Sorption Behavior of Neptunium on Bentonite-Effect of Calcium Ion on the Sorption -
- PDF / 413,055 Bytes
- 8 Pages / 414.72 x 648 pts Page_size
- 65 Downloads / 216 Views
SORPTION BEHAVIOR OF NEPTUNIUM ON BENTONITE - EFFECT OF CALCIUM ION ON THE SORPTION -
Naofumi KOZAI, Toshihiko OHNUKI and Susumu MURAOKA Department of Environmental Safety Research, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken, 319-11 Japan
ABSTRACT The sorption behavior of neptunium on bentonite was studied with batch type sorption and desorption experiments over a pH range of 2 to 8. A series of parallel studies using Na-smectite, 2 Ca-smectite and admixtures of Na-smectite and calcite quantified the capacity of Ca + (which occurs in bentonite as an exchangeable cation of smectite and as a component of calcite) to inhibit the sorption of neptunium. The distribution coefficient (Kd) of neptunium for bentonite was constant from pH 2 to 7, while for pure Na-smectite Kd increased below pH 5 due to "specific sorption" of neptunium on Na-smectite. Specific sorption was defined as occurring when neptunium could be desorbed by a strong acid (1 M HCI) but was stable in the presence of 1 M KCI. It was found that the quantity of neptunium sorbed on Na-smectite was inversely 2 proportional to the concentration of Ca + in solution, an effect most pronounced at pH 7, and maintained a constant value of -5 % from pH 2 to 7 (Fig. 2); this result is similar to the trend in Kd for bentonite versus pH. PI for Nasmectite ranged from 10 - 20 % over the same pH range. On the other hand, the Ps for bentonite maintained a constant value of -5% over the pH range (Fig. 3), while for Na-smectite Ps rose rapidly below pH 5 to approximately 70% at pH 2. These results show the pH dependence in the amount of neptunium sorbed ion-exchangeably on bentonite is similar to that for Nasmectite. In addition, the constant Kd for bentonite from pH 2 - 7 is consistent with little specific sorption of neptunium. The neptunyl ion (NpO 2รท) is believed to be the dominant chemical species under the present experimental conditions [11]. During the sorption experiment, it is supposed that several kinds of bentonitic minerals (calcite, dolomite and pyrite) dissolve in low pH solution to yield CO 2 from calcite and dolomite and Fe2+ from pyrite [4]. However, the chemical form of neptunium should be unaffected by the dissolution of these minerals for two reasons. First, the solubility of CO 2 is too small in low pH solutions to stabilize significant amounts of neptunium carbonate complexes [12,13]. Second, the reduction of Np(V) to Np(IV) by Fe2 + would increase Kd due to low solubility of Np(IV), but this perturbation was not observed. Therefore, it is reasonable to rule out the possibility that differences in the sorption behavior of bentonite and smectite are artifacts arising from valence changes of neptunium. Since the smectite was derived from the bentonite, it might be intuitively expected that specific sorption of neptunium should occur on the bentonite. However, this was not the case (Fig. 3). As Ca 2+ was a major cation of the bentonite (Table I), but not Na-smectite, the effect of Ca 2+ on the sorption capacity was considered worthy
Data Loading...
