was present. Iron oxides are known to occur as cementing materials and partial coatings on the surface of the clay micelles and as discrete oxide particles (Carroll, 1958; Mehra and Jackson, 1960; Jenne, 1968; Anderson and Jenne, 1970). }OOr After Treatment IL Treatment V significantly increased the ?*!Am extractability relative to that ——<- by treatment IV in the acidic pH range below pH = 7.7. Above pH 7.7, the extractability was low as treatments LII and IV. The former effect indicates that the partial removal of free silica and alumina, amorphous alumino-silicates, and residual organic matter had some effect. As to which of these materials After treatment III, the 24. am extractability decrgased at a rapid rate between pH 0.65 to pH = 4 and then at a slower rate to pH = 6.5, Above pH 6.5, ita extractability remained very low (0.01 to 0.09% of dose). Its extractability after treatments IV and V varied to some extent relative to that after treatment I11, but they generally paralleled each other. The low extractability of 24lam in the alkaline range indicated again that its extraction before the removal of organic matter was due to the extraction of alkali-soluble organic matter. PERCENT OF DOSE EXTRACTABLE HNO3-NaOH System. The effects of the HNO3-NaOH system on the extractability of, Am are shown in Fig. 2. The 241am extractability for the control and the soil after treatments I and IE were not appreciably different. In general, the,extractability of *4lam decreased at a very rapid rate between pH 0.65 to pH = 2.2 and then at a slower rate between pH 2.2 to pH = 3.5. Above pH 3654 its extractability remained low to pH = 7.5, increased between pH 7.5 to pH = 12.3, and decreased above pH 12.3. The decrease of its extractability at about pH 12,3 suggests a change of state of the tracer, organic complexing system, and/or the sorption complex. The precise nature of this change is at present not known. The results showed again that 2"!am extractability from the inorganic 86 - 40h — — — - HCI sol, OM, salts OM, Mn oxides Fe oxides Silica, Alum. Amorph alum-sil residual OM L Dose (cpm). e0r 28, 650 + 963 |\OOF Before Treatment II 60 40- fraction of the soil was very low under alkaline condition. On the acidic side of the pt range, the removal of organic matter increased appreciably the extractability of “Am, indicating again that 2am was strongly associated with soil organic matter under acidic condition. Also, below pH 4, 24]am was readily exchanged by H ions from the inorganic fraction of the soil. CH3COGH-NH,OH vs HNO3-NaOH Systems. Fig. 3 shows the relative effect of the two extracting systems used. The HNO3-NaOH system is a strong acid-strong base system that is much less complexing compared to the CH ,COOH-NH,OH system. An overview of the figure shows immediately that wider pH range was covered by the HNO3-NaOH system. S8efore the removal of organic matter, the CH;COOH-NH,OH system extracted more 24lam chan the HNG3-NaOH system on the acidic side roughly to pH 7 and vice versa above pH 7. The greater 24 1am extraction by the CH3COOH-NH,OH system in the acidic range point to the wellknown fact that complexing action ig an important factor in the extractability 4————« ° © o O o——- © 60F played the dominant role was not determined, but in view of the marked influence of organic matter, the partial removal of the residual organic matter must certainly have had some influence. The residual organic C was reduced from 0.43% to 0.28% (Table 1). There was also a reduction of the cation exchange capacity from 17.98 me/100 g to 14.99 me/100 ge. Control x————~-x — H90 sol. OM, salts 20F Fig. 2. Effect of pH on the extractability of ?"1am from contaminated Aiken clay loam chemically treated to remove varfous components (HNO3-NaOH System) 87