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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 time. 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. De sorption 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. ^â¢Unpublished results, Soil and Water Conservation Research Division, Agricultural Research Service, U. S. Department of Agriculture, Beltsville, Maryland.
Strontium is leached slowly through the soil at a rate related inversely to the CEC. Under cropping and fertilizer treatments in soil columns, calcium-45 moved about four inches downward (11), but no detectable movement three inches laterally or four inches downward was observed after 14. 5 inches of rain in 89 days of field ex- periments (12). Strontium-90 from worldwide fallout was located primarily in the upper two inches of uncultivated soil during 1954 and 1955. In 1957, as much as one half of the strontium-90 was found in the two- to six-inch layer of some soils (1, 2). The desorption of cesium is less than that of strontium, possibly because of fixation by micaceous minerals (113). The rate and depth of leaching increases with increments in salt concentration, acidity, and complexing agents, and with a de- crease in base saturation and buffer capacity of the soil. Lime and organic matter also reduce the desorption of strontium and cesium (47). C. Effects of Other Ions The complementary ion exerts a strong effect on the adsorption of a cation. All cations tend to reduce strontium and cesium adsorption if used in large amounts. The order of replacement on soil materials is usually lithium< sodium <potassium < ammonium < rubidium < cesium < hydrogen< magnesium < calcium < strontium < barium <iron <aluminum< lanthanum (48; 60; 62; 124; 128, pp. 158-181; 134). Var- ious exchange equations have been suggested, but none appears universally applicable. The effect of anions cannot be neglected, for it was found that nitrate, chloride, and sulfate reduce the sorption of strontium in that order, whereas oxalate and phosphate tend to increase it (61, pp. 170-190). D. pH Effects In most studies of pH effects, the pH of the leaching solution has been varied. In agriculture, the pH of the soil rather than the contaminating solution is variable. The results of some studies (61, pp. 170-190; 91; 100; 101) indicate that the maximum adsorption of strontium occurs between pH 7 and 9, cesium at 6 and higher, yttrium and cerium above 6, and plutonium from 2. 5 to 9. 0. Since highly acid and alkaline conditions result in the decomposition of the soil minerals, it is expected that under such conditions there would be less fission-product adsorption as a result of com- petition by the products of decomposition, particularly aluminum. Clays differ in their exchange capacity per unit weight and in the energy with which adsorbed ions are held. Exchange capacities in terms of milliequivalents of strontium per g of some representative clays (38) are vermiculite, 1.36; Utah bentonite, 1.28; illite, 0.23; and kaolinite, 0.05. The percentage of water-soluble strontium in a bentonite suspension is 4 compared to 30 for kaolinite (82). Less strontium is adsorbed on illite than on bentonite and a greater uptake of strontium is observed in plants grown in illite (63).
F. Organic Matter Additions of decomposable organic matter can reduce the uptake of strontium (40, 80, 81). A major factor is probably the increased microbial population, although adsorption to the organic matter itself is also important (56, 93, 108, 117). G. Fixation Fixation is a general name for processes occurring in soils that convert ions from forms available to plants into those not available. Soil culture experiments, such as those conducted with the Neubauer technique, have been used to evaluate the amount of applied fertilizer that is available for plant uptake. Neutral normal salt solutions are often used to extract the exchangeable cations, which are considered to constitute the major source of the available quantities of many nutrients. The difference between the applied amount and the available or extractable amount is usually considered fixed. Thus, fixation is an arbitrary term which depends upon the experimental conditions. Proposed mechanisms for fixation include precipitation as slightly soluble materials, physical trapping between clay platelets and in other insoluble precipi- tates, and diffusion into existing crystals (89, 97, 113, 114). Nonexchangeable amounts of strontium (82, 98, 103, 114) and cesium (82, 83, 113, 129) have been found in some soils. In other soils, no fixed strontium was found (43, 47). The magnitude of strontium fixation ranged as high as 20 per cent of fallout strontium-90 in North Carolina soil samples taken in 1958 (103). Labora- tory studies showed that increasing the temperature from room temperature to 60° C tripled the amount fixed in these soils. Increasing the equilibration time from one to two weeks doubled the amount fixed at room temperature but had no effect at 60° C (98). In other laboratory studies with these soils, fixed strontium appeared to be extractable at 80° to 90° C. * H. Erosion Because most fission products are strongly adsorbed to clays, it is expected that any redistribution of surface soil will cause a similar redistribution of fallout. The strontium-90 concentration in runoff from field plots was 10-30 times the con- centration in the soil and was almost entirely associated with the sediment (69). The strontium-90 concentration on cultivated watersheds, where the amount of soil erosion was known, was one third to two thirds of the concentration on watersheds where there had been no erosion (25). Slope and cropping history appeared to be related to the amount of loss.
