292 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



TABLE 1 Effect of Acid Solution on Leachability of Foliar Plutonium 7 Days After 

 Exposure of Bush Bean Plants to Fresh and Hydrated Plutonium Dioxide* 



Foliar plutonium leached,t % 



*Plant fohage was exposed to polydispersed aerosols. The freshly prepared oxide had an 

 AM AD of 1.27 Mm and a GSD of 1.63; the hydrated oxide had an AMAD of 0.73 /Lzm and a GSD 

 of 2.16. Count modes for the aerosols were 0.140 and 0.018 ^m for the fresh and hydrated 

 oxides, respectively. Plants were leached with 200 ml of solution (equivalent to a 7-min rainfall 

 of 0.4 cm). 



t Leachability expressed as microcuries of leachate/(microcuries leached + microcuries 

 remaining on leaves) x 100; four replicate samples, .v ± SE. 



soluble components may result from a solubilization of noncrystalline plutonium on the 

 surface of the particles. The increased leachability of the hydrated oxide (0.019 iim), as 

 compared with the fresh oxide (0.142 jum). may be related to the larger surface area 

 available for reaction. 



Even though much of the foliar-deposited plutonium is unavailable for leaching with 

 weakly ionic pH 5.8 solution, the increased removal of both soluble and insoluble 

 components with acidic solutions may indicate that a portion of the submicron particles 

 intercepted by foliage may be held on the leaf surface by charge phenomena and by 

 physical entrapment and not necessarily buried in waxy plates. Table 2 compares the 

 leaching behavior of plutonium from two plant species with different surface roughnesses. 

 Plants were leached with 800 ml of solution, and the leachate was collected in 50-ml 

 fractions. Since total plutonium in the leachate decreased logarithmically (plutonium 

 activity in the last few leachate fractions approached background levels), reported 

 retention values represent plutonium not readily leachable. 



Scanning-electron-microscope micrographs of the leaf blades show that bush bean 

 leaves have moderate to low surface relief and sugar beet leaves are relatively flat. The 

 difference in surface microtopography between these two species is related primarily to 

 patterns of wax deposition and the presence of trichomes in the bush bean leaves. The 

 surface wax of bush bean leaves is laid down in such a way as to form high longitudinally 

 oriented ridges with deep crevasses, and the surface of the waxy plates is relatively 

 smooth. By comparison, the surface wax of sugar beet leaves forms relatively shallow 

 irregular convolutions, and the surface of the wax deposits is rougher than that of the 

 bush bean. The trichomes of the bush bean leaves, which are approximately 150-/im high 

 and spaced approximately 190 /./m apart, provide additional surface relief. This 

 microtopography and its effect on particle entrapment and leaf-surface wettability may 

 provide a basis for understanding the processes involved in particle retention. 



