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 was 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 highest occupied molecular orbital energy, lowest unoccupied molecular orbital energy, energy gap, ionization potential, electron affinity, absolute electronegativity, hardness, softness, dipole moment, principal polarizabilities, isotropic polarizability, molecular volume, and dielectric constant of uranium(VI) species found in solution with a specific pH. This implies that the speciation of the adsorbate dominates greatly, whereas the adsorbent only plays a minor role in the pH dependence of the adsorption efficiency, which is consistently explained by the Lewis theory of acid–base reactions.