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Biodegradation in soil

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Endpoint:
biodegradation in soil: simulation testing
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
test procedure in accordance with generally accepted scientific standards and described in sufficient detail
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 307 (Aerobic and Anaerobic Transformation in Soil)
GLP compliance:
no
Test type:
laboratory
Specific details on test material used for the study:
- 4 nonlabeled and branched 4-NP isomers: 4-NP111, 4-NP112, 4-NP38 and 4-NP65 (see structures attached)
- 2 isotope-labeled isomers (14C-4-NP111, 13C-4-NP38) synthesized using the Friedel-Crafts alkylation
Radiolabelling:
yes
Oxygen conditions:
aerobic
Soil classification:
not specified
Year:
2011
Soil no.:
#1
Soil type:
other: gleyic hydragric anthrosol
% Clay:
46.7
% Silt:
37.9
% Sand:
15.4
% Org. C:
2.5
pH:
6.31
Details on soil characteristics:
SOIL COLLECTION AND STORAGE
- Geographic location: rice paddy soil collected from the Changshu Experimental Station of the Chinese Academy of Sciences in Jiangsu Province, China
- Collection procedures: not stated
- Soil preparation (e.g., 2 mm sieved; air dried etc.): The soil was air dried, sieved through 2 mm and stored at room temperature shortly before use.
Soil No.:
#1
Duration:
58 d
Soil No.:
#1
Initial conc.:
59.6 other: µmol/kg soil dw
Based on:
other: mixture of 5 nonylphenol isomers (4 branched and 1 linear)
Soil No.:
#1
Initial conc.:
ca. 12 other: µmol/kg soil dw
Based on:
other: of each isomer
Soil No.:
#1
Initial conc.:
3.7 other: MBq/kg soil dw
Based on:
other: mixture of 5 nonylphenol isomers (4 branched and 1 linear)
Parameter followed for biodegradation estimation:
CO2 evolution
radiochem. meas.
Soil No.:
#1
Temp.:
20 ± 1 °C
Humidity:
70% of the max water-holding capacity
Details on experimental conditions:
EXPERIMENTAL DESIGN
- Soil condition: air dried
- Soil (g/replicate): 5 g
- No. of replication treatments: 54
- Test apparatus (Type/material/volume): Serum flask
- Details of traps for CO2 and organic volatile, if any: The 14CO2 released from the soil was absorbed by 1.0 mL of NaOH (1 M) contained in one 6mL vial, which was suspended from the bottom of the stopper.

Test material application
- Volume of test solution used/treatment: A stock solution of a mixture of five 4-NP isomers (14C-4- NP111, 4-NP112, 13C-4-NP38, 4-NP65, and 4-NP1) was prepared in methanol at a concentration of 16.6 μmol/mL with a molar ratio of the five isomers in the mixture at about 1:1:1:1:1 (each isomer at about 3.3 mmol/L). About 18 μL of the stock solution of the 4-NP isomer mixture was added with a microsyringe to 0.2 g of soil.
- Application method (e.g. applied on surface, homogeneous mixing etc.): The soil was mixed and transferred into a 100 mL serum flask containing 4.8 g of soil. The whole soil was then thoroughly mixed and kept overnight to evaporate the methanol solvent. The homogeneity of the 4-NP distribution within the soil was proved by determining the radioactivity of soil subsamples (0.02-0.05 g) from the flask. After solvent evaporation, 1.2 mL distilled water was added to adjust the soil moisture to 70% of the max WHC. The flask was then closed with a rubber stopper.

3. OXYGEN CONDITIONS
- Methods used to create the aerobic conditions: The flasks were opened for 0.5 min each day for exchange of head-space with fresh air.
- Is the co-solvent evaporated: yes

Experimental conditions (in addition to defined fields):
- Continuous darkness: yes

4. SUPPLEMENTARY EXPERIMENTS: Preliminary experiments showed that the applied consecutive extractions with methanol and ethyl acetate was sufficient and exhaustive. Radioactive determinations showed that the freeze-drying and extraction processes had a recovery of 93.1 +- 1.5% for 4-NP isomers.

5. SAMPLING DETAILS
- Sampling intervals: At incubation times of 0, 5, 10, 15, 20, 27, 34, 43 and 58 days, three flasks were sacrificed for analysis of radioactivity in the NaOH trap, concentration of the 4-NP isomers and formation of metabolites and bound residues of 14C-NP111 in the soil.
- Sampling method for soil samples:
> Sterility check, if sterile controls are used: yes, soil sterilization was achieved by autoclavingthe soils at 120 °C for 1h three times in three consecutive days.
> Moisture content: 70% max WHC
Soil No.:
#1
% Total extractable:
38.3
% Non extractable:
54.4
% CO2:
5
% Other volatiles:
0
% Recovery:
97.7
Remarks on result:
other: Test substance: 14C-NP111
Parent/product:
parent
Key result
Soil No.:
#1
% Degr.:
ca. 5
Parameter:
CO2 evolution
Sampling time:
58 d
Remarks on result:
other: 14C-NP-111
Key result
Soil No.:
#1
DT50:
> 60 d
Temp.:
20 °C
Remarks on result:
other: 14C-NP111, based on mineralisation, extrapolated from study results
Key result
Soil No.:
#1
DT50:
> 2.1 - < 10.3 d
Type:
(pseudo-)first order (= half-life)
Temp.:
20 °C
Remarks on result:
other: mean DT50 of the 4 branched isomers; DT50 is mainly based on disappearance due to adsorption (NER) and incorporation in the soil humic acid matrix
Transformation products:
not specified
Details on transformation products:
One metabolite of 14C-4-NP111 was found in the active soil, but no metabolite was detected in the sterilized soil. The metabolite had a higher Rf value (0.75) on TLC than that of the parent 14C-4-NP111 (0.38), indicating that the metabolite was less polar than 4-NP111.
Evaporation of parent compound:
no
Volatile metabolites:
no
Residues:
yes
Details on results:
TEST CONDITIONS
- Aerobicity, moisture, temperature and other experimental conditions maintained throughout the study: Yes
- Anomalies or problems encountered (if yes): no

NON-EXTRACTABLE RESIDUES
- % of applied amount at end of study period: ~ 54% (14C-NP111) -> 31.1% after 5 days

MINERALISATION
- % of applied radioactivity present as CO2 at end of study: about 5% (14C-NP111)

VOLATILIZATION
- % of the applied radioactivity present as volatile organics at end of study: negligible

STERILE TREATMENTS (if used)
- Transformation of the parent compound: <5% mineralized
- Formation of transformation products: no
- Formation of extractable and non-extractable residues: yes
- Volatilization: no

Isomer-Specific Degradation of 4-NP Isomers in Soil. The five (four branched and one linear) 4-NP isomers degraded at different rates in the active rice paddy soil during 58 days of incubation under oxic conditions. The degradation kinetics of the isomers was fitted to the availability-adjusted firstorder
model, the first-order model with one constant parameter, and the simple first-order model. Among the three models, the availability-adjusted first-order model had the best goodness-of-fit and is most rational for describing the gradually increasing unavailability of 4-NP isomers in soil over incubation time. Also, the residual plots of this model show a random distribution of the residues around the
zero line. Therefore, we prefer to apply the availability-adjusted model to fit the
degradation data of the five 4-NP isomers in order to compare their persistence in the soil.


The values of parameters k, a, and t1/2 as well as the goodness-of-fit of the model for the The k ranged from 0.09 to 0.58 per day with the following increasing order: 4-NP1 (0.58 day-1) > 4-NP38 (0.38 day-1) > 4-NP65
(0.13 day-1) > 4-NP112 (0.10 day-1) > 4-NP111 (0.09 day-1), showing that the linear isomer 4-NP1 disappearing most rapidly in the soil (t1/2 = 1.4 days), while the branched 4-NP111, the main component of tNP mixtures,15 disappeared considerably more slowly (t1/2 = 10.3 days). a varied from 0.04 to 0.20 day-1, and 4-NP1 had the highest unavailability factor, which is in good agreement with its highest sorption affinity in soils. The unavailable amounts of the five isomers in the soil ranged from 2.9 to 24.4% of the initial spiking with the following decreasing order: 4-NP111 (24.4%) > 4-NP112 (7.9%) > 4-NP1 (5.6%) > 4-NP65 (3.3%) > 4-NP38 (2.9%). 4-NP111 and 4-NP112, which were more recalcitrant, also had higher unavailable percentage in the soil.
By using defined 4-NP isomers, we clearly showed that the degradation of 4-NP isomers in soil was isomer specific.
The observed higher recalcitrance of the branched 4-NP isomers than 4-NP1 can be attributed to the alkyl chain structure at the benzene ring. The branched isomers have a quaternary a-C on the alkyl chain, and this structure is regarded resistant to .- and ß-oxidation. The length of the side chain at a-C seems to be the most important factor for their degradation.
The three isomers with an ethyl side chain at a-C (4-NP111, 4-NP112, 4-NP65) showed a longer t1/2 than 4-NP38 with two methyl side chains at a-C. 

Conclusions:
Branched nonylphenol quickly diseapper in soil by sorption to and incorporation into the soil matrix. However, mineralisation to CO2 is slow (~ 5% in 58 days).
Executive summary:

Using 14C- and 13C-ring-labeling, degradation of five p-nonylphenol (4-NP) isomers including four branched (4-NP38, 4-NP65, 4-NP111, and 4-NP112) and one linear (4-NP1) isomers in a rice paddy soil was studied under oxic conditions. Mineralisation showed to be in the range of 5%. Over 50% of the parent compound disappeared as NER after 58 days. Thus, branched NP isomers disappear quickly in soil due to sorption and diffusion and form NERs, but mineralisation is low.

Endpoint:
biodegradation in soil: simulation testing
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
Read across justification document is attached in chapter 13.
Reason / purpose for cross-reference:
read-across source
Soil No.:
#1
% Total extractable:
38.3
% Non extractable:
54.4
% CO2:
5
% Other volatiles:
0
% Recovery:
97.7
Remarks on result:
other: 14C-NP111; day 56
Parent/product:
parent
Key result
Soil No.:
#1
% Degr.:
5
Parameter:
CO2 evolution
Sampling time:
56 d
Remarks on result:
other: 14C-NP111
Key result
DT50:
> 60 d
Temp.:
20 °C
Remarks on result:
other: 14C-NP111, based on mineralisation
Transformation products:
not specified

Description of key information

The substance is considered to be persistent in soil

Key value for chemical safety assessment

Additional information

Since no simulation studies assessing the biodegradability of O,O,O-tris(2(or 4)-C9-10-isoalkylphenyl) phosphorothioate (CAS 126019-82-7) in soil are available, in accordance to Regulation (EC) No. 1907/2006 Annex XI, 1.5 Grouping of substances, a read-across to Phenol,4-nonyl-,branched (CAS 84852-15-3) was conducted. The read across is justified by Phenol,4-nonyl-,branched (CAS 84852-15-3) being an impurity and a degradation product of the test substance with the highest degradation potential (see table and Analogue justification document attached).


In the key study, Shan et al. (2011) studied the degradation of five p-nonylphenol (4-NP) isomers including four branched (4-NP38, 4-NP65, 4-NP111, and 4-NP112) and one linear (4-NP1) isomers in a rice paddy soil using  unlabeled as well as 14C- and 13C-ring-labelled test material. The test design was similar to OECD guideline 307. Test substances were incubated up to 56 days at 20 °C. Extractable residues, non-extractable residues as well as CO2 was measured after certain time intervals. Degradation (= mineralization + NERs) followed availability-adjusted first-order kinetics with the decreasing order of half-life 4-NP111 (10.3 days) > 4-NP112 (8.4 days) > 4-NP65 (5.8 days) > 4-NP38 (2.1 days) > 4-NP1 (1.4 days). One metabolite of 4-NP111 with less polarity than the parent compound occurred rapidly and remained stable in the soil. At the end of incubation (56 days), bound residues of 4-NP111 amounted to 54% of the initially applied radioactivity and resided almost exclusively in the humin fraction of soil organic matter, in which chemically humin-bound residues increased over incubation. Mineralization (as CO2 formation) was measured to be 5% after 56 days for the branched 14C-NP111. Thus, branched nonylphenol is mineralized slowly but rapidly integrated into organic matter (humin fraction) in soil. If only considering mineralization for calculating the DT50 value it can be concluded that branched nonylphenol is persistent in soil.


 


Together with additional experimental data and physico-chemical properties of the test substance given in the respective chapters of the dossier one can surely conclude that O,O,O-tris(2(or 4)-C9-10-isoalkylphenyl) phosphorothioate is persistent in soil:



  • A biodegradation in water and sediment simulation test according to OECD 309 shows that O,O,O-tris(2(or 4)-C9-10-isoalkylphenyl) phosphorothioate quickly adsorb to sediment to form NERs (> 50% in 100 days; see study in chapter 5.2.2). This is supported by experimental data on adsorption (log Koc = 3.7 -4.7, see study in chapter 5.4.1). The same rapid adsorption processes will surely also happen (even to a higher degree) in soil.

  • The simulation test further shows that the test substance neither mineralize in the sediment (0.1-02% in 100 days) nor form relevant amounts of metabolites. Only a very low degree of ester cleavage of the alkylphenol moieties was observed (quantification ongoing), which show that if metabolites occur in the timeframe of a simulation test than it will be very low amounts of branched nonylphenol.

  • The given key study clearly shows that even this branched nonylphenol will not mineralize rapidly but need to be judged persistent in soil.

  • In addition, the result of the existing OECD 309 test is already sufficient to assess the test substance as persistent in the environment. Since no relevant degradation products are reasonable and expected, this existing OECD 309 test is considered to be sufficient for the PBT assessment of the test substance.


Thus, O,O,O-tris(2(or 4)-C9-10-isoalkylphenyl) phosphorothioate can surely be judged persistent in soil based on the existing data for the substance itself and a suitable source substance and further simulation testing in soil with the test substance would not generate relevant new data for the substance to register.