Iron(3+) ion dipotassium 2-({2-[bis(carboxylatomethyl)amino]ethyl} (carboxylatomethyl)amino)acetate; hydrate

Biodegradation in water: screening tests

Description of key information

EDTA is not found to be readily biodegradable according to OECD criteria. In standard OECD 301D ready biodegradability tests with natural river water as inoculum it was shown that EDTA complexes with a stability constant lower than 10E13 like EDTA-Na4, EDTA-CaNa2, EDTA-MgNa2 etc. less than 60% biodegradation was observed after 28 days indicating that these substances should indeed not be classified as readily biodegradable. In these same tests however > 60% biodegradation was observed when these tests are prolonged to day 60 (Ginkel, 2018) indicating that these compexes, having stability constants < 10E13, are ultimately biodegradable and should be classified as "not persistent".

EDTA-Fe(OH)k2 however has a stability constant > 10E13 and is therefore under the conditions applied considered to be inherently biodegrable not fulfilling specific criteria.

Key value for chemical safety assessment

Biodegradation in water:

inherently biodegradable, not fulfilling specific criteria

Type of water:

freshwater

Additional information

A large number of degradation tests are available for EDTA (acid form) and its salts, for justification for read-across see IUCLID 6, Chapter 13. In most cases the acid or the Na salts were used as test substances. Results from OECD guideline tests indicate that EDTA is not readily biodegradable [e.g. Gerike & Fischer, 1979 and BASF AG, 1999, 2000, 2001, 2002]. Furthermore different tests on inherent biodegradability result in low biodegradation rates [e.g. BASF AG, 1987].

pH influences

It could be shown that a change of the pH-value have significant influence on the biodegradability of EDTA. In a SCAS test (OECD 302 A) biodegradation of EDTA could be observed at pH 8-9, but not significantly at pH 6.5 [Van Ginkel & al., 1997]. Similar results obtained in a DOC removal test according to the principles of the OECD guideline 301 using natural surface water from the river Rhine as inoculum. After 60 days up to 100 % EDTA was degraded at pH 8.5 but less than 10 % at pH 6.5 [BASF AG, 2000]. These slightly alkaline conditions are realistic in environmental surface water compartments.

Adaptation

An enhanced biodegradability of EDTA could be shown after long adaption. In guideline tests according to OECD 301 EDTA was moderately biodegradable and well eliminable from water using adapted inoculum [BASF AG, 2001, 2002]. The adaptation potential of EDTA degradation shows also an industrial wastewater treatment plant from a Finnish paper mill. Using activated sludge from this plant EDTA was biodegraded about > 80 % CO₂ evolution and about 99 % DOC removal in a laboratory study (OECD 301B) [Kaluza & al. 1998]. This study represents a low-level preadaption test system and can be regarded as an enhanced biodegradation screening test [Guidance for Implementation of REACH, Chapter R.7b, 2008]

Influences of the stability constant

As a chelating agent, EDTA forms complexes with cationic ions. Fundamental EDTA exists naturally as a mixture of chelate complexes. The biodegradability differs between the acid resp. their salts and on the other side the metal complexes. Investigations show, that EDTA complexes with a thermodynamic stability constant below 10E13, like Ca, Mg and Mn, were degraded. On contrast heavy metal EDTA complexes with stability constants above 10E13, such as Cu and Fe, were not significantly degraded [Klüner & al. 1998, Van Ginkel, 1999 and Nörtemann, 2003]. In addition a degradation of Zn-EDTA was observed by Satroutdinov, 2003.

Degradation pathway

Several investigations revealed that it is possible to enrich cultures of EDTA-utilizing microorganisms. Different bacteria strains were isolated which can mineralised EDTA completely [Nörtemann, 1992 and Van Ginkel, 1999]. The degradation pathway of EDTA was described from Klüner & al. (1998) and summarised in the EU Risk Assessment (2004). The first intermediate described is ethylenediaminetriacetate (ED3A). ED3A can react spontaneously to ketopiperazinediacetate (KPDA) by intramolecular cyclisation [Ternes et al., 1996]. KPDA itself is biodegradable which could be shown by Van Ginkel & Stroo (1999).

Conclusion

EDTA is not readily biodegradable according to OECD criteria. It was shown that with natural river water as inoculum, in standard OECD 301D tests, EDTA complexes with a stability constant lower than 10E13 like EDTA-Na4, EDTA-CaNa2, EDTA-MgNa2 etc. less than 60% biodegradation was observed after 28 days indicating that these substances should be not classified as readily biodegradable but in these tests > 60% biodegradation was observed after 60 days in the prolonged (enhanced) tests indicating that these compexes, having stability constants < 10E13, are not persistent. This stability constant threshold of 10E13 will be dependent on the concentration balance between the starting complex and free metal ions (alkali and alkaline earth metals). The lower the starting concentration of the EDTA-metal complex the higher this stability constant threshold for biodegradation.