Is silicone biodegradable

No. Silicone rubber does not break down in the environment, or barely so. The backbone of silicon and oxygen is exceptionally stable chemically, which is exactly what makes the material so useful in moulds and seals, but it also means micro organisms can do nothing with it.

In practice this has three consequences. Never pour silicone down the drain, not even diluted or shredded, because the material is not soluble and passes straight through the treatment plant. Do not work shreds into soil or compost, where they eventually become a source of micro particles. And do not leave moulds in nature or as litter.

At the same time cured silicone rubber is chemically inert and not classified as hazardous. It contains no halogens, releases no plasticisers and causes no soil contamination. The issue is persistence, not toxicity.

The correct route is general or residual waste. From there it goes to an energy from waste plant, where the organic fraction is incinerated with energy recovery and the silicon remains as silicon dioxide in the bottom ash. That is not recycling, but it is controlled and harmless processing.

Read more: where do I dispose of an old silicone mould