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Environmental fate & pathways

Biodegradation in soil

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Reference
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.

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) (Saeger, 1978).

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 for HMDTMP-H 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 water microcosms with HMDTMP-H (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.

The acid, sodium and potassium salts in the HMDTMP category are freely soluble in water. The HMDTMP anion can be considered fully dissociated from its sodium or potassium cations when in dilute solution. Under any given conditions, the degree of ionisation of the HMDTMP species is determined by the pH of the solution. At a specific pH, the degree of ionisation is the same regardless of whether the starting material was HMDTMP-H, HMDTMP.4Na, HMDTMP.7K or another salt of HMDTMP.

Therefore, when a salt of HMDTMP is introduced into test media or the environment, the following is present (separately):

1. HMDTMP is present as HMDTMP-H or one of its ionised forms. The degree of ionisation depends upon the pH of the media and not whether HMDTMP (4-7K) salt, HMDTMP (4-7Na) salt, HMDTMP-H (acid form), or another salt was used for dosing. At pH 5.5 - 6, the HMDTMP anions would be present on average as the HMDTMP trivalent anion according to the pH curves.  At neutral pH (7), the HMDTMP anions would be present on average as the HMDTMP pentavalent anion according to the pH curves. At pH 8, the HMDTMP anions would be present on average as the HMDTMP hexavalent anion according to the pH curves.

2. Disassociated potassium or sodium cations. The amount of potassium or sodium present depends on which salt was added.

3. It should also be noted that divalent and trivalent cations would preferentially replace the sodium or potassium ions. These would include calcium (Ca2+), magnesium (Mg2+) and iron (Fe3+). These cations are more strongly bound by HMDTMP than potassium and sodium. This could result in HMDTMP-dication (e.g. HMDTMP-Ca, HMDTMP-Mg) and HMDTMP-trication (e.g. HMDTMP-Fe) complexes being present in solution.

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