[[(phosphonomethyl)imino]bis[ethane-2,1-diylnitrilobis(methylene)]]tetrakisphosphonic acid

Biodegradation in soil

Link to relevant study record(s)

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:

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.

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 days 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.