Actinide plant uptake

To the extent that diet contributes to internal body burdens, a comparison of the relative plant uptake of actinides from soil is of interest. Several studies have compared the uptake of various transuranium elements by plants (17,18,19). However, studies which include U and Th are not as available. Figure 2 presents recent results for field-grown vegetation of soil originally contaminated in 1944 with soluble forms of U, Th and Pu (and daughter Am) (20.21). The crops examined include soybeans, snapbeans,... 

The limited information on the plant uptake of other actinide elements (U, Np, Am, Cm) indicates that higher CR values can be expected relative to those observed for Pu. Values for Am based on uptake by roots and from deposition on foliage approach or exceed the 10-1 value used in the LMFBR assessment thus, the value used in dose assessments is probably realistic but not conservative. Price (32) reported CR values of 10-1 to 10-2 for 237Np assimilated by the root pathway. Based on these data and on the low K. for Np (Table II), it appears that this element exhibits a higher mobility than the other actinides. A potential CR > 10-1 due to uptake from soil and from direct contamination of foliage is hypothesized for Np. Curium-244 uptake by the root pathway yielded CR values of 10 3 to 10-1, according to pot culture experiments (32, 52). 

Plant uptake of actinide elements varies greatly. Concentration ratios appear to be related to source characteristics, to chemical characteristics of the actinide in environmental... 

Sinha (60,61) has suggested that humic and fulvic acids play a major role in mobilising iron and transporting it from the soil to plant roots. At the normal soil pH it is believed that iron bound by the fulvic acid is partially hydroxylated as Fe(OH)2 (62). These complexes interact with phosphate to give an organicmetallic phosphate which may be taken up by plants (60). It has been suggested that the entire humic-iron-phosphate complex is taken up by the roots of plants and not just the iron and phosphate (60, 63). Jorgensen (64) has observed that soil humates suppress the uptake of Pb2+ into plants it is possible that they will also suppress actinide concentration in plants. 

It is significant that oat plants, which are known to contain the Fe3+ complexor, 2,4-dihydroxy-7-methoxy-l,4-benzoxazin-3-one (128), do not show any significant accumulation of plutonium, or the other actinides. It is possible that this complexing agent is located within plant cells which do not come into contact with the cation transporting mechanisms. Although there is evidence of the existence of microbial hy-droxamates in soil and that hydroxamates do become concentrated in plants (129), there has been no evidence presented yet that hydroxamates are the agents responsible for plutonium uptake into plants. On the other hand there is evidence that EDTA and DTPA can stimulate actinide concentration in plants (See Table 6). 

The two principal methods by which actinides may enter the body are inhalation and penetration through wounds. These two routes of entry are of obvious concern to those individuals working in nuclear fuel reprocessing plants. The principal route of entry by which most of the general public is likely to be exposed to the actinides could be expected to be via the food chain. However, Bennett (176) has indicated that inhalation of 239>(i) 240Pii is more important by a factor of 1000 compared to the uptake by ingestion in contributing to the body burden. 

In Sect. Ill evidence has been presented which indicates that only small quantities of actinides pass from soil into plants. The uptake of the actinides from the gastro-intestinal tract is slow (Tables 7a and b). 

Pu(IV), which forms highly charged polymers, strongly sorbs to soils and sediments. Other actinide III and IV oxidation states also bind by ion exchange to clays. The uptake of these species by solids is in the same sequence as the order of hydrolysis Pu > Am(III) > U(VI) > Np(V). The uptake of these actinides by plants appears to be in the reverse order of hydrolysis Np(V) > U(VI) > Am(III) > Pu(IV), with plants showing little ability to assimilate the immobile hydrolyzed species. The further concentration of these species in the food chain with subsequent deposit in humans appears to be minor. Of the 4 tons of plutonium released to the environment in atmospheric testing of nuclear weapons, the total amount fixed in the world population is less than 1 g [of this amount, most (99.9%) was inhaled rather than ingested]. 

The purpose of this paper is to present data on the environmental behavior of selected actinide elements. Environmental chemistry factors that influence the mobility of these elements are discussed. Some of the variability of data on the uptake of actinide elements by plants is explained. The behavior of manmade actinides in a representative aquatic environment is also discussed. Except for general comparisons, exposure of man by the inhalation pathway is not considered in this paper, even though higher exposures are routinely calculated for this pathway relative to the ingestion pathway. 

An important question is whether chemical transformations in the environment will increase or decrease the uptake of these elements by plants and other organisms that are consumed by man. Longterm projections on changes in availability to biological systems are uncertain. There is no historical geochemical data base to substantiate hypotheses of either increased or decreased uptake and consequent hazards from man-made actinide elements resulting from long-term environmental interactions. 

In nearly all pot culture experiments, the presence of a chelator increased the uptake of Pu and Am by plants (33, 34, 3 , 36). The chelator likely favors mono-dispersed Pu forms and affects sorption of the elements to the solid soil phase. Because the chelator tends to promote mobility, it enables a greater fraction of the actinide to be assimilated by roots. The chelator is also able to complex soil Am directly and increase its uptake by plants (36). 

From new data and from evaluation of published information, it appears that the magnitudes of uptake of actinide elements by plants from contaminated soil generally are less than the value used in the assessment of radiological impact for the LMFBR environmental assessment. The CR value of approximately 10-1 used in the impact assessment exceeds most observed values for Pu (Fig. 3), and appears conservative for incorporation into foods by the root pathway. Even after 30 years of residence time in the biologically active environment of the Oak Ridge floodplain, greater than 99% of the Pu in this ecosystem remains associated with the soil. The observed CR value is 10-3. For this time frame, there is no evidence that ecological or soil processes will cause the soil-to-plant transfer of Pu to approach the 10-1 value used in the LMFBR... 

The array of information available on the uptake of actinide elements by plants permits qualified statements on the probable order of soil-to-plant transport. Plutonium exhibits the lowest level of uptake. Neptunium appears to be assimilated to the greatest extent, and U, Am, and Cm have intermediate values of... 

It appears that both the order of sorption of actinides to colloids and their order of uptake by plants is affected by oxidation state. The postulated order of uptake by plants (Np>Am=CmHI>Pu) is approximately the inverse of their order of hydrolysis and their order of adsorption of oxidation states to colloids [Pu(IV)>U(VI)2Cm(III)=Am(III)>Np(V)]. Based on reported adsorption results for Pu(VI) (48, 49), and on preliminary results from our sorption studies dealing with oxygenated-"yl" species, the position of Pu(VI) would be between U(VI) and Np(V). Results from one study (48) showed that Pu(VI) is assimilated by barley plants more readily than Pu(IV) or Pu(III). 

Uptake of Am, Cm, and Np by plants exceeds that observed for Pu differential uptake 1s postulated to be related to hydrolysis and sorption characteristics of the actinides. The order of uptake for these actinides (Np>Am2CmMJ>Pu) appears to be related to the order of oxidation state species, V>III VI>IV. 

Plutonium is not very available to biota under most circumstances. There is no evidence for biomagnlfication by plants or organisms of terrestrial and aquatic food chains. More information is needed on magnitudes of uptake of non-pluton1um actinides by biological organisms. 

Table 1.1 lists some historical "firsts" with regard to nuclear weapons. The extensive tests in the atmosphere up to 1963 lead to a large global q>read of tritium, fission products and actinides. Scimtists have used this to learn more about global wind and water currents. Radiochemists have studied the migration of deposited radionuclides, as discussed in Chapter 22, radioecologists the uptake of radioactive elements by plants and animals, as described in Chapter 18, etc. 

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