered reliable (Klimisch 2), relevant, and adequate for the assessment. Wetland soils from a sparsely populated wetland area have been pre-incubated with Ammonium (NH4+) salt to induce growth of nitrifying bacteria up to saturation. Such pretreatment was considered to ensure conversion of Ammonium into Nitrate at a linear rate without any lag phase. The test item and Ammonium sulphate (CAS 7783 -20-2) as reference standard were loaded to 40 g soils per replicate by perfusion with 200 mL aqueous solution in concentrations corresponding to 0.00375 mol Nitrogen/L, which is 0.107 mg test item/L perfusion water or 535.5 mg test item/kg soil dw. The transformation was followed by colorimetric determination of nitrate in the elutriation solution after 1, 2, 3, 5, 8, 10, 14 and 16 days. The kinetics were compared with Ammonium sulphate as reference standard. The recovery of the reference substance as Nitrate was 114 %. Under conditions where Ammonium sulphate was completely nitrified in 2 days, Guanidine at equivalent concentrations of nitrogen was not fully nitrified in 16 days, and an initial lag-phase of at least 3 days is visible. Nonetheless the test item showed 40 % mineralization after 10 days and 80 % after 14 days of incubation. Based on this the DT50 roughly estimated as geometric mean of DT40 and DT80 is ca. 11.8 days. It is considered by the authors that a sigmoid rate of Nitrate production found in the perfusion of substance through a soil saturated with nitrifying bacteria is evidence that such substance must first be converted by other (non-stimulated) organisms into Ammonium, which can then be attacked by the nitrifiers. This was found for Guanidine carbonate. On the other hand, if the substance under investigation is converted at an immediate linear rate into nitrate this is evidence that it may be attacked directly by the nitrifying organisms, which is obviously not the case for Guanidine carbonate. In conclusion the nitrification, i.e. full mineralisation of the nitrogen in the test item is evidenced. However due to the enrichment of the nitrifying bacteria in the test soils the results cannot be directly compared to data obtained from OECD TG 307 studies. The presence of a lag-phase indicates nonetheless that the micro-organisms were not actually adapted to the test item and thus the results indicate rapid and ultimate biodegradation of Guanidine carbonate by soil micro-organisms.

Supporting studies

Mitchell (1987 Bull Environ contam Toxicol) studied the biotransformation of radiolabelled Guanidinium (as chloride) in soil(s) Duffield silt loam (pH 6.4 to 7.1), from Maryland, U.S.A. in different concentrations from 10 to 400 mg/kg soil dw (Guanidinium kation, 1 µCi carbon-14) and at 0.22 mg/kg soil dw additionally in Duffield silt loam pH 6.4-7.1 (soil 1) Hagerstown silt loam, pH 7.1 (soil 2) and Collington loamy fine sand, pH 6.3 (soil ). Furthermore the effects of added carbon/energy sources and mineral nitrogen compounds was observed and sterile controls (mercuric chloride or sodium azide or heat sterilization) or were used. The studies are sufficiently published in a sufficiently documented publication, which meets generally accepted scientific standards, and is considered reliable with restrictions (Klimisch 2). Incubation was made for 4 to 24 days under aerobic conditions at 20 ºC, and 60 % soil moisture. Guanidinium was applied at the rate of 0.22 mg a.i./kg soil dw. Samples were analysed to determine the mineralization percentages in the 0.22 mg/kg soil dw level at 0, 1, 2, 3 and 4 days of incubation. Up to 14 samples were taken until day 24 at levels of 10, 25, 50, 100, 200 and 400 mg/kg soil dw to evaluate the concentration dependency of degradation.

In the sterile controls containing the metabolic inhibitors mercuric chloride or sodium azide (0.1 %), mineralisation was inhibited by 73 and 97.5 % respectively. Essentially no guanidinium carbon was mineralised in heat killed controls. The mineralization studies carried out over a range of guanidinium concentrations in Duffield soil showed that the rates (24 hour) of guanidinium carbon mineralization increased with the concentration of the kation. The increases were not in proportion and tended toward saturation of the biodegrading system. The mineralization of guanidinium carbon with extended incubation at concentrations from 10 to 400 mg/kg soil dw were used to estimate the DT50, which was ca. 2.5, 5, 6, 8, 12 and 17 days at 10, 25, 50, 100, 200 and 400 mg/kg soil dw, respectively. In all cases tested, microbial activity ultimately resulted in the conversion of from 62 to 78 % of the guanidinium carbon to carbon dioxide until test end at 10 to 24 days. Mineralization rates showed no tendency to decrease at any concentration until most of the kation was degraded, and the concave nature of the curves apparent at 25 mg/kg soil dw and above indicates that rates could as much as double during continued exposure. Carbon added incubations showed increased 24-h mineralisation and 122.4 and 391.4 % were reached (compared to control without addition) for cellulose and glucose, respectively. Nitrogen added incubations showed decreased 24-h mineralisation and 91.3, 94, 85.5 and 91.7 % were reached (compared to control without addition) for Ammonium, Nitrate, Nitroguanidine and Composite, respectively. In conclusion the guanidinium kation shows mineralisation or ultimate biodegradation in soils with DT50 from 2.5 to 17 days. In presence of carbon/energy sources the mineralisation rate increases while it decreases when alternative nitrogen sources are available.

Praveen-Kumar & Brumme (1999) investigated the mineralization of a various Guanidines including the test item Guanidine carbonate (CAS 593-85-1) in soils. The experimental approach used appropriate elements of the OECD TG 307. No GLP conditions applied, but the experiments are reported with a sufficient level of detail. Accordingly the results are considered reliable (Klimisch 2). The study is relevant and adequate and thus conclusive. The test item was incubated in one sandy loam (soil 1) and two silt loam type soils (soils 2 and 3) with organic carbon levels of 85, 290 and 365 mg/kg dry soil, respectively. It was applied in two concentrations (spacing factor 5) of 100 or 500 mg Nitrogen/kg dry soil, which corresponds to 214 or 1072 mg test item/kg dry soil, plus control in 3 replicates each. The mineralisation was followed by measuring the increase of inorganic nitrogen in the samples taken on days 0, 1, 2, 5 and 7 and thereafter weekly. The decrease of the test item was followed until the nitrogen amount of the test item was completely recovered as inorganic nitrate or ammonium. The time required until complete mineralization is reported and can be regarded as degradation DT90. Furthermore the lag times until onset of degradation and the maximum rates of mineralization were recorded. The Carbon dioxide developed was trapped in the closed test systems, which were opened daily for gas exchange during 10-12 minutes. Carbon dioxide development indicates microbial respiration. The data were used to evaluate microorganism toxicity and are reported elsewhere. The level of 1072 mg test item/kg dry soil is considered toxic, thus excluded for DT90 determination. Lower concentration lag phases were 43.6 ± 1.5, 25.4 ± 0.9 and 19.7 ± 0.3 days in soils 1, 2 and 3, respectively. The values for the higher concentrations were 73.2 ± 1.4, 52.4 ± 0.7 and 40.6 ± 1.1 days in soils 1, 2 and 3, respectively. The maximum rates of mineralisation (Vmax) in mg Nitrogen/kg/week were 23.1 ± 0.6, 29.0 ± 0.6 and 34.2 ± 0.4 in soils 1, 2 and 3, respectively at the lower level and 98.9 ± 1.7, 142.6 ± 0.9 and 176.4 ± 1.9 in soils 1, 2 and 3, respectively at the higher level. The degradation DT90 values derived are 70, 46 and 49 day (lower concentration only). In conclusion the study evidences complete mineralization with DT90 between 49 and 70 days.

Finally in the study of Williams et al (1989) guanidine mononitrate (CAS 506-93-4) was rapidly and extensively mineralized during incubation in soils with adapted microorganisms. Approximately 68 % of the added 14C-Guanidine Nitrate was recovered as 14C-CO2 after 30 hours. The addition of nutrients (glucose or molasses), or incubation at test flask under aerobic versus anaerobic conditions, did not increase the mineralization.

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