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Environmental fate & pathways

Biodegradation in soil

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Endpoint:
biodegradation in soil: simulation testing
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: 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
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/gsoil dwt): 0.20; 0.23; 0.14; 0.06

- Water-holding capacity (gH2O/gsoil dwt): 0.50; 0.45; 0.45; 0.39
Duration:
119 d
Initial conc.:
10 mg/kg soil d.w.
Based on:
test mat.
Parameter followed for biodegradation estimation:
CO2 evolution
Soil No.:
#1
% Degr.:
7.11
Parameter:
CO2 evolution
Sampling time:
119 d
Soil No.:
#2
% Degr.:
9.31
Parameter:
CO2 evolution
Sampling time:
119 d
Soil No.:
#3
% Degr.:
14.16
Parameter:
CO2 evolution
Sampling time:
119 d
Soil No.:
#4
% Degr.:
14.64
Parameter:
CO2 evolution
Sampling time:
119 d
Transformation products:
not measured
Details on results:
Sterilised (sodium azide) soil degradation: Olivette Garden Soil 0.82% 199d; St. Charles Ray-Silt Loam Soil 0.63% 199d.

Though degradation levels were low, breakdown in the soil environment does occur.

Conclusions:
Soil biodegradation of between 7 and 15% over 119 d in four soils was determined in a reliable study conducted according to generally accepted scientific principles.
Endpoint:
biodegradation in soil: simulation testing
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Study well documented, meets generally accepted scientific principles, acceptable for assessment.
Principles of method if other than guideline:
Degradation units, each containing 20 g (dry-weight) soil, spiked with either test or reference substance, sparged with CO2-free air and the off-gas passed through CO2 absorbent scrubbers. The ¹⁴C evolved measured by liquid scintillation counting.
GLP compliance:
no
Test type:
laboratory
Radiolabelling:
yes
Oxygen conditions:
aerobic
Soil classification:
not specified
Details on soil characteristics:
SOIL CHARACTERISTICS: Source: St. Charles Ray-Silt Loam pH: 7.05 % organic carbon: 0.56 Water content: 0.14 g/g (dry-weight basis) Water Holding Capacity: 0.45 g/g
Parameter followed for biodegradation estimation:
radiochem. meas.
Details on experimental conditions:
TEST DETAILS: St. Charles ray-silt loam - was seived through a 2 mm screen and the water content adjusted with either distilled water or a dilute sodium azide solution (to approximate a sterile control). Degradation units, each containing 20 g (dry-weight) soil, were spiked with either test substance or linear dodecylbenzene sulfonate (LAS) at a nominal level of 10 µg/g (for test substance, as active acid). Each soil unit was sparged with CO2-free air and the off-gas passed through two scrubbers each containing 5 ml of the CO2 absorbent (monoethanolamine-ethylene glycol) monoethyl ether 1:7 (v/v) solution. Periodically, the first scrubber was removed, the second scrubber moved to position one and replaced with a fresh scrubber. The C-14 evolved was then measured by liquid scintillation counting using a Mark III Liquid Scintillation Spectrometer (Model 6880, Searle Analytic, Inc.). The percent C-14 evolved was calculated from the disintegrations per minute and the initial C-14 charged to each unit.
Key result
Soil No.:
#1
% Degr.:
14
Parameter:
radiochem. meas.
Remarks:
(¹⁴CO2 generation)
Sampling time:
148 d
Transformation products:
not measured
Evaporation of parent compound:
not measured
Volatile metabolites:
no
Residues:
not measured

Table 1: Percent degradation values (C-14 CO2 evolved, % theory)

Type of suspension

% degradation at sampling time (days)

2

6

12

16

21

28

35

43

58

72

86

100

114

128

148

Reference substance (LAS) Microbial

0.03

0.14

2.20

4.53

7.69

11.46

15.68

20.49

29.44

36.34

41.92

46.59

50.41

53.60

56.97

Reference substance (LAS) Sterile

0

0.02

0.05

0.05

0.05

0.07

0.07

0.07

0.07

0.08

0.08

0.08

0.08

0.08

0.10

 

 

 

 

 

 

 

 

Test sample Microbial

1.87

2.98

4.07

4.74

5.40

6.16

6.86

7.61

8.86

9.89

10.82

11.69

12.48

13.22

14.18

Test sample      Sterile

0.16

0.21

0.24

0.24

0.24

0.24

0.24

0.24

0.26

0.27

0.27

0.27

0.27

0.27

0.28

 

 

 

 

 

 

 

 

 

 

The data suggest that no induction period is required before degradation occurs.

Conclusions:
Soil biodegradation of 14% over 148d in a silt loam soil was determined in a reliable study conducted according to generally accepted scientific principles.

Description of key information

Two reliable studies are available. Biodegradation of 14% over 148 days in a silt loam soil was determined (Saeger 1978). In a separate study, Biodegradation of between 7 and 15% over 119 days in four soils was determined (Saeger 1977).

Although biodegradation in soil has not been demonstrated for ATMP-H and its salts, the role of abiotic removal processes is significant. The key data for soil adsorption are from the study by Michael (1979). 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 ATMP-H and its salts.

Key value for chemical safety assessment

Half-life in soil:
10 d
at the temperature of:
25 °C

Additional information

Some biological degradation in soil takes place, as demonstrated by the higher level of removal in active soils (14% removal in 148 days compared to up to <1% removal in sterile control soil) (Saeger, 1978).

There are degradation modes operative in the environment which could prevent long-term persistence.