Plant roots do not selectively absorb. They absorb anything water-soluble that contacts them.

The reasons why are numerous. Selectivity would require special proteins on the surface for each substance. The proteins would get diseased, slow down growth and limit ecology to defined conditions.

Plants simplify their requirements to every extent possible to allow the widest possible range of environmental conditions. That means they require very few minerals, though they absorb whatever soluble minerals are in the soil around them. Selective absorption by roots would limit their ecology by only allowing those nutrients which have absorption proteins available as evolution produced from previous exposure.

Therefore, plants will pick up toxic substance when water soluble. For example, old, sedimentary soil can have concentrated selenium spots within it. The plants which grow there pick up the selenium, which can harm or kill the cattle that graze on those plants.

That's why polluted water or sewage should not be put on agricultural soil. The plants will pick up toxic substances and transfer those toxins to the food supply.

Some of the toxic substances will not decay or deplete, such as the heavy metals: lead, mercury, arsenic or cadmium. Sewage could have anything for heavy metals, and always will have some.

Organic toxic substances which don't decay can also enter sewage and polluted water, such as PFASs. If placed on agricultural soil, they will end up in the food.