[munk-a]

I feel like "self replicating robots" might be too complex a solution, it might be easier to start investigating enzymes to make the plastic present more plasto-philic to condense the micro-particles and even just "smallish" particles into larger particles - if you could make sure the resultant blob was low enough density to float it might be even better - basically you're removing the issue of trying to sift through water to find the particulate and replacing it with going out in a boat and picking up a few basket balls of conglomerate.