Ferric ammonium citrate

Adsorption / desorption

Additional information

No studies are available for the adsorption/desorption of reaction mass of ammonium iron (III) citrate and ammonium sulfate. However, ammonium iron (III) citrate and ammonium sulfate are both expected to dissociate under environmentally relevant conditions (see Section 1.3 of the CSR). Therefore, it is appropriate to read-across data relevant to the adsorption/desorption of iron, citrate, ammonium and sulfate. Further details may be found in an expert report (Peter Fisk Associates, 2012) attached in Section 13 of the IUCLID 5 dossier.

Iron (III)

Adsorption/desorption as a partitioning process associated with organic carbon is not a relevant endpoint for iron. Environmental fate is dominated by abiotic and physico-chemical processes, including precipitation and settling. Sufficient information for the assessment of the environmental fate of iron is available in the public domain and is discussed in Section 4.2.4.

Citrate

Based on the low log Kow of citric acid, it is not expected to absorb strongly to the organic matter in soil. However, it’s ionic nature and its ability to complex metal ions means that it could absorb strongly via other mechanisms.

A study is not required according to column 2 of REACH Annex VIII because citrate is readily biodegradable and decomposes rapidly in the environment.

Citrates form stable complexes with metal ions due to its multiple-binding capacities; pH will affect the number of binding sites by altering the ionisation state of the substance.

Citrate may form specific complexes with mineral surface functional groups, affecting surface charge characteristics, enhancing metal cation adsorption, competing with other specifically retained substances for surface sites and inhibiting mineral crystallisation and affecting mineral precipitation and dissolution directly. In soils can reside almost completely in the adsorbed phase (Essington 2008).

The log K_ocvalue of citric acid may be read across from EDTA on the principle that they are both complexing agents. EDTA is a stronger complexing agent than citric acid and so the read-across represents a worst case scenario.

The K_ocvalue of citric acid was determined from the Kp-susp of Ethylene Diamine Tetra Acetic Acid (EDTA) by the following equations

Kpsusp = 75

Kpsusp = Foc * K_oc= 0.05 *K_oc

75 = 0.05 *K_oc => K_oc= 1500

The K_ocand Kpsusp values of 1500 and 75 were used for the exposure calculation of citrate respectively.

Where:

Kpsusp = partition coefficient solid-water in suspended matter

Foc = weight fraction of organic carbon in water compartment

0.05 = weight fraction organic carbon sediment solids

Ammonium

Adsorption/desorption as a partitioning process associated with organic carbon is not a relevant endpoint for this inorganic ion.

Ammonium will enter the nitrogen cycle in soil, where it is an important intermediate in the assimilation of nitrogen from the soil by plants.

Ammonium is bound in soil by the attraction of the positive charge on the ammonium ion to the negatively charged soil micelles. In soil, ammonium is adsorbed primarily by four mechanisms: chemical (exchangeable), fixation (non-exchangeable), reaction with organic matter and physical attractive forces (Environment Canada 2001). It is poorly mobile in soil (OECD 2004).

Sulfate

Adsorption/desorption as a partitioning process associated with organic carbon is not a relevant endpoint for this inorganic ion.

Sulfate is ubiquitous in the environment and is not of concern.

The ability of soils to retain sulfate by adsorption or mineral formation is well recognised. The mechanisms are chemically complex and may be non-specific ion-ion interactions or "irreversible" interactions which can involve the accompanying cations and bridging due to SO₄^2-being divalent. Mineral formation may also play some role in sulfate retention in soils, particularly if pH is low and aluminium levels high (Chesworth 2008).