Ammonium 2-amino-4-(hydroxymethylphosphinyl)butyrate

Phototransformation in soil

Link to relevant study record(s)

Reference

Endpoint:

phototransformation in soil

Type of information:

experimental study

Adequacy of study:

key study

Study period:

30.01.1988 - 17.05.1989

Qualifier:

according to guideline

Guideline:

EPA Guideline Subdivision N 161-3 (Photodegradation Studies on Soil)

Deviations:

no

GLP compliance:

yes

Radiolabelling:

yes

Analytical monitoring:

yes

Analytical method:

high-performance liquid chromatography

other: liquid scintillation counting

Details on sampling:

The sampling times for each series were 3, 6, 24, 48, 72, and 120 hours of irradiation in the photoreactor. At each sampling time one of the soil plates was removed for analysis and the absorption media were exchanged. After the last sampling the box was rinsed with 50 mL of water. The same number of plates were analysed for the dark controls.
The following parameter have been determined:
- radioactivity content in:
absorption traps
soil extract
extracted soil
- identity of radioactive residues in soil extract
- estimation of the disappearance time (DT-50) of the test substance

Details on soil:

Sandy loam
Source: Hoechst Aktiengesellschaft
Geschäftsbereich Landwirtschaft
Biologische Forschung
Versuchsfeld Tor Süd, Feld 1, Parzelle 10
Postfach 80 03 20
D-6230 Frankfurt (M) 80, FRG

Properties of the soil:
texture: Sandy Loam SL V
batch: #880721
sand (0.05 - 2 mm): 56.8 %
silt (0.002 - 0.05 mm): 31.6 %
clay (0 - 0.002 mm): 11.6 %
pH-value (0.01 m CaCl2): 6.1
cation exchange capacity: 6.55 meq/100 g soil
bulk density: 1.14 g/cm3
max. water capacity: 37.9 g/100 g soil
organic matter: 1.79 %

Preparation of the soil-covered thin layer plates
The soil (air dried) was passed through a 500 μm sieve and then slurried with water until moderately flowable (120 g soil and 80 ml water). Stainless steel plates (4 cm x 4 cm) were coated with the slurry and then air dried. Prior to the application of the pesticide the soil was sterilized by spraying with chloroform. After evaporation of the chloroform under an infrared radiator, the plates were ready for application.

Light source:

Xenon lamp

Light spectrum: wavelength in nm:

>= 290 - <= 490

Relative light intensity:

>= 0.1 - <= 14.2

Details on light source:

For the simulation of sunlight radiation an ORIGINAL HANAU SUNTEST photoreactor (Hanau Quarzlampen GmbH, D-6450 Hanau) was used. The experimental set-up is essentially the same as described by Parker and Leahey (S.Parker, J.P.Leahey; Development of a Method to Investigate the Photodegradation of Pesticides; Proceedings 1988 British Crop Protection Conference - Festsand Diseases, 663 - 668, 1988.).

Design of apparatus
A xenon burner is installed in the top section of the machine, surrounded by the filter system, comprising selective reflecting mirrors and a quartz glass dish. The selective reflecting mirrors keep the undesirable infra-red radiation from the specimen. The required regions of the radiation such as light and ultra-violet impinge on the surface of the specimen with high intensity. The parabolic reflector ensures uniform radiation on the specimen. The specimen is placed in the bottom section of the machine. A blower is provided for cooling both burner and specimen surface.
Intensity measurements
The radiation produced by the xenon burner and selected by the filter system is close to that of natural sunlight. The radiation is cut off at 290 nm in the ultra-violet range through choice of a supplementary filter made of special ultraviolet glass. The total intensity of light at the irradiation position was measured immediately before and after sample irradiation with an uranyl sulfate/oxalic acid actinometer. From the results of these measurements of the total sum of irradiation and the intensity distribution given by the manufacturer the spectral photon fluxes were calculated for each wavelength and compared to those of natural sunlight with the program REALIN.
Actinometry
At the beginning and the end of the total experiment an actinometric measurement was performed. Based on the emission characteristics of the light source, the UV spectrum of the actinometer solution, and the total number of photons absorbed by the actinometer the spectral photon flux was calculated. The comparison with sunlight data is performed on the basis of total photons available in the range 290 - 490 nm. The calculated fluxes for the different wavelength intervals are added up and divided by the corresponding value for total photon flux at 52° N given in the literature (R.Frank, W.Klöpffer Spectral Solar Photon Irradiance in Central Europe and the Adjacent North Sea; Chemosphere, 17, 985 - 994, 1988). This results in the conversion factor from irradiation hours in the instrument to hours of sunlight. Finally, it must be taken into account that one day under outdoor conditions is equal to only 12 h of sunshine. Based on these considerations the actinometer measurements were evaluated as shown in appendices 1 and 2. One hour of irradiation corresponded to 3.2 h of sunlight. Thus, the 120 h (5 d) experiment simulated a 32 d outdoor irradiation with 12 hours of sunshine per day.

Details on test conditions:

Application of test substance
Application rate: 80 μg/plate (= 7577310 dpm)
The medium weight of the soil on one plate was approx. 3 g, resulting in a concentration of 25 ppm.

Mode of application:
The available stock solution contained 5.9 mg glyphosinate ammonium in 6.4 ml of water. This solution was made up to 10 ml wih bidestilled water. After determination of the the exact concentration by counting with the LSC. 130 μl aliquots of the solution containing 80 μg glyphosinate ammonium were applied with an Eppendorf pipette as spots on the soil surface. The plates were carefully dried under an infrared radiator and then exposed in the photoreactor.
Apparatus
In this experiment the soil-covered plates treated with the pesticide were exposed in a stainless steel box topped with a quartz plate. The box was connected to the absorption unit. For additional cooling this box was placed on a special cooling table. For absorbing volatiles one trap contained 0.5 g XAD-2 resin (Serva) in 10 mL bidistilled water adjusted to pH 3 with sulfuric acid. 14 co2 was absorbed in 10 mL Carbo-Sorb (alkylamine, Packard Instr.). To absorb any substances which might pass both stages a cold trap containing a mixture of 20 mL methanol and 1 mL 1 M sodium hydroxide solution was used which was cooled with dry ice. With the aid of a vacuum pump a slow air stream was passed through the whole system. The air inlet tube was protected by a plug of sterile quartz wool in order to keep the soil plates sterile.


Experimental conditions
The experiments were performed in two series of simultaneous irradiations of six plates each in the quartz-topped box. A seventh plate yielded the data at time t0. An additional series, stored in a closed system excluded from light, served as dark control. The actual temperature of the plates was monitored using an additional plate with a thermocouple embedded in the soil layer. The soil temperature of the irradiated plates was in the range of 25 ± 3 °C, the dark control plates were maintained at a temperature of 21 ± 2 °C.

Duration:

120 h

Temp.:

25 °C

Initial conc. measured:

25 ppm

Reference substance:

no

Dark controls:

yes

% Degr.:

2.3

Sampling time:

120 h

DT50:

>= 3 241 - <= 3 374 h

Transformation products:

yes

No.:

#1

Details on results:

Activity measurements

The results of all activity measurements are stated in tables below. For soil extracts and solutions from soil combustions the individual data are given. Due to the experimental set-up with more than one soil sample present in the irradiation chamber for most of the time, the activity contents of the absorption traps are integrated values for the respective number of soil plates over the time between two sampling intervals. Therefore, the measured data were divided by the soil plate number, and for balancing purposes these normalized data were added to the corresponding data from previous samplings. The normalized data can be derived from the tables as the differences between subsequent sampling times. Total recovery at t - 0 for the respective series was taken as the 100 % value. The mass balances which were normalized for the day zero recovery of the irradiated samples and the dark controls were not significantly different, but they were varying:

irradiation #1: 95.6-107.5%, dark control #1: 99.6-107.7%
irradiation #2: 94.7-101.9%, dark control #2: 96.6-104.5%

Contamination of the irradiation chamber was negligible in all cases. Less than 0.1 % of total applied activity of each series appeared in the final rinse of the whole box.

Analysis of degradation products
The soil extracts were analyzed by HPLC. No degradation products were found in the chromatograms. The extracts consisted exclusively of the test substance glyfosinate-ammonium Cochromatography of a reference solution containing glyfosinate-ammonium and two other known soil metabolites (from biotic soil degradation studies) allowed to exclude the presence of any other compound than the unaltered test substance. The only measurable degradation product was 14-CO2 which amounted 2.3 % of the applied radioactivity after 120 hours of irradiation. The non-extractable residues were low, but increased slightly until study termination (2 - 6 %).

Comparison of irradiation experiments and dark controls
In both series the dark controls did not show the release of 14-CO2 or any other volatile degradation product. The formation of not extracted residues were negligible. This clearly indicates that sterile conditions were maintained during the study. Based on the comparison of irradiated and non-irradiated samples CO2 is considered to be the only photodegradate.

Kinetics of photodegradation
Assuming first order kinetics for the degradation of glyfosinate-ammonium the following times for 50 % disappearance (DT-50) are calculated (amount of glyfosinate-ammonium = extractable part) by

ln c = ln c0 – k * t ; DT-50 = (ln 2) / k
Irradiation #1: k = -0.0002 h^-1 (r = -0.356)
Irradiation #2: k = -0.007 h^-1 (r = -0.576)
Dark Control #1: k = +0.005 h^-1 (r = +0.69was
Dark Control #2: k = -0.004 h^-1 (r = -0.723)
lt was not possible to calculate DT-50 with these values. The regression for both the dark control and the irradiations is poor because of the variation of the very low differences of glyfosinate-ammonium contents in the extracts.
Therefore, the only way to calculate data for the photolytic degradation are to use the values of the CO2 evolution. With the equations
ln (c0 - z) = ln c0 - k' * t ; DT-50 = (ln 2) / k'
where c0 = initial concentration of glyfosinate-ammonium (=100 %),
and z = concentration of 14-CO2 ,


the following DT-50 values have been calculated.
Irradiation #1: DT-50 (Photolysis) = 3241 h
Irradiation #2: DT-50 (Photolysis) = 3374 h

These data can now be converted to half-live times for outdoor conditions at 52° N using the factors quoted in section 5.1. In the first experiment it would take 3241 x 3.19 h = 10400 h of sunshine to degrade 50 % of the initial amount of glyfosinate-ammonium present, i.e. 860 days with 12 hours of sunshine per day. The corresponding values from the second experiment are 3374 x 3.19 h = 10800 h, i.e. 900 days. Thus, the average value for DT-50 under outdoor conditions is 880 ± 20 days or 2.4 years.

Activity measurements

Irradiation Series #1 - material balance and product distribution - Application rate: theor. 7561533 dpm (80 μg) measur. 6852636 dpm (= 100 %)
I R R A D I A T I O N
Time (h)	Soil extract (dpm)	Soil extract (%)	Soil non extr. (%)	Soil non extr. (dpm)	XAD- 2 water (dpm)	XAD- 2 extr.(dpm)	XAD- 2 resin (dpm)	XAD- 2 sum (dpm)	XAD- 2 sum (%)	CO2 (dpm)	CO2 (%)	Cold Trap (dpm)	Sum (dpm)	Rel Sum 0h = 100% (%)
0	7043122	102.8	110109	1.6	0	0	0	0	0.0	0	0.00	0	7153231	104.4
0	6518984	95.1	33057	0.5	0	0	0	0	0.0	0	0.00	0	6552041	95.6
3	7267847	106.1	58503	0.9	158	35	11	204	0.0	1122	0.02	0	7327676	106.9
6	7199425	105.1	56928	0.8	448	87	15	551	0.0	5521	0.08	15	7262439	106
24	6984856	101.9	95556	1.4	1911	177	26	2114	0.0	53163	0.78	321	7136010	104.1
48	7171058	104.6	47575	0.7	4511	230	45	4786	0.0	105560	1.54	463	7329442	107
72	7160900	104.5	59721	0.9	8386	505	92	8983	0.1	136347	1.99	463	7366414	107.5
120	6836842	99.8	171243	2.5	13336	725	232	14293	0.2	157852	2.3	1588	7181818	104.8

Irradiation Series #1 - material balance and product distribution - Application rate: theor. 7561533 dpm (80 μg) measur. 6852636 dpm (= 100 %)
D A R K  C O N T R O L
Time (h)	Soil extract (dpm)	Soil extract (%)	Soil non extr. (%)	Soil non extr. (dpm)	XAD- 2 water (dpm)	XAD- 2 extr.(dpm)	XAD- 2 resin (dpm)	XAD- 2 sum (dpm)	XAD- 2 sum (%)	CO2 (dpm)	CO2 (%)	Cold Trap (dpm)	Sum (dpm)	Rel Sum 0h = 100% (%)
3	6781324	99	46960	0.7	25	35	9	69	0.0	28	0.00	0	6828381	99.6
6	7231568	105.5	71762	1	25	35	9	69	0.0	50	0.00	0	7303449	106.6
24	6869768	100.3	47512	0.7	25	45	13	83	0.0	140	0.00	0	6917504	100.9
48	7208180	105.2	99541	1.5	25	45	13	83	0.0	170	0.00	0	7307975	106.6
72	7157624	104.5	53786	0.8	100	45	13	158	0.0	193	0.00	238	7211999	105.2
120	7307034	106.6	69110	1	100	95	20	215	0.0	348	0.01	738	7377445	107.7

Irradiation Series #2 - material balance and product distribution - Application rate: theor. 7577310 dpm (80 μg) measur. 7228703 dpm (= 100 %)
I R R A D I A T I O N
Time (h)	Soil extract (dpm)	Soil extract (%)	Soil non extr. (%)	Soil non extr. (dpm)	XAD- 2 water (dpm)	XAD- 2 extr.(dpm)	XAD- 2 resin (dpm)	XAD- 2 sum (dpm)	XAD- 2 sum (%)	CO2 (dpm)	CO2 (%)	Cold Trap (dpm)	Sum (dpm)	Rel Sum 0h = 100% (%)
0	7143664	98.8	42731	0.6	0	0	0	0	0.0	0	0.00	0	7186395	99.4
0	7239260	100.1	31751	0.4	0	0	0	0	0.0	0	0.00	0	7271011	100.6
3	7179225	99.3	25000	0.3	5192	443	11	5646	0.1	592	0.01	158	7210621	99.7
6	7339110	101.5	18368	0.3	5192	487	18	5697	0.1	636	0.01	328	7364139	101.9
24	7180432	99.3	38351	0.5	6617	830	111	7557	0.1	7053	0.10	328	7233722	100.1
48	7180000	99.3	50640	0.7	8900	1030	310	10239	0.1	21800	0.30	1212	7263891	100.5
72	6265080	86.7	444115	6.1	8900	3410	833	13143	0.2	117175	1.62	3312	6842824	94.7
120	6937000	96.0	208823	2.9	12700	3770	1022	17495	0.2	166835	2.31	9987	7340136	101.5

Irradiation Series #2 - material balance and product distribution - Application rate: theor. 7577310 dpm (80 μg) measur. 7228703 dpm (= 100 %)
D A R K  C O N T R O L
Time (h)	Soil extract (dpm)	Soil extract (%)	Soil non extr. (%)	Soil non extr. (dpm)	XAD- 2 water (dpm)	XAD- 2 extr.(dpm)	XAD- 2 resin (dpm)	XAD- 2 sum (dpm)	XAD- 2 sum (%)	CO2 (dpm)	CO2 (%)	Cold Trap (dpm)	Sum (dpm)	Rel Sum 0h = 100% (%)
3	7232236	100.00	20620	0.3	0	7	0	7	0.0	10	0.00	0	7252873	100.3
6	7322960	101.3	13959	0.2	220	17	1	237	0.0	36	0.00	60	7337252	101.5
24	7279320	100.7	50494	0.7	220	17	9	246	0.0	102	0.00	60	7330222	101.4
48	7434390	102.8	117736	1.6	220	30	12	262	0.0	247	0.00	60	7552695	104.5
72	6979250	96.5	115299	1.6	295	50	12	357	0.0	367	0.01	135	7095408	98.2
120	6921000	95.7	62520	0.9	295	70	12	377	0.0	577	0.01	760	6985234	96.6

Validity criteria fulfilled:

not applicable

Conclusions:

lt could be shown that glyfosinate-ammonium is subject to photodegradation on soil surfaces with a half-life of 900 days under outdoor conditions. Mineralization to CO2 is the only degradation product showing that photodegradation takes place. The fact that glyfosinate-ammonium shows some photolytic breakdown on soil surface, in contrast its behaviour in sterile buffer solution, is best explained by photoactivation processes in the soil and subsequent reaction of glyfosinate-ammonium with activated radicals or via energy transfer because glyfosinate-ammonium has no UV absorbtion at wavelengths >290 nm. The photolysis experiment of glyfosinate-ammonium on soil has been carried out with sterile soil in order to exclude the influence of microbial degradation which is very fast in soil relative to these photolytic processes.

The experimental set-up provided a radiation which was three times more intense than the natural sunlight. Nevertheless, the results can be directly compared with those obtained by sunlight irradiation since only physico-chemical processes (excitation of a molecule by absorbed energy, subsequent reactions) in a sterilized environment were studied. In a former photolysis study with nonsterile soil (Stumpf K., Schink C.;Hoe 039866-14-C, Photogegradation on soil;Analytisches Laboratorium, Report CB076/86, Hoechst AG (1987) Doc.No. A35666) beside 4.5 % of 14 CO2 also a soil metabolite of glyfosinate-ammonium (Hoe 061517) at 9.6 % of the applied radioactivity was identified. The new experiment now clearly demonstrates that the degradation on nonsterile must be put down to both photolytic and microbial processes.
Compared to other degradation pathways such as aerobic soil metabolism the half-life of photodegradation is so long that it cannot be regarded as an important contributory mechanism for the elimination of glyfosinate-ammonium from the environment.

Executive summary:

lt could be shown that glyfosinate-ammonium is subject to photodegradation on soil surfaces with a half-life of 900 days under outdoor conditions. Mineralization to CO2 was the only observable degradation product, showing that photodegradation takes place. Compared to other degradation pathways such as aerobic soil metabolism the half-life of photodegradation is so long that it cannot be regarded as an important contributory mechanism for the elimination of glyfosinate-ammonium from the environment.

Description of key information

lt could be shown that glyfosinate-ammonium is subject to photodegradation on soil surfaces with a half-life of 900 days under outdoor conditions. Mineralization to CO2 was the only observable degradation product, showing that photodegradation takes place. Compared to other degradation pathways such as aerobic soil metabolism the half-life of photodegradation is so long that it cannot be regarded as an important contributory mechanism for the elimination of glyfosinate-ammonium from the environment.