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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 sieved 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.
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:
Please refer to Annex 4 of the CSR and IUCLID Section 13 for justification of read-across within the DTPMP category.
Reason / purpose for cross-reference:
read-across source
Soil No.:
#1
% Degr.:
64
Parameter:
radiochem. meas.
Remarks:
14CO2 generation
Sampling time:
148 d
Transformation products:
not measured
Details on transformation products:
The registrants consider that the possible benefits to the CSA of conducting further studies of the formation of degradation products are not significant in comparison with the foreseeable difficulties to conduct and interpret the study.

Isolating and identifying degradation products presents a significant analytical challenge. There is substantial evidence across most types of phosphonates of rapid and irreversible binding to solids, particularly inorganic substrates (please refer to Section 4.2.1 of the Category CSR). It is difficult to envisage an analytical system suitable for extracting and analysing the substances which could not be affected by this. Secondly, the relevance of the data must be considered. This CSR discusses the environmental fate of DTPMP and other analogous phosphonates. Whilst there is limited degradation in the environment, it is not extensive or rapid under standard conditions. Removal processes from natural waters are attributed to the typically rapid, irreversible adsorption to solid matrices. As such the chemical safety assessment for the environment focuses on the parent substance. There are no unacceptable risks (please refer to CSR Chapter 10). The substance is not classified for environmental hazard, and is not PBT or vPvB. The organophosphonate impurities are predicted to have the same properties as DTPMP and not be of higher toxicity. Inorganic impurities present are not biodegradable.

Description of key information

A reliable study, measured with DTPMP-H, indicates 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) (Saeger, 1978). This was selected as the key study.

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 (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. (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.

The acid and salts in the DTPMP category are freely soluble in water and, therefore, the DTPMP anion is fully dissociated from its cations when in solution. Under any given conditions, the degree of ionisation of the DTPMP 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 DTPMP-H, DTPMP (1-3Na), DTPMP (5-7Na), DTPMP (4-8K), DTPMP (xNH4) or another salt of DTPMP.

 

Therefore, when a salt of DTPMP is present in test media or the environment, the following is present (separately):

1. DTPMP is present as DTPMP-H or one of its ionised forms. The degree of ionisation depends upon the pH of the media and not whether DTPMP-H, DTPMP (1-3Na), DTPMP (5-7Na), DTPMP (4-8K), DTPMP (xNH4), or another salt was used for testing.

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

3. Divalent and trivalent cations have much higher stability constants for binding with DTPMP than the sodium, potassium or ammonium ions so would preferentially replace them. These ions include calcium (Ca2+), magnesium (Mg2+) and iron (Fe3+). Therefore, the presence of these in the environment or in biological fluids or from dietary sources would result in the formation of DTPMP-dication (e.g. DTPMP-Ca, DTPMP-Mg) and DTPMP-trication (e.g. DTPMP-Fe) complexes in solution, irrespective of the starting substance/test material.