Based on density functional theory calculations, the chemical penetration behaviors and separation properties of hydrogen isotope ions through pristine and fully hydrogenated group-IV monolayer materials are investigated. Both the penetration energy profiles and kinetic isotope effects are studied to evaluate the performance of four group-IV (C, Si, Ge, and Sn) monolayer materials for hydrogen isotope separation. To examine the thermodynamically stable morphologies of these monolayer materials in electrochemical aqueous environment, the Pourbaix diagrams varying with pH and external bias are constructed. The fully hydrogenated monolayer materials are found to be thermodynamically favorable in some conditions, and the proton penetration and hydrogen isotope separation behaviors are different from their pristine counterparts. The silicene is found to be a suitable candidate material for hydrogen isotope separation in an electrochemical environment.