All atoms in the biosphere cycle regularly aside from some very longstanding ones.
If you identify the chemical makeup of the monomer (corroded plastic, micro plastic, whatever you want to call it), there’s nothing stopping you from hypothetically finding or creating enzymes which can digest it. The only reason it hasn’t happened naturally yet is due to a lack of evolutionary pressure to digest the weird compounds we’ve been making up until now.
Set up farms of modified mycelium or bacteria to scrub the plastics, and stop using many or set up required end of life treatment for plastic manufacturers, and you’ll very rapidly make a dent in plastic spread.
Further, modify wild biota, such as mushrooms, bacteria, etc to have the ability to produce the same enzymes for assisting in cleanup.
Big project, yes, but technically feasible. We’ve done more extreme things.
ValiantDust