Tetraammonium ethylenediaminetetraacetate

Biodegradation in water: screening tests

Description of key information

EDTA is not readily biodegradable according to OECD criteria, but ultimately biodegradable under special environmental conditions.

Additional information

A large number of degradation tests are available for EDTA (acid form) and its salt,for justification for read-across see IUCLID 5, 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 a great impact on the biodegradability of EDTA. In a SCAS test (OECD 302 A) biodegradation of EDTA could be observed at pH 8-9, but not 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 % CO₂evolution 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 a lot of 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 10E12, like Ca, Mg and Mn, were degraded. Complexes with higher stability constants like Pb, Zn and Co are slowly degraded via the weaker complexes present in equilibrium. The degradation of complexes containing very bacteriotoxic metals like Cu or Cd stops as soon as sufficient metal is liberated [Klüner & al. 1998 and Van Ginkel, 1999, Nörtemann 2003, Satroudinov 2000, 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 andVan 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 under special conditions like slightly alkaline pH or adaptation the biodegradability of EDTA is considerably improved. EDTA was biodegradable in an enhanced test using preadapted activated sludge. Therefore it can be concluded that EDTA is ultimately biodegradable under such conditions.