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Environmental fate & pathways

Biodegradation in soil

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Tabatabai & Bremner (1975) studied the decomposition of trisodium nitrilotriacetate monohydrate in soils by performing analyses for Na3NTA and inorganic nitrogen after incubation of NTA-treated soils (200 mg/kg soil d.w.) for various times (>= 7 - <= 42 days) at 30 °C.
Tiedje & Mason (1974) measured the biodegradation of nitrilotriacetate (NTA) at room temperature in a variety of soils as measured by total CO2and14CO2production from14C-carboxyl-NTA for up to 50 days. 
Shimp et al. (1994) assessed the biodegradation of nitrilotriacetic acid (H3NTA) in an established septic tank system and adjoining shallow sand aquifer (Cambridge, Ontario). Studies were conducted on soil and aquifer sediment samples from a transect of the septic tank effluent plume with initial concentrations of radiolabelled [1-14C]-nitrilotriacetate (specific activity of 35µCi/mg, radiochemical purity > 95 %) between 0.025 and 0.1 mg/kg soil. Biodegradation measurements in soil were based on the mineralization of radiolabeled NTA to14CO2. Assays were conducted using composite samples from five to 10 depths throughout each 1.5 m soil core. Biodegradation was measured, and rates were compared in samples from outside and within the discharge zone.
Dunlap et al. (1971) determined the biodegradability of radiolabeled trisodium nitrilotriacetate (conc. ca. 50 mg/l of both BOD and Na3NTA, specific activity of 900 pCi/ml) in soil column experiments under aerobic and anaer. conditions using three different soil types (sand, loam, and clay-loam soil) for a total duration of 77 days. Degradation was followed by test material analysis and radiochemical measurement of selected effluent samples.
The overall conclusion from the studies is that Na3NTA can be quickly degraded under aerobic conditions in previously adapted soils. The degradation is limited under O2 deficient conditions and in non-adapted soils.
A possible degradation intermediate of Na3NTA degradation in soils is iminodiacetate.

Key value for chemical safety assessment

Half-life in soil:
56 d
at the temperature of:
25 °C

Additional information

The results obtained by Tabatabai & Bremner (1975) show that trisodium nitrilotriacetate monohydrate (Na3NTA * H2O) is readily decomposed by soil microorganisms under aerobic or anaerobic conditions (≥ 50 % degradation after 5 days of incubation at 30 °C). NTA-N was converted to nitrate and ammonium under aerobic and anaerobic conditions, respectively.

The results of the soil column studies conducted by Dunlap et al. (1971) also indicate rapid and complete biodegradation of Na3NTA under aerobic conditions. In contrast, the results Dunlap et al. (1971) with anaerobic sand, loam, and clay-loam soil columns clearly demonstrate slow and incomplete biodegradation of Na3NTA, indicating that Na3NTA infiltrating through saturated soil systems such as flooded septic tank percolation fields or under waste water lagoons, will likely undergo at most only slow degradation (half-lives were not reported).

Further study results are available for nitrilotriacetic acid (NTA acid, H3NTA) and nitrilotriacetate. NTA acid, Na3NTA, 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 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 using read-across. 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.

One additional study was conducted by Tiedje & Mason (1974). In contrast to the findings of Tabatabai and Bremner (1975), results from Tiedje & Mason (1974) indicate that14CO2production from nitrilotriacetate (NTA) did not occur anaerobically and was severely limited under microaerophilic conditions. This finding is in line with the results obtained by Dunlap et al. (1971). In accordance with Tabatabai & Bremner (1975) and Dunlap et al. (1971), Tiedje & Mason (1974) found rapid degradation of NTA under aerobic conditions, with iminodiacetate as possible degradation intermediate.

Besides, the results of Tiedje & Mason (1974) show that NTA degradation rates did not correlate with pH, drainage, texture, or plant cover. Rates of degradation increased with increasing substance concentrations. 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 (at 40 ppm NTA: 8 to 10 ppm/day which is equivalent to 0.2 to 0.25 d-1and half-lives of 2.8 to 3.5 days) while the degradation in mineral surface soils ranges from 0.5 to 6 ppm/day (at 40 ppm NTA; equivalent to 0.0125 to 0.15 d-1and half-lives of 4.6 to 55.5 days).

The assumption that adaption seems to be key process is further supported by the results determined by Shimp et al. (1994), as illustrated by the rapid biodegradation of nitrilotriacetic acid (NTA acid) near the tile field (half-lives ≥ 1 - ≤ 3 days in soils and sediment samples, and ≤ 1 day in groundwater samples) and limited biodegradation at locations far downgradient or ungradient of the system, where little or no NTA acid loading occurred (half-life of approximately 1.7 days for groundwater samples in 60 m distance from the tile field, almost no biodegradative activity in soil samples collected from sites 20m downgradient form the tile field).

From these studies it can be concluded that Na3NTA can be readily degraded under aerobic conditions in previously adapted soils. The degradation is limited 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 cannot be regarded as persistent in soils.

 

For aerobic mineralisation of Na3NTA, half-lives between 1 and 56 days were determined. For the exposure calculations, a half-life of 56 days is used as a worst case.