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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:
key study
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 C-14 evolved measured by liquid scintillation counting.
GLP compliance:
no
Test type:
laboratory
Radiolabelling:
yes
Soil no.:
#1
Soil type:
other: St. Charles Ray Silt Loam
Soil no.:
#2
Soil type:
other: Meramec river bank
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

Source: Meramec river bank soil
pH: 7.70
% organic carbon: 0.70
Water content: 0.06 g/g (dry-weight basis)
Water Holding Capacity: 0.39 g/g
Soil No.:
#1
Duration:
148 d
Soil No.:
#2
Duration:
148 d
Soil No.:
#1
Soil No.:
#2
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.
Soil No.:
#1
% Degr.:
60.68
Parameter:
radiochem. meas.
Remarks:
14CO2 evolution
Sampling time:
148 d
Soil No.:
#2
% Degr.:
76.24
Parameter:
radiochem. meas.
Remarks:
14CO2 evolution
Sampling time:
148 d
Soil No.:
#1
% Degr.:
32.42
Parameter:
radiochem. meas.
Remarks:
14CO2 evolution (sterile control)
Sampling time:
148 d
Soil No.:
#2
% Degr.:
5.57
Parameter:
radiochem. meas.
Remarks:
14CO2 evolution (sterile control)
Sampling time:
148 d
Transformation products:
not measured
Evaporation of parent compound:
no
Volatile metabolites:
no

Table 1: Percent degradation values (14CO2 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

 

 

 

 

 

 

 

 

HMDTMP - St Charles Ray silt loam - active

23.19

28.32

32.04

34.37

36.77

39.36

41.79

44.41

48.26

51.26

53.72

55.85

57.65

59.18

60.68

HMDTMP - St Charles Ray silt loam -

Sterile

21.97

27.22

29.38

30.12

30.66

31.13

31.46

31.70

32.00

32.15

32.23

32.29

32.34

32.36

32.42

 

 

 

 

 

 

 

 

 

 HMDTMP - Meramec river bank - active  22.45 29.63   35.31  39.53  43.34  47.91  52.13  56.03  61.46  65.23  68.35  70.62  72.63  74.30  76.24
 HMDTMP - Meramec river bank - sterile  2.73  3.57  4.03  4.22  4.38  4.44  4.56  4.67  4.86  5.01  5.10  5.22  5.33  5.43  5.57

 

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

Conclusions:
Soil biodegradation rate of 61 - 76% over 148d in two active soils was determined in a reliable study conducted according to generally accepted scientific principles.
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:
See IUCLID Section 13 and Annex 7 of the CSR for justification of read-across within the HMDTMP category
Reason / purpose for cross-reference:
read-across source
Soil No.:
#1
% Degr.:
60.68
Parameter:
radiochem. meas.
Remarks:
¹⁴CO2 evolution
Sampling time:
148 d
Soil No.:
#2
% Degr.:
76.24
Parameter:
radiochem. meas.
Remarks:
¹⁴CO2 evolution
Sampling time:
148 d
Transformation products:
not measured

Description of key information

Some biological degradation in soil takes place, as demonstrated by the higher level of removal in active soils (60 - 76% removal in 148 days compared to up to 32% removal in sterile control soil).

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

Key value for chemical safety assessment

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

Additional information

Biodegradation of 76.2% over 148 d in a river bank soil and 60.7% in silt loam soil over the same period was determined (Saeger et al., 1978). There are degradation modes operative in the environment which could prevent long-term persistence.