Investigation of Radionuclide ⁶³Ni(Ⅱ) Sequestration Mechanism and Model Study on Kaolinite

The uptake of radionuclide on kaolinite has been investigated extensively due to its low penetrability and high sorption capacity. Herein, kaolinite was characterized by acid-base titration. The uptake behavior of ⁶³Ni(Ⅱ) on kaolinite as a function of pH and ionic strength was studied by using batch technique under ambient conditions. The experimental data of ⁶³Ni(Ⅱ) sorption on kaolinite was obtained using the diffuse layer model combined with the FITEQL 4.0 program and the consecutive acidity constants as pK_a were optimized simultaneously. The uptake of ⁶³Ni(Ⅱ) on kaolinite contains ≡XONi⁺ species at low pH values and ≡YONi⁺ species at high pH values. Macroscopic experiment results show that the uptake of ⁶³Ni(Ⅱ) is greatly dependent on ionic strength at pH<8.5, and independent of ionic strength at pH>8.5. At low pH, the uptake of ⁶³Ni(Ⅱ) is predominated by ion exchange and/or outer-sphere surface complexation with Al³⁺/H⁺ on kaolinite surface, whereas inner-sphere surface complexation is the primary uptake mechanism at high pH-values. The macroscopic uptake data of ⁶³Ni(Ⅱ) surface speciation at the kaolinite/water interface should be significant to evaluate and predict the migration and uptake behavior of ⁶³Ni(Ⅱ) in environment mediums.