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Water solubility data for iron (hydr)oxides, including zinc ferrite, indicate a poor solubility. Furthermore, iron is relatively immobile under most environmental conditions, mainly due to the very low solubility of iron (III) hydroxide in its various forms (see “Background paper on iron in the aquatic environment, 2010” attached in section 6).

 

Zinc ferrite can be considered environmentally and biologically inert due to the spinel structure in which atoms are tightly bound and not prone to dissolution in environmental and physiological media. This assumption is supported by available transformation/dissolution data (Klawonn, 2021) that indicate a very low release of metal ions at pH 6 and 8. Transformation/dissolution tests (according to OECD Series on Testing and Assessment Number 29) of zinc ferrite at a loading of 1 mg/L and pH 6 resulted in dissolved zinc concentrations of 9.2 µg Zn/L and 12.5 µg Zn/L after 7 and 28 days, and at pH 8 in 3.0 µg Zn/L and 8.0 µg Zn/L after 7 and after 28 days, respectively. Iron ion release of zinc ferrite after 7 and 28 days at 1 mg/L loading and pH 6 and 8 resulted in concentrations below the respective limit of detection (< 2.6 µg Fe/L). Thus, the rate and extent to which zinc ferrite produces soluble (bio)available ionic and other ion-bearing species in environmental media is limited. Hence, zinc ferrite can be considered as environmentally and biologically inert during short- and long-term exposure. The poor solubility of zinc ferrite is expected to determine its behaviour and fate in the environment, including the partitioning in soil, sediment and water.

 

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). Regarding the partitioning of zinc in the environment, the median total zinc concentration of European stream sediment expressed as Zn is 60 mg/kg ranging from 7 to 1251 mg/kg, whereas zinc concentrations of the < 45 µm fraction of European stream waters are highly variable ranging from 0.09 to 310 μg/L with a median of 2.6 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 iron (hydr)oxides.

 

Biodegradation is not relevant for metals and metal compounds that are not biodegradable, including iron (hydr)oxides.

 

For iron and zinc as essential, homeostatically controlled elements, the bioaccumulation potential is considered to be low. A similarly low potential is assumed for the poorly soluble iron (hydr)oxides (see “White paper on waiving for secondary poisoning for Al and Fe compounds, 2010” attached in section 6).

 

The main environmental processes determining the environmental fate of iron (hydr)oxides separated in four categories are of different importance as tabulated below (see Table).

 

Table: Importance of environmental processes.

 

Environmental process

Low

Medium

High

Chemical processes

Solubility/dissolution

X

-

-

Physical processes

Aggregation/agglomeration

-

-

X

 

Sedimentation

-

-

X

Adsorption/desorption

Soil retention

-

-

X

 

Retention in sewage treatment plants

-

-

X

Biologically mediated processes

Biodegradation

Not relevant

-

-

 

Additional information

Environmental solubility

Transformation/dissolution tests (according to OECD Series on Testing and Assessment Number 29) of zinc ferrite (Klawonn, 2021) indicated that the rate and extent to which zinc ferrite produces soluble (bio)available ionic and other ion-bearing species in environmental media is limited. Hence, zinc ferrite can be considered as environmentally and biologically inert during short- and long-term exposure. The poor solubility of zinc ferrite is expected to determine its behaviour and fate in the environment, including the partitioning in soil, sediment and water.

 

Biodegradation

Biotic degradation does not need to be assessed, as all members of the category are inorganic. In the environment, the ratio of iron(II) oxides to iron(III) oxides will be influenced by the availability of oxygen, and will also depend on the presence of microorganisms, nutrients, organics and many other environmental factors.

 

Transport and Distribution

Iron oxides exist in crystalline form as uncharged, solid substances and no adsorption to suspended solids and sediment is expected. Due to their high density, iron oxides will be deposited on the ground of environmental waters. The hydrated form of diiron trioxide exists in an amorphous form when being precipitated from iron hydroxide. This form crystallises after ageing and drying. For soluble forms of Iron (III) a mean log Kd of 4.9; 6.6 and 2.7 for sediment, suspended particles and soil, respectively, is reported. Additionally, a log Kd, observed range of 3.97-5.66 for sediment and a log Kd of 4.50 for sediment is reported. Those studies are not reliable or not sufficiently described and are not taken into account for assessment.

The Henry’s law constant (HLC) and the distribution of iron oxides in the environment are not calculated according to the Mackay fugacity model, because the substances are inorganic and have an extremely low vapour pressure at ambient temperature.

Iron oxides are not volatile from aqueous suspensions.

In the atmosphere, iron oxide substances will exist solely in the particulate phase and may be removed from the air by wet and dry deposition.

 

Ubiquitousness and environmental chemistry of iron and zinc

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. The median total zinc concentration of European stream sediment expressed as Zn is 60 mg/kg ranging from 7 to 1251 mg/kg, whereas zinc concentrations of the < 45 µm fraction of European stream waters are highly variable ranging from 0.09 to 310 μg/L with a median of 2.6 µg/L (Salminen et al. 2005).

 

Iron and zinc 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.

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

A study on bioaccumulation does not need to be conducted, based upon the low bioavailability of the category members. The substances are highly insoluble in water, out of toxic response to aquatic organisms, and based upon its physico-chemical properties the category members do not bind to biological ligands. The essentiality of iron, the evidence of absence of biomagnification is demonstrated as follows:

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 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).

The cellular uptake of zinc is actively regulated by a homeostatic control system, which suggests a low potential for zinc bioaccumulation. Furthermore, zinc does not biomagnify (WHO, 2001 and references therein).

 

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.

WHO (2001) Environmental Health Criteria 221 – Zinc.