Iron sinter

Short-term toxicity to aquatic invertebrates

Description of key information

A large number of acute toxicity studies with aquatic invertebrates have been identified for iron, but due to one or more significant deficiencies in the testing procedure and/or limited reporting of relevant data, none of these studies can be considered as a key study on its own. Hence, a weight of evidence approach was used for assessing this endpoint.

 

Similar findings were reported on fish. The main deficiency of all these studies is a limited characterization of exposure concentrations. All tests were conducted at nominal concentration levels that exceed the solubility of the ferrous/ferric free ion, and therefore exposure concentrations represent a mix of free iron ions, colloidal iron forms (low and high molecular weight), iron bound to dissolved organic matter, and precipitated Fe-species. None of the studies report a proper characterization of the Fe-speciation, but it is important to note that Fe-speciation is defined by a dynamic equilibrium that is governed by several processes with (short) transformation half-lives. Therefore, it is virtually impossible to properly qualify and quantify the time-dependent Fe-speciation profile during a standard 48h invertebrate toxicity test.

 

Studies with L(E)C50-values that are based on nominal values with no indication of analytical measurements are therefore considered unreliable. Several of these studies investigated iron toxicity to standard invertebrate species (Daphnia, Ceriodaphnia) and reported EC50s were between 5.3 and 73.2 mg/L (Lillius et al (1994), Khangaroth and Ray (1989), Calleja et al (1994), Biesinger and Christyensen (1972), Bosnir et al (2013)).  

 

The indication of measured data, however, does not ensure that the reported values are meaningful as it is not always reported whether samples were filtered prior to analysis, therefore not enabling a distinction between total and dissolved iron levels (e.g. Rathore and Khangarot, 2002,2003 ; Buikema et al, 1977 ; Maltby et al, 1987). Other studies reported measured iron data, but samples were acidified prior to analysis without any indication that a filtration step was included in the sample treatment (Shuhaimi et al (2012)).

 

The one study that clearly identified a measured dissolved fraction (LISEC, 1999), also reported a 48h-EC50 of 0.99 mg/L which is well above the solubility of iron; observed effects could therefore be related to precipitation onto the test organisms.

 

In addition, three acute studies with D. magna, equivalent to OECD 202, were conducted with insoluble iron compounds (iron(II,III) oxides, iron hydroxide oxide) and no effects were observed at the highest test concentrations which ranged between 10-100 g/L test material (nominal values).

Additional information

Available data suggest that iron salts are relatively non-toxic and this was sufficient for the EU Classification and Labelling Committee to determine that there was no need for classification of iron salts. It was also concluded that iron massive and sparingly soluble forms of iron are highly insoluble and non-hazardous.

 

All of the identified studies used test solutions with iron concentrations above that of its solubility limit. Due to physical effects of precipitated material, some of these studies are meaningless for the investigation of intrinsic toxicity. Iron ions released to surface waters quickly form insoluble iron hydroxides in mixing zones. These positively charged iron colloids will react with the negatively charged mucus that lines the respiratory system, or could result in fouling of feeding appendages.

 

Iron has complex redox chemistry. In very special conditions transient iron species can be formed that cause toxicity. These conditions, however, are not typical of most ambient conditions and are more representative of specific mixing zones. In ambient conditions, the dissolved natural background concentrations of iron, in most cases, are at equilibrium, therefore addition of iron would lead to the precipitation of iron compounds from solution. Adverse effect caused by these precipitates (e.g., smothering, shading, etc) do not represent the intrinsic toxicity of iron (Jackson, Versfeld & Adams 2010, Peters, Brown & Merrington 2010).