AIRBORNE PLUTONIUM 291 



Total 



t ■ • ■] Insoluble 

 I I Soluble 



rh 



* 



* 



^ 



1^ 



21 



TIME OF LEACHING AFTER CONTAMINATION, days 



Fig. 1 Leachability of plutonium from bush bean foliage. Sets of four plants each were 

 leached at 1, 7, 14, or 21 days after contamination; v ± SEM (« = 4). (a) Fresh 

 plutonium dioxide, (b) Water-aged plutonium dioxide. 



Data on the retention of plutonium by foliar surfaces are limited. The data that do 

 exist are based on laboratory studies in which a low-windspeed exposure chamber was 

 used to contaminate plant canopies (Cataldo, Klepper. and Craig, 1976). Figure 1 

 illustrates the leachability of two forms of '^^^Pu dioxide as a function of residence time 

 on the foliage of the bush bean following a simulated rainfall of 0.4 cm in 7 min. The 

 particles deposited on the foliage had aerodynamic sizes (activity median aerodynamic 

 diameter ± geometric standard deviation) of 1.274 ^(m± 1.63 and 0.734 /im ± 2.16 for 

 freshly prepared and water-aged oxides, respectively. The count modes for the log-normal 

 distributions were 0.142 and 0.019 /jm, respectively. These latter values represent the 

 particle diameter (absolute size) with the highest frequency within the family of particles. 

 The plutonium retained on foliage after mild leaching ranged from 92 to 99%. These data 

 are qualitatively similar to those obtained for 1- to 3-ium lead particles (Carlson et al., 

 1976) but are contrary to data obtained with larger simulated fallout particles 

 (Witherspoon and Taylor, 1969; 1970; 1971). Both the fresh and the hydrated oxide 

 exhibit a reduced leachability with increased residence time on the leaf. The retention 

 mechanism may be related to physical entrapment of the submicron-size particles in small 

 fissures on the leaf surface or to charge adsorption between particles and the leaf surface. 

 The inability to readily remove plutonium from foliar surfaces has been noted (Hanson, 

 1975; Romney et al., 1975; Iranzo, 1968); the mechanisms controlling retention, 

 however, are not clear. 



Little (1973) used weakly acidic solutions to study the physical processes of ion 

 exchange involved in the retention of heavy metal particles, such as lead, on foliage. 

 Table 1 compares the leachability of foliar plutonium by synthetic rainwater with and 

 without 0.1% HNO3. Leaching with acidic solution results in a moderate increase in 

 insoluble plutonium leached from leaves contaminated with fresh PuO^ but a substantial 

 increase from leaves contaminated with the hydrated oxide. The large increase in the 



