Harnessing the best of both worlds in an ultralight crystalline hybrid composite aerogel to tackle nuclear waste contamination

Constructing effective radioiodine adsorbents is difficult when there is no proper material design approach. We have successfully created a multifunctional hierarchically porous nanocomposite that exhibits efficient iodine and polyiodide species trapping under both static and dynamic conditions.
Published in Ecology & Evolution and Materials
Harnessing the best of both worlds in an ultralight crystalline hybrid composite aerogel to tackle nuclear waste contamination

The continual global expansion of nuclear power, to some extent, is fostered by the downsizing of greenhouse gases while delivering steady power output. Howbeit, this amplification has created a cloud on the horizon that demands regulation and safe, long-term disposal of a substantial volume of radioactive isotope contaminated water used to cool these plants. On the flip side, incremental contamination of water bodies is a global, local problem that has come to a head as the majority of the world population is facing the crisis of water scarcity. Volatile radioactive iodine isotopes (e.g., 129I and 131I), with significant abundance, contaminate watery environments, creating an imminent threat to all living beings that requires immediate mitigation.

The crux of our paper is driven by the strategic amalgamation of necessary fundamental features in a singular material toward achieving par excellence radioiodine sequestration. A detailed study of the recent literature served as the ideal guide for us to chalk out the a priori necessities. We have judiciously chosen the covalent threading of discrete cationic cage molecules (Zr-based metal organic polyhedra (MOPs)) in a dual pore covalent organic framework (COF) matrix to construct a novel hybrid crystalline aerogel material. The choice of MOP was based on its inherent iconicity combined with abundant Zr-secondary building units (SBU) as well as hydroxyl groups and amine functionality that enable selective interactions with I2/I3- analytes. Additionally, it is well established that high surface area (pore size and pore volume) and heteroatoms can also act as a boon toward augmented iodine sequestration, which led us to choose a dual pore COF that satisfies the criterion. We also envisioned that the strategic construction of the hybrid composite aerogel as opposed to its powder counterpart could significantly enhance its performance. Overall, the composite material, given the name IPcomp-7 (“IP” stands for IISER Pune, and “comp” stands for composite), featured an ultralight nature, macro-micro hierarchical porosity, various interacting sites, and exchangeable anions to allow swift and efficacious biphasic (vapor & aqueous) segregation of I2 and I3- ions. A combination of conventional characterization techniques along with advanced X-ray computed tomography and microscopic studies were employed to elucidate the molecular level structure of the hybrid material.

The potency of IPcomp-7 to sequester iodine from both the vapor and aqueous phases was evaluated, which, to our delight, illustrated remarkable activity. The results, as obtained from the experiments, showed >99%  removal efficiency within only 60 seconds with a high capacity of 9.98 gg-1, which stands as one of the highest among most of the other reported iodine adsorbing materials. The conceptual feasibility of the material was also verified by performing the adsorption under industrially relevant conditions, as well as its high dynamic I2 sorption capacity and reusability up to 5 cycles. Aqueous phase iodide capture (I3-) further illustrated exceptionally high affinity (Kd ~106 mL.g-1) and removal efficiency (>99%), along with a high capacity of 5.16 g.g-1. In addition, the potency of IPcomp-7 was revealed by >97% I3- sequestration efficiency in the presence of a 100 fold excess amount of interfering ions. Lab-scale trials with spiked solutions from various real world samples (lake, river, sea) along with dynamic flow-through capture established the potential of IPcomp-7 as a superior sorbent material towards radioiodine sequestration. We were also able to show the easy recovery of iodine from vapor as well as organic solvent by the hybrid composite material.

The plausible underlying mechanism was disseminated by employing a combination of theoretical & experimental techniques. The abundance of various interacting sites prevailing in IPcomp-7 with I2 and I3- ions was corroborated by additional advanced experiments, which were further supported by DFT studies. Overall, IPcomp-7 can serve as an ideal sorbent material in terms of swift kinetics, remarkable capacity, high selectivity, and dynamic sequestration efficiency in a recyclable manner.

In a nutshell, IP-comp 7 constructed strategically through molecular engineering to manifest a periodic arrangement of spatially isolated cationic MOP “Nanotraps” in cooperation with a porous COF scaffold, enabling exceptional performance in radioiodine capture toward combating air and water pollution. The hybrid crystalline composite can be obtained as a low density ultralight aerogel featuring high mechanical stiffness and can be employed to effectively rid air and water of radioiodine. A synergistic combination of hierarchical macro-microporosity, decorated cationic Zr-MOP and various interaction sites allows IPcomp-7 to illustrate striking performance towards rapid and selective uptake with outstanding capacities for I2 and I3- ions under both static and dynamic conditions. Satisfyingly, the material shows intact performance in both the vapor and aqueous phases, as well as in industrially relevant conditions. Unaltered efficiency in complex real water samples and recyclability further advocate the potential of IPcomp-7 as a water-cleaning agent. Our work highlights the strategy-driven rational design of novel innovative hybrid composites that enable successful amalgamation of various task-specific functionalities in a single sophisticated material to combat water pollution.

As a last but very important note, we wish to thank the esteemed reviewers for their comments, which led to great improvements to the article.

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