They could potentially do pyrolysis of the biomass (after harvest) and then extract the heavy metals from the resulting char.

e.g. https://www.sciencedirect.com/science/article/abs/pii/S09619...

https://www.sciencedirect.com/science/article/abs/pii/S09213...

> No clue how they would have harvested and disposed of the tainted flax.

Sounds like a good basis for a NileRed[1] episode, say making paint[2] from flax seeds.

[1]: https://nile.red/

[2]: https://en.wikipedia.org/wiki/Red_pigments#Cadmium_red

> No clue how they would have harvested and disposed of the tainted flax.

It's flax. Harvest it before it goes to seed, ret it, break it, scutch it, spin it, weave it, make it into expensive garments. Unless you eat your shirt it's going to be perfectly safe.

And when said shirts are washed, the cadmium rich fibers in the effluent water go where?

Probably burned it - hence releasing it all into the air. But hey, out of sight, out of mind?