Plants have an amazing ability in polluted environments: They tend to concentrate pollution into their bodies, which then concentrates more in the bodies of things that eat them, and so on. This is normally bad. You don't want cadmium, mercury, or lead in your food. But, this same principle, applied intelligently, can lead to cleaner soil so that this ceases to be a problem. I think I read about this idea in a magazine before, but like most things I read, I cannot for the life of me remember where.
The original article suggested growing key plants (different species prefer to absorb different chemicals), then burning the plants to recover the pollution. The recovered pollution is disposed of, safely this time.
But I think modern chemists can do better than just burning. I think they can mash the plants (say, with a pestle), and chemically separate the plant from the pollution, siphon the pollution off into some safe (or at least safely disposable) form, and compost the organic parts.
As an example of this in action, chocolate plants have a major affinity for airbourne lead. They concentrate this in their shells. Bad news for chocolate fans, because some half the world's supply of chocolate is grown in countries that have legal leaded gasoline, and hence a rich supply of lead in their air, which winds up in the chocolate pods, and some of it leaches into the final chocolate. Good news for cleaning that lead, since you can grow sacrificial chocolate to remove the lead already in the air.
This could even have economic benefits. Lead may be the cheapest heavy metal, but people still mine for it, because it's useful as a cheap radiation absorber, in certain dyes, and a few other safe uses. Lead claened from the air not only ceases to poison humans, but can be sold to the medical scanner company as a radiation shield.
(I have a nagging feeling like I wrote this before. )