Heavy metals- big, black book question

In the Permaculture Designer's Manual (aka the big, black book), Mollison talks about using plants to eliminate heavy metals from feces. Then, he suggests harvesting the plants and using them as mulch around food-producing plants or feed for chickens.

It seems to me that this would just put the heavy metals back into your food. What am i missing here?

Heavy metals aren't very labile, so you can get by using them as mulch or chicken feed, but it's kind of silly. Ideally, the best thing to do when you have it sequestered in a plant is to bury them deep. Take them out of the top 8" of soil completely.

There are patents (5,296,146 and 5,120,441) to use Datura innoxia (jimsonweed) for the removal of barium and plutonium from soil. Now if you are trying to get rid of plutonium contamination (like say at Los Alamos), I would definitely not use it as mulch, but find some stable geologic formation below the water table to bury it.