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Environmental fate & pathways

Additional information on environmental fate and behaviour

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Endpoint:
additional information on environmental fate and behaviour
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment

Data source

Reference
Reference Type:
publication
Title:
The sulfur cycle of freshwater sediments: Role of thiosulfate
Author:
Jørgensen, BB
Year:
1990
Bibliographic source:
Limnol. Oceanogr., 35(6), 1990, 1329-1342

Materials and methods

Test guideline
Qualifier:
no guideline followed
Principles of method if other than guideline:
The study investigated the fate of thiosulfate (S2O32-) in anoxic river and lake sediments using radio isotopic labelling with focus on the metabolic process of S2O32-/ sulfite disproportionation by sulfate-reducing bacteria.
GLP compliance:
no

Test material

Specific details on test material used for the study:
Radiotracers: H235S was prepared from 35S elemental S (Amersham) by Cr reduction just before the experiments. S2O32- tracer was obtained from Amersham with either the inner or outer S atom labelled with 35S. The radiochemical purity was 99% for the outer label and 96-98% for the inner label. Carrier-free 35SO42- was obtained from the Isotope Laboratory, Risø, Denmark.

Results and discussion

Any other information on results incl. tables

SO42-concentrations on Odder River and Brabrand Lake water at the time of sampling amounted to approx. 450 µM with a maximum (>550 µM) at a depth of 1-2 cm, indicating a zone of intense sulfide oxidation, presumably caused by microbial activity. At deeper levels, i.e. at a depth of 4-10 cm, SO42-was gradually depleted, presumably due to SO42-reduction. Neither S2O32-nor free sulfide was detected in pore water (< 1 µM).

In both sediments investigated, S2O32-was consumed at a constant rate and depleted after 5-6 h.

 

- Odder river experiments:

In river sediments, S2O32-was consumed at a constant rate and depleted after 5-6 h, however with minor contribution of oxidative processes (approx. 7%). About 50% of the consumed S2O32-got reduced to SO42-, presumably by SO42-- reducing bacteria. Upon S2O32-depletion, the SO42-pool started to decrease again due toSO42-reduction. SO42-reduction processes, however, were relatively slow.

Regarding the H235S tracer addition, rapid conversion into both SO42-and S2O32-occurred by immediate oxidation after addition of the tracer. However, a total of only 10% of the added H235S tracer appeared in oxidized pools, whereas the rest was bound iron as FeS/FeS2.

 

- Lake Brabrand:

In lake Brabrand sediments, S2O32-was consumed at a constant rate and depleted after 2 h, again with minor contribution of oxidative processes (approx. 6%). During S2O32-consumption, the labelled sulphur was equally distributed between SO42-and reduced sulphur, with about 50% of the inner (oxidized) S atom of S2O32-truly being reduced to sulphide. Regarding SO42-, reduction proceeded at a constant, however relatively slow rate as seen in sediments of the Odder river.

 

Total conversion percentages of S2O32-and SO42-were further calculated as the sum of all relevevant processes, including oxidative and reductive processes as well as disproportionation:

S2O32- -> 72% H2S + 28% SO42-

H2S -> 47.5% recycled + 52.5% oxidized (i.e. SO42-), with “recycled” referring t H2S that was converted to S2O32-and then recycled to H2S by reduction and disproportionation.

It is hypothesized that continuous cycling of new and regenerated H2S will ultimately lead to oxidation to SO42-percentages of >28%. Regarding the H235S tracer addition, rapid conversion of H235S into both SO42-(34%) and S2O32-(66%) occurred.

Applicant's summary and conclusion

Conclusions:
The fate of different sulphur compounds, i.e. H2S, S2O32- and SO42- was monitored in freshwater sediments using radiotracers. Regarding the H235S tracer addition, rapid conversion of H235S into both SO42- (34%) and S2O32- (66%) occurred. In addition, thiosulfates were converted into 72% H2S and 28% SO42- with the resulting 72% H2S again being subject to oxidation into SO42- in subsequent reactions. It is therefore hypothesized that continuous cycling of new and regenerated H2S will ultimately lead to SO42- oxidation percentages of >>28% and an ultimate transformation to sulphate is likely to occur in freshwater sediments under anoxic conditions. The study however only focussed on the turnover to SO42-, S2O32- and total reduced sulphur pools (reduced sulphur = H2S, FeS, FeS2, S0), therefore ignoring potential other products such as sulphites or polythionates.