Radioactive Strontium Removal from Seawater by a MOF via Two-Step Ion Exchange

Radioactive waste, such as 90Sr, 134Cs, and 131I, from the Fukushima nuclear spill highlighted the need to find effective adsorbents for scrubbing radioactive ions from seawater. In this issue of Chem, Wang and colleagues report a remarkably 90Sr-selective metal-organic framework (SZ-4) that operates with a two-step ion-exchange mechanism and at a wide pH range while being active and intact when tested in actual seawater. Radioactive waste, such as 90Sr, 134Cs, and 131I, from the Fukushima nuclear spill highlighted the need to find effective adsorbents for scrubbing radioactive ions from seawater. In this issue of Chem, Wang and colleagues report a remarkably 90Sr-selective metal-organic framework (SZ-4) that operates with a two-step ion-exchange mechanism and at a wide pH range while being active and intact when tested in actual seawater. Even after 8 years, the Fukushima nuclear waste spill into the Pacific Ocean is still an issue given that the cleanup methods require elemental selectivity, particularly to common ions, such as Cs+, I−, and Sr2+.1Manos M.J. Ding N. Kanatzidis M.G. Layered metal sulfides: exceptionally selective agents for radioactive strontium removal.Proc. Natl. Acad. Sci. USA. 2008; 105: 3696-3699Crossref PubMed Scopus (213) Google Scholar, 2Aguila B. Banerjee D. Nie Z. Shin Y. Ma S. Thallapally P.K. Selective removal of cesium and strontium using porous frameworks from high level nuclear waste.Chem. Commun. (Camb.). 2016; 52: 5940-5942Crossref PubMed Google Scholar, 3Zeng M.-H. Wang Q.-X. Tan Y.-X. Hu S. Zhao H.-X. Long L.-S. Kurmoo M. Rigid pillars and double walls in a porous metal-organic framework: single-crystal to single-crystal, controlled uptake and release of iodine and electrical conductivity.J. Am. Chem. Soc. 2010; 132: 2561-2563Crossref Scopus (590) Google Scholar, 4Vellingiri K. 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Reticular chemistry in action: a hydrolytically stable MOF capturing twice its weight in adsorbed water.Chem. 2018; 4: 94-105Abstract Full Text Full Text PDF Scopus (187) Google Scholar In this issue of Chem, Shuao Wang and his team tackle an outstanding radioactive contamination problem by employing a target-oriented approach (Figure 1). Given that removing an earth alkali metal has to include selectivity over more strongly binding transition metals, they knew that traditional adsorptive media wouldn’t work. Ion exchange, they determined, would at least give them selectivity over weakly binding, abundant cations. But the more challenging task was to specifically favor strontium. For that, they turned to crystal engineering of metal-organic frameworks (MOFs), where one could pinpoint functionalities without yielding mass transfer features. The only problem for such a MOF design is stability. For that, they used zirconium MOFs, where zirconium-based frameworks are known to be stable in a wide variety of conditions. They started with the complexation of ZrOCl2·8H2O and methylenediphosphonic acid with N,N-dimethylacetamide in a solvothermal method full of acids, which resulted in the single crystal of anionic, layered coordination polymers (CPs) of SZ-4, [(CH3)2NH2][ZrCH2(PO3)2F].9Zhang J. Chen L. Dai X. Zhu L. Xiao C. Xu L. Zhang Z. Alekseev E.V. Wang Y. Zhang C. et al.Distinctive two-step intercalation of Sr2+ into a coordination polymer with record high 90Sr uptake capabilities.Chem. 2019; 5: 977-994Abstract Full Text Full Text PDF Scopus (95) Google Scholar The single-crystal analysis revealed that SZ-4 crystallizes in the triclinic space group P-1. The asymmetric unit of SZ-4 contained one Zr4+ ion, one F atom, one methylenediphosphonate ligand, and one protonated dimethylamine (DMAH+). This was particularly attractive because the DMAH+ cations in the channels of SZ-4 could enhance the ion-exchange mechanism, mainly with the Sr2+ ion. These zirconium fluorophosphonate layers feasibly propagated along the b axis with interlayer distances of 3.6 Å each. The octahedral ZrO5F unit in coordination with diphosphonate ligands predominantly formed these layers. The shorter distances of N···F (2.797 Å) and N···O (2.856 Å) were the main driving forces for the layered packing structure, which would eventually help the exclusive ion-exchange mechanism. Stability was the first to be checked. Powder X-ray diffraction (PXRD) patterns confirmed the structural integrity of crystalline SZ-4 in water and, most importantly, seawater; surprisingly, the authors found that SZ-4 could resist the radiation even at large doses up to 200 kGy of 60Co γ and β irradiation. Then, SZ-4 was found to withstand a wide pH range from 3 to 11. At pH over 12, crystals dissolved in solution and lost structural integrity. But when the pH went down to 0, SZ-4 crystals remained intact throughout the entire course of the pH, making it possible to determine single crystal structures. It is important to note that only a handful of reported MOFs show such remarkable stability. At pH 2, SZ-4 transformed into a crystalline phase of [(CH3)2NH2]0.5[ZrCH2(PO3)(PO3H0.5)F] (SZ-4-a). The single crystals of SZ-4-a exhibited the same space group as SZ-4 with an anionic layered structure. As a result of a partial substitution of DMAH+ with H3O+, the interlayer distance shrunk from 3.6 to 2.7 Å. When the pH was further lowered down to 0, SZ-4-a morphed into another crystalline phase of Zr2F0.5(NO3)1.5[CH2(PO3)(PO3H)]2·1.5H2O (SZ-4-b) with the same space group and 2D layered topology. With a further increase in acidity (pH < 0), asymmetric units changed upon removal of DMA by water. At the same time, partial substitution of F− occurred by NO3− exchange, and the phosphonate ligand was partially protonated to form a neutral layer without any ion-exchange capability. Once the stability was in the right place, SZ-4 was put to good use. To date, conventional techniques (adsorption, ion exchange, and precipitation) have mostly been used for wastewater treatment (Figure 1). Among them, adsorption is usually the most effective because of the strong binding, chemical stability, and heterogeneous practices. Precipitation is the ultimate cleanup method, but only if you can let go of the selectivity. Ion exchange offers the most targeted option and is used mostly for specific capture purposes, such as in household water filters. In toxic and radioactive wastewater treatment, ion exchange works the best given that we’re always after a certain ion. Capacity might not be as high as adsorption, but the kinetics of the exchange make up for the losses. For example, we reported that the nanoporous amidoxime PIM-1 showed high uranium uptake capacity from seawater with fast kinetics (Kd = 8.02 × 103 mL/g).10Sihn Y.H. Byun J. Patel H.A. Lee W. Yavuz C.T. Rapid extraction of uranium ions from seawater using novel porous polymeric adsorbents.RSC Advances. 2016; 6: 45968-45976Crossref Google Scholar SZ-4 showed a fast and efficient uptake capacity for 90Sr2+ in comparison with other ion-exchange materials, even in very acidic conditions (Kd = 4.06 × 106 mL/g). It is normally very challenging for adsorptive media because of the competition with H+ ions. In a wide range of pH and in the presence of competing cations, SZ-4 removes 90% of 90Sr2+ from wastewater as quickly as within 20 min. The uptake capacity at pH 2 is 121 mg/g, very close to the theoretical capacity (133 mg/g), and is almost four times larger than that of KMS-1 (ca. 23 mg/g) under the same conditions.1Manos M.J. Ding N. Kanatzidis M.G. Layered metal sulfides: exceptionally selective agents for radioactive strontium removal.Proc. Natl. Acad. Sci. USA. 2008; 105: 3696-3699Crossref PubMed Scopus (213) Google Scholar In addition, SZ-4 captures 59.6 mg/g at pH 1. It’s important to note that no such activity at highly acidic media has ever been reported before. Real seawater samples also showed very high selectivity for Sr2+, another first to be reported. Because SZ-4 is a single crystal, in situ Sr2+ ion exchange could be visualized by single-crystal-to-single-crystal (SCSC) transformation, which illustrates a unique two-step intercalation mechanism and exclusive soft cation (Sr2+) uptake selectivity. This is particularly useful because it is critical to know what exactly is taking place when such remarkable selectivities are observed. After interacting with Sr2+ for 12 h, parent crystal SZ-4 formed an intermediate complex [((CH3)2NH)Sr(H2O)3]0.5[ZrCH2(PO3)2F]·0.5H2O, (SZ-4-Sr-1). SZ-4-Sr-1 maintained the parent crystal’s space group. The single-crystal analysis revealed that polyhedral Sr2+ coordinated with basic [ZrCH2(PO3)2F]− anionic layers and showed a significant contraction of the interlayer from 3.6 Å in SZ-4 to 2.9 Å in SZ-4-Sr-1. As a result of the incorporation of Sr2+ in SZ-4, DMAH+ molecules were deprotonated via the ion-exchange mechanism, a thermodynamically stable process that can prove the selective Sr2+ uptake under lower pH value. A lower Sr–N bond distance (2.69 Å) indicates a strong interaction between Sr2+ and DMA in SZ-4-Sr-1. This, in turn, supports uptake selectivity toward soft Sr2+ over the hard divalent cations according to Pearson hard and soft acid-base theory. After 24 h of ion exchange, DMA molecules were fully removed, but SZ-4-Sr-2 was still crystallized with the same space group. Its interlayer distance expanded again (3.8 Å) close to the parent crystal. Each Sr2+ cation was surrounded by one O atom from the p=O bond, two F atoms, and five coordinated water molecules, suggesting that Sr2+ is thermodynamically stable in SZ-4-Sr-2. Time-dependent PXRD analysis proved the Sr2+ uptake phenomenon through observed peak shifting. In addition, density functional theory calculations confirmed the dynamic structural changes. Lastly, the pH of nuclear waste and contaminated ground water can vary from strongly acidic to extremely alkaline. Adsorption capacities are mainly studied in a wide range of pH. One of the best-performing adsorbents, the non-oxidic inorganic KMS-1, shows high selectivity at pH 12.8 because of its soft negatively charged sulfide layers. Biomineral-based GO–HAp (hydroxyapatite) nanocomposites are capable of 90% Sr2+ uptake in a wide pH range (5–11), and 76% Sr2+ was adsorbed at low pH (pH 2–4). But still, SZ-4 shows faster sorption kinetics, better performance under acidic conditions, increased refinement depth over a wide pH range, and upgraded Sr2+ remediation efficiency from wastewater. As a bonus, single crystals of SZ-4 allow in situ studies, which could reveal more adsorptive mechanisms of other important ions and lead to effective sorbents for economical seawater mining of valuable resources such as uranium. C.T.Y. acknowledges support from a National Research Foundation of Korea grant (NRF-2017M3A7B4042140) funded by the Korean government. Distinctive Two-Step Intercalation of Sr2+ into a Coordination Polymer with Record High 90Sr Uptake CapabilitiesZhang et al.ChemMarch 18, 2019In BriefHere, Zhang et al. report an atypical oxidic ion-exchange material SZ-4 that can remove 90Sr2+ with record-high removal efficiencies in both acidic solution and seawater, where a distinctive two-step intercalation mechanism is directly visualized by single-crystal structures during in situ Sr2+ sorption process, revealing a new type of Sr2+ uptake selectivity achieved through collaborative coordination by the oxidic layer and the dimethylamine interlayer as a soft N-donor ligand. Full-Text PDF Open Archive