Pyrithione zinc

Biodegradation in water: screening tests

Key value for chemical safety assessment

Biodegradation in water:

inherently biodegradable

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 Regulation EC 1907/2006.

Zinc pyrithione was not readily biodegradable under the conditions of the OECD 301B CO₂evolution test (Table 4.4). Degradation to CO₂reached 17% after 8 days, 39% after 28 days, and 54% after 43 days. This was below the pass level of 60% within a 10-day window within the 28-day test period. This data indicate that zinc pyrithione may be inherently biodegradable. 

Sodium pyrithione, which is completely ionized in water, was shown to be readily biodegradable under the conditions of OECD 301B (see Table 4.4). This implies that zinc pyrithione would also be readily biodegradable at concentrations lower than those required for the OECD 301B test because zinc pyrithione is mostly dissociated to the free pyrithione and zinc ions at low concentrations in water. Refer to the low-level photolysis study for a comparison of the UV spectra of sodium pyrithione and zinc pyrithione at low concentration.

The major non transient metabolite, pyridine sulphonic acid, was also shown to be readily biodegradable under the conditions of OECD 301B.   

The results of these studies suggest that zinc pyrithione would be at least inherently biodegradable; however, a guideline inherent biodegradability test was not done because other data on the degradation of zinc pyrithione exist from higher-level simulation tests conducted with radiolabelled test substance in natural water and natural water/sediment systems. 

Table 4.4: Ready Biodegradability of ZnPT, NaPT, and the primary non-transient degradation product 2-Pyridine sulphonic acid (PSA)

Guideline / Test method	Test type^1	Test parameter	Inoculum			Addit-ional substr.	Test substance concentr.	Degradation		Reference
			Type	Concen­tration	Adap­tation			Incubation period	Degree [%]
OECD guideline 301B	ready	CO2evolution	Activated sludge	30 mg dry material per litre	No	No	ZnPT 13.2 mg/L (5 mg C/L)	28 d	17 % degradation after 6 days, 49 % degradation after 29 days	Clarke (2002) Unpublished study
92/69/EEC, C.4-C. OECD 301B.							ZnPT 10 mg/L	43 d	17 % degradation after 8 days, 54 % degradation after 43 days	Jenkins (1998) Unpublished study
OECD guideline 301B							PSA 26.5 mg/L (10 mg C/L)	29 d	49 % degradation after 6 days, 64 % degradation after 14 days 73 % degradation after 28 days	Clarke (2002) Unpublished study
OECD guideline 301B							NaPT 30.2 mg/L (5 mg C/L)	28 d	0 % degradation after 6 days, 24 % degradation after 20 days 79 % degradation after 28 days	Clarke (2002) Unpublished study

¹Test oninherentorreadybiodegradability according to OECD criteria