Adsorption behavior of Re(VII) and Mo(VI) on mesoporous silica materials functionalized with nitrogen-containing groups

The deep removal of molybdenum (Mo) from rhenium (Re)-containing solutions is critical for mitigating molybdenum toxicity and enabling sustainable recycling of strategic rhenium resources. Selective adsorbents play a unique and critical role in this process, where the functional groups serve as decisive factors governing the Mo (VI)/Re(VII) separation efficiency. Herein, MCM-41 was grafted with primary amine, quaternary ammonium, and imidazole groups to systematically compare their Mo/Re adsorption and separation performance. The XRD patterns show a broadened (110) diffraction peak centered at approximately 2.35 degrees. The grafting amounts of primary amine groups, quaternary ammonium groups, and imidazole groups measured by thermogravimetric analysis were 22.81 %, 19.08 %, and 16.66 %, respectively. MCM41 grafted by imidazole groups retained the highest specific surface area of 710.98 m2/g and mesoporous structure with 2.40 nm pore size, although its grafting amount was low, attributed to its aromatic stacking-driven pore preservation. In single and binary adsorption systems, imidazole-modified adsorbents demonstrated superior Mo(VI) selectivity with a separation factor of 1171.57, outperforming other amine counterparts. Langmuir isotherm modeling coupled with pseudo-second-order kinetic analysis demonstrated a chemisorption-dominated monolayer adsorption mechanism, achieving maximum Mo(VI) adsorption capacities of 169.60 mg/g for primary amine-functionalized materials, 183.20 mg/g for quaternary ammonium-modified systems, and 191.33 mg/g in imidazole-grafted adsorbents. Notably, while density functional theory calculations indicated a reduced adsorption energy for imidazole groups at-362.32 kJ/mol compared to-406.46 kJ/mol for quaternary ammonium counterparts, the hierarchical pore architecture of imidazole-modified composites provided abundant ion-accessible sites, underscoring the dual necessity of structural accommodation and chemical driving forces. This work establishes a mechanistic framework for designing selective adsorbents, offering a viable solution for deep Mo(VI) removal from Re-rich industrial effluents.