Lignin-based porous carbon/palygorskite composites doped with different metals for efficient iodine capture: Structure, performance, and mechanism

Radioactive iodine waste from the nuclear power industry will cause air and water pollution. Here, a series of metal (Bi, Zn and Fe)-doped lignin-based porous carbon/palygorskite composites (ELC-P-X) were prepared by wet impregnation and carbonization method for enhancing the iodine capture. Their morphology, porosity, and surface functional groups of ELC-P-X were characterized in detail, these metal species showed different sizes of nanoparticles with the oxide form. Zn, Fe doping enhanced its porosity of ELC-P (247.5 m(2)/g), and up to 359.5 m(2)/g, while Bi doping had slight negative influence on the porosity. Adsorption experiments showed that the iodine vapor adsorption capacity of ELC-P-X followed an order of ELC-P-Zn > ELC-P-Fe > ELC-P-Bi> ELC-P, the highest adsorption capacity can reach 650.0 mg/g. The results suggested that above metal doping can promote iodine vapor adsorption. Meanwhile, the adsorption capacity of ELC-P-X for iodine in n-hexane solution only showed an increase on ELC-P-Fe (362.2 mg/g), compared to ELC-P (332.0 mg/g). ELC-P-Fe still had good adsorption stability, acid and alkali resistance and cycling performance. We found that the adsorption of iodine vapor could mainly depend on the porosity of ELC-P-X materials, while the surface functional groups, iodine-affinity metal species, and micro-nano structure made a major contribution synergistically to the enhanced adsorption for the iodine solution. The adsorption mechanism study revealed that the Lewis acid-base interaction, electrostatic interactions, and charge transfer action accompanied by weak chemisorption were the main driving force for the iodine molecule adsorption on ELC-P-Fe. This work provided an important reference for the rational preparation of lignin based iodine adsorbents and proposed a kind of universal strategy for enhancing iodine adsorption.