Tc-99 has drawn widespread concern because of its long half-life, high fission yield and high mobility in research of radioactive waste disposal and environmental remediation. TcO₄⁻ through-diffusion experiments in Opalinus Clay (OPA) were performed under air and under argon No noticeable Tc-breakthrough was observed for over one year while the total Tc concentration in the source reservoir was steadily decreasing under both air and argon atmosphere. The total Tc activity distribution in the clay sample along the diffusion direction was obtained by slicing the OPA clay samples retrieved from the diffusion cells, using the abrasive peeling technique. In the case of diffusion under air atmosphere, almost no Tc was measured in that part of the sample close to source reservoir, while much more Tc was measured under argon atmosphere. A reasonable explanation for this observation is that the reductive retention of Tc plays a significant role during transport. A reactive transport model was constructed to simulate the diffusion process whereby the diffusion of Tc was coupled with redox reactions. Even though reduction of TcO₄⁻ by aqueous Fe²⁺ is thermodynamically feasible, it was not observed in the experiment. Furthermore, Fe²⁺ associated with solid phase was demonstrated to be more active than aqueous Fe²⁺. Therefore, surface complexation redox reaction was proposed. Dissolution rate of pyrite, equilibrium constant and diffusion coefficient of TcO₄⁻ were considered as possible factors controlling the redox reaction. Modeling results showed that TcO₄⁻ diffused into the clay and was partially reduced into surface complexed Tc(IV) by pyrite. When TcO₄⁻ transported under air atmosphere, O₂ competitively consumed Fe²⁺ and pyrite, resulting in no Tc immobilization in the related zone.