Trisodium hydrogen 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.

Key value for chemical safety assessment

Biodegradation in water:

inherently biodegradable

Type of water:

freshwater

Additional information

Since no studies investigating the biodegradation potential of trisodium hydrogen EDTA (CAS 150-38-9) are available, in accordance to Regulation (EC) No 1907/2006 Annex XI, 1.5 a read-across to structurally related EDTA species was conducted. This read-across is justified in detail in the overall summary (IUCLID Section 6.1) and within the analogue justification in IUCLID Section 13.

 

A large number of biodegradation tests are available for different EDTA species. In most cases the acid or the Na salts were used as test substance. 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 several tests on inherent biodegradability result in low biodegradation rates [e.g. BASF AG, 1987].

Influence of pH

It was shown that a change of the pH-value has 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 6.7 (van Ginkel et al., 1997). Similar results were obtained in a DOC removal test according to the principles of the OECD guideline 301 A 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. Thus, a degradation of EDTA species under environmental conditions can be expected.

Adaptation

Enhanced biodegradability of EDTA was shown after long adaption periods of the inoculum. 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 was also shown in an industrial wastewater treatment plant from a Finnish paper mill. When using activated sludge from this plant, EDTA was biodegraded by > 80-90% based on CO2 evolution and by 90-100% based on DOC removal in a laboratory study according to OECD 301B (Kaluza et al., 1998). This study represents a low-level pre-adaption test system and can be regarded as an enhanced biodegradation screening test (ECHA, 2017).

Influences of the stability constant

As a chelating agent EDTA forms complexes with a variety of cationic ions. EDTA occurs naturally as a mixture of chelate complexes in the environment. The biodegradability differs between the corresponding acids and the respective salts, and on the other hand, between the metal complexes. Investigations show, that EDTA complexes with a thermodynamic stability constant below 10E+12, like Ca2+, Mg2+ and Mn2+, were degraded. Complexes with higher stability constants like Pb2+, Zn2+, Cu2+, Fe3+ and Co2+ 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 released (Klüner et 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 which are able to mineralise EDTA completely were isolated (Nörtemann, 1992 and Van Ginkel, 1999). The degradation pathway of EDTA was described by Klüner et al. (1998) and summarised in the EU Risk Assessment (2004). The first intermediates described are glyoxylic acid and ethylenediamine triacetate (ED3A). EDTA can react spontaneously to ketopiperazinediacetate (KPDA) by intramolecular cyclisation (Haberer & Ternes, 1996). KPDA itself is biodegradable which could be shown by van Ginkel & Stroo (1999).

Conclusion

EDTA is not readily biodegradable according to OECD criteria. However, a considerably improved biodegradability of EDTA was demonstrated under special conditions like slightly alkaline pH (like in most environmental compartments) or slight adaptation. EDTA was biodegradable in an enhanced test using pre-adapted activated sludge (80-90% CO2 evolution; 80-100% DOC removal). Moreover, using natural freshwater from the river Rhine resulted in 80-90% degradation after 60 d. Therefore it can be concluded that EDTA is ultimately biodegradable under such conditions.