# Efficient Capture of Sr\({}^{2+}\) Ions by a Layered Potassium Neodymium Phosphate
## 1 Introduction  
As a clean energy, nuclear energy has developed rapidly, accompanied by the aggravation of spent fuel containing uranium, plutonium, and fission products [1]. Spent fuel generates highly radioactive waste during reprocessing, of which \({}^{90}\)Sr is one of the main components and one of the main radioactive sources [2]. \({}^{90}\)Sr, with a long half-life (\(t_{1/2}=29\) y), can emit highly energetic \(\beta\) particles (0.546 MeV) [3]. It is often present in ionic form and has high environmental mobility [4, 5]. After the leakage accident in the Fukushima Daiichi Nuclear Power Plant (FDNPP) in 2011, it was found that the activity of \({}^{90}\)Sr was two orders of magnitude higher than the environmental background level in offshore waters 100-200 km from Fukushima [6]. \({}^{90}\)Sr is chemically similar to calcium, and once ingested by the human body through the food chain, it will accumulate in bones and teeth and participate in the metabolism, which will lead to bone cancer or leukemia [2, 7, 8, 9]. Part of the concern about nuclear safety is precisely the fear of the dreaded radioactive disease. The Japanese government's decision in 2021 to discharge nuclear wastewater from the nuclear accident at FDNPP into the Pacific Ocean has raised global concerns about the safety and environmental consequences of nuclear power [10, 11]. Thus, it is of vital significance to study Sr\({}^{2+}\) removal from radioactive waste streams for the sake of human safety and the sustainable development of nuclear energy. To date, methods for the removal of strontium from contaminated aqueous solution mainly include chemical precipitation, reverse osmoses, adsorption, solvent extraction,and ion exchange [12; 13]. Among these methods, ion exchange receives much concern due to its advantages of low cost, simple operation, high selectivity, and less secondary pollution [12; 14; 15]. In this regard, various ion exchange agents have been investigated for the elimination of strontium from aqueous liquid radioactive wastes, such as zeolites [16], clays [17], silicotitanates [18], metal organic frameworks [19], and metal sulfides [20; 21]. Among these adsorbents, inorganic ion exchange agents are favored because of their high thermal stability, high radiation resistance, chemical stability, and good selectivity [22]. In recent years, the application of metal phosphates in nonlinear optics, catalysis, proton conduction, and batteries has received extensive attention [23; 24; 25; 26; 27; 28]. Practically, metal phosphates are also a class of momentous ion exchange materials [29]. Benefitting from the rigid structure of PO4 tetrahedron, metal phosphates have the features of high chemical and thermal stabilities and low radiation damage, which make them have broad prospects in the removal of radionuclides [30; 31]. Thus far, numerous studies have proven that alkali metal ions or protons as exchangeable ions in metal phosphates can be exchanged for the effective separation of radioactive ions from aqueous solution environments [31; 32; 33; 34; 35; 36]. As early as in the last century, classical metal phosphate cation exchange materials with a two-dimensional layered structure were developed, namely M(HPO4)2: H2O (M = Zr, Ti, Hf, Ge, Sn, Pb) [37]. Among them, Zr(HPO4)2: H2O (\(\alpha\)-ZrP) has been widely studied as the representative for the efficient removal of metal ions, including Sr2+, Cs+, and rare earth cations, which have high thermal stability [38]. In addition, metal phosphates containing alkali metal ions with a layered or porous three-dimensional structure are also considered to be potential materials for the selective separation of radioactive ions [39]. It is worth noting that rare earth metal phosphates are rarely reported for the removal of Sr2+. A tetravalent cerium phosphate with a three-dimensional structure, namely K2 Ce(PO4)2, was studied with the Sr2+ ion exchange capacity of 45.65 mg/g [32; 39].