Migration mechanisms of Cs, U(VI), and Th(IV) in granite-filled fractures under competitive sorption conditions: A kinetic analysis integrating transport and desorption dynamics.

Accurate prediction of co-transport of multiple radionuclides within geological barriers remains a core challenge in the long-term safety assessment of high-level radioactive waste (HLW) repositories. This study investigates the co-transport of three representative nuclides-Cs, U(VI), and Th(IV)-through granite-filled single fracture, integrating breakthrough experiments with sequential desorption tests to establish a kinetic framework for their transport-desorption process. A three-site nonequilibrium kinetic model was applied to inversely simulate breakthrough curves, yielding quantitative retardation factors, sorption site distributions, and desorption rate constants for each nuclide. Results demonstrate that Cs migration is governed by reversible ion exchange coupled with interlayer diffusion, exhibiting a "high-retardation-high-recovery" signature (retardation factor = 1.77-2.64; recovery = 87.4-97.9%). U(VI) transport is dominated by energetically heterogeneous surface complexation, with occupation of strong-binding sites kinetically controlled by solid-liquid contact time; the ratio of fast-to-slow desorption rate constants during acid elution (k1/ k2 ≈ 86-384) directly reflects site energy heterogeneity. Th(IV) retention is controlled by contact time-dependent surface precipitation, manifesting extreme flow velocity sensitivity (recovery = 57.4-93.4%) and pronounced irreversibility. In the ternary system, the distribution of U(VI) between strong- and weak-binding sites varies concurrently with Th(IV) interfacial speciation transitions (precipitate versus adsorbed phase), whereas Cs migration remains relatively unaffected owing to the spatial and chemical isolation of its primary sorption domain (interlayer sites of phyllosilicates). The coupled transport-desorption kinetic framework established herein, together with the quantified parameters and mechanistic insights, provides critical experimental constraints for developing nonequilibrium reactive transport models capable of predicting multicomponent radionuclide migration in fractured geological media.