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Environmental fate & pathways

Phototransformation in water

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Description of key information

Iron exposure to light gives reduction of the ferric form. Equilibrium speciation rather than unidirectional reaction applies under environmental conditions. Organically bound iron species were transformed to ionic species under light influence. 

Key value for chemical safety assessment

Additional information

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 publications (Barbeau 2006, Emmenegger et al. 2001, Hong & Kester 1986, Johnson et al. 2007, Kopácek et al. 2005, Rose & Waite 2003) is metal species which may be formed from the submission item, which is in conclusion considered appropriate for the assessment of the submission item.


Photolysis is considered not applicable for the naturally occurring species of iron according to the U.K. Environment Agency evaluations (Johnson et al. 2007). Anyhow phototransformation of metal species rather than photolysis should be considered as environmentally reversible light-induced speciation rather than cleaving of chemical bounds are relevant for metals. Light may have influence of the speciation of iron in water under environmental conditions.


Photoreduction of ferric (Fe 3+) kations to ferrous (Fe 2+) kations is evidenced and subject to a number of publications (Barbeau 2006, Emmenegger et al. 2001, Hong & Kester 1986, Kopácek et al. 2005, Rose & Waite 2003). The process overcompensates the aerobic oxidation resulting in the presence of ferrous (Fe 2+) ions during daylight and disappearing during the night (Kopácek et al. 2005).


Organically bound iron (chelated in organic complexes) was under the influence of 300 nm light in a laboratory experiment transformed to ionic species and thus photoliberated with a half-life of less than 12 h (Kopácek et al. 2005).

  • 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.