Iron sulphide

Endpoint summary

Description of key information

It is concluded that at relevant concentrations iron sulfide has no adverse effects on the aquatic organisms.

Additional information

Iron sulfide is a natural mineral occurring in practically all freshwater (Bott 2002) and marine sediments (Posfai and Dunin-Borkowski 2006). Iron sulfate is an important mineral in the redox-processes in the oxic- anoxic transition zone. The typical iron sulfide content in sediments is > 1 mg/g sediment (Yan 2006).

For maintaining water ways harbors sediments - including the naturally occurring iron sulfide - are dredged and dumped into the sea. In the following a rough estimation of the amount of re-suspended iron sulfide caused by man-made sediment dumping is made. The total annual amount of dumped sediments within the EU during the last decade is greater 100 Million tons (dry weight)/year (OSPAR 2009 and 2010a). Assuming a conservative average iron sulfide content of 0.1 mg/g and a conservative dumping activity of 100 million tons dry weight/year, this results in a total dumping of at least 100.000 t FeS/year in Europe. Turbidity and burial of sediment dwellers, heavy metals (not Fe) and organic pollutants were identified as problems associated with dumping of dredged sediments (OSPAR 2010 b). During the monitoring programs of the dumping areas iron sulfide was not identified as problem. Iron and sulfide are not among the parameters to me measured prior to dredging and dumping. Therefore, it must be concluded that iron sulfide are apparently not considered to be parameters of concern. There is no evidence for toxic effects resulting from iron sulfide when sediments are re-suspended either naturally (e.g., storms, currents, sediment slides, black smokers) or by man (e.g., dredging and sediment dumping; iron sulfide is not on the list of parameters of interest in the Revised OSPAR Guidelines for the Management of Dredged Material 2004). Hence, it can be concluded that there is evidence from "monitoring" of the environment that FeS has no adverse effects on aquatic organisms on the short-term or long-term basis. In large oceanic areas iron is considered to be the limiting factor for the primary production as it has been shown by several iron enrichment tests. These iron deficient areas are typically located far away from the costs and hence the effect of fertilization by anthropogenic iron salts is considered to be of relative minor importance for the freshwater and marine environment.

The OECD made an assessment of the environmental impact of different iron salts, mainly sulfates and chlorides (OECD 2007). Iron sulfide was not part of that analysis. In the SIAM report the OECD came to following conclusion (OECD 2007) : "The members of the category are currently of low priority for further work. The hazard profile of iron salts is dependent on the environmental conditions and the necessary conditions for harmful effects to be expressed are very specific (low pH and low dissolved oxygen) and are, in themselves, intrinsically unfavorable to many aquatic species." "Acutely toxic levels of the iron salts to aquatic organisms are observed in nominal exposure concentrations in the range equivalent to 1 – 1000 mg/l salt, with the majority of the results being in the 10 – 100 mg/l range. Chronic effects on aquatic organisms are also observed at nominal concentrations in the range 1 – 1000 mg/l for each individual salt with the majority of the results being >10 mg/l."

Following effect concentrations were reported based on the Fe moiety:

·        "Green algae (Pseudokirchneriella subcapitata) E_rC₅₀ (72 h) = 18 mg/l (growth rate)

·        Invertebrates (Daphnia magna, 4 results): EC₅₀ (48 h) = 1 - 10 mg/l

·        Fish (various fish species): LC₅₀ (96 h) = 0.41 - >28 mg/l

·        Micro-organisms (Vibrio fischeri) EC₅₀ (15 min) = 40 mg/l"

 

Iron sulfide was not included in this OECD assessment. Based on the low water solubility of iron sulfide in relation to the concentration range where acute and chronic effects were observed for iron salts, no acute or chronic effects can be expected from the exposure of aquatic organisms to iron sulfide.

 

The assessment from the OECD is supported by a short-term fish test with FeS where an LC₅₀ of >10000 mg/L was determined, i.e., a value which is several orders of magnitude above the water solubility level. In aquaculture, iron-rich soils are used to reduce toxic effects of H₂S by adding iron ions to the water resulting in a subsequent precipitation of FeS (Lahav et al. 2004). This is a further evidence that toxic effects of FeS on fish does not need to be expected.

Some marine worms (e.g.Halicryptus spinulosus) can live for a short time in the anoxic zone. During that time iron sulfide is formed within their blood. it is speculated that this mechanism assists in the sulfide detoxification (Oeschger and Vetter 1992). After getting into oxic areas again, the iron sulfide and the black color disappears. This iron sulfide formation is fully reversible in these worms. Since the anoxic zone in marine and freshwater sediments starts typically few mm - cm below the sediment surface, other worms (e.g. Tubifizidae) and sediment dwellers are also adapted to live under conditions where iron sulfide is typically found. The roots of aquatic plants are normally in an mostly anoxic sediment and hence they are exposed to FeS under natural conditions as well.

Based on  

- the ubiquitous occurrence of FeS in natural aquatic systems,

·        its very low water solubility which is significantly below the concentrations where effects for other Fe-salts were observed, 

·        the finding that no toxic effects of FeS were reported in monitoring programs,

·        experimental evidence from a short term fish test

·        experience from aquaculture

it is concluded that tests with aquatic organisms will not provide further evidence for the lack of toxicity of iron sulfide. Therefore further tests are not needed and were hence waived.

It is concluded that at relevant concentrations iron sulfide has no adverse effects on the aquatic organisms.