Fenamiphos

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

EPA Guideline Subdivision N 161-1 (Hydrolysis)

Deviations:

no

GLP compliance:

yes

Analytical monitoring:

yes

Details on sampling:

Samples were taken on days 0, 2, 4, 7, 14, 22 and 30 after application.

Buffers:

steril aqueous buffer solutions of pH 4

Estimation method (if used):

not specified

Details on test conditions:

The hydrolysis of [phenyl-UL-14C]fenamiphos, [phenyl-UL-14C]fenamiphos sulfoxide (M01) and [phenyl-UL-14C]fenamiphos sulfone (M02) was investigated in a steril aqueous buffer solutions of pH 4. The test solutions were prepared at concentrations of 0.5 mg fenamiphos/L, 0.5 μg fenamiphos sulfoxide/L and 0.5 μg fenamiphos sulfone/L. The solutions were incubated under sterile conditions in the dark at 24.5 °C.

Transformation products:

yes

Remarks:

Fenamiphos sulfoxide was found in quantities > 10% up from day 14 after application, reaching 14.9% of the applied radioactivity on day 30. The unidentified radioactivity did not exceed 3.5% during the time course of the study.

No.:

#1

% Recovery:

>= 99.9 - <= 104.5

pH:

4

Temp.:

24.5 °C

Duration:

30 d

Key result

pH:

4

Temp.:

24.5 °C

DT50:

192.5 d

Type:

(pseudo-)first order (= half-life)

Table: Distribution of radioactivity [% of applied] after hydrolysis of fenamiphos in water at pH 4 (mean of two replicates)

Interval	Fenamiphos	Fenamiphos	Unidentified	Total
[days]		sulfoxide	radioactivity	recovery
0	94.0	6.0	1.8	101.8
2	95.8	6.7	2.0	104.5
4	93.2	5.9	3.5	102.6
7	93.7	8.2	1.8	103.7
14	90.1	10.1	1.2	101.4
22	90.7	11.0	0.0	101.7
30*	82.9	14.9	2.1	99.9

* 30-day values are results of only one analysis (sample).

Validity criteria fulfilled:

yes

Remarks:

The information provided was taken from the original plant dossier submitted in 2002 for inclusion of fenamiphos to Annex I of Directive 91/414/EEC and has been previously evaluated in the DAR and subject to peer review by EFSA and Member States.

Conclusions:

The hydrolysis of fenamiphos-phenyl-1-14C/13C/12C was determined in sterile aqueous buffers at pH 5, 7, and 9. Recalculated (by RMS) DT50 values (25 °C) for fenamiphos by non-linear fitting of first order kinetics were 252 (r2 0.76), 304 (r^2 0.85) and 236 days (r^2 0.94) at pH 5, 7 and 9, respectively. No major hydrolysis products were formed, but the maximum formation of fenamiphos-sulfoxide was reached on the last day of the study at pH 5 (9.9 % of applied radioactivity). Therefore, this compound is considered to be a major hydrolysis product since it cannot be concluded that the % of applied radioactivity of this compound would have remained < 10 % during the entire incubation period. Fenamiphos-sulfone was detected only at pH 9 and as a minor hydrolysis product with a maximum of 5.2 % of applied radioactivity at the end of the study.

Executive summary:

The hydrolysis of [phenyl-1-14C]fenamiphos, [phenyl-1-14C]fenamiphos-sulfoxide and [phenyl-1-14C] fenamiphos-sulfone was tested at pH 4 in accordance with US EPA Test Guideline 161-1. The information was taken from the original plant protection product dossier for Fenamiphos submitted in 2002 for inclusion of Fenamiphos to Annex I of Directive 91/414/EEC and has been previously evaluated in the Draft Assessment Report and was subject to peer review by EFSA and Member States. The study data is regarded valid and suitable for assessment. Samples were incubated in the dark at 24.5 °C. Under these conditions, hydrolysis DT50 of fenamiphos calculated by non-linear fitting of first order kinetics are 205 days for fenamiphos, 151 days for fenamiphos-sulfoxide, and 134 days for fenamiphos-sulfone (r2 values 0.74, 0.69, and 0.76 respectively). In the fenamiphos treatment fenamiphos-sulfoxide was found as a major metabolite (14.9 % of applied radioactivity after 30 days). In the fenamiphos-sulfoxide and fenamiphos-sulfone treatment, only unidentified metabolites were detected at a maximum formation rate of 11.1 % and 10.8% of applied radioactivity, respectively (both on day 30). The RMS and EFSA concluded that: "As both unidentified metabolites are also identified in the water-sediment study that lasted 100 days, further information from a prolonged hydrolysis study is not considered necessary." The total recovery ranged from 99.9 to 104.5 % of the applied radioactivity in the test with the parent compound. Fenamiphos sulfoxide was found in quantities > 10 % up from day 14 after application, reaching 14.9 % of the applied radioactivity on day 30. The unidentified radioactivity did not exceed 3.5 % during the time course of the study. The hydrolytic degradation of fenamiphos in water at pH 4 calculated according to 1st order kinetics is DT50 = 192.5 days (DT90 = 640 days). Fenamiphos can be classified as slightly degradable in aqueous solutions of pH 4.

Description of key information

The DT50 values (25 °C) for the test substance were 245, 301 and 235 days at pH 5, 7 and 9, respectively (EFSA Conclusion - Appendix A: List of endpoints, 2019).

Key value for chemical safety assessment

Half-life for hydrolysis:

301 d

at the temperature of:

25 °C

Additional information

Experimental information on the hydrolysis of the test substance is available and summarized below.

 

Table: Overview about experimental data on the hydrolysis

Author(s)	Year	Title Company Report No. Source (where different from company) GLP or GEP status Published or not	Adequacy	Result
Mulford	1987	Stability of Nemacur in sterile aqueous buffer solutions Report no. MR94535 Bayer AG Non-GLP Unpublished	Key	DT50 = 245 days at pH 5, DT 50 = 301 days at pH 7 DT50 = 235 days at pH 9
Arthur, Parker, and Shepherd	1999	Hydrolysis of Fenamiphos, Fenamiphos-sulfoxide and Fenamiphos-sulfone in sterile pH 4 aqueous buffer solution Report no. 109471 Bayer AG GLP Unpublished	Supporting	DT50 = 192.5 days at pH 4

 

Key information

The hydrolysis of fenamiphos-phenyl-1-14C/13C/12C was determined in sterile aqueous buffers at pH 5, 7, and 9 in accordance with US EPA Test Guideline 161-1 (Mulford, 1987). The information was originally submitted the EU in 2002 for inclusion of the test substance to Annex I of Directive 91/414/EEC and has been previously evaluated in the Draft Assessment Report (DAR) and subject to peer review by EFSA and Member States. The active substance was applied at a concentration of 35 mg/L and the samples incubated at 25°C in the dark under sterile conditions. Duplicate samples were taken and analysed on days 0, 5, 14, 10, 18, 24 and 31 after application. Control samples containing only the buffer solutions were taken on days 10, 18 and 31. After determination of the total radioactivity by liquid scintillation counting, each sample was extracted twice with dichlormethane and the radioactivity of both phases (water and solvent) determined by LSC. The extracts were analysed for degradation compounds via co-chromatography (TLC) using standards of fenamiphos and metabolites. Radioactive regions were scraped of the plates and the compounds quantified by liquid scintillation counting. HPLC and MS/GC methods were also used for separation and structure elucidation of the degradation products. The total recovery for the experiments with the three different aqueous solutions ranged from 93 to 100.1 % of the applied radioactivity. Loss of radioactivity, especially in the experiment performed at pH 5 (loss of approximately 7%) can be attributed to adsorption (nylon filter). At the end of the experiment, after 31 days still 86.7 to 90.2 % of the applied radioactivity was identified as parent compound . Fenamiphos sulfoxide (M01) amounted to 9.8 % of the applied radioactivity after 31 days in the test performed at pH 5. In the three experiments the concentration of fenamiphos sulfoxide (M01) increased up to the end of the study (day 31). At more basic conditions (pH 9) the formation of fenamiphos phenol (M11) was also observed, reaching 5.2% of the applied radioactivity on day 31. Unidentified radioactivity did not exceed 1% of the applied amount at any time in all tests. Based on the hydrolytic degradation of fenamiphos, the half-life was calculated according to 1st order kinetics for each pH value. The DT50 values calculated were 245 days at pH 5, 301 days at pH 7 and 235 days at pH 9. However, because of the short duration of the study, these half-lives can be regarded only as an estimation. Nevertheless, fenamiphos has to be considered as slightly degradable in water at environmentally relevant conditions. The DT50 values (25 °C) for fenamiphos by first order kinetics were 245, 301 and 235 days at pH 5, 7 and 9, respectively. The data is considered reliable and acceptable for the risk assessment under Regulation (EC) No 1907/2006 (REACH).

 

Supporting information

The hydrolysis of [phenyl-1-14C]fenamiphos, [phenyl-1-14C]fenamiphos-sulfoxide and [phenyl-1-14C] fenamiphos-sulfone was tested at pH 4 in accordance with US EPA Test Guideline 161-1 (Arthur, Parker and Shepherd, 1999). The information was taken from the original plant protection product dossier for Fenamiphos submitted in 2002 for inclusion of Fenamiphos to Annex I of Directive 91/414/EEC and has been previously evaluated in the Draft Assessment Report and was subject to peer review by EFSA and Member States. The study data is regarded valid and suitable for assessment. Samples were incubated in the dark at 24.5 °C. Under these conditions, hydrolysis DT50 of fenamiphos calculated by non-linear fitting of first order kinetics are 205 days for fenamiphos, 151 days for fenamiphos-sulfoxide, and 134 days for fenamiphos-sulfone (r2 values 0.74, 0.69, and 0.76 respectively). In the fenamiphos treatment fenamiphos-sulfoxide was found as a major metabolite (14.9 % of applied radioactivity after 30 days). In the fenamiphos-sulfoxide and fenamiphos-sulfone treatment, only unidentified metabolites were detected at a maximum formation rate of 11.1 % and 10.8% of applied radioactivity, respectively (both on day 30). The RMS and EFSA concluded that: "As both unidentified metabolites are also identified in the water-sediment study that lasted 100 days, further information from a prolonged hydrolysis study is not considered necessary." The total recovery ranged from 99.9 to 104.5 % of the applied radioactivity in the test with the parent compound. Fenamiphos sulfoxide was found in quantities > 10 % up from day 14 after application, reaching 14.9 % of the applied radioactivity on day 30. The unidentified radioactivity did not exceed 3.5 % during the time course of the study. The hydrolytic degradation of the test item in water at pH 4 calculated according to 1st order kinetics is DT50 = 192.5 days (DT90 = 640 days). The test substance can be classified as slightly degradable in aqueous solutions of pH 4.

 

Conclusion

Based on worst case considerations and in compliance with the EFSA Conclusion on Pesticides Peer Review for the test substance (approved 10 December 2018), the key value for the risk assessment in accordance with REACH is determined to be DT50 (25 °C) = 301 days at a pH of 7.