IV. SOIL REACTIONS 



A. Adsorption 



The adsorption of cations by soil particle surfaces from solution can occur by 

 several processes; ion exchange is one of the most important. It was found that ion 

 exchange increases the sorption of calcium and strontium by a volume of soil 10 

 times greater than that held in solution in the pore space (88, pp. 191-211). The 

 adsorption of plutonium, cesium, strontium, yttrium, and cerium ions from solution 

 was found to be nearly complete up to amounts equal to 0. 01 times the saturation 

 capacity of the soil (61, pp. 170-190; 62). 



Strontium has a slightly higher adsorption energy than calcium (38, 48, 59). 

 Leaching and uptake experiments indicate sites of differential adsorption (15, 38). 

 The rate of exchange from solution to surface is rapid. -^ For soils of high "cation 

 exchange capacity" (CEC), the reaction is essentially complete in one minute, 

 whereas for soils of lower CEC there is a significant rise in adsorption over a longer 

 period of tLme. The equilibration of strontium-89 and calcium-45 with labile soil 

 calcium is complex, and the differential behavior of strontium and calcium increases 

 up to 70 days (59). 



Leaching soil columns with mixed-fission product solutions resulted in 80- to 

 85-per cent adsorption of the total radioactivity in the first few centimeters of the 

 soil (47). This accumulation in the top few centimeters agrees with analyses of soils 

 from test sites (108, 117, 120). Much work has been done on the adsorption of fission 

 products from solution in relation to the disposal of waste products. In such experi- 

 ments, the concentrations of radioisotopes and salts are usually in excess of those 

 expected in agricultural soils, but some of the results at lower concentrations may 

 be applicable. 



B. Desorption 



Rare earth isotopes contribute one half to three fourths of the activity in some 

 soils contaminated by fallout (117, 120). In one soil, 50 volumes of water, corre- 

 sponding to 320 inches of rain, were required to leach 10 per cent of the beta activity 

 from one soil volume. The rate of leaching was nearly constant after the first 20 

 volumes. About four per cent of the radioactivity in fallout from Operation Hurricane 

 was leached through 20 cm of soil in a 12-week field experiment (108). The leached 

 radioactivity was mainly ruthenium- 106 and rhodium-106. The activity of an equilib- 

 rium mixture of the soil, 405 days after the blast, was due mainly to ruthenium- 106, 

 rhodium-106, cerium-144, and praseodymium- 144. 



■•^ Unpublished results. Soil and Water Conservation Research Division, Agricultural 

 Research Service, U. S. Department of Agriculture, Beltsville, Maryland. 



