Paraquat-dichloride

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

Study period:

07 Sep 1988 to 18 Oct 1989

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Qualifier:

no guideline followed

Principles of method if other than guideline:

14C-pyridyl labelled test substance was applied to a sandy loam soil (field sample) at a nominal rate of 1.05 kg/ha, and incubated in darkness at 20 ± 2°C under aerobic conditions. At intervals (0, 3, 7, 30, 61, 90 and 180 days after treatment), complete pots of soil were analysed by sequential extraction with methanol, with an aqueous solution of unlabelled test item and with 6M hydrochloric acid under reflux. The extracts were analysed by LSC, TLC and HPLC.

GLP compliance:

yes

Test type:

laboratory

Radiolabelling:

yes

Remarks:

[14C]-labelled at both pyridine rings (positions 2 and 6)

Oxygen conditions:

aerobic

Soil classification:

USDA (US Department of Agriculture)

Soil type:

sandy loam

% Clay:

14

% Silt:

22

% Sand:

64

% Org. C:

1.57

pH:

6.5

CEC:

10.8 meq/100 g soil d.w.

Details on soil characteristics:

SOIL COLLECTION AND STORAGE
- Pesticide use history at the collection site: The soil has received no pesticide treatment since October 28, 1985. Previously to this there were several treatments, specifically on February 26, 1985 "Gramoxone" (with the test substance as active substance) was applied. From this report it can be seen that no appreciable degradation of the test substance occurred. Therefore previous treatment has not resulted in enhanced degradation. Given that the soil had a microbial biomass of 3.19mg carbon per 100g of soil, biomass carbon was therefore equivalent to 2.9% organic carbon content of the soil, thus the soil was considered microbially active and typical for this soil type. Therefore the lack of degradation cannot be attributed to low microbalial activity.
- Sampling depth (cm): 10 after removal of turf
- Storage length: 20 days
- Storage conditions: In order to maintain its microbial viability, the sieved soil was adjusted to 40% of its moisture holding capacity (MHC) and kept at this level at 20 ± 2°C for 20 days until radiochemical application.
- Soil preparation: After drying sufficiently to facilitate sieving, the soil was passed through a 5 mm mesh, followed by a 2 mm mesh sieve before its physical and chemical characteristics were determined.

PROPERTIES OF THE SOILS
- Moisture at 1/3 atm (%): 14.7

Duration:

180 d

Initial conc.:

1.05 kg/ha d.w.

Based on:

test mat.

Parameter followed for biodegradation estimation:

CO2 evolution

radiochem. meas.

Temp.:

20 ± 2 °C

Humidity:

40 % (at zero suction)

Microbial biomass:

31.9 mg C/100 g soil

Details on experimental conditions:

TREATMENT OF SOIL
An aliquot of 106 μL of an aqueous solution of the radiochemical (containing 108.05 μg cation and 79.37 KBq) was applied dropwise by syringe to the surface of the soil in each pot, (equivalent to a rate of 1.005 kg/ha). The treatment solution was analysed by LSC before, during and after application to the soil.

INCUBATION OF SOIL
After the addition of the radiolabelled test substance solution, the treated pots of soil were assembled into glass columns supported on PTFE coated wire racks and incubated in darkness in a controlled temperature room (monitored at 20±2°C). The effluent air from each incubation unit was passed through one tube of 0.1 M hydrochloric acid (to absorb organic bases), one tube of 2- methoxyethanol (to absorb other organic volatiles) and two tubes of ethanolamine (to absorb 14-CO2). The level of radioactivity in the traps was determined by LSC at each sampling time or every two weeks, whichever occurred sooner. In order to prevent saturation of the 14-CO2 absorbing traps, atmospheric CO2 was removed from the ingoing airstream by passage through soda lime (self indicating 'Carbosorb', 10-16mm mesh). The CO2 free air was then moistened by bubbling through a column of distilled water.

Throughout the incubation period, the aerobic soils were maintained at 40% of their moisture holding capacity (at zero suction) by the weekly addition, as necessary of distilled water. This moisture level, rather than the 75% of 1/3 bar level recommended by the EPA Guidelines, was used in an attempt to optimise the microbial activity of the soil and, thereby to give the greatest potential for degradation.

% Recovery:

98.2

Key result

% Degr.:

< 5

Parameter:

radiochem. meas.

Sampling time:

180 d

Remarks on result:

other: The test substance accounted for over 93% of the applied radiolabelled carbon after 180 days incubation and no degradation products were detected

Remarks on result:

not measured/tested

Transformation products:

no

Evaporation of parent compound:

no

Volatile metabolites:

no

Remarks:

<0.1% as 14-CO2

Residues:

not specified

Remarks:

Only very small amounts of material (<5%) were not associated with paraquat and none of this chromatographed as a discrete band.

Details on results:

An overview of the analytical results is presented in 'Any other information on results incl. tables'.

EXTRACTION OF RADIOACTIVITY
Throughout the study, the extractability of the radioactive residues was essentially quantitative. At all sampling times less than 0.2% of applied activity was extracted in the initial methanol shake for each soil pot. At zerotime, 95.3% of the radioactivity was extracted with the aqueous 7440 ppm solution of the test substance cation. By day 30 this had decreased to 83.3% and after 180 days was 73.5%. The 6M hydrochloric acid reflux accounted for the majority of the remainder of applied activity. At zerotime, this extract contained 7.4% of applied radioactivity, at day 30 this had increased to 10.4% and after 180 days was 21.7%. Where third (acid) extracts were processed prior to TLC analysis, the mean recoveries from this were 96.1% (SD 2.7%).

HPLC OF SOIL EXTRACTS AND DERIVATISED SOIL EXTRACTS
The chromatograms of the extracts from Day 0 and Day 180 all showed one single radioactive component. This component co-chromatographed with a standard solution of the test substance under the same conditions. The mean recovery of the radiolabelled test substance from this HPLC procedure was 93.4% (SD - 3.4%) of the radioactivity injected. Derivatisation of soiI extracts to form 1,1'-dimethyl-4,4'bipyridyl- 2,2'-dione also yielded only one radioactive component. This radioactive component co-chromatographed with a standard solution of the "dione" under the same conditions. The dione was found to represent a mean value of 96.0% (SD 0.9%) as of the radioactivity injected.

Table: Distribution of Radioactivity in Soil Treated with 14C-pyridyl labelled test substance

Portion analysed	Radioactivity recovered (a) (as % applied of)
	Day 0	Day 3	Day 7	Day 30	Day 61	Day 90	Day 180
Extract 1	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2
Extract 2	95.3	98.6	81.7	83.3	83.6	80	73.5
Extract 3	7.4	3.3	10.5	10.4	12.2	12	21.7
14-CO2	N/A	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Unextracted	4.1	1.9	4.1	1.2	1.4	0.5	0.7
Total	106.8	103.8	96.3	94.9	97.2	92.5	95.9

N/A - not analysed

(a) mean of duplicate pots

(b) Includes soil on filter papers used

 

Table: Characterisation of radioactive residues in soil extracts

Sampling Interval (Days)	Extract (a)	Radioactive residues (c) (as % of applied to soil)
		Test substance	Baseline	Remainder(b)
0	2	92.3	1.8	1.2
	3	7.2	<0.5	<0.5
3	2	93.0	4.8	0.8
7	2	79.8	<0.5	2.5
	3	9.9	<0.5	0.5
30	2	82.1	<0.5	0.9
	3	10.2	<0.5	<0.5
61	2	81.4	1.0	1. 3
	3	11.7	<0.5	<0.5
90	2	77 .3	0.7	1. 9
	3	11.5	<0.5	0.5
180	2	72. 9	<0.5	0.5
	3	20.5	<0.5	1.0

a) 3rd extracts representing <5% of applied radiocarbon were not analysed by TLC

b) Remainder of activity on chromatogram

c) Mean of duplicate soil pots in solvent systems I and III

 

Table: HPLC of Soil Extracts

Sample	Bq Injected	Total Bq Collected	Bq under peak co-chromatographing with the test substance standard peak	Percent of Injected as test substance	As percent applied to soil	TLC data: Test substance residue as % of applied to soil (mean if the two solvent systems)
Day 0 Extract 2	57.4	57.0	56.4	98.2%	91.4	91.2
Day 180 Extract 2	54.8	51.2	50.1	91.5%	69.8	75.6
Day 180 Extract 3	36.5	35.0	33.0	90.6%	18.0	18.0

 

Table: HPLC of derivatised soil extracts

Sample	Bq Injected	Total Bq Collected	Bq under peak co-chromatographing with “dione” standard peak	Percent of Injected as “dione”
Derivatised Day 0 Extract 2	48.5	47.0	46.0	94.7%
Derivatised Day 180 Extract 2	43.1	42.9	41.6	96.6%
Derivatised Day 180 Extract 3	46.2	45.5	44.6	96.6%

 

Conclusions:

No clear evidence of test substance degradation was detected in this study.

Executive summary:

14C-pyridyl labelled test substance was applied to a sandy loam soil (field sample) at a nominal rate of 1.05 kg/ha, and incubated in darkness at 20 ± 2°C under aerobic conditions. At intervals (0, 3, 7, 30, 61, 90 and 180 days after treatment), complete pots of soil were analysed by sequential extraction with methanol, with an aqueous solution of unlabelled test substance and with 6M hydrochloric acid under reflux. The extracts were analysed by LSC, TLC and HPLC. The analytical results demonstrate very little radioactivity was evolved from the soil, as 14CO2 was less than 0.1% of applied activity being evolved over the 180 day incubation period. Radiochemical recoveries from soil extracts, extracted soil and volatile products were within the range 92.5 to 107% (mean 98.2%). The test substance accounted for over 93% of the applied radiolabelled carbon after 180 days incubation and no degradation products were detected. These data are entirely consistent with reported data which show that the test substance has a half-life of the order of 10 years in soil.

Description of key information

The test substance accounted for over 93% of the applied radiocarbon after 180 days incubation and no degradation products were detected (Vickers 1989). All available data was assessed and the study by Vickers is considered representative of the soil degradation properties of the test substance. Other studies are included as supporting information.

Additional information

Table: Overview of available data on biodegradation in soil of the test substance

Method				Guideline / GLP	Endpoint	Value (% deg)	Comment	Reference
Test medium	pH	CEC (meq/100g soil dw)	%OC
Crop field soil	6.5	10.8	1.57	No guideline specified / GLP	Disappearance (% Degradation) after 180 days	<5%	Almost no degradation observed. In line with previous soil studies (field), providing a half-life of the order of 10 years.	Vickers, 1989

Biodegradation in soil microorganism cultures

Once the test item enters the environment, it is rapidly and strongly bound to clay minerals and organic matter due to it’s adsorptive characteristic. The binding processes biologically deactivate the compound and generally account for >99.9% of the test item residues in soil. Several studies have used micro-organisms extracted from the soil to elucidate the degradative pathways of ring-labelled test item. The test item can be completely decomposed by micro-organisms: Corynebacterium fasciens (Tilford), Unidentified anaerobic species from soil and Clostridium pasteurianum Winogradsky that display similar efficiency in decomposing 20-40% of the test item administered. Bacterial isolated by Cahaba loamy fine sand, showed to degrade the test item and one of the metabolites was found. The yeast, Lipomyces starkeyi, has been isolated from soils which can utilize the test item as a sole source of nitrogen.Lipomyces starkeyi cultures or cultures originating from two sandy loam soils extensively metabolized test item. No test item was identified in any of the incubation solutions where mineralisation had taken place. Oxalic acid does not appear to be a major end product of normal metabolism in the yeast, and the 2 and 3 ring carbons of test item are the direct precursors of the oxalate. By Neocosmospora casinfecta as isolated by Cahaba loamy fine sand, was no degradation of test item. Evidence of biodegradation of one of the possible (photo)degradation products of the test substance was provided in literature. It was shown that extracts of Achromobacter D were able to cleave the ring structure of the degradation product between the C-2 and C-3 atoms. (Baldwin 1966; Baldwin 1971; Ricketts 1997; Funderburk 1967; Wright & Cain 1972).