Hydrogen sulphide

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.03 µg/L

Assessment factor:

50

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

0.19 µg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.003 µg/L

Assessment factor:

500

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

1.33 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

no exposure of sediment expected

Sediment (marine water)

Hazard assessment conclusion:

no exposure of sediment expected

Hazard for air

Air

Hazard assessment conclusion:

PNEC air

PNEC value:

7 mg/m³

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no exposure of soil expected

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

Sediment and soil toxicity

The PNEC sediment for hydrogen sulphide (H₂S) is not calculated for two reasons, as mentioned in REACH Annex XI §2 and §3: 1) no significant exposure of sediment is expected from industrial uses under REACH, especially in light of other natural and anthropogenic source of sulphides, and therefore it is not deemed to be a compartment of concern for this risk assessment, 2) testing toxicity to benthic organisms is technically not feasible.

1. No significant exposure of sediment to H₂S is expected

Migration of H₂S from water column to sediments is unlikely, due to its high volatility (Henry constant > 17000 Pa.m³.mol^-1) and very reactivity: H₂S can be oxidized in less than an hour in seawater according to Treatise on Geochemistry 2003; and according to Hansen et al., 1978 and Jorgensen, 1982, 1984 “In sediments, about 90% of the sulfide generated from sulfate reduction is oxidized, mostly through microbial activity” cited from Bagarinao (1992).

Moreover, sulfides concentration of 300 to 1900 µg/L have been measured in freshwater sediment pore water, and 300 µg/L to 120 mg/L in marine sediment pore water (as reported in review from Bagarinao, 1992)

Assuming that this minimum concentration of 300 µg/L in sediment pore water corresponds to the background concentration of sulphides, it may be concluded that the unlikely contribution to sediment compartment from H₂S uses under REACH (less than 100 tpa H₂S used) will be negligible compared to these levels. Furthermore, one can reasonably assume that the benthic species are already adapted and resistant to H₂S potential toxicity. Indeed several adaptation mechanisms of benthic species have been described by Bagarinao (1992).

2. Testing toxicity to benthic organisms is technically not feasible

It is technically not feasible to measure toxicity to benthic organisms using standard methods (like OECD guidelines), because of its high volatility (Henry constant > 17000 Pa.m³.mol^-1) and because it is readily oxidized in water, as well as the lack of oxygen induced by the presence of sulfides, as explained by Wang and Chapman (1999). It is therefore not possible to maintain sulfide and/or dissolved oxygen concentrations during a test, even under flow-through conditions.

 

Soil toxicity

The PNEC soil for hydrogen sulphide (H₂S) is not calculated, based on REACH Annex XI §2, because no significant exposure of soil is expected from industrial uses under REACH. Indeed, H₂S is highly volatile (Henry constant > 17000 Pa.m³.mol^-1) so it is expected to volatilize completely during WWTP aeration treatment. Even assuming unlikely presence of H₂S in sludge residues, it can reasonably be assumed that it would volatilize before being spread on soil via sludge spreading. And in the unlikely eventuality that some H₂S reach the terrestrial compartment, it can equally be assumed that it would volatilize rapidly before causing any toxicity.

References:

-       Bagarinao, T. (1992). Sulfide as an environmental factor and toxicant: tolerance and adaptations in aquatic organisms. Aquatic Toxicology 24(1-2), 21-62.

-       Hansen, M.H., K. lngvorsen and B.B. Jorgensen, 1978. Mechanisms of hydrogen sulfide release from coastal marine sediments to the atmosphere. Limnol. Oceanogr. 23.68-76.

-       Jorgensen. B.B., 1982- Ecology of the bacteria of the sulphur cycle with special reference to anoxic/oxic interface environments. Phil. Trans. R. Sot. London B 298, 543-561.

-       Jorgensen B.B., 1984. The microbial sulfur cycle. In: Microbial geochemistry, edited by W.E. Krumbein. Blackwell Science Publishers, Oxford. pp. 9 I-I 24.

-       Regulation (EC) No 1907/2006 of the European Parliament and of the Council of 18 December 2006 concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH)

-       Treatise on Geochemistry, 2003, Pages 257–288, Volume 7: Sediments, Diagenesis, and Sedimentary Rocks, 7.10 – Sulfur-rich Sediments

-       Treatise on Geochemistry, 2003, Pages 645–682, Volume 8: Biogeochemistry, 8.14 – The Global Sulfur Cycle

-       Wang, F. and Chapman, P. M. (1999), Biological implications of sulfide in sediment—a review focusing on sediment toxicity. Environmental Toxicology and Chemistry, 18: 2526–2532.

Conclusion on classification

Hydrogen sulphide is highly toxic to aquatic organisms with 96 hour - LC50 values <1 mg/L on fish. Hence, it should be classified as acute 1 according to CLP (M factor of 10, because LC50 (96 h, fish): 0.01 < LC50 <= 0.1 mg/L).

Concerning the chronic toxicity, hydrogen sulphide should not be classified according to CLP as it is neither bioaccumulable nor persistent.