Phytoextraction: pulling contaminants out of soil with the harvest
Phytoextraction is a relocation, not a clean-up. A plant draws a contaminant out of the soil through its roots, stores it in stem, leaf or fruit, and once the crop is cut and hauled away the substance leaves the site. Leave the crop lying and nothing has been gained.
That sets the yardstick: soil gets cleaner exactly to the degree that mass is removed. Not to the degree that the plant takes up well.
Why the substance does not disappear
The plant destroys nothing. With heavy metals that is obvious, since a chemical element cannot be broken down and can only change place. With PFAS it comes down to a particularly firm link inside the molecule, the one between carbon and fluorine: neither plants nor the bacteria in their root zone generally break it. Whatever is taken up therefore stays intact.
A soil problem turns into a harvest problem, and the question moves to disposal. A biogas plant does not degrade absorbed PFAS; they sit afterwards in the digestate, the residue left after fermentation, which usually goes back onto the field.
What decides whether it works
Three quantities: how much substance the crop takes up per area and year, how much is present in the rooted soil at all, and how many years follow from the two.
In a greenhouse comparison of sunflower, mustard and hemp, removal ranged from 0.2 to 33 % of the soil load per growing cycle. Short-chain compounds sat at the top end, long-chain ones such as PFOS at the bottom with roughly 1 % at most. The same work extrapolates: to remove 90 %, sunflower needs six growing cycles with perfluoropentanoic acid and 232 with perfluorooctanoic acid – one summer against several human lifetimes.
Heavy metals look similar. A German field trial put removal at 126 g of cadmium per hectare and growing season; whether that is much is decided only by the stock in the soil, and that figure belongs beside it every time.
That is why the type of molecule matters more than the choice of crop. Where contamination consists mainly of long-chain compounds, the responsible authority in Baden-Württemberg considers the method severely limited.
Distinction from related terms
Three words sound alike and lead to opposite recommendations. Phytostabilisation aims at the reverse: the substance should be locked into the soil so it does not seep into groundwater, and specifically should not move into the plant. Rhizoremediation relies on bacteria in the root zone dismantling the molecule rather than it being carted off. Phytoremediation is the umbrella term covering all three.
Anyone assessing a site therefore has to know which of the three is meant. Otherwise a recommendation gets followed that was written for a different goal.
Link to cannabis
Industrial hemp looks like an obvious crop because it is annual, grows fast and builds a lot of mass. It has no special capacity for uptake: cadmium levels in the stem were around a hundred times lower in an Italian trial than in a known hyperaccumulator species, meaning a species that concentrates a substance many times above the soil level. What hemp contributes is volume, not concentration. How much actually adds up in the field and where it ends up inside the plant is covered in the article on contaminant uptake in hemp.