Regulating Pore Space Occupancy via Fe0 Content in Amino-Functionalized Silica for Stable Uranium Immobilization

Incorporating Fe0 into the porosity of silicon-based materials can overcome the shortcomings of nanozero-valent iron (nZVI), such as aggregation, passivation, and secondary release of reduction products. The mass transfer resistance due to iron oxidation expansion and consequent pore blockage during uranium reductive coprecipitation remains unclear. The present work employed amino-functionalized mesoporous silica (NH2-SGB) as the support to precisely regulate the spatial occupancy rate of Fe0 in the pores, and synthesized a series of Fe0@NH2-SGB (x-PV, x = 0.00, 0.33, 0.50, and 0.67) composites. Batch experiments and structural characterization results illustrated that Fe0@NH2-SGB (0.33-PV) achieved 97.0% U(VI) removal and an equilibrium immobilization capacity of 48.3 mg·g-1 from 10 mg·L-1 initial U(VI) solution at pH 5.0 with 0.2 g·L-1 added, and a Langmuir-fitted theoretical maximum adsorption capacity of 449.43 mg·g-1 at room temperature, which is better than that of 0.50-PV and 0.67-PV. The reserved redundant pore of Fe0@NH2-SGB (0.33-PV) can effectively accommodate the iron-uranium coprecipitation products, maintain the unobstructed mass transfer channels, and avoid the orifice blockage effect, resulting in an effective utilization rate of Fe0 to 98.2%, which exceeded that of 0.67-PV (46.7%), the uranium fixation product achieved a spatial fit of up to 89.1% pore volume and realized confined encapsulation in the pore. Mechanistic analysis revealed that the immobilization of U(VI) first took place on the surface of the sample via selective trapping by -NH2 groups. Subsequently, the Fe0 and Fe(II) in the pores efficiently reduced U(VI) to U(IV) and achieved stable self-sealing immobilization. Meanwhile, continuous 30-day static desorption tests and acid stress tests showed uranium leaching rates consistently below 0.5%, confirming the long-term stabilization effect of the confinement encapsulation strategy. It provides a novel material design strategy for the rational design of confined nanomaterials and the long-term immobilization of uranium in contaminated groundwater.