I have seen knotweed talked about for metals removal as well. it grows quickly so you can cut and carefully discard the growth and the patch will keep working. Might not cure our soils overnight but having an invasive battle industrial waste is makes for a cool antihero story.
Why is it that S. viminalis and other mentioned Salix species are so effective at this? Is it that the Salix genus is more effective or are more genuses (geni?) also effective but just haven't been studied yet?
But good question. There's actually been a ton of plants identified as "hyperaccumulators" (which is distinguished from regular "accumulators") of various heavy metals. You can actually find a good list on Wikipedia[0] organized by the contaminant. There's entire journals dedicated to this research. If you peruse that Wikipedia list you'll see Salix spp in general listed as being a noted accumulator of a large variety of contaminants.
Given that there's over 400 species of willows though I doubt this holds uniformally for all species and I also doubt that all 400 species have been rigorously tested for their phytoremediative potential. Common osier (aka S. viminalis) is a common tree in Europe so it's likely that it's just a readily available type species to study
I once heard willow was used as an important component of the medieval land reclamation projects. The tree was good at sucking out and evaporating water, and grows quickly. It was used to turn overly wet land into usable farmland.
Maybe that same mechanism helps here? If it pulls out water, and the water is contaminated, the willow evaporates the water and keeps the contaminants.
You're left with a much smaller volume of willow wood than the original soil. Wood that could be then be dried to reduce its weight even more.
It could then be used as a biofuel in a fluidized bed reactor specifically created to handle such heavy metal dense fuel, as suggested in this paper.[1] Most of the heavy metals remain in the ash, the cadmium needs to be trapped in the exhaust.
