Pyrithione zinc

Endpoint summary

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.

Adsorption and desorption in soil and sediment

In a study of the adsorption and desorption of zinc [pyridine-2,6-¹⁴C]pyrithione in two sediments and two terrestrial soils (Wysocky & Reynolds, 1996) conducted according to U.S. EPA Pesticide Assessment Guidelines, Subdivision N, §163-1, it was shown that zinc pyrithione has a high affinity for soil and sediment with adsorption Koc values ranging from 784 to 10,633. The corresponding desorption Koc values were even higher. The Koc values were not proportional to the percent organic matter in the soils and sediments, indicating other mechanisms for adsorption. Mobility was classified as low and slight in the two soils and slight and immobile in the two sediments. This was consistent with the results from a soil column leaching study discussed below (Ritter, 2001), in which zinc pyrithione was found to be immobile in four different soil types. 

In a study on the soil sorption and desorption of zinc pyrithione in five European soils by Haberhauer (2002) (Ritter, 2001) conducted according to OECD Guideline 106, very high sorption (Kd > 550) was obtained for a clay loam. Similar sorption (Kd of 20 to 79) was determined for the other soils. Desorption is much less favoured than sorption, with desorption coefficients of one order of magnitude greater than that of sorption. The mobility of the test substance in soil is therefore low. .

Soil column leaching study

A soil column leaching study(Ritter, 2001)according toU.S. EPA Pesticide Assessment Guidelines, Subdivision N, §163-1, was conducted to further study and evaluate the mobility of zinc pyrithione in soils. The leaching potential of zinc pyrithione by four different soils types was investigated using the radiolabelled test substance. The leaching potential of pyrithione degradation products was also investigated after aging zinc pyrithione in sandy loam soil for a period of approximately one half-life (13 - 15.5 hours). 

Most of the radioactivity in the unaged soil was retained in the top 6-cm soil segment. In the aged soil columns, most of the radioactivity remained in the top 0.6-cm layer of aged soil. Degradation of zinc pyrithione in the soil was extensive during the period over which the columns were leached. 

The data from this study confirm the large K_adsvalues found in the batch equilibrium studies on zinc pyrithione. Zinc pyrithione binds strongly to soil and is immobile. 

In summary, zinc pyrithione is adsorbed and rapidly degraded under conditions presented in a broad range of soil types. The half-life for pyrithione during aging in sandy loam was calculated as 13 - 15.5 hours. Significant degradation also occurred in unaged soil on the soil columns. The major terminal metabolites are humic-fraction incorporated bound residues, pyrithione sulfinic acid, and pyrithione sulfonic acid. 

The leaching behavior of zinc pyrithione in four soils shows pyrithione to be immobile, which is consistent with the results from the adsorption/desorption study in two soils and two sediments. 

Table 4.8: Adsorption and desorption of zinc pyrithione

Guideline / Test method	Soil/sediment	Adsorbed a.s. [average %]	Ka^1	KaOC^2	Kd^3	KdOC^4	Ka/ Kd^5	Degradation products Name                 [%] of a.s.		Reference
U.S. EPA Pesticide Assessment Guidelines, Subdivision N, §163‑1	sandy loam loam Saltwater sediment Freshwater sediment	92.2 69.3 46.2 37.8	50.37 11.37 98.68 47.99	2347 784 10633 3597	70.67 12.66 202.45 87.04	3293 873 21814 6524	0.71 0.90 0.49 0.55	OMSiA OMSoA OMDS PSoA MxDS PDS 2-MPY	nd - 22.3 % nd - 0.7 % 5.3 - 39.6 % nd - `0.4 % 1.7 – 3.9 % nd – 14.9 % nd -1.8 %	Wysocky, Reynolds (1996) Unpublished study
OECD Guideline 106	Silty loam Clay loam Loam Clay Sand	28.7 92.7 56.6 46.8 45.4	21 780 71 45 42	860 22500 2250 1700 8200	2300 7200 2300 260 390	97000 205000 74500 9600 75500	Not reported in study	Degradants not identified		Haberhauer (2002) Unpublished study
U.S. EPA Pesticide Assessment Guidelines, Subdivision N, §163‑1	Sandy loam Sand Loam Clay Loam	71.5 62.7 85.0 68.9	Not calculated	Not calculated	Not calculated	Not calculated	Not calculated	OMSiA OMSoA PSiA PSoA OMDS MxDS	7.5 – 22.8 % nd – 21.4 % nd – 8.6 % nd – 3.7 % 0.1 – 6.8 % 1.1 – 8.8 %	Ritter (2005) Unpublished study

¹Ka= Adsorption coefficient

2KaOC= Adsorption coefficient based on organic carbon content

3Kd= Desorption coefficient

4KdOC= Desorption coefficient based on organic carbon content

5Ka/ Kd= Adsorption / Desorption distribution coefficient

Conclusion

Zinc pyrithione are slightly (Koc=784) or very slightly (Koc=2347) mobile in soils and very slightly mobile (Koc= 3597-10633) in sediments.

 Volatilisation

Henry’s Law constant

The calculated Henry’s Law constant of <5 x 10^-5Pa-m³-mol^-1indicates that volatilisation from surface waters is not expected to be an important process.