Chelates, those metal-grabbing crab claw molecules which enfold metal atoms (or ions?) Like EDTA, but you'd want it to have a polymer backbone, to keep it stuck to your nets. You want it to have a reversible let-it-go-now cycle to reclaim the metal, capable of lots of repetitions before the material would have to be refreshed.
I think anything that would catch mercury would also catch all the heavier metals. Maybe it can't bee all that selective. But of course, after mercury, the heavier the more valuable the metal, so who's bitching.
turning science fact into <<science fiction>>