AIRBORNE PLUTONIUM 293 



In general, the leaching data for sugar beets and bush beans suggest that both surface 

 roughness and particle size affect the retention of particles on foliar surface. With the 

 larger fresh-oxide particles (count mode approximately 0.142 /im), substantially more of 

 the plutonium is leachable from smooth leaf surfaces under both leaching conditions. 

 This may be the result of physical entrapment of particles in comparatively deep fissures 

 or crevices contributing to surface roughness in the bush bean leaf, especially if it is 

 assumed that a particle must be suspended in a water droplet to be removed from the leaf 

 surface. Similarly, the effect of acid leachate may be in alleviating the attractive forces 

 holding particles to leaf surfaces, particularly in the case of the sugar beet. The retention 

 behavior of the smaller hydrated oxide particles (count mode, approximately 0.019 jum) 

 is slightly different from that of the fresh oxide. The synthetic rainwater was about 

 equally effective in removing particles from both the bush bean and sugar beet; the acid 

 leach was slightly more effective with the bush bean. 



Although the gross surface structure of bush bean and sugar beet leaves is obviously 

 different, the microtopography of the surface itself may not be as different with respect 

 to the retention of very small particles (0.02 jum). This may explain similarities in the 

 retention behavior of plutonium deposited onto the foliage of sugar beets and bush beans. 

 It is impossible with limited data to generalize as to mechanisms controlling the fate of 

 particles on foliar surfaces. For the small hydrated-oxide particles, however, it appears 

 that leachability and retention are not only dependent on particle size with respect to leaf 

 topography and physical attraction, such as charge, but also on the ability of a water 

 droplet to contact the particle; thus wettability and contact angle become important 

 (Gregory, 1971). along with other environmental factors (Hull, Morton, and Wharrie, 

 1975) that influence the physical and chemical nature of the leaf surface. 



Aside from our lack of understanding of mechanism, it is important to note that the 

 behavior of small particles, such as that of plutonium on leaf surfaces with respect to 



TABLE 2 Effect of Continuous Leaching Regimes on the Removal of 

 Plutonium Particles from Leaves of Bush Bean and Sugar Beet*t 



*D. A. Cataldo, unpu Wished data. 



t Plant foliage was exposed to polydispersed aerosols. Particle-size data for bush bean 

 are given in Table 1. For sugar beet, fresh oxide had an AMAD of 1.59 ^m and a GSD of 

 1.76; the hydrated oxide had an AMAD of 0.75 /um and a GSD of 1.84. Count modes for 

 the aerosols were 0.130 and 0.048 mhi for the fresh and hydrated oxides, respectively. 

 Plants were leached with 800 ml of solution (equivalent to a 28-min rainfall of 1.7 cm) 7 

 days after exposure. 



|Four replicate samples, ;c ± SE. 



