Decahydronaphthalene

Biodegradation in water and sediment: simulation tests

Link to relevant study record(s)

Reference

Endpoint:

biodegradation in water: simulation testing on ultimate degradation in surface water

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

2016-08-01 to

Reliability:

3 (not reliable)

Rationale for reliability incl. deficiencies:

significant methodological deficiencies

Remarks:

- Decahydronaphthalene is an extremely volatile substance that is very difficult to maintain in water. It may also sorb to glassware and other surfaces, and it does not appear that efforts have been made to extract these surfaces. A lack of mass balance measurement in this study is therefore very concerning, since an incomplete mass balance would imply that any amount of the test substance that would have been lost from the system is not accounted for and that half-lives based on mineralisation would be underestimating the degradation of the substance. - OECD 309 requires, if necessary, stock solutions of non-volatile substances with poor water-solubility should be prepared by use of a volatile organic solvent and the solvent should be stripped off to an extremely small quantity. Decahydronaphthalene is volatile what makes the accomplishment of the study complicated and requires additional modifications. The chosen solvent silicone oil instead is non-volatile and remained in the system. - By dosing the test compound in silicone oil, a third phase was introduced into the test system that the test compound would preferentially partition into. When solvents are used in the OECD 309 test, they should be water miscible in order to ensure that the test substance is effectively dissolved in the water. This is a critical issue with the study, as the amount of decahydronaphthalene partitioned into silicone oil would be unavailable for biodegradation. The use of silicone oil most likely led to a reduction of the overall degradation and mineralization rate measured in the study. - The degradation rate is based on mineralisation rather than parent degradation – this would also lead to overestimating the half-life of the test substance. - Suitability of ethylene glycol traps seems very unlikely given the volatility of the test substance. The trap was composed of ethylene glycol (log Kow = -1.36), which is not expected to be a suitable absorber of decalin. Therefore, it is not surprising that decalin was not found in these traps, and this along with the lack of mass balance means that the fate of the decalin in this experiment is highly uncertain. - The high agitation rate of 150 rpm potentially impacts partitioning into water phase and further volatilization into headspace, it remains unclear to what extent. - The treatment and storage of surface water was not in line with the guideline and at summer temperatures you cannot exclude impact on the inoculum e.g. further increase of temperature during storage or shortly anaerobic conditions.

Qualifier:

according to guideline

Guideline:

OECD Guideline 309 (Aerobic Mineralisation in Surface Water - Simulation Biodegradation Test)

GLP compliance:

yes (incl. QA statement)

Specific details on test material used for the study:

Batch Number 160708
Specific activity 55 mCi/mmol
Test concentrations 10 µg/L and 1 µg/L
Stock solution 10 µg/L: 147.8 kBq test item/mL silicone oil AR 20
1 µg/L: 13.8 kBq test item/mL silicone oil AR 20
Activity in the test solution 11.83 kBq / 80 mL and 1.174 kBq / 80 mL

Radiolabelling:

yes

Oxygen conditions:

aerobic

Inoculum or test system:

natural water: freshwater

Details on source and properties of surface water:

Natural surface water (field fresh sampled) of the river ALTE LEINE.

Origin Sampling address: K224 Redener Strasse, 30982 Pattensen, Germany
Coordinates: 52°16’50.312'' N 9°47’28.513'' E

The sampling site was selected with respect to the regional biological and chemical water quality maps (interactive online version) of the Lower Saxony Water Management, Coastal Defence and Nature Conservation Agency (German: NLWKN). The sampling site is classified as unpolluted and is located in a water protection area. The surface water meets the recommendations of the guideline.

Collection / Sampling The water was collected from the surface of the river at an undisturbed recess. The depth of sampling and the appearance of the water sample were documented at field sampling. Furthermore temperature, pH value and O2 concentration of the water were measured at field sampling.

Handling The water was clear, further pretreatment was not necessary.

Date of sampling 2016-08-02

Storage Storage at 4 ± 2 °C was not necessary as the study was started within one day after collection of the water.

Duration of test (contact time):

76 d

Initial conc.:

10 µg/L

Based on:

other: measured applied radioactivity

Initial conc.:

1 µg/L

Based on:

other: measured applied radioactivity

Parameter followed for biodegradation estimation:

radiochem. meas.

Details on study design:

Due to the physico-chemical nature of the test substance, the test design was modified:

Test vessels
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. This test design is successfully used for the OECD 310 test and proved suitable for volatile substances. Furthermore the headspace of 45 mL ensures that sufficient oxygen is available within a max. test duration of up to 90 days.

Application Procedure
Due to the low solubility and small test item amount required per replicate the set-up of a suitable application procedure was challenging. Direct application was not feasible due the required small test item volume per replicate.
Initially it was considered to prepare the intended test concentrations of 1 µg/L and 10 µg/L with an aqueous stock solution of 500 µg/L. 500 µg/L is below the water solubility, but corresponding investigations showed that the preparation was not feasible with the obligatory small test item amounts and volumes.

For that reason, the use of a solvent or inherent carrier was obligatory.
At first, the use of a solid carrier like silica gel or organic material (sterilized soil) was investigated. Due to the small amounts of silica gel or soil necessary per replicate, the use of a solvent was required for the application of cis,trans-Decahydronaphthalene [1-14C] on the silica gel and soil, respectively. The subsequent removal of the solvent was considered not practicable, as the test item would be stripped off as well, while removing the solvent.
However it was investigated to apply the test item with solvent but not to remove the solvent from the silica gel and soil after application. These investigations revealed that the mineralization of cis,trans-Decahydronaphthalene [1-14C] was significantly retarded by the solvent.

Eventually the use of an inert carrier was evaluated. Recent research to improve the bioavailability of volatile and poorly soluble substances established the use of non-toxic, inert silicone oil (2). Silicone oil AR 20 (Polyphenyl-methylsiloxane) is non-biodegradable and therefore not affecting the biodegradation kinetic of the test item.
The suitability of silicone oil AR 20 as solvent for application and as carrier to retain the test item in the test solution and therefore minimizing volatilization, was investigated.
The investigations indicated that volatilization from the surface water was negligible. Therefore silicone oil AR 20 was used as carrier in the definite study.
As the test item was not completely dissolved in the surface water separate test replicates were prepared for each sampling.

Reference substance:

aniline

Remarks:

As a solvent control the reference item was treated with the same amount of silicone oil AR 20 as used for the application of the test item.

Test performance:

Measurements of pH and oxygen concentration in the test system (blank control) was done at least every 14 days until day 54 and at test end on day 76. The oxygen concentration was > 7.8 mg O2/L throughout the duration of the study, the pH remained in the range 7.92 – 8.38.

Key result

% Degr.:

>= 26.1 - <= 39.8

Parameter:

CO2 evolution

Sampling time:

76 d

Remarks on result:

other: Due to the analysis of separate (independent) replicates at each sampling, variations between sampling days are higher than sampling from the same test solution.

Remarks:

Test item concentration 1 µg/L

Key result

% Degr.:

>= 11.8 - <= 37.3

Parameter:

CO2 evolution

Sampling time:

76 d

Remarks on result:

other: Due to the analysis of separate (independent) replicates at each sampling, variations between sampling days are higher than sampling from the same test solution.

Remarks:

Test item concentration: 10 µg/L

Key result

Compartment:

natural water: freshwater

DT50:

73 d

Type:

(pseudo-)first order (= half-life)

Temp.:

20 °C

Other kinetic parameters:

first order rate constant

Transformation products:

not measured

Evaporation of parent compound:

no

Volatile metabolites:

no

Results with reference substance:

The mineralisation of the reference item Aniline sulfate, [14C(U)]- in the reference control and solvent control was > 75 % already on day 5. This high mineralization indicates that the surface water contained an active microbial population and that the solvent silicone oil AR 20 did not cause adverse effects on the activity of the microbial population.

Validity criteria fulfilled:

yes

Remarks:

Calculation of the total recovery (mass balance) was not applicable due to the necessary modifications of the test design. As the mineralization was determined directly by 14CO2 analysis, the calculation of the half-life (t1/2) was not affected.

Conclusions:

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.

Executive summary:

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.

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 evolved14CO2in 1 mol/L NaOH. The radioactivity of the NaOH solution was determined by LSC.

Mineralisation of the test item started after a lag phase of 20 days (1 µg/L) and 26 days (10 µg/L), respectively. The mineralisation 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. The test item did not volatilize and was bioavailable in the test solution throughout the duration of the study. Mineralisation 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 evolved14CO2and the applied radioactivity (AR).

Additional information

A simulation test in surface water according to OECD 309 was requested by ECHA within the context of a CORAP evaluation for PBT properties.

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 evolved ¹⁴CO₂ in 1 mol/L NaOH. The radioactivity of the NaOH solution was determined by LSC.

Mineralisation of the test item started after a lag phase of 20 days (1 µg/L) and 26 days (10 µg/L), respectively. The mineralisation 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. Mineralisation 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 evolved ¹⁴CO₂and the applied radioactivity (AR).