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Sediment toxicity

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Reference
Endpoint:
sediment toxicity: long-term
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
other:

Description of key information

Iron (hydr)oxides in powder and nano-form are not classified as harmful, toxic or very toxic to aquatic life or may cause long lasting harmful effects to aquatic life. Iron (hydr)oxides in powder and nano-form is also not an unclassified hazard to the aquatic environment. Based on the poor solubility, bioavailability, lack of a potential for bioaccumulation and toxicity to aquatic organisms and considering ubiquitousness of iron (hydr)oxides in sediment and essentiality of iron, iron (hydr)oxides in powder and nano-form are also not considered an unclassified hazard to the sediment compartment.

 

Further, a comprehensive evaluation of whether the relative contributions of anthropogenic iron to the existing natural iron pool in soils and sediments are relevant in terms of added amounts and in terms of toxicity was performed (see "White Paper on exposure based waiving for iron and aluminium in soil and sediments, 2010", attached in section 6.0) and concludes as follows: The justification for exposure based waiving of conducting additional soil and sediment tests should be based on information on absence of exposure or in the case of metals on information showing that the contribution of the anthropogenic emissions are overruled by the already present natural background. Exposure based waiving is justified for iron since results indicate that the relative contribution of anthropogenic iron to the already present natural iron pool in soils and sediments is not relevant in terms of added amounts and in terms of toxicity.

Key value for chemical safety assessment

Additional information

Ubiquitousness: Iron is a major constituent of the lithosphere, comprising approximately 5.1%.The median total iron content of European stream sediment expressed as Fe (XRF analysis) is 2.50% ranging from 0.08 to 12.80% (Salminen et al. 2005). In primary minerals iron occurs largely as ferromagnesium minerals (Salminen et al. 2005). These minerals dissolve during weathering and released iron precipitates as ferric oxides and hydroxides in sediments. The solubility of iron in sediments is largely governed by iron (hydr)oxides with redox, pH and hydroxide formation being key modifying factors. Organic matter in sediments is stabilised by sorption to iron (hydr)oxides suggesting that iron phases serve as sink for organic carbon involved in its long-term storage and contribute to the global cycles of carbon, oxygen and sulfur. The median total zinc concentration of European stream sediment expressed as Zn is 60 mg/kg ranging from 7 to 1251 mg/kg (Salminen et al. 2005).

Iron exposure, bioavailability and uptake from porewater by soil organisms are under environmentally relevant and tolerable conditions (pH and redox) limited by the solubility of naturally occurring ferric (hydr)oxides. Poorly soluble synthetic iron oxides are not expected to influence any soil parameter, including dissolved and bioavailable iron concentrations.

 

Essentiality: Iron is essential for almost all living organisms as it is involved in a wide variety of important metabolic processes including oxygen and electron transport, gas sensing and DNA repair and replication and regulation of gene expression. Thus, iron is critical to the survival of living organisms, including terrestrial bacteria, plants and invertebrates. Due to its poor solubility under environmentally relevant conditions, iron is not readily available, and soil organisms have developed sophisticated pathways to import, chaperone, sequester, and export iron. Thus, iron is an essential element that is homeostatically controlled by all organisms.

Zinc as the key element of zinc finger domains of proteins is required for membrane stability, protein-protein and protein-DNA interactions (WHO, 2001 and references therein).

 

Bioaccumulation: The existence of saturable uptake mechanisms, the presence of significant amounts of stored metal in organisms, and the ability of some organisms to regulate bioaccumulated metal within certain ranges are all thought to be responsible for the inverse relationship that has been frequently reported between bioaccumulation factors (BAFs) and metal exposure concentrations. In these cases, higher BAFs are associated with lower exposure concentrations and also can be associated with lower tissue concentrations within a given BAF study. This is contrary to the implicit assumption that higher BAFs indicate higher metal hazard. Nearly all metals, including iron, have BAFs >1000 in natural, healthy ecosystems with aqueous iron concentrations at background. Bioaccumulation factors for metals are clearly inversely related to water, sediment and soil concentrations (Adams, 2011).

For iron and zinc, essential, homeostatically controlled elements, the bioaccumulation potential is considered to be low. Differences in iron uptake rates are related to essential needs, varying with the species, size, life stage, seasons etc. Iron homeostatic mechanisms are applicable across species with specific processes being active depending on the species, life stages. The available evidence shows the absence of iron biomagnification across the trophic chain both in the aquatic and terrestrial food chains. The existing information suggests that iron does not biomagnify, but rather that it tends to exhibit biodilution. Differences in sensitivity among species are not related to the level in the trophic chain but to the capability of internal homeostasis and detoxification (see "White Paper on waiving for secondary poisoning for Al and Fe compounds, 2010" attached in section 6).

 

References:

Adams B, 2011. Bioaccumulation of metal substances by aquatic organisms, OECD meeting, Paris September 7-8, 2011.

Lindsay WL, 1979. Chemical equilibria in soils. The Blackburn Press.

Salminen R et al. 2005. Geochemical Atlas of Europe. Part 1: Background Information, Methodology and Maps. http://weppi.gtk.fi/publ/foregsatlas/index.php.