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Environmental fate & pathways

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The ready biodegradability of dicyandiamide was investigated in an OECD 301E test (“Ready Biodegradability: Modified OECD Screening Test“). No DOC removal could be observed within 28 days. Therefore, dicyandiamide is not readily biodegradable based on screening level test results.

In contrast, soil degradation studies (laboratory and field soil degradation experiments) clearly demonstrate that dicyandiamide is degradable in soil through abiotic and biotic mechanisms.

Chemical breakdown of dicyandiamide is catalysed by metal oxides. Furthermore, dicyandiamide can be decomposed by soil bacteria.

Generally, degradation of dicyandiamide in soils depends on soil moisture and temperature. The main degradation products are guanylurea, guanidine, urea and ammonia.

Based on the experimental data a geometric mean of normalised DT50 soil values was derived, equalling 11.1days (20°C).

Due to its physicochemical properties (high water solubility, hydrophilicity, log Pow -1.0) dicyandiamide will not adsorb to the organic matter fractions of soil or sediment. Therefore, there is no potential for exposure of sediment, even in the event of direct release of dicyandiamide to aqueous systems (dicyandiamide will remain in the aqueous phase). Therefore, no sediment simulation testing is required.

The biological degradation of 1-cyanoguanidine (dicyandiamide) in natural surface water was studied using filtered pond water (pelagic test design) over a period of 61 days, according to OECD guideline 309. At study termination, evolved14CO2accounted for 0.5–1.1 % of the applied test material, and the concentration of parent compound was still close to 100 %. The mass balance ranged between 100.9 and 101.3 %. The degradation half-life (DT50) in surface water was calculated to be > 1000 days.

Overall, there is currently no evidence that 1-cyanoguanidine (dicyandiamide) is biologically degraded in the aquatic compartment. Nevertheless, metal oxide catalysed breakdown of dicyandiamide, as established for soil, is likely to take place also in surface waters, in view of the general presence of inorganic suspended matter in water bodies.

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