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Element Concentration in Samples at Each Treatment Step

Figure 12.3 shows the element concentrations in samples at each treatment step. Co, Mn, Ni, and Sr concentrations in RL were lowered at the AR step. The removal rates at the AR step for Co, Mn, Ni, and Sr were 87.3 %, 99.1 %, 83.9 %, and 93.2 %, respectively. However, Cs concentration was not changed at any treatment steps.

As already described, except for Cs, all the target elements were removed from the RL at the alkali removal step. To obtain the degree of precipitation, the distribution coefficient (Kd) for each element was calculated by the following equation:

Kd = Csolid / Cliquid

where Csolid is an element concentration in the solid phase (precipitate) and Cliquid is an element concentration in the liquid phase (AR sample). Table 12.2 shows the Kd values for the target and major elements of the precipitate at the AR step. At this step, Na2CO3 was added as a reagent for removing alkalinity. Na concentration was

1.5 g/L in the RL and then rose to 2.2 g/L, which implied at least 1.5 × 10−2 mol/L

2−

CO3 ion was added at the AR step. Ca and Sr had high Kd values and were present mostly as cationic forms in the RL sample. The solubility products of CaCO3 and SrCO3 are quite low (10−8.3 for Ca and 10−10 for Sr) [15]; therefore, these elements in the RL must be precipitated by the following reactions:

Na2CO3 + Ca2+ ® CaCO ¯ +2Na+

Na2CO3 + Sr2+ ® SrCO ¯ +2Na+

Table 12.2 Kd values of the precipitate at the alkali removal (AR) step for each element

In addition, particulate fraction could be coprecipitated by the increasing pH at the AR step, which would explain why Co, Mn, and Ni also had high Kd values.

Although the zeolite adsorption step introduced for Cs adsorption was not effec-

tive for Cs removal, Takano et al. [16] succeeded in developing a radiocesium removal treatment system from a leachate in a general waste disposal landfill site, and in fact, the system has been introduced to a landfill site in Gunma Prefecture [16]. At the Iwate Clean Center landfill site, use of zeolite was begun in an adsorption tower in May 2012, and the zeolite had not been exchanged by the collection of the last samples in the present study (December 2012). Adsorption sites on the zeolite must be occupied by not only Cs but also other major cations such as K and Ca. K, especially, which is most like Cs with similar chemical properties, had a concentration in the AC that was 1.5 × 105 times higher than the Cs concentration. From this fact, little Cs could be adsorbed by the zeolite. Consequently, frequent changing of zeolite or introduction of another new type of treatment system may be necessary for effective Cs removal.

 
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