Plant uptake of plutonium

Plant uptake of actinides is well documented, particularly from past research on human health impacts of nuclear weapons use and testing. We know far more about plutonium uptake into plants from these past studies than we know about other actinides. Relative plant uptake availability for actinides is neptunium > uranium > curium > americium > plutonium, which is a reflection of their solubility under environmental conditions. Grasses produce phytosiderphores (PS) to acquire the iron (Fe) they need. Could these phytosiderophores also be used to trick plants into taking up plutonium?

Many variables have been shown to affect plutonium uptake into plants: plutonium form and concentration, soil chemistry, soil type, number of successive plantings, plant species, and plant surface area (when fallout and resuspended soil are major accumulation contributors).

As with most metals, it seems that the predominant factor in plutonium uptake from soil by plants depends on chelation of plutonium to increase plutonium solubility. Application of synthetic chelators (EDTA and DPTA), to either soil or hydroponic solutions, has been shown to increase uptake of plutonium up to 1,300 timesÑresulting in a 20-fold concentration. Ranking of various ligands and counterions for enhancing uptake of plutonium has been shown to be nitrate < acetate < glycolate < oxalate < citrate < EDTA < DPTA, which is a reflection of ligand affinity for plutonium.

Plutonium is translocated from the plant roots into the higher parts of the plants. Plants reduce plutonium(VI) upon uptake. Various studies have shown plutonium is present only as plutonium(IV) (greater than 90 percent) in plant xylem, even if supplied as plutonium(VI). This reduction process may be similar to the purported requirement for reduction of iron(III) to iron(II) prior to root membrane translocation.

These studies also indicate plutonium is transported as an anionic organic complex that is different than the form supplied to root-bathing solutions. In hydroponic studies in soybeans using plutonium(IV) nitrate, xylem exudates analysis of samples with plutonium added both in vivo and in vitro (xylem exudates collected then plutonium added) using anion and cation exchange columns and thin-layer electrophoresis showed that both iron and plutonium are present primarily as organic acid complexes (instead of amino acid or peptide).

The plutonium phytoremediation process is illustrated.

Nickel(II) and cadmium(II) were present primarily with components of the amino acid/peptide fraction, again showing the differing uptake and transport systems for hard and soft metals. The forms of plutonium transported in the xylem appear to change with plant age, and the amounts of plutonium parallel essential ion concentrations.

The evidence suggests that plutonium may track the normal plant ligands used for metal uptake and growth. Indeed, it has been suggested that after absorption in the plant, trace contaminants are translocated, metabolized, and stored generally analogously to nutrient elements. In support of this, nutrient elements, especially iron, zinc, and copper, that compete for reaction sites and organic ligands affect the chemical form of plutonium in the plant.

Phytosiderophores-mediated uptake could explain some of the observations made on plutonium uptake into grasses

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