First-principles study of hydrated and hydrolyzed Nd(III) adsorption on kaolinite (001) and 001¯ surfaces

Ion-adsorption deposits (IADs) are important sources of heavy and selected light rare earth elements (REEs), yet the adsorption mechanisms of hydrated and hydrolyzed Nd(III) on clay-mineral surfaces remain incompletely resolved. Structural refinement of adsorbed Nd by extended X-ray absorption fine structure (EXAFS) is challenging because the Nd L 3 -edge region is susceptible to interference from nearby Ce and Pr absorption edges. In this study, first-principles calculations were performed to investigate the adsorption of [Nd(H 2 O) 9 ] 3+ and [Nd(OH)(H 2 O) 8 ] 2+ on the (001) and ( 00 1 ¯ ) basal surfaces of kaolinite. Optimized isolated complexes show that nine-coordination represents the saturated first hydration shell of Nd 3+ . Upon adsorption, [Nd(H 2 O) 9 ] 3+ remains outer-sphere on both basal surfaces. Partial dehydration occurs on the (001) surface, whereas the dehydration behavior on the 00 1 ¯ surface depends on the adsorption geometry. For [Nd(OH)(H 2 O) 8 ] 2+ , adsorption remains outer-sphere on the (001) surface without dehydration. On the 00 1 ¯ surface, the adsorption mode of [Nd(OH)(H 2 O) 8 ] 2+ is influenced by the Nd–O OH bond orientation, with a toward-surface orientation leading to partial dehydration and inner-sphere complexation. Electronic-structure analyses reveal localized interfacial charge redistribution on the (001) surface, pronounced Si-O framework polarization on the 00 1 ¯ surface, and more pronounced overlap between Nd-d and surface O-p states in the inner-sphere complex. DFT + U tests for [Nd(H 2 O) 9 ] 3+ indicate no substantial changes in adsorption geometries or net interfacial charge redistribution. These results indicate that Nd hydrolysis, Nd–O OH bond orientation, and basal-surface chemistry jointly influence the adsorption mode of Nd on kaolinite, providing molecular-level constraints on pH-dependent light REE retention in IADs.