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Environmental fate & pathways

Hydrolysis

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Description of key information

A hydrolysis value of 70 % and 90 % removal from solution is calculated to be achieved in 19 and 36 hours, respectively.

Key value for chemical safety assessment

Additional information

Testing for this endpoint has been waived in accordance with Annex XI, 1 and 2 as an experiment is considered scientifically unjustified.

Justification of read-across

The environmental fate assessment of the submission item must be based on the metal kations considering their speciation, while the anions can be considered nontoxic ubiquitary present species in the environment. The subject of the available studies (Cotton & Wilkinson 1972, Johnson et al. 1997, Kotrly & Sucha 1985, Lide 2009, Lindsay 1979, Morel 1983, OECD 2007, Schnitzer 1969, Sillén & Martell 1971, Sillén et al. 1964, Skeaff 2004 and Stumm & Morgan 1981) is metal species which may be formed from the submission item, which is in conclusion considered appropriate for the assessment of the submission item.

Testing for this endpoint has been waived in accordance with Annex XI, 1 and 2 as an experiment is considered scientifically unjustified.

Irrelevance of hydrolysis as decomposition process

Since hydrolysis changes the chemical form but does not decompose metal species and since characterization of total metal concentration considers all chemical forms, the concept of degradation of metals by hydrolysis is not relevant in the consideration of their environmental fate. Nonetheless several hydrolysis reactions of metal kations are known and of importance as the formed hydroxides play an important role with regard to precipitation and bioavailability.

Oxidation of iron(II) under environmental conditions forms iron(III) rapidly

Iron(II) ions may be oxidised to iron(III) under most environmental conditions. The iron(II) ion can be oxidised by common oxidants such as nitrate (Johnson et al. 2007 with reference to Schnitzer 1969). Ferrous ion, iron(2+) is unstable when exposed to air, it oxidised to ferric ion, iron(3+), which in turn forms the insoluble, hydrated, amorphous and gelatinous precipitate of ferric hydroxide Fe(OH)3. The expected rapid oxidation of iron(II) to iron(III) is the precondition to treat their hydrolysis behaviour together and to consider them a category.

Hydrolysis of Iron(III)

Johnson et al. (2007) summarize the hydrolysis of iron(III) with reference to Schnitzer (1969) as follows: “In solution, iron(III) ions are expected to hydrolyse or form complexes. At pH <1, the hexa-aqua ion ([Fe(H2O)6]3+) is the predominant species. As the pH increases above 1, a stepwise hydrolysis occurs. Between pH 1–2, various species of hydroxo- and oxo-iron compounds may be formed. Above pH 2, colloidal gels are formed, giving a precipitate of the red–brown gelatinous hydrous iron oxide. In the presence of complexing anions, such as chloride, the hydrolysis of iron(III) can result in chloro-, aqua- and hydroxo-species.

Iron kations at equilibrium in water

When ferric ion is added to water the hexa-aquo kation is formed. This is strongly acidic with a pKa of 3.05 (Cotton & Wilkinson 1972). Thus:

[Fe(H2O)6]3+ → [Fe(H2O)6](OH)2+ + H+(aq)

The complete hydrolysis of Fe(III) follows the reaction:

Fe3+ + 3 H2O <=> Fe(OH)3 (s) + 3 H+

The ferrous iron(II) kation is in a redox equilibrium with iron(III), and only under non-oxygenated conditions stable. And is not acidic in solution. The importance of pH is further emphasised by consideration of the solubility product (Ksp) values of the hydroxides. The equations defining solubility product are:

Ksp = [Fe2+][OH-]2 for ferrous hydroxide, and

Ksp = [Fe3+][OH-]3 for ferric hydroxide.

Thus ferrous ion Fe(OH)2, can be formed, it is moderately insoluble, with

Ksp = 1.6 x 10^-14 (Lide 2009).

Ferric hydroxide (Fe(OH)3) is highly insoluble with

Ksp = 1 x 10^-36 (Lide 2009).

Formation of ferric hydroxide at pH levels above 5.0 limits the presence of iron in aqueous systems.

The significance of pH on the solubility of ferrous and ferric kations can be seen in the respective Table in the section on water solubility. The implication of this analysis is that under conditions of very low oxygen concentration, the ferrous kation is freely soluble but the ferric kation is not. Under conditions of high concentration and low oxygen, ferric ion could acidify the water, thereby having environmental consequences. These conditions would not apply in the normal direct uses, although could occur during major accidental leakage.

The redox potential of the iron(III)-iron(II) couple, 0.77 V, is such that molecular oxygen can convert ferrous to ferric kations in acid solution or basic solution (Cotton & Wilkinson 1972).

An in-depth analysis of the oxidation and precipitation of iron was carried out by CEFIC as part of the European Chemicals Bureau classification process of ferrous sulphate (Skeaff 2004).

In conclusion at environmental pH (5-9, ECHA R.7a 2014, p 59), colloidal gels are formed, giving a precipitate of the red-brown gelatinous hydrous iron oxide. These insoluble species are thus predominant at environmentally relevant pH.

  • Cotton FA, Wilkinson G eds (1972). Advanced Inorganic Chemistry. A comprehensive Text. 3rd edn. ISBN 0-471-17560-9. Interscience publishers, a division of John Wiley and Sons, New York, U.S.A. 1145 p.
  • Johnson I, Sorokin N, Atkinson C, Rule K, Hope S-J (2007). Proposed EQS for Water Framework Directive Annex VIII substances: iron (total dissolved). ISBN: 978-1-84432-660-0. Science Report: SC040038/SR9. SNIFFER Report: WFD52(ix). Product Code SCHO0407BLWB-E-E. Self-published by Environment Agency, Bristol, U.K. 65 p.
  • ECHA R.7a: European Chemicals Agency (2014). Guidance on information requirements and chemical safety assessment. Chapter R.7a: Endpoint specific guidance. Version 3. Document Reference ECHA-14-G-03-EN. ISBN 978-92-9244-749-6. Self-published, Helsinki, Finland, in August. 395 p.
  • Lide DR ed (2009). CRC Handbook of Chemistry and Physics. 90th print run. Taylor & Francis, ISBN 978-1-4200-9084 -0
  • Schnitzer M (1969). Reactions between fulvic acid, a soil humic compound, and inorganic soil constituents. DOI 10.2136/sssaj1969.03615995003300010022x Soil Science Society of America Journal 33(1):75-81.
  • Skeaff JM (2004). Review of the Oxidation of Ferrous Ion in Aqueous Media. ECB European Chemicals Bureau ECBI/14/01 Add. 13. Report no.: CANMET-MMSL 04-035. Owner company: CANMET-Mining & Mineral Science Laboratories. Report date: 2004-08-01.