Overcoming structural collapse in stable zirconium phosphonate materials for strontium removal

Abstract

Zirconium phosphate materials are promising adsorbents for removing metal ions from harsh conditions such as nuclear wastewater. However, the layered structure of zirconium phosphate distorts or even collapses under strong acidic conditions, lowering the industrial efficiency of zirconium phosphate in spent fuel reprocessing. We address this problem and show two different methods for improving acid tolerance of zirconium phosphate materials by intercalating aza-18 crown 6 ethers. These zirconium materials retain their layer-structure by guest crown ether molecules supporting the planes of host α-ZrP, and exhibit excellent stability in acidic conditions. In particular, D-DZ-ZrP possesses excellent stability even in 3 mol L-1 HNO3, HCl and H2SO4 solutions, due to the diaza-18 crown 6 ether can support the bilateral planes simultaneously by forming the stronger P-C covalent bond. More importantly, the existence of aza-18 crown 6 ethers in interlayer spacing, which produce the wide layer distances and abundant adsorption sites, make these samples exhibit the strong ability for selectively capturing of Sr2+ from mixed solutions. Remarkably, D-AZ-ZrP and D-DZ-ZrP especially showed the excellent adsorption properties in highly concentrated HNO3 solutions because of their excellent stability. In addition, the substitution of –NH groups in the crown ether ring favors the coordination interaction with Sr2+, which is confirmed by DFT calculations. Sr2+ adsorption capacities onto D-DZ-ZrP reached 88.34 mg g−1, which is also 2.5 times than that on the traditional α-ZrP. Our work poses a new way to overcome the structure collapse of the layered materials, and improve their applicability for use in actual industrial process.