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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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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.

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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.