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.