An Inquiry into the Oxidative-Reductive Actions of 79Se4+

The long-term disposal risks associated with the key radioactive isotope ⁷⁹ Se ⁴⁺ remain a subject of debate, with the specific mechanism of containment yet to be determined. This study, reveals a novel observation in the field, showing that ⁷⁹ Se ⁴⁺ can be efficiently reduced and sequestered by precipitating as Se ⁰ . Our findings indicate that the reduction of Se ⁴⁺ is controlled by the "H ⁺ inhibition" mechanism. The rate at pH ~ 8.0 (𝑘 = 4.22×10 ⁻³ mol·L ⁻¹ ·h ⁻¹ ) is 1131 times faster than the rate at pH 6.3 (𝑘 = 3.73×10 ⁻⁶ mol·L ⁻¹ ·h ⁻¹ ). Granite's influence is more nuanced than pH, establishing a distinctive reaction environment at the solid-liquid interface distinct from the bulk solution, leading to reactions occurring above a pH of 5.7 (pHpzc). Additionally, we examined the long-term retention mechanism of Se ⁴⁺ in the solid phase, confirming that adsorbed Se ⁴⁺ undergoes continuous reduction instead of dissociating into the solution. This validates the effectiveness of the engineering design in the prolonged immobilization of ⁷⁹ Se ⁴⁺ . The data and conclusions presented in this study offer crucial support for the safety assessment of geological disposal repositories and relevant environmental research.