Trisodium nitrilotriacetate

Biodegradation in water: screening tests

Description of key information

Biodegradation studies according to the Sturm test (based on CO2 evolution), 301 E (Ready biodegradability: Modified OECD Screening Test), 302 B (Inherent biodegradability: Zahn-Wellens Test), and a die-away test with activated sludge, as well as a combined CO2/DOC test including a test according to OECD 301 F (manometric respirometry test), were conducted to determine the biodegradability of trisodium nitrilotriacetate (Na3NTA).

From the results obtained in these tests it can be that NTA is readily biodegradable after lag phases between 1 and 16 days.

Key value for chemical safety assessment

Biodegradation in water:

readily biodegradable

Additional information

Short description of key information:

Biodegradation studies according to the Sturm test (based on CO₂evolution), 301 E (Ready biodegradability: Modified OECD Screening Test), 302 B (Inherent biodegradability: Zahn-Wellens Test), and a die-away test with activated sludge, as well as a combined CO2/DOC test including a test according to OECD 301 F (manometric respirometry test) were conducted to determine the biodegradability of trisodium nitrilotriacetate (Na3NTA).

From the results obtained in these tests it can be that Na3NTA is readily biodegradable after lag phases between 1 and 16 days.

Discussion:

According to the Sturm test, Na3NTA has been degraded to 100 % without any lag phase. In tests conducted according to OECD 301 E for a total duration between 7 and 14 days, Na3NTA has been degraded to 75-100 %. Observed lag phases ranged between 1 and 11 days. In OECD 302 B, 96 % of NTA have been degraded within 28 days after a 7 days lag phase. The die-away test revealed 100 % degradation within 23 days after a 14 day lag period. In the combined CO2/DOC test, Na3NTA degradation based on DOC removal and CO₂evolution was > 95 % and 91 %, respectively. Lag phases for degradation based on DOC removal and CO₂evolution were 2 and 5 days, respectively. In the manometric respirometry test (OECD 301 F), 92 % of NTA have been degraded after 28 days after a 16 days lag phase.

A short outline of these results is presented in the table given below.

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)
Modified OECD Screening Test	OECD 301 E	28	BASF WWTP effluent	70	90 - 100	-	BASF (1983)

 

From these results it can be concluded that Na3NTA is readily biodegradable after lag phases between 1 and 16 days.

Biodegradation can be considered to be an important removal process of Na3NTA in surface water and treatment plants.

The test substances used in the studies conducted by Strotmann et al. (1995) and Takahashi (1997) were nitrilotriacetate and nitrilotriacetic acid (NTA acid), respectively. Both studies are considered in the assessment of NTA asNTA acid, NTA, and nitrilotriacetate display the same behaviour in the environment: splitting of sodium ions or protons (in case of NTA and NTA acid) and uptake of multivalent metal ions withsubsequent 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 line with the Canadian ‘Draft Screening Assessment for Nitrilotriacetic acid (CAS 139-13-9)’ from January 2010,which also consideredinformation relating toNa3TAand nitrilotriacetateintheassessmentof NTA acid. This is due to the fact that the toxicological endpoints, as stated in the Canadian ‘Screening Assessment for Nitrilotriacetic acid’, of NTA acid and Na3NTA are similar. Moreover, the dissociation of NTA acid and Na3NTA leads to the common moiety nitrilotriacetate.

Data from studies with salts formed with various cations such as calcium, magnesium, aluminum, zinc and iron were not included. Canada and the European Union also similarly did not include these other NTA salts in the ‘Draft Screening Assessment for Nitrilotriacetic acid’ and the ‘Draft Risk Assessment Report (EURAR 2008)’, respectively.