55r 50+ ions formed complexes with them. At pH = 4.8, the common ion effect became effective and the sorption of the hydrolysis products of **!Am became the more dominant factor. According to the ion adsorption model of James and Healy (1972) conditions favorable to the sorption of hydrolysis products may be brought to fore as hydrolysis proceeds. Concomitant with the hydrolysis of oO 7"lam, increasing amounts of the pH-dependent sorption sites (cation exchange 5 Treatment 40+ SJ r 35+ a <I - or 30 WwW 25F 3 - : Lo uj Isr Me uJ A A-HCI sol. OM, salts o o-Fe oxides X-H20 sol, OM, salts o- OM, Mn oxides o———o- Silica, Alum. Amorph alum-sii A\G \ . # Control ° q - oF o \ capacity) probably were formed with increasing pH and thus favored the sorption. The latter condition arises from increasing fonization of mineral hydroxyls and hydrous oxides and humus colloids with increasing pH (Buckman and Brady, 1969), At the higher pH levels, americium hydroxide probably was formed, but « X - GS residual OM ing extractability of 741Am above pH 10 was due mainly to the extraction of the alkali-soluble organic matter. The extractability of soil organic matter The influence of by alkalies is well known (Mortensen, 1965; Kononova, 1966}. organic matter became more evident after treatment ITI. The 24am extractability above pH = 7.7 ranged from 0.0 to 0.04% of dose. Although a portion of manganese oxides were removed (Table 1), the major cause of the enhanced extractability was the removal of the organic matter. This is evi- denced by the very low 7"!Am extractability above pH 10 after the removal of soil organic matter. Furthermore, manganese oxides that were formed under natural conditions are likely to be only slightly soluble under alkaline conditions. The results indicate that the high 2*!Am extractability above pH 10 before the removal of organic matter was due to the association of 24 lam with alkali-soluble organic matter and that its extractability was very low from the inorganic fraction of the soil and/or the precipitate of americium hydroxide under strongly alkaline condition, Part of the enhanced extractability below pH 7.7 may be due to the reduction of the sorption sites with the removal of the organic fraction. The cation exchange capacity was decreased from 20.45 me/100 g to 16.53 me/100 g (Table 1) by the treatment. The major cause of the effect, however, was considered to be due to the strong association of 24lam with the organic fraction of the soil before its removal. From ancther point of view, it appears that 24lam ions are quite readily exchanged off from the inorganic fraction by H tons under acidic condition. 5 L - T q qT q 1OF qT qT y Before Treatment I a es 5h - Oe1 tp 2 4 6 8 10 le 14 pH Fig. 1. solid phase is not known, As evidenced by the increasing coloration cf the soil extract from pale brown to very dark brown with increasing pH, the increas- range below pH = 7.7 andsignificantly reduced it at pH 11.6. Dose (cpm): - 0 ita state of polymerization and the nature of its association with the soil Treatment III significantly increased (at the 95% significance level) the Am extractabjlity relative to the control and treatments I and II in the pH o 1OFr 0 The NH,* ions exchanged a fraction of 74!Am ions off the ion exchange complex, while the CH C00 \A 45r lu increasing amounts of HNy* and CH4CoO- ions were formed. After Treatment I o Treatment IV significantly reduced the 2" 1am extractability relative to that by treatment IIL in the pH range below pH = 7.7. Above this pH level, there was no significant difference between the two treatments. The latter effect has been discussed above. Not all of the reactions causing the marked reduction of 2*lam extractability in the acidic pH range are known, but part of it might be due to the exposure of additional inorganic and organic sorption sites by the removal of free iron oxides. This is supported by the fact that the cation exchange capacity of the soil was increased from 16.53 me/100 g to 17.98 me/100 g (Table 1) and that some residual organic matter (0.43% organic C) ted Effect of pH on the extractability of 24lam from contamina components Aiken clay loam chemically treated to remove various (CH,COOH-NH,OH System) 84 85