Zircon, cadmium yellow

Categories

Category name:

Cadmium zircon pigments

Justifications and discussions

Category definition:

Cadmium zircon pigments

Category description:

Cadmium zircon pigments

Category rationale:

Zircon, cadmium yellow is a zircon-type oxide/pigment with the main structural element ZrSiO4 (Zr in an dodecahedral O environment) which forms a strong and inert matrix. Zr and Si are the main components with percentages of ~46% w/w and 15% w/w respectively. The amount of Cd as component is about 2% w/w.

The read-across substance Silicic acid, zirconium salt, cadmium pigment encapsulated is a zircon-type pigment as well with an identical crystallographic structure and a very similar composition. Zr content is ~46% w/w and the Si content is ca. 14% w/w. Cd is present in about 4.7% w/w. Minor amounts of Se and S (1.53 and 1.04% w/w, respectively) are present as well.

Besides the obvious structural analogy the solubility of both pigments in aqueous and physilogical media are as follows (determination of Zr and Cd):

Solubility of Cd from the pigment Zircon, cadmium yellow in physiological media was in a range of 0.180 µg/L (PBS) and 0.398 µg/L (ALF) after 2 hours. After 24 hours a dissolution range from 0.140 µg/L (GMB) - 0.722 µg/L (GST) was measured.

Solubility of Zr from the pigment Zircon, cadmium yellow in physiological media was in a range of below LOD and 0.812 µg/L (ALF) after 2 hours. After 24 hours a dissolution range from below LOD (ASW) - 1.44 µg/L (ALF) was measured.

The read-across substance Silicic acid, zirconium salt, cadmium pigment encapsulated afforded after 2 hours a solubility for the element Cd in range of 0.07 µg/L (GMB) 2.58 µg/L (GST) and after 24 hours a solubility of 0.07 µg/L (GMB) to 6.13 µg/L (GST).

Solubility of Zr from the pigment Silicic acid, zirconium salt, cadmium pigment encapsulated in physiological media was in a range of below LOD and 0.52 µg/L (ALF) after 2 hours. After 24 hours a dissolution range from below LOD - 1.25 µg/L (ALF) was measured.

T/D testing of the pigment Zircon, cadmium yellow afforded the following solubility at 1mg loading after 28 days:
Cadmium: below LOD (<0.0048) and 0.2077 ± 0.0993μg/L at pH 8 and pH 6, respectively
Zirconium:
T/D testing of the read-across substance Silicic acid, zirconium salt, cadmium pigment encapsulated afforded the following solubility at 1mg loading after 28 days:
Cadmium: 0.34 ± 0.09 μg/L and 0.86 ± 0.39 μg/L at pH 8 and pH 6, respectively
Zirconium:
In sum, the two zircon type pigments are structurally identical with very similar composition and both show a very low solubility in different artificial and aqueous media. In fact, the read-across substance Silicic acid, zirconium salt, cadmium pigment encapsulated shows slightly higher dissolution and bioaccessibility without showing any signs of systemic or local toxicity in various studies (acute inhalation, skin/eye irritation, sensitisation). No toxicity on algae, fish and invertebrates could be observed in respective studies.
Hence, read-across to Silicic acid, zirconium salt, cadmium pigment encapsulated is fully justified.