In this study, hydroxycalix[4]arene, hydroxycalix[5]arene, hydroxycalix[6]arene, and their complexes with uranyl and thorium ions were analyzed by density functional theory. The coordination ability of hydroxycalixarene ligands to uranyl and thorium ions could be analyzed by simulating the infrared and ultraviolet–visible spectra, the Wiberg bond energy levels, the energy gaps, and the binding energies. We found that various hydroxycalixarene complexes with uranyl or thorium ions had significant differences in their properties, as well as in their infrared vibrational and ultraviolet absorption spectra. More importantly, different hydroxycalixarenes can be selected based on the adsorption requirements in environments with coexisting uranium and thorium ions. For example, hydroxycalix[4]arene and hydroxycalix[5]arene had a significantly higher binding capacities for thorium than uranyl ions, while hydroxycalix[6]arene showed less of a difference for the two ions but had a good binding ability for both uranyl and thorium ions at the same time. Thus, the use of hydroxycalixarene can achieve the removal of both uranyl and thorium ions in the environment, and the use of specific hydroxycalixarenes may be able to achieve the separation of these ions. These results provide important guidance for the future research of hydroxycalixarene without requiring the synthesis of hydrophilic derivatives, and they may be directly applied to the field of heavy metals.

The molecular structural characteristics of the complexes formed by the substitution of the central atom of heme (iron, Fe) by mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr), or arsenic (As) were studied using density functional theory (DFT). The simulated infrared and ultraviolet-visible spectra, Wiberg bond levels, frontier molecular orbital energy levels, and the binding capacity of the heme ligand to the central atom were assessed for all complexes. The results showed that when Fe was replaced by Hg, Cd, Pb, Cr, or As, the resulting complex was stable, with unique spectroscopic characteristics.Interestingly, the binding capacity of the heme ligand to Pb was about 10 times greater than that of the heme ligand to Fe and more than 10 times greater than that of the heme ligand to any of the other four heavy metals. The complex in which Fe was replaced by Cd had the greatest chemical stability. These results will provide an important reference for the future pollution control programs and will support efforts to enrich the five studied heavy metals using heme ligands, especially the specific capture of Pb and the enrichment of Cd.