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Environmental fate & pathways

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According to the RAR for EDTA (CAS 60-00-4), EDTA is resistant to hydrolysis. Neither strong acids nor alkalis cause any degradation. Based on structural similarity (for read-across justification see IUCLID section 13), this is also applicable for EDTA-Mn(NH4)2.

Concerning phototransformation, especially Fe(III) is susceptible to photodegradation in surface water. When EDTA-Mn(NH4)2 is released into the environment, the Fe(III)-EDTA complex may be formed (see also read-across justification in section 13 for more information). Therefore, some information on photolytic degradation of the Fe(III)-EDTA complex in aqueous solution as reviewed in the EU RAR for EDTA (2004) is provided here.

Photodegradation will occur in surface water, in particular during summer and in the upper water layer. 

Photodegradation does not result in complete mineralization, but as the degradation products ED3A, EDDA and EDMA are readily biodegradable the combined effect of daylight UV and bacteria will be a complete mineralization (Nowack and Baumann 1998 in Bucheli-Witschel, Egli 2001).

 

Frank and Rau (1990) determined the quantum yields and made model calculations for the half-life value in surface water in central Europe. Between summer and winter the half-lives varied between 5 and 480 hours. Svenson et al (1989) estimated a half-live of 11 minutes at the top layer of a water body in early summer.

In field studies at the Swiss river Glatt half-lives varied between 20 – 100 minutes for summer and winter respectively (Kari 1994).

 

Other metal complexes like Co(III) and Mn(II) showed much slower rates of photodecomposition, whereas complexes with Zn, Cu and Ca appear inert to photodegradation (Kari, 1994).

 

In the EU RAR a worst case approach is used with a half-life of 20 days for the Fe(III)-EDTA complex only. Based on studies of Kari (1994) a separation of photolytical stable and unstable EDTA species can be made and 25- 50% of EDTA is discharged as the unstable Fe(III)-complex. In the estimation for the regional PEC it is assumed that in the environment 75 % will end in a stable form (mainly Ca), whereas 25% is degraded with a half-live of 20 days.

 

This approach is conservative because it does not take in consideration that after degradation of Fe(III)-EDTA, the Fe(III) can become available again for displacement of stable Ca-complexes. The rate of displacement will of course depend mainly on the pH and the relative concentrations of various metal ions in the specific surface water. The above figures show that at variations of pH between pH 7 and 8 there will be almost equal amounts of Ca- en Fe EDTA complex, so molecules Fe(III)-EDTA that are degraded will be replaced until all EDTA has disappeared.