Efficient and Ultrafast Adsorption of Rhenium by Functionalized Hierarchically Mesoporous Silica: A Combined Strategy of Topological Construction and Chemical Modification

Radioactive Tc-99 released by nuclear accidents threatens the environment and human health due to its long half-life and strong transportability. A combined strategy synergizing topological construction and chemical modification was proposed for the synthesis of high-performance adsorbents for Re as an analogue to Tc. On the one hand, hierarchically mesoporous SiO₂ with a fibrous structure (F-SiO₂), a peculiar topology integrating wrinkled open mesopores around 12 nm and on-wall mesopores around 3 nm, was adopted as the substrate of adsorbents. The larger mesopores can act as the superhighway for mass transfer, while the abundant smaller mesopores provide numerous adsorption sites. On the other hand, a series of dicationic pyridine (DCP) derivative groups (−Py⁺CₙH₂ₙN⁺Me₃) were designed to functionalize F-SiO₂ for improving the adsorption performance toward ReO₄– anions, the dominating form of Re in aqueous solution. Density functional theory (DFT) calculation combined with batch adsorption experiments revealed that the ReO₄– adsorption on −Py⁺C₅H₁₀N⁺Me₃ was the most favorable when the length of the spacer between the two positively charged N atoms ranged from 2 to 7 carbons (n = 2–7). However, −Py⁺C₅H₁₀N⁺Me₃ exhibited a much slower adsorption rate than −Py⁺C₂H₄N⁺Me₃. The stronger interaction between ReO₄– and −Py⁺C₅H₁₀N⁺Me₃ suppresses the adsorbate diffusion. The two positive charges of −Py⁺C₅H₁₀N⁺Me₃ may be perpendicularly distributed, sterically hindering ReO₄– transport in smaller mesopores. The longer and flexible carbon chains may be aggregated to form the hydrophobic region, repulsing the hydrated ReO₄– anions. Therefore, the efficient and ultrafast Re adsorption was achieved by synergizing the unique topology of F-SiO₂ and functionalization by −Py⁺C₂H₄N⁺Me₃ with a shorter spacer and weaker affinity ReO₄–. The detailed investigation demonstrated that −Py⁺C₂H₄N⁺Me₃ possessed exothermic adsorption nature, adequate radiation-resistance, and excellent reusability. Meanwhile, −Py⁺C₅H₁₀N⁺Me₃ exhibited stronger salinity tolerance and higher selectivity. The DCP groups are promising in decontamination of radioactive Tc, as they can meet specific requirements by manipulating the length of spacers.