Magnesium dihydrogen disulphite

Additional information on environmental fate and behaviour

Administrative data

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

Test material information

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

SO₄^2-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, SO₄^2-was gradually depleted, presumably due to SO₄^2-reduction. Neither S₂O₃^2-nor free sulfide was detected in pore water (< 1 µM).

In both sediments investigated, S₂O₃^2-was consumed at a constant rate and depleted after 5-6 h.

 

- Odder river experiments:

In river sediments, S₂O₃^2-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 S₂O₃^2-got reduced to SO₄^2-, presumably by SO₄^2-- reducing bacteria. Upon S₂O₃^2-depletion, the SO₄^2-pool started to decrease again due toSO₄^2-reduction. SO₄^2-reduction processes, however, were relatively slow.

Regarding the H₂³⁵S tracer addition, rapid conversion into both SO₄^2-and S₂O₃^2-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, S₂O₃^2-was consumed at a constant rate and depleted after 2 h, again with minor contribution of oxidative processes (approx. 6%). During S₂O₃^2-consumption, the labelled sulphur was equally distributed between SO₄^2-and reduced sulphur, with about 50% of the inner (oxidized) S atom of S₂O₃^2-truly being reduced to sulphide. Regarding SO₄^2-, reduction proceeded at a constant, however relatively slow rate as seen in sediments of the Odder river.

 

Total conversion percentages of S₂O₃^2-and SO₄^2-were further calculated as the sum of all relevevant processes, including oxidative and reductive processes as well as disproportionation:

S₂O₃^2- -> 72% H₂S + 28% SO₄^2-

H₂S -> 47.5% recycled + 52.5% oxidized (i.e. SO₄^2-), with “recycled” referring t H2S that was converted to S₂O₃^2-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 SO₄^2-percentages of >28%. Regarding the H₂³⁵S tracer addition, rapid conversion of H₂³⁵S into both SO₄^2-(34%) and S₂O₃^2-(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.