Sodium 2-mercaptoethanolate

Biodegradation in water and sediment: simulation tests

Link to relevant study record(s)

Reference

Endpoint:

biodegradation in water: simulation testing on ultimate degradation in surface water

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 309 (Aerobic Mineralisation in Surface Water - Simulation Biodegradation Test)

Deviations:

no

GLP compliance:

yes

Specific details on test material used for the study:

Unlabelled- Appearance: Liquid- Batch: 2015-08-0281- Storage: At room temperature- Expiry date: 06 August 2017Radiolabelled Substance 206877/A- Appearance: Aqueous solution- Batch: 150814- Storage: In refridgerator (2-8°C)- Expiry date: Not reported- Specific activity: 1.85 GBq/mmol (50 mCi/mmol)- Concentration: 3.7 MBq/mL (0.1 mCi/mL)Radiolabelled Substance 206877/D- Appearance: Aqueous solution- Batch: 160701- Storage: In refridgerator (2-8°C)- Expiry date: Not reported- Specific activity: 2.035 GBq/mmol (55 mCi/mmol)- Concentration: 3.7 MBq/mL (0.1 mCi/mL)

Radiolabelling:

yes

Oxygen conditions:

aerobic

Inoculum or test system:

natural water: freshwater

Details on source and properties of surface water:

- Details on collection: Water was sampled from the upper layer (up to 1 meter depth) at the bank of a small pond "Schoonrewoerdse Wiel" located in Zuid-Holland, Leerdam, the Netherlands (N51.9168, E005.1331). The appearance of the water sample (colour and turbidity) was slight brown and very clear.- Storage conditions: The water was transported to the laboratory at ambient temperature.- Storage length: The water was used on the day of sampling.- Temperature (°C) at time of collection: 18.5°C- pH at time of collection: 8.6- Oxygen concentration: 10.1 mg/L (106%) at sampling- Hardness: 132 mg/L as CaCO3- Dissolved organic carbon: 12.6 mg/L- Electrical conductivity: 395 µS/cm- Water filtered: Yes. Upon arrival in the laboratory, the water was sieved through a 150 μm sieve.

Details on source and properties of sediment:

Not applicable

Details on inoculum:

Not applicable

Duration of test (contact time):

21 d

Initial conc.:

10 µg/L

Based on:

test mat.

Initial conc.:

50 µg/L

Based on:

test mat.

Parameter followed for biodegradation estimation:

radiochem. meas.

Details on study design:

TEST SYSTEM- Culturing apparatus: 1 L amber-coloured conical flasks- Volume of test solution/treatment: 300 mL- Agitation: Continuous by magentic stir bar- Method used to create aerobic conditions: Humidified air was gently passed through the flasks just above the water surface- Aeration: Continuous- Test temperature: 19.3-20.5°C in the climate controlled room and 22.8-23.6°C measured in a dedicated flask.- pH: 8.3-8.4- Dissolved oxygen concentration: 8.3-8.7 mg/L- Illumination: Constant darkness- Test set up: Flasks were connected to a series of traps: a polyurethane foam (PUF) plug inserted in the neck of the metabolism flask, a liquid trap containing ethylene glycol monoethyl ether (EGME) and two liquid traps containing 2N NaOH.- Number of culture flasks/concentration: 3 (plus 2 further flasks for mass balance)- Control: Yes. Sterile water (autoclaved for 20 minutes at 121°C)- Number of culture flasks/control: 1 per test concentration- Reference substance: Benzoic acid at target concentration of 50 µg/L- Number of culture flasks/reference substance: 2SPIKING- Composition of medium: A spike solution was prepared on day of spiking by diluting radiolabelled test item with Milli-Q water- Sampling: In the beginning, halfway through and at the end of the spiking procedure, the same volume of spike solution was analysed on LSC.- Analysis: The radiochemical purity of the spike solutions was determined by HPLC on the day of spiking.SAMPLING- Sampling frequency: Low concentration, high concentration, sterile control and reference control flasks were sampled at 0 and 4 hours and 1, 2, 6, 9, 14 and 21 days after spiking. The high concentration flasks were additionally sampled 2 and 8 hours after spiking. The mass balance flasks were sampled 0, 14 and 21 days after spiking. STATISTICAL METHODS:- Calculation: Radioactivity was expressed as % of applied activity. The choice of models to which the data were fitted was based on the FOCUS guidance document on estimating degradation kinetics. Optimisations were performed using the program CAKE version 3.2. Single first order kinetics (SFO) and to the Gustafson and Holden model (FOMC) were fitted to the parent data (not averaged). If the SFO fit was acceptable and better than the FOMC fit (based on visual assessment, t-test, and χ2 test), no further work was done. If the SFO fit was not acceptable or the FOMC fit was better, exclusion of outliers, constraining M0 or weighting was tried provided this could be justified by the data. If the SFO results were not improved by these adjustments, the data were also fitted to the hockey stick model (HS) and the bi-exponential model (DFOP). In this study, modelling beyond the FOMC model was not required.- Half-life: The DT50 and DT90 calculations for the decrease of the substance are based on the individual HPLC results. The SFO and FOMC model were fitted to the data. The fits obtained by the SFO and FOMC models were equally good based on visual fit and residuals. Therefore further modelling beyond the FOMC model was not performed. Considering that the p-values for estimated FOMC model parameters are statistically not significant or higher compared to SFO model, the results from the SFO fits are reported.

Reference substance:

other: Benzoic acid

Compartment:

natural water: freshwater

DT50:

0.049 d

Type:

(pseudo-)first order (= half-life)

Temp.:

22 °C

Remarks on result:

other: Nominal concentration: 10 µg/L

Compartment:

natural water: freshwater

DT50:

0.079 d

Type:

(pseudo-)first order (= half-life)

Temp.:

22 °C

Remarks on result:

other: Nominal concentration: 50 µg/L

Transformation products:

yes

No.:

#6

Details on transformation products:

TRANFORMATION PRODUCTS- Tranformation products: Three major transformation products were detected which exceeded 10% of applied activity at both test concentrations. Attempt were made to identify the transformation products by comparison with available reference substances.- Transformation product M-1: This reached a maximum of 26% after 4 hours (0.17 days) at the low test concentration and a maximum of 13% after 8 hours (0.33 days) at the high test concentration, and then decreased to non-detectable amounts after 6 days of incubation (both concentrations). M-1 could not be identified.- Transformation product M-3: This reached a maximum of 36% (low test concentration) and 30% (high test concentration after 2 days and then decreased to 14% after 9 days of incubation (low test concentration) and to <5% after 14 days of incubation (high test concentration). M-3 could not be identified.- Transformation product M-6C: This reached a maximum of 41% (low test concentration) and 39% (high test concentration after 4 hours (0.17 days) and then decreased to non-detectable amounts after 6 days of incubation. M-6C was identified as 2-Hydroxyethyl disulphide based upon retention time on HPLC (both chromatographic methods).

Details on results:

SURFACE WATER- Test vessels: The amount of radioactivity recovered in the spiked surface water quickly decreased in the test systems. After 2 days of incubation approximately 60% of applied activity was recovered in the water layers of both test systems, which decreased further to 16% (low test concentration) and 12% (high test concentration) of applied activity at the end of the incubation period (21 days). There was a significant difference between the radioactivity measured directly in the water layer and the activity measured in the water layer after addition of acid indicating that significant amounts of CO2 could be stripped off from the water layer.- Control: In the sterile controls, more than 90% of applied activity was recovered in the surface water after 2 days of incubation, which then decreased to 44% (low test concentration) and 29% (high test concentration) of applied activity at the end of the incubation period (21 days).- Reference: The activity in the water layer of the test systems spiked with the reference control decreased to less than 10% of initially applied activity within 14 days of incubation.VOLATILISATION- Test vessls: In the NaOH traps, approximately 20% of applied activity was recovered in the low concentration after 2 days of incubation. Activity increased further to 65% at the end of the incubation period (21 days). For the high concentration, approximately 17% of applied activity was recovered after 2 days of incubation. Activity increased further to 63% after 14 days of incubation. At the end of the incubation period (21 days) 57% of applied activity was recovered in the NaOH traps. - Control: In the NaOH traps, negligible amounts of activity was recovered during the first 2 days of incubation (≤ 0.2% of applied activity), which then increased to 16% (low test concentration) and 30% (high test concentration) of applied activity at the end of the incubation period (21 days).- Reference: In the NaOH traps, significant amounts of activity were detected, increasing to 62% of initially applied activity after 14 days of incubation.- Confirmation of radioactivity source: The radioactivity in the traps was confirmed to be 14CO2 after precipitation with barium hydroxide. - Organic volatiles: No significant amounts of radioactivity (< 2%) were detected in the polyurethane foam plugs (sampled at day 14 only) and in the ethylene glycol monoethyl ether traps, indicating that no significant organic volatiles were formed.MASS BALANCE- Mass balance: The mean values of the total amounts of activity recovered in the incubated test systems were between 69% and 95% (both test concentrations). Loss of activity occurred which was partially due to the fact that activities in the subsamples (sampled at previous sampling intervals) were not taken into account. Decrease in total amount of activity was also observed for the reference control.- Low test concentration: The mean total amount of activity recovered in the mass balance flasks was 91% of applied activity after 14 days of incubation, indicating that no activity was lost. - High test concentration: The mean total amount of activity recovered in the mass balance flasks was 81% after 14 days of incubation, indicating that some activity was lost, which was expected to be due to insufficient trapping of CO2 because activity in the NaOH traps connected to the regularly sampled flasks was 9% higher.DEGRADATION AND MINERALISATION- Degradation: The substance quickly degraded in the surface water. After 1 day of incubation, no parent could be detected in the water layers of both test concentrations. - Mineralisation: Mineralisation was a significant route of degradation. Activity recovered as CO2 increased to more than 60% after 14 days of incubation at both test concentrations.

Table 1          DT₅₀ and DT₉₀ in surface water

Test system	Initial concentration (µg/L)	Kinetics	DT50(days)	DT50 (hours)	DT90(days)	DT90(hours)
Surface water	10	SFO	0.049	1.2	0.16	3.9
	50	SFO	0.079	1.9	0.26	6.3

 

Table 2          Distribution of radioactivity in test system at low test concentration (% of applied radioactivity; mean values)

Time (days)		PUF		EGME		NaOH		Water layer		Total
0		na		na		na		100.9		100.9
0.17		na		0.5		0.3		93.8		94.6
1		na		1.0		2.0		68.5		71.6
2		na		0.1		20.2		60.1		80.4
6		na		0.1		45.3 *		34.9		80.9
9		na		0.1		51.3 *		28.6		80.3
14		0.0		0.1		61.8 *		21.0		82.6
21		na		0.1		66.5 *		16.2		82.8
14^		0.0		0.1		67.6		23.3		91.0
21^		na		0.0		65.5		18.3		83.9

na:     not applicable

*         average of 2 replicates

^         Mass balance replicates

 

Table 3          Distribution of radioactivity in test system at high test concentration (% of applied radioactivity; mean values)

Time (days)		PUF		EGME		NaOH		Water layer		Total
0		na		na		na		100.9		100.9
0.08		na		na		na		97.8		na
0.17		na		0.8		0.2		93.3		94.3
0.33		na		na		na		90.7		na
1		na		1.8		5.5		70.3		77.7
2		na		0.1		16.9		57.3		74.4
6		na		0.1		49.2 *		28.8		77.8 *
9		na		0.1		50.9 *		22.8		73.5 *
14		0.0		0.1		62.6 *		15.6		78.2 *
21		na		0.1		57.0 *		11.9		69.0 *
14^		0.0		0.1		54.4		26.5		81.1
21^		0.0		0.1		32.4		15.5		48.0

na:     not applicable

*         average of 2 replicates

^         Mass balance replicates

 

Table 4          Distribution of radioactivity in sterile control at low/high test concentration (% of applied radioactivity)

Time (days)		PUF		EGME		NaOH		Water layer		Total
0		na / na		na / na		na / na		102.3 / 100.8		102.3 / 100.8
0.17		na / na		0.2 /0.0		0.0 / 0.0		99.6 / 99.7		99.8 / 99.7
1		na / na		0.5 / 0.5		0.1 / 0.1		95.4 / 96.1		96.0 / 96.8
2		na / na		0.5 / 0.2		0.1 / 0.2		92.5 / 94.1		93.1 / 94.5
6		na / na		0.1 / 0.1		3.1 / 11.7		74.8 / 70.7		78.0 / 82.5
9		na / na		0.0 / 0.1		8.2 / 22.9		60.5 / 53.8		68.7 / 76.8
14		0.2 / 0.2		0.1 / 0.1		11.5 / 32.4		52.2 / 34.0		63.9 / 66.8
21		na / na		0.1 / 0.0		16.0 / 30.3		44.0 / 28.5		60.1 / 58.8

na:     not applicable

 

Table 5          Distribution of radioactivity in test system with reference control (% of applied radioactivity; mean values)

Time (days)		PUF		EGME		NaOH		Water layer		Mass balance
0		na		na		na		98.9		98.9
0.17		na		0.0		0.1		98.0		98.1
1		na		0.0		4.7		51.0		55.7
2		na		0.0		19.5		47.4		66.9
6		na		0.0		42.3		23.1		65.4
9		na		0.0		53.7		13.6		67.3
14		0.0		0.0		62.1		5.4		67.5
21		na		1.5		50.2		2.4		54.1

na:  not applicable

Table6          Parent and metabolite in in test system at low test concentration (% of applied radioactivity)

Time (days)	Parent	M-1	M-3	M-6C
0	100	0.0	0.0	0.0
0.17	8.9	26.2	17.9	40.9
1	0.0	12.0	35.8	6.8
2	0.0	5.8	35.9	6.7
6	0.0	0.0	18.5	0.0
9	0.0	0.0	9.9	0.0

Table 7          Parent and metabolite in in test system at high test concentration (% of applied radioactivity)

Time (days)	Parent	M-1	M-3	M-6C
0	100	0.0	0.0	0.0
0.08	60.7	2.9	0.0	28.1
0.17	16.5	9.3	8.6	38.7
0.33	4.8	12.5	14.0	38.3
1	0.0	8.7	28.6	18.0
2	0.0	5.4	30.3	0.0
6	0.0	0.0	11.3	0.0
9	0.0	0.0	7.0	0.0
14	0.0	0.0	2.5	0.0

Table 8          Goodness of fit assessment(parent)

	Model	visual fit	c^2	p-value*	residuals
Low concentration	SFO	good	0.0001	<0.05	good
	FOMC	good	0.0004	N/A	good
High concentration	SFO	good	16.7	<0.05	good
	FOMC	good	17.7	N/A	good

* p<0.05: estimated parameter(s) statistically significant (t-test) at 0.05 level. N/A: not applicable

 

 

Table 9          Parameter estimates(parent)

	Model	M0	k (d^-1)	α	b	DT50(d)	DT90(d)
Low concentration	SFO	100.0 ± 0.3	14.25 ± 0.11	na	na	0.049	0.16
	FOMC	100.0 ± 0.3	na	60.6 ± 44500	4.2 ± 3120	0.048	0.16
High concentration	SFO	106.4 ± 4.0	8.74 ± 0.52	na	na	0.079	0.26
	FOMC	108.8 ± 4.2	na	3700 ± nd	400 ± nd	0.075	0.25

na:     not applicable

Table 10       Individual HPLC results of surface water in sterile controls (% of applied radioactivity)

Time (days)	Flask	Parent	M-1	M-3	M-6C
0	15	100	-	-	-
	16	100	-	-	-
0.17	15	76.3	-	-	12.4
	16	82.1	-	-	10.8
1	15	49.9	1.9	-	23.6
	16	57.1	2.7	-	24.4
2	15	*	*	*	*
	16	38.6	3.7	2.3	35.7
6	15	-	5.0	16.2	23.2
	16	-	3.3	13.0	34.3
9	15	-	-	11.2	2.8
	16	-	1.5	15.3	14.8
14	15	**	**	**	**
	16	-	0.9	7.2	-

Transformation products are named M- and transformation product fractions MF-. Transformation product fractions contain multiple transformation products.

- no peak detected
* Injection failure
** Not performed

 

Validity criteria fulfilled:

not specified

Conclusions:

2-mercaptoethanol quickly degraded in the surface water, with a half-life (DT50) of 0.049 days at 10 µg/L and 0.079 days at 50 µg/L. After 1 day of incubation, no parent could be detected in the water layers of both test concentrations. Under biotic conditions, Ethanol, 2-mercapto degraded into the major metabolites, M-1, M-3 (both unidentified) and converted into disulphide M-6C (identified as 2-Hydroxylethyl disulphide). These intermediate compounds degraded to form CO2 (ultimate degradation) in less than 21 days.

Executive summary:

The mineralisation of 2-mercaptoethanol in surface water was determined in a GLP-compliant study following OECD guideline 309. Radiolabelled substance was added to surface water at concentrations of 10 and 50 µg/L and incubated for 21 days under aerobic conditions. Samples of the test solutions were analysed throughout the test by Liquid Scintillation Counting and High Performance Liquid Chromatography to determine the distribution of radioactivity between the surface water and the volatilised radioactivity in the traps and identify where possible the transformation products. Three major transformation products were detected, only one of which could be identified (2 -hydroxyethyl disulphide). It was concluded that 2-mercaptoethanol degrades quickly in water, with no parent compound detected in the water layers of both test concentrations after 1 day of incubation.

Description of key information

No data on the biodegradation in water and sediments is available for sodium 2 -mercaptoethanolate (target chemical). Read across can however be made to 2- mecaptoethanol (CAS 60-24-2). The (sodium) 2-mercapthoethanolate ion is a conjugated base, the read across substance 2-mercaptoethanol its undissociated acid. With a pKa value of ca. 9.5 for both substances, in an aqueous environment with pH 7, they will be present almost exclusively as 2-mercaptoethanol. Furthermore, as sodium is inorganic degradation does not occur.

Reliable data on the degradation in surface water of 2 -mercaptoethanol is available from one study. 

2-mercaptoethanol quickly degraded in the surface water, with a half-life (DT50) of 0.049 days at 10 µg/L and 0.079 days at 50 µg/L. After 1 day of incubation, no parent could be detected in the water layers of both test concentrations. Under biotic conditions, Ethanol, 2-mercapto degraded into the major metabolites, M-1, M-3 (both unidentified) and converted into disulphide M-6C (identified as 2-Hydroxylethyl disulphide). These intermediate compounds degraded to form CO2 (ultimate degradation) in less than 21 days.

Key value for chemical safety assessment

Half-life in freshwater:

0.079 d

at the temperature of:

22 °C

Additional information

No data on the biodegradation in water and sediments is available for sodium 2 -mercaptoethanolate (target chemical). Read across can however be made to 2- mecaptoethanol (CAS 60-24-2). The (sodium) 2-mercapthoethanolate ion is a conjugated base, the read across substance 2-mercaptoethanol its undissociated acid. With a pKa value of ca. 9.5 for both substances, in an aqueous environment with pH 7, they will be present almost exclusively as 2-mercaptoethanol. Furthermore, as sodium is inorganic degradation does not occur.

The mineralisation of 2-mercaptoethanol in surface water was determined in a GLP-compliant study following OECD guideline 309. Radiolabelled substance was added to surface water at concentrations of 10 and 50 µg/L and incubated for 21 days under aerobic conditions. Samples of the test solutions were analysed to determine the distribution of radioactivity between the surface water and the volatilised radioactivity in the traps, and to identify, where possible, the transformation products.Mineralisation was a significant route of degradation and activity recovered as CO2 increased to more than 60% after 14 days of incubation at both test concentrations. Three major transformation products were detected which exceeded 10% of applied activity at both test concentrations. Transformation product M-1 reached a maximum of 26% after 4 hours (0.17 days) at the low test concentration and a maximum of 13% after 8 hours (0.33 days) at the high test concentration, and then decreased to non-detectable amounts after 6 days of incubation (both concentrations). Transformation product M-3 reached a maximum of 36% (low test concentration) and 30% (high test concentration after 2 days and then decreased to 14% after 9 days of incubation (low test concentration) and to <5% after 14 days of incubation (high test concentration). Transformation product M-6C reached a maximum of 41% (low test concentration) and 39% (high test concentration after 4 hours (0.17 days) and then decreased to non-detectable amounts after 6 days of incubation.It was concluded that 2-mercaptoethanol degrades quickly in water, with a half-life of 0.079 days, and no parent compound was detected in the water layers of both test concentrations after 1 day of incubation.