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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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The degradation of 14C-glufosinate-ammonium was investigated under the conditions of two water /sediment studies at 20 °C and 8 °C or 10°C in contrasting sediments and their associated water. The degradation under conditions of a water/sediment study was also investigated following separate dosing of metabolite AE F061517 (MPP).

Route of degradation:

Glufosinate-ammonium was degraded via a similar route as found for aerobic soil, i.e. via oxidative deamination to AE F065594 (PPO) as a minor metabolite, followed by the formal but multistep loss of methylene groups each to result in AE F061517 (MPP, maximum 79.8% at day 14) and AE F064619 (MPA, maximum 19.9% at day 50) by oxidative processes. Alternatively, the transformation of AE F061517 (MPP) resulted via dehydrogenation (formal loss of hydrogen) in the formation of 3-[hydroxy(methyl)phosphoryl]acrylic acid (P-X, AE 0015081, maximum 12.5% at day 50). Metabolite N-acetylglufosinate (NAG) was observed at a maximum of 13.7% at day 1 thus demonstrating the transient character.

Rate of degradation:

Water/sediment studies were kinetically evaluated following the FOCUS kinetics guidance. For total systems the kinetic evaluation according to FOCUS guidance (Level P-I) resulted in geometric mean degradation half-lives of 8.7 days for glufosinate-ammonium and 219.4 days for MPP (AE F061517). For MPA (AE F064619) a DegT50 of 33.3 days was derived for total systems.

According to Echa guidance R.16, equation R.16 -9, the DT50 for the whole system of 8.7 days at 20°C resulted in a half-life of 16.5 days or a degradation rate of 0.042 d-1 at 12°C.

Additionally the degradation of 14C-glufosinate-ammonium was investigated in a pelagic-water study at 20 °C and the DT50 was estimated to be 54.3 days.

Key value for chemical safety assessment

Additional information

Three studies on the distribution and degradation in sediment/water-systems of glufosinate-ammonium and metabolites under aerobic conditions are available. All studies were conducted according to GLP and different international guidelines, were applicable. Additionally the degradation of 14C-glufosinate-ammonium was investigated in a pelagic-water study at 20 °C. As this study did not recogniize the effect of increased microbial activity induced by sediment, the result of the study are not used for further caonclusion about biodegradation of glufosinate-ammonium.

The dissipation from the water phase and the degradation of 3,4-14C- AE F039866 [glufosinate-ammonium, purity 98%] was studied in a sediment/water system (gravel-pit, sand) in accordance with the Candian Environmental Fate Guideline, Chapter 6.2, C-2, and US EPA Guidelines, Subdivision N, 162-4 (Oct. 1982) (Stumpf, 1993;M-132665-02-1). The test was performed at two different temperatures under aerobic conditions and a concentration of approx. 0.1 mg a. s. /kg water. The ratio of water/sediment was 10:1 based on dry sediment. The samples were treated with the test substance and incubated in the dark at 21 °C and 8 °C. The incubation flasks were equipped with glass wool and soda lime traps to collect organic volatiles and14CO2. Single samples were taken for analysis at 0, 1, 4, 7, 14, 21, 29, 60, 90, 120, 238 and 361 days after application. For each sample supernatant and sediment were separated by centrifugation. The sediment was extracted with water. Sediment extracts and the water phase were analysed by radio-HPLC using a strong basic anion exchange phase, isocratic elution and radio-TLC (normal phase). Metabolites were characterised and identified by co-elution with reference standards. Non-extractable residues were quantified by combustion of the dried sediments after extraction with water. The overall recovery of radioactivity in both series for all samples was in the range between 96 - 108% of applied radioactivity. The main proportion remained in the water phase (21 °C series: 68 - 97%, 8 °C series: 70 - 97% of applied). Approximately half of the radioactivity remaining in the sediment after centrifugation was extractable with water. The DT50/DT90 values for glufosinate-ammonium were calculated according to 1st order kinetics. The DT50/DT90 values for total system were 3/9 days at 21 °C and 20/65 days at 8 °C. DT50/DT90 for dissipation from the water phase, gave practically the same values (21 °C: r2=0.95), (8 °C: r2=0.96). The study was performed in accordance with the referred guidelines. For the major metabolites in the systems, MPP (AE F061517) and P-Y, no declination pattern could be established at the end of the study in the 8 °C system, and at 20 °C, degradation was slow. The metabolite NAG (AE F099730) reached maximum levels in the water phase after 1 day at 21 °C; 12% of applied (2% in the sediment). In the 8 °C system, the maximum level of 14% was reached after 7 days. The declination DT50/DT90 of this metabolite at 21 °C could be calculated to be 11/35 days (r2=0.98), based on first order kinetics and data from days 1 – 60 of incubation. MPA (AE F064619) and P-X were only found at <10% of applied radioactivity.

The dissipation from the water phase and degradation of 3,4-14C- AE F039866 [glufosinate-ammonium, purity 94%] were studied in two water/sediment system (Nidda, loam; gravel-pit, sand) under aerobic conditions in accordance with German BBA Guideline IV, 5-1 and US EPA Guidelines, Subdivision N, 162-4 (Oct. 1982) (Stumpf, 1994;M-133889-03-1). The water/sediment samples were treated at application rates of approx. 0.1 and 1 mg a. s. /kg water. Three series of samples were treated with the test substance and incubated in the dark at 20 °C. The incubation flasks were equipped with appropriate traps to collect organic volatiles and14CO2. Samples were taken for analysis at 0, 1, 3, 7, 14, 21, 30, 50, 77, 91 and 130 days after application. For each sample supernatant and sediment were separated by centrifugation. The sediment was extracted with water. Sediment extracts and the water phase were analysed by radio-HPLC using a strong basic anion exchange phase and isocratic elution and radio-TLC (normal phase). Metabolites were characterised and identified by co-elution with reference standards. Non-extractable residues were quantified by combustion of the dried sediments after extraction with water. The DT50/DT90 values for glufosinate-ammonium were calculated based on 1st order kinetics. DT50/DT90 values for the whole systems were 11/34 days in the Nidda system treated at 1 mg/kg, 2/5 days in the gravel pit system treated at 0.1 mg/kg and 42/>130 days in the gravel pit system treated at 1 mg/kg. Calculation of the dissipation rate of glufosinate-ammonium in the water phase results in DT50/DT90 values of 13/44 days (r2=0.98, days 0 - 30) in the Nidda system treated at 1 mg/kg, 1.4/5 days (r2=1.0, days 0 - 14) in the gravel pit system treated at 0.1 mg/kg and 72/>200 days (r2=0.83, days 0 – 90) in the gravel pit system treated at 1 mg/kg. For the later system, declination of glufosinate-ammonium was only seen during the first period of incubation. Based data from days 0 – 50, DT50/DT90 for the water phase was 42/140 days (r2=0.92). However, the ceased degradation indicate that the microbial activity in that system was almost depleted already after 30 days and therefore, this data is considered to be less relevant for the environmental assessment. In one of the systems (Nidda, 20 °C, 1.0 mg a.s./L), the proportions of glufosinate-ammonium in the sediment reached a maximum of 13% of applied radioactivity during the first week of incubation. The degradation of14C-glufosinate-ammonium [purity >91%] was studied in an additional experiment to determine the degradation pathway and to elucidate the structure of unknown degradation products. The study was performed in accordance with the German BBA Guideline IV, 5-1 and US EPA Guidelines, Subdivision N, 162-4 (Oct. 1982). Three samples of14C-glufosinate-ammonium, each labelled in a different position of the molecule (C1, C3,4 and H3C-P, part A, B and C of the study) were applied to a surface water/sediment system (Nidda, loamy sediment) and incubated under aerobic conditions in the dark at 20 °C. The water/sediment samples were treated at an application rate of approx. 15 mg a. s. /kg water and sediment. The water/sediment ratio was 9:1. The incubation flasks were equipped with traps to collect organic volatiles and14CO2. Aliquots of the water phases were taken for analysis at 0, 1, 3, 7, 14, 21, 30, 36, 45, 55, 77, 92, 215 and 318 days after application. The mineralisation rate was measured at the same sampling time points. Only at day 325 (study termination), the sediments were separated from the water phases and extracted with water. The water phases and the sediment extracts were analysed separately by radio-HPLC (strong basic anion exchange column, SAX) and in addition the water phases of day 318 by radio-TLC (silica gel). Metabolites were characterised and identified by co-elution with reference standards. The unknown metabolites were identified with NMR and HPLC/ESI-MS. Non-extractable residues were quantified by combustion of the dried sediments after extraction with water. The overall recovery of radioactivity was in the range 94 - 106% of applied. The main proportion of applied radioactivity remained in the water phase (1 mg/kg series: 56 - 94%; 0.1 mg/kg series: 79 - 89% of applied). The extractable radioactivity from the Nidda sediment was 9 - 19% and from the gravel-pit 5 - 11% of applied. In one of the systems (Nidda, 20 °C, 1.0 mg as/L), the proportions of glufosinate-ammonium in the sediment reached a maximum of 13% of applied radioactivity during the first week of incubation. Identified metabolites were MPP (max 13% after 92 days in system B), NAG (max 5.9% after 45 days in system C, MPA (max 9.3% after 215 days in system B) and P-Y (MPF, methylphosphinico-formic acid, AE F130947, maximum 65% of applied after 325 days in system C) formed by a further shortening of the carbon chain, i. e., the next homologue of MPP and MPA. In sample C treated with glufosinate-ammonium labelled in the H3C-P position, two additional metabolites were identified at low levels; methylphosphinic acid AE F078091 (P-Y(A)) and methylphosphonic acid AE F130948 (Hoe 130948).

The dissipation from the water phase and degradation of 2,3-14C-labelled 3-methylphos-phinico-propionic acid (AE F061517, MPP, purity 94%), a metabolite of glufosinate-ammonium, were investigated in two water/sediment systems at 10 ± 2 °C and 20 ± 2 °C under aerobic conditions in the dark (Zumdick, 1997; M-141125-01-1). The study was conducted according to EU Annex II, 7.2.1.3.2 (July 1995), SETAC (March 1995) and German BBA Guideline IV, 5-1 using two water/sediment systems. Following an acclimatisation period, the test substance was applied onto the water surface at a total concentration of 0.383 mg/L water. Two series of samples were treated with the test substance and incubated in the dark at ca. 20 °C and 10 °C. The incubation flasks were equipped with appropriate traps to collect organic volatiles and14CO2. Samples were taken for analysis at 0, 3, 7, 14, 28, 59, 90 and 100 days after application. Water phase and sediment were separated by centrifugation. The sediment was extracted with water. Sediment extracts and the water phase were analysed by HPLC and TLC. Total recoveries accounted for 99 - 102% of applied radioactivity for both systems and temperatures. At the end of the incubation period the amount of MPP was above 50% of applied radioactivity in the systems at 10 °C or 20 °C. DT50 values were estimated to be ca 150 days for the water phase and >200 days for the whole system. The maximum levels of radioactivity in the sediment were observed after 28 - 59 days and amounted to 21 – 32% of applied.

The mineralisation of 3,4-14C-labeled active substance glufosinate-ammonium was investigated in non-sterile natural water at pH 8.0 at test concentrations of 9.0 µg a.s./L(Stupp, H.-P. & Kasel, D., 2015; M-519185-01-1). An exaggerated high dose of 90 µg/L was investigated as well, but not summarized herein, as the concentration of 9.0 µg/L is regarded as more representative for the environment. Samples were incubated at 20 ± 2°C in the dark for 66 days in maximum. Microbial activity of the test water was demonstrated by incubation of phenyl-UL-14C-labeled benzoic acid serving as reference. The mean material balances were 101.8% ± 1.8% AR. Values of the test substance in the test water decreased from 98.2% of AR at time zero to 70.0% after 66 days of incubation. Metabolites AE F065594 (PPO), NAG, AE F061517 (MPP) and AE 0015081 (P-X) were observed at maximum values of 8.1% AR (day 44), 26.3% (day 30), 8.3% (day 44) and 5.2% (day 44), respectively, in the course of the study. The observation of AE F065594 (PPO), NAG, AE F061517 (MPP), AE 0015081 (P-X) and carbon dioxide clearly indicated that the transformation of glufosinate-ammonium under the ‘pelagic’ conditions of the test was driven by microbial processes. Formation of14C-carbon dioxide accounted for 8.1% of AR in maximum by day 55. A decline of microbial activity was indicated by a lower level of carbon dioxide formed (5.2% AR) at the end of the test (day 66). Values of the DT50 of glufosinate-ammonium under conditions of mineralization testing were calculated to 54.3 days following simple first order (SFO) kinetic evaluation as best fits to measured data.

The kinetics of degradation in total water/sediment systems was evaluated for glufosinate-ammonium and its metabolites (Preuss, T. & Mikolasch, B., 2015; M-522823-01-1) from three water/sediment studies mentioned above (Stumpf, 1993; Stumpf, 1994, Zumdick, 1997). The kinetic evaluation followed FOCUS guidance to derive values for the degradation in total sediment/water systems from best fits to measured data for use as modelling endpoints in aquatic exposure assessments. Analysis was performed for glufosinate-ammonium at Level I for total systems with results summarised in the table below.

The predominant number of datasets was evaluated by the ‘all-SFO’ approach for all compounds while some datasets for the active substance for metabolite AE F061517 (MPP) resulted in DFOP kinetics as best-acceptable fits.

For the active substance glufosinate-ammonium the kinetic evaluation resulted in a geometric mean value for the DegT50 of 8.7 days in total systems (see Table below).

Table: Total system DegT50 values for glufosinate-ammonium according to FOCUS Level I

Water/Sediment system

Total system DegT50
(days)

Gravel-pit*

4.3a)

Nidda, 1 mg/L**

12.3

Gravel-pit, 0.1 mg/L**

1.4

Gravel-pit, 1 mg/L**

78.2a)

Nidda***

-

Gravel-pit***

-

Geometric meana)

8.7

* Stumpf, 1993; ** Stumpf, 1994; ***Zumdick, 1997

a)Conservative evaluation on the basis of second, lower rate of degradation following DFOP kinetic model

For metabolites AE F061517 (MPP) and AE F064619 (MPA) the kinetic evaluation resulted in a geometric mean values for the DegT50 in total systems of 219.4 days and 33.3 days (see Table below).

Table: Total system DegT50 values for metabolites AE F061517 (MPP) and AE F064619 (MPA) according to FOCUS Level I

Water/Sediment system

Total system DegT50
(days)

Compound

AE F061517
MPP

AE F064619
MPA

Gravel-pit*

234.9

n.d.

Nidda, 1 mg/L**

n.d.

n.d.

Gravel-pit, 0.1 mg/L**

463.2

n.d.

Gravel-pit, 1 mg/L**

60.5

n.d.

Nidda***

314.3

n.d.

Gravel-pit***

245.6

33.3

Geometric meana)

219.4

33.3

* Stumpf, 1993; ** Stumpf, 1994; ***Zumdick, 1997

a)geomean in case of more than one value

 

Overall conclusion on biodegradation in water/sediment

The degradation of14C-glufosinate-ammonium was investigated under the conditions of two water /sediment studies at 20 °C and 8 °C or 10°C in contrasting sediments and their associated water. The degradation under conditions of a water/sediment study was also investigated following separate dosing of metabolite AE F061517 (MPP).

Route of degradation:

Glufosinate-ammonium was degraded via a similar route as found for aerobic soil, i.e. via oxidative deamination to AE F065594 (PPO) as a minor metabolite, followed by the formal but multistep loss of methylene groups each to result in AE F061517 (MPP, maximum 79.8% at day 14) and AE F064619 (MPA, maximum 19.9% at day 50) by oxidative processes. Alternatively, the transformation of AE F061517 (MPP) resulted via dehydrogenation (formal loss of hydrogen) in the formation of 3-[hydroxy(methyl)phosphoryl]acrylic acid (P-X, AE 0015081, maximum 12.5% at day 50). Metabolite N-acetylglufosinate (NAG) was observed at a maximum of 13.7% at day 1 thus demonstrating the transient character.

Rate of degradation:

Water/sediment studies were kinetically evaluated following the FOCUS kinetics guidance. For total systems the kinetic evaluation according to FOCUS guidance (Level P-I) resulted in geometric mean degradation half-lives of 8.7 days for glufosinate-ammonium and 219.4 days for MPP (AE F061517). For MPA (AE F064619) a DegT50 of 33.3 days was derived for total systems.