Decahydronaphthalene

Endpoint summary

Description of key information

The biodegradability of decahydronaphthalene was evaluated in studies of van Ginkel (2015), CITI (1992), Biodynamics (1986), Evonik (2010) and Hüls (1997). Results showed that this substance is not readily biodegradable. However, in a prolonged Closed Bottle test (enhanced biodegradability test) according to ISO 10707 and OECD 301D using natural surface water the test substance is biodegraded in excess of 63% at day 42.

Several other studies report degradation rates by marine bacteria of up to 25%. The study of Prince et al (2008) also reports a fast degradation of all the components of the biodiesel B20, with estimated half-lives for decalins of < 30 days. Therefore, although not readily biodegradable according to the guideline requirements, decahydronaphthalene can be metabolized by microorganisms and it is expected to biodegrade in the environment. Furthermore, predictions based on the Biowin models indicate the possibility of the test item to degrade fast for the models Biowin 1, Biowin 2 and Biowin 6. The models Biowin 3 and Biowin 4 predict degradation within days to weeks and the model Biowin 5 and 7 predicted a non-fast degradation of the test item. Due to the uncertainties related to the above-mentioned available information, it was requested within the substance evaluation process (SEv) for PBT properties, a simulation test on ultimate degradation in surface water to access the properties of the test item and its biodegradation. The prolonged Closed Bottle test according to ISO 10707 and OECD 301D was not available at that time.

The aerobic mineralization of cis,trans-Decahydronaphthalene [1-14C] in surface water was determined in a simulation biodegradation test over a test period of 76 days according to OECD guideline 309.

Due to the inherent test item properties (poor water solubility, high volatility and adsorption potential) the standard OECD 309 test design was not applicable. For that reason, modifications had to be applied to set-up a reliable test design. However, due to critical test item properties and significant methodological deficiencies the study has to be regarded non reliable (Klimisch 3, see IUCLID study entry). To minimize volatilisation the study was carried out in closed vessels with reduced headspace.125 mL gastight headspace bottles with 80 mL surface water were used. Silicone oil AR 20 was used as solvent for the application. Furthermore, the silicone oil functioned as carrier to retain the test item in the test solution and to minimize volatilization of the test item.

The test replicates were incubated in the dark under aerobic conditions for 76 days under controlled laboratory conditions on an orbital shaker. The aerobic mineralization was determined by trapping and analysis of the evolved¹⁴CO2 in 1 mol/L NaOH. The radioactivity of the NaOH solution was determined by LSC.

Mineralization of the test item started after a lag phase of 20 days (1 µg/L) and 26 days (10 µg/L), respectively. The mineralization proceeded steadily until day 54 (1 µg/L) and day 62 (10 µg/L), respectively. Afterwards the plateau was reached. No radioactivity (all measured values << LOQ) was determined in the ethylene glycol traps of the test item replicates. Mineralization in the sterile controls was ≤ 0.14 % of applied radioactivity, indicating that abiotic degradation of the test item is negligible.

For the kinetic evaluation, the residual activity was calculated from the evolved¹⁴CO2 and the applied radioactivity (AR). The calculated half-life was 73.8 days for 1 µg/L and 72.5 days for 10 µg/L. The differences of the degradation first order rate constants k were statistically not significant; k is independent of the test concentration. As said before the study has severe methodological deficiencies and is regarded not reliable (Klimisch 3). Based  on the prolonged Closed Bottle test according to ISO 10707 and OECD 301D, decahydronaphthalene is not persistent.