Thermodynamics of Se(IV) Sorption onto Ca-type Bentonil-WRK Montmorillonite

Seonggyu Choi^1,*, Ja-Young Goo¹, Jeonghwan Hwang¹, Yongheum Jo², Jae-Kwang Lee¹, Jang-Soon Kwon¹

Received 2024-06-04 ; Revised 2024-07-03 ; Approved 2024-08-12

Abstract

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) 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+ + SeO32− ⇌ ≡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.

Key Words : Bentonite, Montmorillonite, Bentonil-WRK, Selenium, Thermodynamic sorption modeling, Surface complexation