Pyrithione zinc

Biodegradation in water and sediment: simulation tests

Description of key information

Degradation rates: water (20degrees C) = 0.176d-1, sediment (20 degrees C) = 0.63d-1

Additional information

The information contained within this robust summary document comes from studies which are in the ownership of Arch Chemicals Inc and which are protected in several regions globally. This information may not be used for any purpose other than in support of the Chemical safety Report submitted by Arch Chemicals Inc. under RegulationEC 1907/2006

A study conducted according to the OECD 303A Simulation Test - Aerobic Sewage Treatment guideline is available(Itrich 2006), the results from which provide information on the degradation of zinc pyrithione under simulated sewage treatment plant conditions. Most of the work on the simulation test has been completed, and preliminary results are provided in the discussions that follow. The higher-level simulation tests conducted using natural water includes a freshwater die-away study and a seawater die-away study. The higher-level simulation tests conducted using natural sediment/water systems include: two aerobic aquatic metabolism studies; two anaerobic aquatic metabolism studies; a saltwater microcosm study; and a freshwater microcosm study.

Aerobic Biodegradation

Refer toTable12for a summary of the results.

Seawater/sediment studiesRitter (1999a) and Ritter (1999b)

These studies were conducted in the dark with seawater and sediment. Mineralization to CO₂was insignificant during the timeframe of the study; however the percentage of the dose converted to bound residues in the sediment increased to 29% of the dose by day-30.

Pyrithione thiosulfate (OTS) was seen in the sediment extracts. 

The results from the aerobic aquatic metabolism studies with zinc pyrithione and copper pyrithione showed that pyrithione (including OTS) degraded in the water/sediment with an initial half-life (DT50) of 0.89 days. 

Initial degradation was biphasic and fit a two-compartment model consisting of the water compartment and the sediment compartment. The derived rate constant for degradation in the water compartment was 11.4 day^-1(1.5 hour half-life). This is much faster than the measured hydrolysis rate and, since the study was done in the absence of light, was not the result of photolysis. Because biodegradation is unlikely to occur quite this rapidly, the initial loss is attributed to abiotic sediment mediated degradation. 

Seawater die-away study(Fenn, 1995)

This study was conducted in the dark with seawater without sediment. 

A 2-3 day induction period was observed, during which the pelagic organisms presumably adapted to pyrithione as food source, followed by rapid first-order degradation. The average degradation rate of 0.2014 day^-1(22º C) for ZnPT was adjusted to 0.176 day^-1for 20º C.

Freshwater die-away study(Fenn 1996)

This study was conducted in the dark with river water without sediment. Zinc pyrithione degraded according to first-order kinetics with a half life of 17 hours at room temperature. The degradation half life of 17 hours may be regarded as biological since light was excluded and hydrolysis in sterile water occurs at a much slower rate.

Seawater microcosm study(McLaughlin, 2003)

This study was an outdoor study conducted with seawater and sediment with exposure to sunlight. 

 

Analyses for pyrithione and degradation products were done on the water and sediment over a period of 30 days. Degradation of pyrithione was first order after dosing during the day. A rate constant of 1.16 hr^-1, corresponding to a half life of 35.8 minutes, was obtained. Pyrithione was not detected in the water or sediment 4 hours after dosing during the daytime.

Freshwater microcosm study(Fenn, 2008)

This study was an outdoor study conducted with fresh water and sediment with exposure to sunlight. 

Analyses were done on the water and sediment over a period of 55 days for pyrithione and degradation products. No pyrithione was found in the sediment. The concentration of pyrithione in the water decreased from 74.0% of the dose at time 0 to 0.4% of the dose at 6 hours. The half life from time 0 to 4 hours was calculated to be 44 minutes.

 

Anaerobic degradation

Refer toTable13for a summary of the results.

Seawater/sediment studies(Ritter, 1999c and d)

After doses of 50 ng/g (0.05 ppm) to a water/sediment system in the dark, zinc pyrithione degraded rapidly following bi-phasic kinetics. The initial half-life was 0.5 hours, with 70% of the dose degraded at the first sampling time (<1 hour) and over 90% at the second time point (six hours). After 3 days, only 1% of the dose remained. Mineralization to CO₂was insignificant during the timeframe of the study; however the percentage of the dose converted to bound residues in the sediment increased to 52.7% of the dose by day-182.

Table12:Biodegradation of ZnPT and CuPT

Guideline / Test method	Test type^1	Test parameter	Inoculum			Addi-tional substr.	TS conc.	Degradation		Reference
			Type	Concen­tration	Adap­tation			Incubat-ion period	Degree [%]
No guidelines for this study	Aerobic marine water system (dark)	Pyrithione concentration over time Metabolite formation	Natural seawater	Not applicable	Not applicable	No	ZnPT 33 ng/mL	14	T day 0: 100% remaining T day 1: 96.2% T day 2: 94.6% T day 3: 93.7% T day 7: 50.9% T day 14: 10.4% DT50= 3.4 days	Fenn (1995) Unpublished study
No guidelines for this study	Aerobic freshwater system (dark)	Pyrithione concentration over time Metabolite formation	Natural river water	Not applicable	Not applicable	No	ZnPT 8 ng/mL	24 hours	T hour 0: 66.1% remaining T hour 2: 55.0% T hour 8: 46.9% T hour 23.5: 24.4% DT50= 17 hours	Fenn (1966) Unpublished study
U.S. EPA §162-4	Aerobic marine water sediment system (dark)	pyrithione dissipation in aqueous phase and sediment; metabolite formation; CO2evolution; bound residues accumulation	Natural marine water and sediment	5g (dw) marine sediment in 10ml water	Not applicable	No	ZnPT 52.2 ng/g	30 days	ZnPT DT50- 0.03 days DT90 - 1.3 days ZnPT + OTS DT50 - 0.89 days DT90 - 34 days	Ritter (1999a) Unpublished study
U.S. EPA §162-4	Aerobic marine water sediment system (dark)	pyrithione dissipation in aqueous phase and sediment; metabolite formation; CO2evolution; bound residues accumulation	Natural marine water and sediment	5g (dw) marine sediment in 10ml water	Not applicable	No	CuPT 46.5 ng/g	84 days	CuPT DT50- 0.03 days DT90 - 1.3 days CuPT + OTS DT50 - 0.89 days DT90 - 34 days	Ritter (1999b) Unpublished study
U.S. EPA §162-3	Anaerobic marine water sediment system (dark)	pyrithione dissipation in aqueous phase and sediment; metabolite formation; CO2evolution; bound residues accumulation	Natural marine water and sediment	5g (dw) marine sediment in 10ml water	Not applicable	No	ZnPT 50 ng/g	30 days	ZPT DT50water- 0.52 hours DT90water - 2.8 hours   DT50sediment - 17 hours	Ritter (1999c) Unpublished study
U.S. EPA §162-3	Anaerobic marine water sediment system (dark)	pyrithione dissipation in aqueous phase and sediment; metabolite formation; CO2evolution; bound residues accumulation	Natural marine water and sediment	5g (dw) marine sediment in 10ml water	Not applicable	No	CuPT 46 ng/g	182 days	CuPT DT50water0.52 hours DT90water 2.8 hours   DT50sediment 17 hours	Ritter (1999d) Unpublished study
No guidelines for this study	Aerobic marine water and sediment microcosm (one dosed in the dark and one dosed during daylight)	- pyrithione dissipation in aqueous phase and sediment - metabolite formation - bound residues accumulation	Natural marine water and sediment	3 cm of sediment with 25 - 30 cm of overlying water	Not applicable	No	ZnPT 53 ng/g	30 days	Day time dosing: DT50- 36 minutes No significant accumulation in sediment Night dosing: DT50- ~20 hours during darkness Pyrithione reached a maximum of ~3% of the dose in the sediment, but was not persistent	McLaughlin (2003) Unpublished study
No guidelines for this study	Aerobic fresh water and sediment microcosm (dosed during daylight)	- pyrithione dissipation in aqueous phase and sediment - metabolite formation - bound residues accumulation	Natural marine water and sediment	3 cm of sediment with 25 cm of overlying water	Not applicable	No	ZnPT 50 ng/g	55 days	DT50- 44 minutes No significant accumulation in sediment	Fenn (2006) Unpublished study
OECD 303A	Simulation test - Aerobic Sewage Treatm-ent	Removal of parent in effluent; distribution of radiolabel in CO2, effluent, and solids; % of parent and metabolites in effluent and solids	Activated sewage sludge	Not applicable	No adaptation other than that which occurs during normal operation of the plant from which the substrate was obtained	No	ZnPT 4 ng/mL	hydraulic retention time (HRT) - 6 hours	~99% removal of zinc pyrithione was attained	Itrich 2006 Unpublished study Key study

Table13: Maximum amount (%) of degradation products identified and detected more than 10% at one time and maximum amount of bound residues in water-sediment degradation studies of copper pyrithione (CuPT) and zinc pyrithione (ZnPT)

Degradation Products	Aerobic water/sediment system				Anaerobic water sediment/system
	Marine			Freshwater	Marine			Freshwater
		EZPTF 7012‑221 ZnPT (30 days)	EZPTF 7012‑222 ZnPT (30 days)			EZPTF 7012‑224 ZnPT (30 days)	EZPTF 7012‑225 ZnPT (90 days)
OTS		25% (day-0)	-	-		-	-	-
OMSiA		45% (day-3)	17% (day-30)	16% (day-1)		10% (day-0)	-	13% (day-0.25)
OMSA		22% (day-30)	15% (day-30)	-		-	-	-
PSoA		15% (day-30)	-	-		-	11% (day-7)	23% (day-0.75)
PSiA		-	-	-		-	57% (day-90)	31% (day-90)
OMDS		-	33% (day-7)	17% (day-7)		-	30% (day-0)	15% (day-14)
PDS		-	-	-		-	12% (day-3)	11% (day-90)
2-MP						70% (day-7)	20% (day-14)
Mixed disulfide		-	-	-		-	-	15% (day-2)
Unidentified metabolites		-	-	16% (day-7)		-	10% (day-90) 13% (day-0.5) 12% (day-2)	23% (day-1) 11% (day-7)
CO2		-	-	12% (day-30)		-	-
Bound residues		29% (day-30)	37% (day-21)	64% (day-21)		22% (day-30)	11% (day-30)	35% (day-60)

(OMSiA= pyrithione sulfinic acid, OMSoA=pyrithione sulfonic acid, PSoA=2-pyridine sulfonic acid, PSiA=2-pyridine sulfinic acid, OTS= pyrithione thiosulfate, OMDS=pyrithione disulfide, 2-MP=2-mercaptopyridine, mixed disulfide=the disulfide of 2-mercaptopyridine and 2-mercaptopyridine N-oxide)