Fe-doped HKUST-1 for light rare earth recovery from industrial solid waste leachates: Adsorption mechanisms and interpretable machine learning

Recovering light rare earth elements (LREEs) from industrial solid waste leachates requires adsorbents with high adsorption capacity, selectivity, and aqueous stability. However, the susceptibility of pristine HKUST-1 to water-induced degradation limits its application in aqueous separation. Here, Fe-doped HKUST-1 adsorbents were synthesized via a one-pot solvothermal route to improve aqueous stability and LREE adsorption performance. The optimal HKFe5 exhibited equilibrium adsorption capacities of 364.33, 386.33, 352.67, and 353.67 mg·g -1 for La(III), Ce(III), Pr(III), and Nd(III), respectively, representing increases of 19.62%–26.12% over HKFe0. In water stability tests, Cu(II) leaching and total organic carbon release were reduced by approximately 35% and 26%, respectively. Competitive adsorption tests and experiments with fly ash and red mud leachates demonstrated preferential LREE uptake in complex multi-ion matrices. Spectroscopic analyses and density functional theory calculations supported an adsorption mechanism involving primary Cu(II)-node cation exchange and auxiliary axial Fe–OH coordination, with geometric matching contributing to preferential Ce(III) capture. An interpretable XGBoost model combined with SHAP analysis predicted adsorption capacity with a test-set R 2 of 0.9814 and quantified the contributions of experimental variables. These findings demonstrate the potential of Fe modification to improve the aqueous stability and adsorption performance of HKUST-1 for LREE recovery from industrial solid waste leachates.