Hierarchical porous carbon with an ultrahigh surface area for high-efficient iodine capture: Insights into adsorption mechanisms through experiments, simulations and modeling

The removal of radioactive iodine species during nuclear accidents and nuclear fuel reprocessing is crucial to nuclear safety, public health and environmental protection. Here, a series of nitrogen-doped porous carbon derived from benzimidazole (NBCs) with well-developed pore structure were prepared. During KOH activation, the potassium salt formed at 600–700 °C is mainly K2CO3 and K2O, and the pore size obtained is mainly microporous. With the temperature increases, the K2CO3 and K2O are converted to KOCN, which generates mesopores. NBCs were subsequently tested for gaseous iodine and solution iodine. The results show that NBC800-2 exhibits the highest iodine vapor adsorption capacity (7873 mg/g) at 75 ℃. Theoretical calculations and experimental data show that the iodine adsorption of NBCs for iodine vapor is mainly determined by pore structure, especially micropores and narrow mesoporous. The effect of nitrogen and oxygen doping on the adsorption capacity of gaseous iodine is limited. In the cyclohexane solution, the adsorption capacity of NBCs for iodine is mainly determined by micropores. In KI/H2O solution, the iodine adsorption mainly depends on micropores and narrow mesopores, which is completely different from the result of iodine adsorption in cyclohexane. This is mainly due to the fact that I3- has a larger diameter than I2, and the adsorption energy of I3- (82.8 kJ/mol) on the carbon surface is higher than that of I2 (58.1 kJ /mol). Moreover, Langmuir model can better describe the adsorption process of iodine in cyclohexane and KI/H2O solution than Freundlich model, which proves that the process is mainly monolayer formation. This study provides insights for the further design and development of adsorbents for iodine capture from nuclear waste and nuclear accidents.