Iron sulphide

Endpoint summary

Description of key information

Anthropogenic iron sulfide at relevant amounts will not adversely affect the terrestrial organisms.

Additional information

Iron sulfide might get into the terrestrial environment by different ways:

 

1 . natural iron sulfide, e.g., from volcanic activity, marine sediments which were raised to the surface or by mining activity

2. anthropogenic iron sulfide, e.g., by disposing sludge from sewage treatment plants.

 

Natural iron sulfide: As described for the aquatic toxicology, iron sulfiide can be found in larger amounts in freshwater and marine sediments. When these sediments are lifted up or the water level is decreased, these sediments will get over the water level and might form new land. In Europe iron sulfide containing soils are mainly reported from Finland and Sweden (Österholm et al 2010) and Spain and Portugal (Leistel et al 1998). The sulfide layers in Spain and Portugal are mainly volcanic origin. The amount of sulfides in the Spanish Iberian Pyrite Belt it is estimated to be about 1.700.000.000 tons (Leistel et al. 1998) (from which about 90 % are pyrite). As long as the iron sulfide containing layers are not disturbed, they are not typically not considered as problematic since the oxidation is slow and hence the production of acid is typically minor. Once the horizon is disturbed either by natural or (predominantly) human activities like mining, the iron sulfide gets into contact with oxygen and the sulfide is oxidized to sulfate (Thomas et al 2003 page 13). This process is often enhanced by bacteria, e.g.,Thiobacillus ferrooxidans (Schippers et al. 1996). When the sulfate dissolves in water with low buffering capacity this causes a drop of the pH to less than pH 4. This low pH might bring other metals into solution. The decline of the pH and the subsequent potential leaching of metal ions is considered to be the main problem associated with acid sulfate soils and the resulting acid mining drainage. To understand the relevance of the description of the natural processes for the assessment of the anthropogenic iron sulfide addition, it is important to keep the dimensions in mind: to identify a sulfuric horizon (in Australia) as such, this horizon needs a minimum thickness of 15 cm (Thomas et al. 2003 page 63). Another example where natural iron sulfide causes environmental problem is the coal mining where iron sulfide rich strata are disturbed or the groundwater level is reduced so that these strata become get into contact with oxygen. In Germany this is observed for example in the German Lausitz area where intensive coal mining was performed.

 

When anthropogenic iron sulfide is disposed on soil in small amounts under oxic conditions, the sulfide will be oxidized to sulfate as described for the natural iron sulfide. The amount of acid produced will depend on the amount of iron sulfide disposed. Since the amounts are minor when compared to the natural iron sulfide horizons, it can be expected that the effects as described above for natural iron sulfide can be neglected on normal buffered soils. Furthermore, they need to be put into relation with natural acids which enter the soil. Therefore, the US EPA comes to following conclusion for the assessment of iron sulfates (EPA-738-F-93-002, February 1993): "No adverse effects to avian, mammalian or aquatic populations are anticipated from the use of iron salts. Iron is one of the earth's most abundant elements, and it is immobilized at the pH range of 5-9. Runoff to aquatic systems is unlikely since the parent compounds convert very rapidly to less soluble forms in the environment. Furthermore, the oxidized iron compounds bind tightly to soil under turf. No adverse effects to endangered species are anticipated from the use of iron salts." Since the sulfide from iron sulfide will be oxidized to sulfate, this statement is is also valid for iron sulfide.

 

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, for more details see attached SIDS_Iron salt category report) : "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."

 

Under oxic conditions, iron and sulfide will be oxidized. Iron oxide is almost water insoluble. The sulfate will decrease the pH. Based on the buffering capacity of the majority of European soils and the expected relatively small amounts of iron sulfide to be added to the soil, the decline of the pH caused by FeS will have only a very minor impact on the pH of the soil.

 

Iron salts are listed as fertilizer (section E1.4) in the Regulation 2003/2003/EC of the European Parliament and the Council of 13 October 2003 relating to fertilizers. Fe II and Fe III salts are used as fertilizers in Europe (Spaey et al 2012a). Moreover, it is proposed by JRC that iron salts will be added to the positive list of additives to digestate (Spaey et al 2012 b, page 78 f). Iron is not listed on the list of regulated heavy metals in the EEC directive 86/278/EEC on the protection of the environment, and in particular of the soil, when sludge is used in agriculture.

 

Based on this assessment and the argumentation from OECD and US EPA, it is considered that anthropogenic iron sulfide at relevant amounts will not adversely affect the terrestrial organisms. Based on the role of iron as essential micro nutrient and its role in nutrient cycles it is highly unlikely that iron sulfide will bioaccumulate. Therefore, testing with terrestrial organisms is waived as it will not provide additional information on the toxicity of iron sulfide.