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Monitoring data for nitrilotriacetate (NTA) are available for primary effluents of the Glatt sewage treatment plant near Zürich. Alder et al., 1990, investigated the behaviour and diurnal load of (NTA) in winter and in summer in this sewage treatment plant. The plant had a sludge age of 3.6 d in winter and 4.8 d in summer and was in winter only partly nitrifying.

The average daily loads of NTA in the primary effluent in the investigated lane was during the two investigated periods 13 +/- 7 kg NTA per day (0.5 +/- 0.3 g NTA/(person * day)).

The influent concentration ranged between 300 and 1500 µg NTA/l. In both seasons NTA was biologically degraded up to 97%.

The influent concentrations increased by a factor of 4 between 1984 and 1987, whereas the effluent concentrations rose only by a factor of 1.5.

 

Field studies (see Shannon et al. 1974) were undertaken to assesses the degradation of nitrilotriacetic acid (NTA acid) in the natural river environment downstream of a wastewater treatment plant with NTA acid input at two NTA acid input levels (16-20 mg/L and 8-16 mg/L) in winter and summer.

NTA acid removal by the treatment works at 13 - 14 ºC (aeration tank temperature) was > 95 % compared to < 45 % at 10 ºC. The effects of dilution and degradation gave downstream NTA acid concentrations of less than 10 µg/L at higher temperatures (16.5 - 21 ºC), and 125 µg/L at lower temperatures (0.5 - 3.0 ºC). Levels of NTA acid it the mouth of Grindstone Creek averaged 40 - 50 µg/L at the time of the study.

There was evidence of in-stream biodegradation even at the lower temperatures. Whilst dilution and dispersion were obviously major factors in removal of NTA acid from the system, downstream observed NTA acid concentrations were found to be lower than expected from these processes alone, suggesting (together with the temperature effects) that in-stream biodegradation had occurred.

Waterdown wastewater treatment works processes domestic waste, therefore metal concentrations from plant effluent were relatively low, limiting the formation of NTA-metal complexes. Therefore, the majority of the NTA acid leaving the plant as effluent would be either uncomplexed, or as calcium or magnesium complexes (rapidly biodegradable forms). The authors considered it unlikely that resistant heavy metal complexes contributed significantly to the degradation of NTA acid in Grindstone Creek. However, the formation of NTA-metal complexes may contribute to the degradation pathway of effluent from treatment plants handling industrial wastewaters.

 

From these studies it can be concluded that NTA / NTA acid is removed in municipal treatment plants with rates generally above 95% under normal operation conditions.

Contradicting results were obtained for measurements in the winter season: Alder et al., 1990, were not able to determine differences in degradation between summer and winter, the results of Shannon et al., 1974, considerable decrease of NTA degradation at low temperatures.

 

These study results obtained for NTA acid and NTA are used to assess the environmental behaviour of trisodium nitrilotriacetate (Na3NTA). NTA acid, 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-<-> NTA acid <-> 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 considered information relating to Na3NTA and nitrilotriacetate in the assessment of 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.

 

In accordance with the EU RAR 2005 on trisodium nitrilotriacetate, for exposure calculations a removal of 95% is proposed; having in mind that in harsh winters the Na3NTA releases can be increased.