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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. 

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Additional information


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 (Fe3+) ions to ferrous (Fe2+) ions 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 overcompensate the aerobic oxidation resulting in the presence of ferrous (Fe2+) 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 than12 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, Almondsbury, Bristol BS32 4UD, U.K. 65 p.