Se sorption onto Ca-type montmorillonite purified from Bentonil-WRK—a new research bentonite introduced by Korea Atomic Energy Research Institute—was examined under ambient conditions (pH 4−9, pe 7−9, I = 0.01 M CaCl2, and T = 25°C). Se(IV) was identified as the oxidation state responsible for weak sorption (Kd < 22 L∙kg−1) by forming surface complexes with edge functional groups of the montmorillonite. Thermodynamic modeling, considering reaction mechanisms of outer-sphere complexation (≡AlOH2 + + HSeO3 − ⇌ ≡AlOH3SeO3, log K = 0.50 ± 0.21), inner-sphere complexation (2≡AlOH + H2SeO3(aq) ⇌ (≡Al)2SeO3 + 2H2O(l), log K = 7.89 ± 0.51), and Ca2+-involved ternary complexation (≡AlOH + Ca2+ + SeO3 2− ⇌ ≡AlOHCaSeO3, log K = 7.69 ± 0.28) between selenite and aluminol sites of montmorillonite, acceptably reproduced the batch sorption data. Outer- and inner-sphere complexes are predominant Se(IV) forms sorbed in acidic (pH ≈ 4) and near-acidic (pH ≈ 6) regions, respectively, whereas ternary complexation accounts for Se(IV) sorption at neutral pHs under the ambient conditions. The experimental and modeling data generally extend a material-specific sorption database of Bentonil-WRK, which is essential for assessing its radionuclide retention performance as a buffer candidate of deep geological disposal system for high-level radioactive waste.

1. Introduction
2. Experimental
    2.1 Chemical Reagents and Clay Adsorbent
    2.2 Batch Sorption Experiment
    2.3 Geochemical Speciation and ThermodynamicSorption Modeling
3. Results and Discussion
    3.1 Aqueous Species Distribution of Se Underthe Experimental Conditions
    3.2 Sorption Thermodynamics of Se(IV) OntoCa-type Bentonil-WRK Montmorillonite
4. Concluding Remark
Conflict of Interest
Acknowledgements
REFERENCES

• Seonggyu Choi(Korea Atomic Energy Research Institute, 111, Daedeok-daero 989beon-gil, Yuseong-gu, Daejeon 34057, Republic of Korea) Corresponding author
• Ja-Young Goo(Korea Atomic Energy Research Institute, 111, Daedeok-daero 989beon-gil, Yuseong-gu, Daejeon 34057, Republic of Korea)
• Jeonghwan Hwang(Korea Atomic Energy Research Institute, 111, Daedeok-daero 989beon-gil, Yuseong-gu, Daejeon 34057, Republic of Korea)
• Yongheum Jo(Hanyang University, 222, Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea)
• Jae-Kwang Lee(Korea Atomic Energy Research Institute, 111, Daedeok-daero 989beon-gil, Yuseong-gu, Daejeon 34057, Republic of Korea)
• Jang-Soon Kwon(Korea Atomic Energy Research Institute, 111, Daedeok-daero 989beon-gil, Yuseong-gu, Daejeon 34057, Republic of Korea)