There are a whole range of phytoremediation technologies now available
or under consideration.There is quite a lot of interest in
phytovolatilisation, which involves the plant taking up a toxic
substance from the soil, and then losing it from the leaves. In the
case of mercury, one obviously would not want to do this in an enclosed
space! But my guess is that it would be released fairly slowly, and
that the amounts present in the atmosphere near the plants at any one
time would be quite small. It would then become quite dispersed, and
problems would decrease further still. One might still want to ask
whether it was desirable to increase the flow of mercury in the
biogeochemical cycle in this way, but local toxicity will probably be
negligible.
Best Wishes,
Martin