I quite like the idea. I ...

I quite like the idea. I would rather have something slowly disintegrated by bacteria than something that never breaks down, continuously creating microplastic contamination. I would consider that the pistachio shells are floating, the almond shells are sinking. They have considerable surface, but let's not forget that only the inside counts: they will continuously rub the outside clean. 

I've never heard of using any of these materials (maybe as they are byproducts, and no profit in using them).