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Herein, lanthanides (Samarium: Sm, Lanthanum: La, and Erbium: Er) adsorption mechanism over copper trimesate metal-organic framework (Cu-BTC MOF) was studied. The MOF crystallized as [Cu(HBTC)(H₂O)₃] and [Cu₃(BTC)₂] with a rod-like structure and abundant free single bondCOOH groups. The Langmuir adsorption capacity of 1007.1, 656.5, and 410.5 mg g⁻¹ was recorded for Sm, La, and Er, respectively. The adsorption process was driven by rapid ion exchange with crystal structure transformation in the first hour of mixing, mediated by the surface ion-exchange phenomenon and free single bondCOOH groups. After 1 h, the process was driven by a slow ion-exchange mechanism where Ln ions drifted internally, and Cu ions moved out towards the surface. Among the three lanthanides studied, Er/Cu-BTC system was more sensitive to structural reorganization with the increasing Ln/Cu ratio. The work …