Trisodium nitrilotriacetate

Endpoint summary

Additional information

Trisodium nitrilotriacetate (NTA) was tested for ready and inherent biodegradability according to various guidelines (OECD 301 E, OECD 302 B, Sturm Test, combined CO2/DOC test). These test resulted in 75 -100 % degradation after lag phases ranging between 1 and 16 days (see Table 1).

Table 1: Results of laboratory biodegradation tests

Type	Method	Duration [d]	Inoculum^1)	Na3NTA conc. [mg/l]	Degradation [%]	Lag phase [d]	Reference
Modified OECD Screening Test	OECD 301 E	14	River water	70	100	5	BASF (1983b)
Modified OECD Screening Test	OECD 301 E	14	Industrial WWTP effluent	70	100	5-11	BASF (1983b)
Modified OECD Screening Test	OECD 301 E	7	Adapted AS	70	100	1	BASF (1983c)
Modified OECD Screening Test	OECD 301 E	12	Adapted AS	140	75-90	2-5	BASF (1983c)
Sturm Test	CO2evol.	9	Effluent from stand test	10/20	100	-	BASF (1983d)
Manometric Respirometry Test	OECD 301 F	28	Industrial AS	250-360	92	16	Strotmann et al. (1995)
Combined CO 2 /DOC Test	Other	28	Industrial AS	140	> 95 (DOC) 91 (CO2)	2 (DOC) 5 (CO2)	Strotmann et al. (1995)
Modified Zahn-Wellens Test	OECD 302 B	28	Industrial AS	1400	96	7	BASF (1983a)
Die-away Test	Other	23	Municipal AS	210	100	14	Takahashi et al. (1997)

 

According to results from ready biodegradation tests, Na3NTA can be regarded as readily biodegradable. 

 

Apart from studies conducted with trisodium nitrilotriacetate (Na₃NTA) in ready biodegradation tests, studies on the biodegradation of nitrilotriacetic acid (H3NTA) are available for surface water simulation studies. For studies determining biodegradation in soils, data for the biodegradation of H3NTA and nitrilotriacetate were used in this assessment.

H3NTA, Na3NTA, and nitrilotriacetate display the same behaviour in the environment: splitting of sodium ions or protons (in case of Na3NTA and NTA acid) and uptake of multivalent metal ions with subsequent formation of 1:1 or 1:2 complexes.

Since sodium salts are generally considered to be completely dissociating, a solution of Na3NTA in water yields the tribasic anion nitrilotriacetate. Nitrilotriacetic acid is a weak acid, and in such a solution, the NTA will therefore exist as an equilibrium mixture of several species:

NTA^{- - -}<-> HNTA^{- -}<-> H₂NTA^-<-> H₃NTA <-> H₄NTA⁺

with the last species occurring when, in a very acidic environment, the central nitrogen atom is protonated.

Due to pH differences, the NTA speciation equilibrium will be different for Na3NTA and for NTA acid, unless dissolved in a buffered solution (controlled pH). A solution of NTA acid will be (slightly) acidic, whereas a Na3NTA solution will be alkaline (‘basic’). Toxicologically, this is not assumed to be significant, since it can be presumed that ‘in vivo’ systems are buffered systems. The chelating behaviour of Na3NTA and NTA acid will be slightly different, but this is not a significant effect for the relevant endpoint under REACH with regard to environmental fate and behaviour, ecotoxicology and toxicology.

Therefore, also results on NTA acid and nitrilotriacetate are considered for the assessment of trisodium nitrilotriacetate.

This is in accordance with the Canadian ‘Draft Screening Assessment for Nitrilotriacetic acid (CAS 139-13-9)’ from January 2010. In this assessment, the evaluation of nitrilotriacetic acid was supplemented by information on the trisodium nitrilotriacetat, which also exists in a monohydrate form, commonly known as trisodium nitrilotriacetate monohydrate due to similar toxicological effects of NTA and its sodium salts.Moreover, under environmentally relevant pH Values (5-9) the dissociation of H3NTA and its sodium salts leads to the common moiety nitrilotriacetate. Therefore, also results on nitrilotriacetate, nitrilotriacetic acid, and trisodium nitrilotriacetate monohydrate are considered for the assessment of trisodium nitrilotriacetate (for simplification all forms referred to as NTA).

In surface water simulation tests, the biodegradation of NTA was measured in the laboratory using procedures and bacterial inoculates that simulate potential degradation in specific environmental compartments. Simulation tests for the biodegradation of NTA are available for the environmental compartments freshwater, estuarine water and freshwater sediment, information relating to anaerobic sediment is limited, therefore studies on similar conditions in water logged soil and activated sludge are included. In addition, there are references to biodegradation pathways of NTA in open literature.

The results of the simulation tests demonstrate that NTA is readily degraded under aerobic conditions in fresh waters (Larson & Davidson, 1982; Shannon et al., 1974) and freshwater sediments (McFeters et al., 1990). Biodegradation is enhanced by increased temperature, although was still found to occur even at low temperatures, albeit at a slower rate (Shannon et al., 1974). Acclimation of bacteria to NTA was demonstrated even at low test concentrations. In saline/estuarine conditions, NTA degradation was found to be rapid over a range of salinities (up to 19%, see Larson & Ventullo, 1986), however the rate of NTA removal decreases at higher salinities and the effect of high salinity was found to be compounded under high NTA dose conditions (Hunter et al., 1986). This suggests that the role of microbes in degradation of NTA may be limited in marine conditions if initial doses are high; dilution and other degradation processes may be dominant in the marine system. Data relating to anaerobic sediments is limited, however NTA was shown to be degraded in anaerobic sludges and waterlogged soils (Tabatabei & Bremner, 1975), which have been acclimatised, as such this is considered to be an important factor. NTA degradation rates in aerobic sludge are found to be reduced at low temperature; demonstrated in field rather than laboratory conditions.

Microbial degradation of NTA leads to intermediates such as iminodiacetate (IDA), glyoxylate, glycine and ammonia, with final metabolic end products including ammonia, nitrates, and carbon dioxide (Egli, 1992).

 

Biodegradation of NTA in soils was followed at temperatures between 2 °C and 30 °C by either test substance analyses in combination with analyses of inorganic nitrogen, or¹⁴CO₂ production from¹⁴C-carboxyl-NTA.

The study results indicate that NTA is readily decomposed by soil microorganisms under aerobic conditions (Tabatabei & Bremner, 1975; Dunlap et al., 1971; Tiedje & Mason, 1974; Shimp et al., 1994), but may be limited under anaerobic conditions (Dunlap et al., 1971). Iminodiacetate is a possible degradation intermediate of NTA biodegradation (Tiedje & Mason, 1974). Biodegradation rates of the test substance did not correlate with pH, drainage, texture, or plant cover (Tiedje & Mason, 1974). NTA was found to be degraded also at low temperatures (2 °C) in previously acclimatized soils. At room temperature degradation rates were highest in soils receiving sewage effluent and in muck soils as well as in soil and sediment samples taken near tile fields (half-lives: ≥ 1 – 3.5 days), while reduced biodegradation rates in mineral surface soils ranging from 4.6 to 55.5 days could be observed. Therefore, it can be assumed that adaption is a key process in NTA biodegradation.

From these studies it can be concluded that NTA can be readily degraded under aerobic conditions in previously adapted soils. The degradation is reduced under O2 deficient conditions and in non-adapted soils. As reported half-lives in non-adapted soils range between 4.6 and 55.5 days, NTA can be regarded as readily biodegradable.

For aerobic mineralisation of NTA, half-lives between 1 and 56 days were determined. For the exposure calculations, a half-life of 56 days in soil is used as a worst case.

 

The reported results demonstrate that NTA is readily biodegraded in all environmental compartments even at low environmental temperatures. Therefore, biodegradation can be considered to be an important removal process of NTA in soil, sediment, surface water, and treatment plants.