Speciation and Valence of Np in Balanced System of Np-groundwater-bentonite

The speciation and valence distributions and effect of pH on the speciation and valence distributions of Np in the balanced system of Np-groundwater-bentonite were studied by batch experiments and solvent extraction methods. The results indicate that the valence of Np in groundwater is penta-valent state, and tetra- and penta-valent states were obtained on bentonite. With the increase of pH, the percentage of Np(Ⅴ) in bentonite increases at first and then decreases at pH>9. With the increase of CO^2-₃, the percentage of Np(Ⅴ) in bentonite decreases. The calculation results of PHREEQCI show that NpO⁺₂ is the main speciation of Np at pH<9, and NpO₂OH at pH>9. NpO₂(CO₃)^3-₂ and NpO₂(CO₃)^5-₃ are the main speciations of Np at high concentration CO^2-₃ solution. The valence of Np is not affected by pH.