They accumulate it in a specific organ, but they aren't magical alchemy machines. It stays in their bodies until they die, then they decompose and it goes back into the ground.
I don't recommend anyone buy any of them, but one of the books (or possibly short stories? It's been a while) in the Ender series by OSC kind of touches on that.
While isopods can sequester heavy metals within their bodies, this is a temporary solution. Upon their death and decomposition, the heavy metals are released back into the soil, so they do not permanently remove these contaminants from the environment.
Armadillidium vulgare is a common species. But there are probably hundreds of species that share the same nickname because most people don't know the differences.