Diiron trioxide

Endpoint summary

Description of key information

Solubility data indicate that diiron trioxide in powder and nano-form is poorly soluble (< 2 µg/L). Furthermore, iron is relatively immobile under most environmental conditions, mainly due to the very low solubility of iron (III) hydroxide in its various forms. Thus, the rate and extent to which diiron trioxide in powder and nano-form produces soluble (bio)available ionic and other iron-bearing species in environmental media is limited. Hence, dissolution is not considered to be an important process for diiron trioxide in powder and nano-form in the environment. Further, the poor solubility of diiron trioxide in powder and nanoform is expected to determine its behaviour and fate in the environment.

Regarding the partitioning of iron in the environment, the median total iron content of European stream sediment expressed as Fe (XRF analysis) is 2.50% ranging from 0.08 to 12.80% whereas iron concentrations of the < 45 µm fraction of European stream waters are highly variable ranging from < 5 to > 3600 μg/L with a median of 67 µg/L (Salminen et al. 2005). A similarly high potential to partition into the sediment (or other solid phases) may be assumed for the poorly soluble diiron trioxide in powder and nanoform.

Biodegradation is not relevant for metals and metal compounds that are not biodegradable, including diiron trioxide in powder and nano-form.

For iron an essential, homeostatically controlled element, the bioaccumulation potential is considered to be low. A similarly low potential is assumed for the poorly soluble diiron trioxide in powder and nano-form.

The dispersion stability alpha-Fe2O3 nanoparticles was screened in a study performed similar to the OECD TG 318. The stability of alpha-Fe2O3 nanoparticle in dispersion after 6 h varied depending on the composition of the medium, i.e. alpha-Fe2O3 nanoparticles can be considered of condition-dependent intermediate dispersion stability (10% < x < 90%) according to OECD 318. After 72 h, the dispersion stability of alpha-Fe2O3 nanoparticles further decreased to < 50% at pH 4, 7, 9 and 0, 1, 10 mM Ca(NO3)2, respectively, indicating intermediate low to low dispersion stability after 72 h.

The main environmental processes determining the environmental fate of diiron trioxide in powder and nano-form separated in four categories are of different importance (see Table).  

 

 Table: Importance of environmental processes 

	Environmental process	Low	Medium	High
Chemical processes	Solubility/dissolution	powder/nano	-	-
Physical processes	Aggregation/Agglomeration	-	-	powder/nano
	Sedimentation	-	-	powder/nano
Adsorption/desorption	Soil retention	-	-	powder/nano
	Retention in sewage treatment plants	-	-	powder/nano
Biologically mediated processes	Biodegradation	Not relevant	-	-

 

Additional information

Environmental solubility

Poor water solubility of diiron trioxide (EC 215-168-2) was determined at a loading of 10 g/L at pH 8 (Daphnia medium, ISO 6341) with dissolved iron concentrations being < 1 µg/L (OECD TG 105; Prüm, 2009). A poor solubility (fraction < 3 kDa) without any increase in dissolved Fe concentrations was observed for two nanoforms of diiron trioxide in water over 25 days with 1.50 and 1.77 μg/L for Ferroxide® 212P E172 (SSA: 11.7 m²/g; D50: 82 nm) and SICOVIT® Red 30 E172 (SSA: 9.4 m²/g; D50: 85 nm), respectively (OECD TG 105; Klawonn, 2018).

 

Ubiquitousness and environmental chemistry of iron

Iron is the fourth most abundant element with a crustal average of 7%. It has lithophile and chalcophile properties, forming several common minerals, including pyrite FeS₂, magnetite Fe₃O₄, haematite Fe₂O₃and siderite FeCO₃. It is present in many rock-forming minerals. Secondary hydrous oxides are the dominant Fe phases of sedimentary rocks although primary oxides may account for some of the iron.

Iron has two main oxidation states (2+ and 3+). Iron is relatively immobile under most environmental conditions, mainly due to the very low solubility of iron (III) hydroxide in its various forms. Its solubility is strongly influenced by redox conditions. The Fe²⁺ion is more soluble in strong acid or reducing conditions. However, dissolved Fe precipitates rapidly with increasing pH or Eh and forms hydrous oxide (coatings on particles) in aerobic environments (Salminen et al. 2005).

Iron speciation in the simple system Fe-O-H without (left) and with (right) the effect of sulfur are presented in the attached Figure (Eh-pH diagrams for F-O-H and fe-S-O-H systems.pdf) . Hematite (Fe₂O₃) is shown as the stable Fe(III) species, since Fe(OH)₃ and FeO·OH will eventually age to Fe₂O₃ although the kinetics for this aging may be very slow.

Ferrous iron (Fe²⁺) is reasonably soluble at neutral pH in anoxic environments, but in the presence of oxygen aqueous Fe²⁺is rapidly converted to relatively insoluble ferric (Fe³⁺) oxide-hydroxide. Ferric iron (Fe³⁺) is almost insoluble at neutral pH but can be solubilized by acidification (< pH 3).

Significant levels of H₂S and CO₂ in solution influence the pH-Eh conditions for mineral stability, decreasing the solubility of Fe under more reducing conditions particularly at near-neutral pH. The complexation with chloride, fluoride, nitrate, phosphate, sulfate and natural organic materials further affects dissolved Fe concentrations of stream water. The median total iron content of European soils expressed as Fe (XRF analysis) is 2.45% ranging from 0.11 to 15.60% in topsoil. The median total iron content of European stream sediment expressed as Fe (XRF analysis) is 2.50% ranging from 0.08 to 12.80% whereas iron concentrations of the < 45 µm fraction of European stream waters are highly variable ranging from < 5 to > 3600 μg/L with a median of 67 µg/L (Salminen et al. 2005).

Iron 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 plants, bacteria, animals and humans, to transport oxygen through the haemoglobin in animals and humans and to produce energy through electron transfer in the mitochondrial respiratory chain. Iron is a major constituent of the cell redox systems such as haeme proteins (e.g. cytochromes, catalase, peroxidase, leghaemoglobin) and iron sulfur proteins (e.g. ferredoxin, superoxide dismutase).

Due to its poor solubility under environmentally relevant conditions, iron is not readily available, and organisms have developed sophisticated pathways to import, chaperone, sequester, and export iron. Microorganisms, for example, employ various iron uptake systems, and there is considerable variation in the range of iron transporters and iron sources utilised by different microbial species. Iron as essential element for all plants has many important biological roles in biochemical processes including photosynthesis, chloroplast development and chlorophyll biosynthesis. Also, vertebrates have high requirements for iron, the majority of which is used by red blood cells for hemoglobin production.

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 an essential, homeostatically controlled element, 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 in 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.