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Environmental fate & pathways

Biodegradation in soil

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Reference
Endpoint:
biodegradation in soil: simulation testing
Type of information:
migrated information: read-across based on grouping of substances (category approach)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Remarks:
The study was well documented and meets generally accepted scientific principles, but was not conducted in compliance with GLP.
Qualifier:
no guideline followed
Principles of method if other than guideline:
Soil CO2 evolution test. Degradation chamber consisted of modified 60 ml Buchner-type funnel. 20g soil in each of 12 funnels. Test substance added as aqueous solution to give 10µg/g.
GLP compliance:
no
Test type:
laboratory
Radiolabelling:
yes
Year:
1976
Soil no.:
#1
Soil type:
other: Iowa Farm Soil
% Org. C:
3.31
pH:
5.6
Soil no.:
#2
Soil type:
other: Olivette Garden Soil
% Org. C:
3.03
pH:
7.65
Soil no.:
#3
Soil type:
other: St Charles Ray-Silt Loam
% Org. C:
0.56
pH:
7.05
Soil no.:
#4
Soil type:
other: Meramec River Bank Soil
% Org. C:
0.7
pH:
7.7
Details on soil characteristics:
SOIL COLLECTION AND STORAGE

- Storage conditions: sealed polyethylene bags

- Soil preparation (e.g., 2 mm sieved; air dried etc.): 2 mm sieved

PROPERTIES OF THE SOILS (in addition to defined fields)

Water content (gH2O/g soil): Soil #1: 0.20, Soil #2: 0.23, Soil #3: 0.14, Soil #4: 0.06.

Water holding capacity (gH20/g soil): Soil #1: 0.50, Soil #2: 0.45, Soil #3: 0.45, Soil #4: 0.39.
Duration:
119 d
Initial conc.:
10 mg/kg soil d.w.
Parameter followed for biodegradation estimation:
CO2 evolution
Details on experimental conditions:
1. PRELIMINARY EXPERIMENTS: none reported

2. EXPERIMENTAL DESIGN
- Soil (g/replicate): 20g

- Control conditions, if used (present differences from other treatments, i.e., sterile/non-sterile, experimental conditions): approximation of a sterile control acheived by addition of sodium azide to selected soil samples at the 0.1% level (1mg/g soil)

- No. of replication controls, if used: Samples of Iowa Farm Soil and of Meramec River Bank Soil were prepared as sterile controls.

- No. of replication treatments: 5

- Test apparatus (Type/material/volume): Degradation chamber: modified 60ml Buchner-type funnel (pyrex with coarse frit)

- Details of traps for CO2 and organic volatile, if any: radiolbelled carbon dioxide given off was trapped in scrubbers containing 5ml ethanolamine-ethylene glycol monoethyl ether, 1:7 solution.

Test material application
- Volume of test solution used/treatment: one ml of aqueous solution (200µg/ml)

Any indication of the test material adsorbing to the walls of the test apparatus: none reported

Experimental conditions (in addition to defined fields)
- Moisture maintenance method: soil was continuously purged with air saturated with water and free of carbon dioxide at a rate of 10ml/min.

Other details, if any:

3. OXYGEN CONDITIONS (delete elements as appropriate)
- Methods used to create the an/aerobic conditions: soil was continuously purged with air saturated with water and free of carbon dioxide at a rate of 10ml/min.
- Evidence that an/aerobic conditions were maintained during the experiment (e.g. redox potential): none reported

4. SUPPLEMENTARY EXPERIMENTS: none reported

5. SAMPLING DETAILS
- Sampling intervals: 5, 7, 9, 12, 16, 21, 28, 35, 42, 49, 63, 77, 91, 105, 119 days

- Sampling method: Periodically the first scrubber was removed, the second scrubber moved to position one and replaced with a fresh scrubber. The 14CO2 evolved was then measured by liquid scintillation counting. Duplicate 1 ml aliquots of the monoethanolamine-ethylene glycol solution were pipetted into glass liquid scintillation counting vials and 19 ml of LSC cocktail (9/1, methanol/Fluoralloy TM by volume) added. The vials were then capped, mixed, and equilibrated at 10°C prior to counting. Each sample vial was counted in triplicate for a 20 minute period with a Mark III Liquid Scintillation Spectrometer (Model 6880, Searle Analytic, Inc.) using the preset isotope quench corrected program 3 ( l4C, low activity) mode. 14C counting efficiencies were determined via the external standard pulse method using a certified set of 8 sealed 14C quench standards (Amersham/Searle, cat#180060). All CPM values were automatically converted to DPM values via the Mark III DPM calculation accessory. The percent theoretical 14CO2 evolved was then calculated from these DPM values.

- Sampling intervals/times for:
> Sterility check, if sterile controls are used: 5, 7, 9, 12, 16, 21, 28, 35, 42, 49, 63, 77, 91, 105, 119 days
Soil No.:
#1
% Degr.:
6.7
Parameter:
CO2 evolution
Sampling time:
119 d
Soil No.:
#2
% Degr.:
25.2
Parameter:
CO2 evolution
Sampling time:
119 d
Soil No.:
#3
% Degr.:
28.2
Parameter:
CO2 evolution
Sampling time:
119 d
Soil No.:
#4
% Degr.:
26
Parameter:
CO2 evolution
Sampling time:
119 d
Transformation products:
not measured
Details on results:
The Iowa Farm soil (Soil #1) shows the least activity; this may be related to its low pH. The two soils with the lowest organic carbon content (Soil #3 & #4) were the most active.
Results with reference substance:
The sterile controls evolved no significant 14CO2 (Soil #1(NaN3 treated): 3.29%, Soil #4(NaN3 treated): 3.75% . However, the results indicate that there was probably an incomplete sterilisation of the soil.

Table 1: Soil Biodegradation (Cumulative 14CO2 Evolution - % of theory)

Days of Exposure

5

7

9

12

16

21

28

35

42

49

63

77

91

105

119

Soil #1

0.22

0.40

0.60

0.71

0.82

1.42

2.14

2.80

3.38

3.96

4.32

5.07

5.72

5.98

6.68

Soil #2

1.20

2.04

2.90

3.37

3.88

6.54

9.64

12.34

14.66

16.25

17.99

21.04

22.93

23.75

25.16

Soil #3

4.04

6.00

7.45

9.15

10.15

12.26

14.58

16.67

18.64

20.68

22.89

24.31

25.86

26.55

28.16

Soil #4

1.00

1.50

1.96

2.43

3.41

6.80

9.32

10.99

14.88

17.66

19.96

22.08

23.55

24.30

25.98

Soil #1 (NaN­3treated)

0.42

0.55

0.67

0.76

0.86

1.06

1.32

1.69

2.07

2.47

2.61

2.80

2.99

3.07

3.29

Soil #4 (NaN3treated)

0.88

1.19

1.41

1.53

1.65

1.96

2.23

2.43

2.58

2.68

2.90

3.07

3.24

3.38

3.75

Conclusions:
Soil biodegradation rates of between 6.7 and 28.2% over 119d in four soils were determined in a reliable study conducted according to generally accepted scientific principles.

Description of key information

Although biodegradation in soil has not been demonstrated for HEDP-H and its salts, the role of abiotic removal processes is significant. The key data for soil adsorption are from the study by Michael (undated) (refer to Section 5.4.1 for further information about this test). There is no evidence for desorption occurring. Effectively irreversible binding is entirely consistent with the known behaviour of complexation and binding within crystal lattices. The high levels of adsorption which occur are therefore a form of removal from the environment. After approximately 40-50 days, the phosphonate is >95% bound to sediment with only 5% extractable by ultrasonication and use of 0.25N HCl-xylene solvent (based on radiolabelling) in river and lake water microcosms (Monsanto internal report, cited by Gledhill and Feijtel, 1992). 66-80% removal (binding) is seen after 11 days in the same test. In the context of the exposure assessment, largely irreversible binding is interpreted as a removal process; 5% remaining after 40 - 50 days is equivalent to a half-life of 10 days which is significant for the environmental exposure assessment in the regional and continental scales. This abiotic removal rate is used in the chemical safety assessment of HEDP-H and its salts.

Key value for chemical safety assessment

Half-life in soil:
10 d
at the temperature of:
12 °C

Additional information

Two reliable studies of biodegradation in soil are available. Soil biodegradation rates of between 6.7 and 28.2% over 119 days in four soils was determined (Saeger et al., 1977). A soil biodegradation rate of 47% over 148 days in silt-loam soil was determined (Saeger 1978).