Adsorption mechanisms of UO22+ on lignocellulosic biopolymers: a DFT study

Uranium is the main raw material for nuclear energy, which is an alternative to renewable energy sources. In nature, hexavalent uranium (U(VI)) is the most stable and highly soluble form. U(VI) present in deep repositories created for the disposal of radioactive waste or nuclear reprocessing can leach into water systems, posing environmental and human health risks. Therefore, uranium species must be removed from the environment. Adsorption is considered an efficient technique for removing uranium from water resources based on the ease of adsorption on bioadsorbents such as cellulose, hemicellulose, and lignin compounds. Therefore, this study evaluated the adsorption mechanisms of uranyl ions ( UO 2 2 + ) from an aqueous medium onto cellulose (CE), hemicellulose with arabinose groups (Hm1) and without arabinose groups (Hm2), and lignin with coniferyl and sinapyl groups (GS) matrices via computer simulations. The adsorbate-adsorbent systems were optimized, and it was observed that among the matrices, UO 2 2 + exhibited the highest interaction energy with CE owing to its abundant hydroxyl groups, followed other matrices with increasingly weaker interactions. All evaluated interactions were exothermic (Δ r H < 0) and almost all were spontaneous (Δ r G < 0), except for Hm1– UO 2 2 + 1 and GS– UO 2 2 + 1 . The topological parameters obtained by the quantum theory of atoms in molecules showed that all types of interactions are non-covalent, in which the interactions between uranium atoms and the oxygen atoms in the matrices are partially covalent, and those between the oxygen atoms of UO 2 2 + and the hydrogen atoms of the matrices are electrostatic. These trends were confirmed by the isosurface graphs obtained from non-covalent interaction analyses, which identified strong adsorbent U O interactions and weak Van Der Waals O adsorbate –H adsorbent interactions. Thus, it was concluded that unmodified cellulose, hemicellulose and lignin are good adsorbent matrices for removing UO 2 2 + from water resources. The theoretical studies on the adsorption mechanisms of cellulose, lignin, and hemicellulose matrices shown in this work can provide insights into the interaction mechanisms and enhance our understanding of UO 2 2 + ∙∙∙biopolymer interactions during environmental remediation processes. • DFT study of uranyl ion UO₂ 2+ adsorption on cellulose, hemicellulose, and lignin. • Identification of the most stable adsorption sites and coordination modes for U(VI). • Lignocellulosic biopolymers are proven effective eco-friendly adsorbents for uranium recovery. • Calculated Gibbs free energies ($\Delta G < 0$) confirm the spontaneity of the uranium removal process in aqueous media.