Adsorption of cesium and neptunium(V) on bentonite

The adsorption of cesium and Np(V) on a bentonite of calcium type was studied by a batch method. The distribution coefficient (Kd) of Cs decreased with increasing ionic strength and was independent of pH. The adsorption of cesium is well explained by the cation exchange reaction of montmorillonite and expressed by a Langmuir adsorption isotherm: Kd = 162/([Na+] + 170[Cs+]). The adsorption of Np(V) was measured at the pH range of 2 - 12 in the absence of carbonate. The Kd value of NpO2+ was determined to be 100.986±0.043ml/g in the pH range of 2 - 8. The Kd increased steeply at higher pH (>8) due to the formation of NpO2OH which was confirmed by measuring the solubility. The stability constant of NpO2OH obtained from the solubility measurement is 102.58±0.11, which is close to the result from adsorption experiment, 102.48±0.20. The Kd value of NpO2OH is evaluated to be 105.09±0.17ml/g. The distribution coefficient became lower in the carbonate solution at high pH (>10). The formation of the complex species Np(OH)2(CO3)22- and Np(OH)4(CO3)24- were suggested to explain the obtained data. The distribution coefficient of Np(OH)2(CO3)22- and Np(OH)4(CO3)24- are calculated to be 103.11±0.07ml/g and 102.356±0.046ml/g, respectively. (author)