(Though, I'm not too clear on why this is a hard problem—couldn't they just, say, throw them into some nitric acid to destroy any organics, like you'd do with chem-lab glassware?)
There was an oil spill on the East River in NYC a month ago, about 5000 gallons of oil[0]. Since each nanoparticle binds to some small amount of point, then would we need some close fraction of 5000 gallons worth of charged-polymer coated nanoparticles to extract it all?
If the production of this awesome substance is even close to as straining on the environment (aren't most polymers oil-based?) then I don't see it as a magic bullet for solving all of our water-contamination problems.
Still super freaking cool though.
[0] http://www.amny.com/news/east-river-oil-spill-thousands-of-g...
The expensive and tricky part of these (like any) magnetic nanofluid particles is the surface covering that makes them soluble in water but also attracted to oil droplets. Any harsh chemicals would destroy that, and you're left with just fancy rust.