Bioaccumulation factor: how strongly a plant concentrates a substance

The bioaccumulation factor is a fraction with two measured values. On top is how much of a substance sits in a kilogram of plant, underneath how much sits in a kilogram of the soil it grows on. A result above one means the plant carries more than the ground beneath it; below one it is the other way round.

The value therefore answers a narrower question than its name suggests. It says how well a plant takes a substance up, and nothing about how much mass adds up in the end. That is exactly where the obvious conclusion falls apart, the one that a high factor makes a good crop for cleaning soil.

What drives it up and down

For PFAS the build of the molecule decides. PFAS are industrial chemicals whose carbon backbone is densely occupied by fluorine; they differ mainly in how long that backbone is. In the hemp field trial the factor fell as the chain grew longer – perfluoropentanoic acid was the exception, sitting above the shorter perfluorobutanoic acid.

The chemical end group counts too. At equal chain length, carboxylic acids accumulated more strongly than sulfonic acids, in the field as in the greenhouse, where the difference ranged from 0.4-fold to 360-fold. Long sulfonic acids such as PFOS, by contrast, stay mostly in the root and barely move into stem and leaf.

And it matters which part of the plant is measured. One greenhouse trial found over 45 % of the absorbed PFAS load in the pollen, at a factor above 20.8 – the pollen there carried more than twenty times what the soil did. No field measurement exists for this.

Why a high value settles nothing on its own

For the question of whether a site can be relieved through the harvest, what counts is mass per hectare and year, not the ratio. A small plant with a high factor can carry off less than a large one with a low factor, simply because it weighs less. In the greenhouse comparison mustard reached the highest tissue levels, while sunflower and hemp took up more in total through their greater mass.

The factor remains a definition all the same: a plant counts as a hyperaccumulator when it concentrates a substance many times above the soil level. Hemp does not qualify. Cadmium levels in the shoot were around a hundred times lower than in a known hyperaccumulator species.

The factors measured for hemp in the field trial sit in the range reported for other plant species. The crop therefore holds no special position as a collector, neither for PFAS nor for heavy metals. What follows from that for growing on contaminated land is covered in the article on contaminant uptake in hemp.

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