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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:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Qualifier:
no guideline followed
Principles of method if other than guideline:
Test type: soil degradation test.
20-unit biodegradation test using 14CO2 evolution to determine degree of biodegradation. Sterilised soils used as a control.
Two soil types - Meramec River bank and St. Charles ray-silt loam - were 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, equivalent to 5 µg/g 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.
GLP compliance:
no
Test type:
laboratory
Radiolabelling:
yes
Oxygen conditions:
aerobic
Soil classification:
not specified
Soil no.:
#1
Soil type:
other: Meramec river bank
% Org. C:
0.7
pH:
7.7
Soil no.:
#2
Soil type:
other: St. Charles Ray Silt Loam
% Org. C:
0.56
pH:
7.05
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
Parameter followed for biodegradation estimation:
radiochem. meas.
Key result
Soil No.:
#1
% Degr.:
64
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
Results with reference substance:
Linear dodecylbenzene sulfonate used as control substance.

Table 1. Degradation of test substance over 148 days in two soils in the presence and absence of sterilising agent

Day of exposure   Silt loam (microbial)  Silt loam (sterile)  River bank (microbial)  River bank (sterile)
Day   2   18.60  12.79  15.89  1.81
Day   6   25.11  15.46  21.62  2.32
Day  12   30.38  17.00  26.24  2.60
Day  16   33.22  17.55  29.19  2.72
Day  21   36.00  17.97  32.93  2.82
Day  28   39.03  18.23  36.86  2.94
Day  35   41.75  18.45  40.65  3.01
Day  43   44.37  18.61  44.18  3.08
Day  58   48.64  18.83  49.50  3.20
Day  72   51.86  18.96  53.14  3.30
Day  86   54.60  19.04  56.01  3.37
Day 100   56.96  19.10  58.36  3.45
Day 114   58.88  19.14  60.30  3.51
Day 128   60.50  19.17  61.92  3.58
Day 148   62.55  19.23  63.97  3.67 

Table 2. Degradation of LAS over 148 days in two soils in the presence and absence of sterilising agent

Day of exposure   Silt loam (microbial)  Silt loam (sterile)  River bank (microbial)  River bank (sterile)
Day   2    0.03  0.00   0.05  0.02
Day   6    0.14  0.02   0.20  0.05
Day  12    2.20  0.05   1.90  0.08
Day  16    4.53  0.05   4.51  0.08
Day  21    7.69  0.05   9.20  0.08
Day  28   11.46  0.07  15.79  0.08
Day  35   15.68  0.07  24.02  0.08
Day  43   20.49  0.07  33.92  0.08
Day  58   29.44  0.07  51.04  0.08
Day  72   36.34  0.08  61.98  0.10
Day  86   41.92  0.08  68.46  0.10
Day 100   46.59  0.08  72.36  0.10
Day 114   50.41  0.08  74.86  0.10
Day 128   53.60  0.08  76.80  0.10
Day 148   56.97  0.10  79.12  0.15


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

The study report states that the high C-14 evolution observed in the sterilised St. Charles ray-silt loam soil samples is likely to be due to the time required for the sodium azide sterilant to be distributed throughout the soil.

Conclusions:
Biodegradation of 64% in a river bank soil and 62.6% in silt loam soil in a 148 d time period was determined in a reliable study conducted according to an appropriate test methodology.
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:
In an environmental context the speciation will be controlled by the prevailing conditions and ions present, and will be the same regardless of the starting form. The properties of the members of the category are consistent across all endpoints.
The category hypothesis is that all the members are various ionised forms of the same parent acid. The main assumption is that sodium and potassium are not significant in respect of all the properties under consideration. For ammonium salts the properties and role of ammonia are given due consideration. In dilute aqueous conditions of defined pH a salt will behave no differently to the parent acid, at identical concentration of the particular speciated form present and will be fully dissociated. Hence some properties (measured or expressed in aqueous media, e.g. ecotoxicity) for a salt can be directly read-across (with suitable mass correction) to the parent acid and vice versa, and from one salt to another.
Reason / purpose:
read-across source
Soil No.:
#1
% Degr.:
64
Parameter:
radiochem. meas.
Remarks:
14CO2 generation
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 (62-64% removal in 148 days compared to up to 19.2% removal in sterile control soil).

Although biodegradation in soil has not been demonstrated for DTPMP-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 DTPMP-H 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 64% over 148 d in a river bank soil and 62.6% 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.