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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Reference
Endpoint:
biodegradation in water: sediment simulation testing
Remarks:
Degradatio in in water and in sediment tested
Type of information:
experimental study
Adequacy of study:
key study
Study period:
no data
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions
Remarks:
Meets generally accepted scientific standards and acceptable for assessment. Degradation was investigated in marine water and sediment samples from a range of different sites. No standardised test methods were used. Any conclusions regarding degradation rates cannot be drawn. Extrapolation to freshwater habitats is not possible.
Qualifier:
equivalent or similar to guideline
Guideline:
other: no guideline reported
Principles of method if other than guideline:
Degradation in nano- to microgram range.
Two phases:
Phase 1: Determination of degradation under environmentally relevant conditions (samples were taken from several sampling points along the river Elbe, North Sea and Baltic Sea).
Phase 2: Determination of ultimate biodegradation and of degradation at lower temperatures, varying levels of salinity and at different test substance concentrations.
14C radiolabelled test material
The river Elbe was chosen since representing a biotope with anthropogenic influence reflected by the concentration of heavy metals, chlorinated pesticides, polychlorinated biphenyls (PCB) and other organic substances. The Baltic Sea was chosen due the more pristine character of this ecosystem with less industrial influence. Contaminants predominantly originate from agricultural use. Therefore, different degradation processes and timescales were expected.
Non-labelled and labelled thiourea was tested.
GLP compliance:
not specified
Radiolabelling:
yes
Oxygen conditions:
aerobic
Inoculum or test system:
natural water / sediment
Details on source and properties of surface water:
- Details on collection (e.g. location, sampling depth, contamination history, procedure):
1) Locations:
* 3 sampling points along the river Elbe: Hamburg-Teufelsbrück, Mühlenberger Loch, Cuxhaven
* 2 sampling points in the North Sea: Elbe I, Helgoland
* Baltic Sea sampling points: - Kiel Fjord (shipping pier of the Institute of Oceanography, and a sampling point in the proximity of Bülk close to the STP Bülk), Kiel Bay, Gabelsflach, Schlei estuary (relatively high eutrophication), Trave estuary, Lübeck Bay, Oderbank

2) Sampling: sampling was done using sterilized equipment

3) Contamination history:
The river Elbe and its estuary served as example for a highly contaminated habitat.
Details on source and properties of sediment:
- Details on collection (e.g. location, sampling depth, contamination history, procedure):
1) Locations:
* 3 sampling points along the river Elbe: Hamburg-Teufelsbrück, Mühlenberger Loch, Cuxhaven
* 2 sampling points in the North Sea: Elbe I, Helgoland
* Baltic Sea sampling points: - Kiel Fjord (shipping pier of the Institute of Oceanography, and a sampling point in the proximity of Bülk close to the STP Bülk), Kiel Bay, Gabelsflach, Schlei estuary (relatively high eutrophication), Trave estuary, Lübeck Bay, Oderbank

2) Sampling depth: surface sediments (-> aerobic conidtions)

3) Sampling procedure: Van Veen grab for sandy sediments, and "Rumohr-Lot" for silty sediments

4) Contamination history:
The western Baltic Sea can be regarded as less polluted.
Details on inoculum:
Naturally occurring microorganisms in water/sediment samples.
Duration of test (contact time):
>= 24 - <= 83 d
Initial conc.:
1 µg/L
Based on:
test mat.
Parameter followed for biodegradation estimation:
CO2 evolution
Details on study design:
A) CONDITIONS for TESTS on biodegradation in WATER:
- Volume of test solution/treatment: 200-1000 mL (dependent on bacterial count)
- Test temperature:
* samples from Elbe and North sea: preliminary test at 20 °C; main test at 10 °C (with radiolabelled material)
* Baltic sea samples: generally 20 °C (to enhance degradation reaction)
- pH adjusted: no
- Continuous darkness: yes (to avoid photdegradation and algal growth)
- 3 to 5 replicates

TEST SYSTEM
- Method used to create aerobic conditions: Agitation

CONTROL AND BLANK SYSTEM
- Inoculum blank: no data
- Abiotic sterile control: Yes (1-3 replicates)
- Toxicity control: no
- Other:

STATISTICAL METHODS:
- The possibility of nonlinear least-squares regression for estimation of bacterial growth and initial population density is briefly discussed. However, no corresponding results are presented.

B) CONDITIONS for TESTS on biodegradation in SEDIMENT:
- Sediment volume: 4 cm³ (7.34 ± 0.3 g dry weight) with 100 mL sterile water from the same sampling site.
- Amount of test substance added to sediment: 8 ng
- Test apparatus: Biometer flasks (Bellco Glass) consisting of an incubation vessel and a side arm. The side arm has a seperate opening that is closed with a stopper and a cannula and can be used to change the NaOH solution and measure the production of CO2.
- Ventilation was provided via a short cannula closed with cotton wool in the stopper at the top of the incubation vessel.
Parent/product:
parent
Compartment:
sediment
Key result
% Degr.:
>= 40 - <= 70
Parameter:
CO2 evolution
Sampling time:
83 d
Remarks on result:
other: Sediment samples from Elbe estuary: Hamburg-Teufelsbrück, Cuxhaven, Elbe I, Helgoland.
Parent/product:
parent
Compartment:
sediment
Key result
% Degr.:
75
Parameter:
CO2 evolution
Sampling time:
30 d
Remarks on result:
other: Sediment sample from Oderbank (D, Sediment; Baltic Sea).
Key result
Compartment:
other: marine sediment
DT50:
>= 17 - <= 28 d
Type:
not specified
Temp.:
10 °C
Remarks on result:
other: DT50 based on mineralisation to CO2; sampling sites: Helgoland (DT50 = 17 d) and Elbe I (DT50 = 28 d; marine location); DT50 estimated based on degradation graphs (see "Illustration" below)
Key result
Compartment:
other: marine sediment (Baltic Sea)
DT50:
>= 9 - <= 33 d
Type:
not specified
Temp.:
20 °C
Remarks on result:
other: DT50 based on mineralisation to CO2; Baltic Sea sampling sites: Oderbank (D, DT50 = 9 d) and Lübeck Bay (C, DT50 = 33 d); DT50 estimated based on degradation graphs (see "Illustration" below)
Transformation products:
yes
No.:
#1
Details on transformation products:
not applicable
Evaporation of parent compound:
not measured
Volatile metabolites:
yes
Residues:
not specified
Details on results:
No additional information reported.
Results with reference substance:
Not applicable.

1) Elbe estuary


Thiourea biodegradation is more extensive and faster in habitats with lower nitrogen (N) content. Therefore higher thiourea biodegradation can be found in the saltwater samples from the Elbe estuary (e.g. annual average of the total N-content at the area “Großer Vogelsand/Scharhörn”: 1.6 mg/L, Elbe I and Helgoland: < 0.5 mg/L) compared to highly eutrophic freshwater sites (e.g. annual average of the total N-content at Hamburg-Teufelsbrück, 1988: 6.2 mg/L). Thiourea is utilised as N source. Its mineralization proceeds slowly and continuously and takes place in parallel to degradation of other carbon (C) sources. The same relationship could be observed for the sediment samples: Thiourea biodegradation increased with increasing distance from the coast.


2) Baltic Sea


Biodegradation of thiourea strongly differed between different sampling points: In the surface water samples from the Trave estuary (A, 1m) thiourea was completely and rapidly mineralised. The test substance was presumably consumed by bacteria that otherwise degrade urea. These bacteria exist predominantly in wastewater and are regarded as tracers for water pollution, as urea is considered to be an anthropogenic pollutant. Thiourea competitively inhibits the absorption of urea, thus the same transport mechanism is used.


In the water samples taken close to the sediment layer from the Trave estuary (A, sw) biodegradation of thiourea was much slower compared to the water sample A, 1m. This may be attributed to either a reduced number of degrading bacteria that could immediately utilise thiourea (however, after 40 days thiourea biodegradation increased), or to the higher amount of N-sources in the water layer close to the sediment.


In the water sample from the Lübeck bay (B, 1m) thiourea was almost as rapidly used as in the underlying sediment sample (B, sediment). However, the extent of mineralisation was much higher in the water sample due to a lower N-content.


In the sediment sample B only 24 % of thiourea was mineralised, but only 41 % of thiourea could be found in the water phase above the sediment layer. This indicates adsorption of 35 % in the silty sediment.


In sediment sample C, 52 % of thiourea was mineralised, with a maximum of 13 % adsorbed to the sediment, as 35 % of the test substance could be found in the water phase.


At station D (Oderbank) which is influenced by the river Oder 72 % mineralisation of thiourea could be measured in the respective sediment sample (D, sediment). Unfortunately, no water sample could be analysed for thiourea biodegradation. Sediment sample D showed almost exactly the same biodegradation behaviour as water sample A, 1m. Analysis of the water layer above the Oderbank sediment revealed high nutrient contents. However, in this case the high NH4-N content did not inhibit thiourea mineralisation.

Validity criteria fulfilled:
not applicable
Conclusions:
The biodegradability of thiourea differed strongly between habitats. Overall, the biodegradation of thiourea (i.e. complete mineralisation to CO2) ranged between 28 % (after 70 d of incubation) and 87.3 % (after 14 days of incubation).
The experiment demonstrates that thiourea serves as nitrogen (N) source for degrading microorganisms. Thiourea is more easily biodegraded in N-limited environments.
Executive summary:

The department of microbiology of the Institute for Physical Oceanography in Kiel conducted a three year research project on the microbial degradability of xenobiotics at low, environmentally relevant concentrations in the aquatic compartment. One of these chemicals was thiourea.


In the estuary of the river Elbe as well as in the North Sea and the Baltic Sea the mineralisation of thiourea by natural microbial communities in water and sediment samples was studied at a concentration of 1 µg/L. The river Elbe and its estuary served as an example for a highly contaminated habitat, whereas the western Baltic Sea can be regarded as less polluted.


Water and sediment samples (3 to 5 replicates) from the Elbe estuary and from the Baltic Sea spiked with 1 µg/L radiolabelled 14C-thiourea were incubated in the dark at 10 and 20 °C, respectively, for up to 85 days. The biodegradation of thiourea was measured by quantitative determination of CO2 released from the test vessels. An abiotic sterile control sample was included in the test setup to account for abiotic degradation processes.


The biodegradability of thiourea differed strongly between habitats. Overall, the biodegradation of thiourea ranged between 28 % (after 70 d of incubation) and 87.3 % (after 14 days of incubation).


The experiment demonstrates that thiourea serves as nitrogen (N) source for degrading microorganisms. Thiourea is more easily biodegraded in N-limited environments. Therefore, in the Elbe estuary and in the North Sea thiourea biodegradation increases with increasing distance from the coast. At all sampling points of the Elbe estuary and the North Sea the mineralisation of thiourea proceeded slowly and continuously and took place in parallel to degradation of other carbon (C) sources.


This correlation between availability of N and biodegradation of thiourea is further supported by the results from the Baltic Sea samples: Biodegradation decreases with increasing content of N-sources. In addition, the water sample from the Trave estuary and the sediment sample from Oderbank showed high biodegradation of 87.3 % and 75 % within 14 days and 30 days, respectively. This rapid and extensive biodegradation may be attributed to bacteria that previously degraded urea and which predominantly occur in polluted wastewater.

Description of key information

The biodegradability of thiourea differed strongly between habitats.


Overall, the biodegradation of thiourea ranged between 28 % (after 70 d of incubation) and 87.3 % (after 14 days of incubation).
The experiment demonstrates that thiourea serves as nitrogen (N) source for degrading microorganisms. Thiourea is more easily biodegraded in N-limited environments.
Microorganisms in activated sewage sludge are able to completely biodegrade thiourea after adaption to the substance. Degradation products are nitrate and sulphate.

Key value for chemical safety assessment

Half-life in marine water:
29.5 d
at the temperature of:
20 °C
Half-life in marine water sediment:
22.5 d
at the temperature of:
20 °C

Additional information

In a three year project the department of microbiology of the Institute for Physical Oceanography in Kiel investigated the microbial degradability of thiourea at low, environmentally relevant concentrations (1 µg/L radiolabelled 14C thiourea) in samples taken from the estuary of the river Elbe as well as from the North Sea and the Baltic Sea. The river Elbe and its estuary served as an example for a highly contaminated habitat, whereas the western Baltic Sea can be regarded as less polluted.


Water and sediment samples from the Elbe estuary and from the Baltic Sea were incubated in the dark at 10 and 20 °C, respectively, for up to 85 days. The biodegradation of thiourea was followed by quantitative determination of CO2 released from the test vessels. An abiotic sterile control sample was included in the test setup to account for abiotic degradation processes.


The biodegradability of thiourea differed strongly between habitats. Overall, the biodegradation of thiourea ranged between 28 % (after 70 d of incubation) and 87.3 % (after 14 days of incubation).


The experiment demonstrates that thiourea serves as nitrogen (N) source for degrading microorganisms. Thiourea is more easily biodegraded in N-limited environments. Therefore, in the Elbe estuary and in the North Sea thiourea biodegradation increases with increasing distance from the coast. At all sampling points of the Elbe estuary and the North Sea the mineralisation of thiourea proceeded slowly and continuously and took place in parallel to degradation of other carbon (C) sources.


This correlation between availability of N and biodegradation of thiourea is further supported by the results from the Baltic Sea samples: Biodegradation decreases with increasing content of N-sources. In addition, the water sample from the Trave estuary and the sediment sample from Oderbank show a high biodegradation of 87.3 % and 75 % within 14 days and 30 days, respectively. This rapid and extensive biodegradation may be attributed to bacteria that had previously degraded urea and which predominantly occur in polluted wastewater.


Degradation half-lives (DT50) based on mineralisation to CO2 were derived from the degradation graphs presented in the study. The DT50 in marine water and sediment samples taken from the North Sea ranged from 20 to 39 days and 17 to 28 days, respectively. These samples were incubated at 10 °C and are therefore considered to be more representative for environmental conditions as the results obtained with Baltic Sea samples that were incubated at 20 °C. The hazard assessment is based on the mean value of the mineralisation half-lives reported above (marine water: t1/2 = 29.5 d, marine sediment: t1/2 = 22.5 d).


In a supporting study (Tomlinson et al., 1966), long-term inhibitory effects of thiourea on activated sludge microorganisms were investigated using automatic fill-and-draw activated sludge units for 110 days in total. With increasing thiourea concentrations applied in the test the effluent concentrations of inorganic nitrogen and sulphur increased also. The authors additionally established a detailed mass balance for these elements when the units were receiving 76 mg/L thiourea and compared it with mass balances for the controls and units receiving less thiourea. They were able to demonstrate that the increases of inorganic nitrogen and sulphur could be attributed to all of the thiourea having been biologically oxidised, yielding equivalent amounts of sulphate and nitrate. These results demonstrate that also sewage sludge microorganism are capable of degrading thiourea.

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