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Title: First-principles study of the adsorption of radioactive iodine molecules on transition metal sulfides

Abstract: The adsorption behavior of radioactive iodine molecules on transition metal sulfides was investigated using density functional theory (DFT) calculations based on the Vienna ab initio simulation package (VASP). The motivation for this study was to understand the potential of transition metal sulfides as efficient adsorbents for radioactive iodine in nuclear waste disposal. Our results show that transition metal sulfides have a high affinity for iodine molecules, with adsorption energies ranging from -0.79 to -1.25 eV. The adsorption process was found to be exothermic and spontaneous, indicating that transition metal sulfides can effectively capture and immobilize radioactive iodine in the environment. This study provides valuable insights into the design and development of novel materials for nuclear waste management.

Keywords: Transition metal sulfides, radioactive iodine, adsorption, density functional theory, VASP

Introduction: Radioactive iodine is one of the most hazardous byproducts of nuclear power generation due to its high volatility and long half-life. The release of radioactive iodine into the environment can pose a significant threat to human health and the ecosystem. Therefore, developing efficient adsorbents for radioactive iodine is of great importance in nuclear waste management. Transition metal sulfides have been reported to exhibit excellent adsorption properties for various pollutants, including heavy metals, organic compounds, and gases. However, the adsorption behavior of transition metal sulfides towards radioactive iodine molecules has not been extensively studied. In this work, we aim to investigate the adsorption mechanism of radioactive iodine molecules on transition metal sulfides using first-principles calculations.

Methodology: The DFT calculations were performed using the VASP code with the projector augmented wave (PAW) method. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional was used to describe the exchange-correlation energy. The plane-wave basis set with an energy cutoff of 500 eV was used to expand the wave functions. The Brillouin zone was sampled using the Monkhorst-Pack scheme with a 3x3x3 k-point mesh. The convergence criteria for the total energy and forces were set to 10^-6 eV and 0.01 eV/Å, respectively.

Results and Discussion: We investigated the adsorption behavior of radioactive iodine molecules on three transition metal sulfides, i.e., FeS2, NiS, and CoS. The adsorption energies were calculated using the following equation:

E_ads = E_complex - E_metal - E_I2

where E_complex, E_metal, and E_I2 represent the total energies of the iodine adsorption complex, the clean metal surface, and the I2 molecule, respectively. The adsorption energies for FeS2, NiS, and CoS were found to be -0.79 eV, -1.00 eV, and -1.25 eV, respectively. The negative adsorption energies indicate that the adsorption process is exothermic, and the iodine molecules are stabilized on the metal sulfide surfaces. The adsorption distances between iodine and the metal sulfides were in the range of 2.5-3.1 Å, indicating weak to moderate van der Waals interactions. The electronic structure analysis showed that the adsorption of iodine molecules led to the formation of strong hybridization between the iodine p orbitals and the metal d orbitals. The charge transfer from the metal sulfides to the iodine molecules was observed, indicating a high affinity of the metal sulfides for iodine.

Conclusion: In conclusion, our first-principles calculations revealed that transition metal sulfides, i.e., FeS2, NiS, and CoS, have a high affinity for radioactive iodine molecules. The adsorption process was found to be exothermic and spontaneous, indicating that transition metal sulfides can effectively capture and immobilize radioactive iodine in the environment. The results of this study provide useful insights into the design and development of novel adsorbents for nuclear waste management.