In the context of ocean plastic recovery/harvesting, I don't know that the purity is all that important - the more important factor is, collection. Being able to take plastic bottles and turn them into a kind of string, for example, seems more viable - if a hopper could be designed which takes a plastic bottle, rotates it around a stripping knife, and the output is a long twine - this could then be fed into the knitting machine.

I imagine this rube-goldberg'esque strandebeest-like contraption sitting out there harvesting wind and waves, slowly turning every bottle it gorges on into a finely woven matte of materials .. maybe even reproducing itself, who knows ..

EDIT: I asked Grok to design a self-replicating ocean weaver, and I have to say .. it seems like a viable idea to me. Perhaps we will see this kind of plastic harvesting in the near future .. at the very least, were I to be stranded on a plastic-laden island, I'm pretty sure I could work out a way to build a raft with sails ..