Combined Experimental and Quantum Chemistry Studies on the pH Dependence of the Adsorption Efficiency of Uranium(VI): Lewis Theory of Acid–Base Reactions in Adsorption Processes

The pH dependence of the adsorption efficiency of uranium (VI) ions by adsorbents was investigated experimentally. Moreover, quantum chemistry calculations were subsequently performed to reveal the vague adsorption mechanisms not clarified experimentally in detail. The highest uptake of uranium (VI) on alginate acid were commonly observed at pH 5 and all the adsorption efficiencies decreased with decreasing pH, whereby uranium (VI) species took on a variety of chemical forms. The tendency was reasonably predicted from the calculations of the EHOMO (highest occupied molecular orbital energy), ELUMO (lowest unoccupied molecular orbital energy), energy gap (EGAP), ionization potential (IE), electron affinity (EA), absolute electronegativity (χ), hardness (η), softness (S), dipole moment (μ), principal polarizabilities (αxx, αyy, αzz), isotropic polarizability (α), molecular volume (VM), and dielectric constant (e) of uranium (VI) species found in solution with a specific pH. This implies that the speciation ...