chlorate, arsenate, and fluoride, leads to increased ecological concerns because of their toxicity toward humans and animals at trace levels. [4] Furthermore, the maintenance of low levels of inorganic anions, including nitrogen (nitrate) and phosphorus (phosphate), a nonrenewable resource used mainly in fertilizers, is crucial for ensuring food safety. [5,6] The separation of ionic species through adsorption-based or membrane-based technologies has been studied using porous materials like clays, [7] zeolites, [8,9] metal oxides, [10] metal-organic frameworks, [11] nanocarbons, [12] and microporous polymers. [13] In these systems, the process of separation is primarily driven by hostguest charge interactions, nucleophilic reactions, and the principles of size exclusion. Inorganic anions, especially nucleophilic anions such as arsenate, phosphate, and fluoride, can be efficiently separated by exploiting these interactions and principles. However, the key mechanistic factors associated with the separation of low nucleophilicity anions, such as nitrate, chlorate, borate, and bromide, are unclear because anionic compound classes are limited. Among them, nitrate ion (NO 3
−) is weakly adsorbed on various substances due to its planar symmetrical (D 3h ) resonance structure. Hence it is an important example of a low nucleophilicity anion.Layered double hydroxides (LDHs), 3+ , and A n− are divalent metal cations, trivalent metal cations, and anions, respectively; the molar ratio (x) = 0.166-0.33] [14][15][16] -the only class of anionic clay-have attracted extensive attention in catalysis, [16,17] battery engineering, [18,19] gas separation, [20] and biomedical engineering [21] and serve as promising candidates for adsorption-based anion separation. Even for nitrate ions, LDHs exhibit better ionstorage capacities on a volumetric basis than various materials, such as carbonaceous, inorganic, and polymetric materials (Figure S1, Supporting Information). [4] However, the ionstorage capacity correlates inversely with the x value (Figure 1a), despite the number of interlayer anions (that is, the maximum capacity) increasing proportionally with increasing x, which indicates that a significant portion of the nitrate ion-storage capacity remains inaccessible. [22][23][24] This trend is opposite to that observed for other anions, such as fluoride and sulfate. [25] Host-guest chemistry principles and mechanisms have been exploited to separate harmful and valuable ionic species. However, separating low nucleophilicity anions using these protocols is difficult due to weak host-guest interactions. Herein, using layered double hydroxides (LDHs), it is demonstrated that guest-guest interactions considerably influence the separation of a low nucleophilicity nitrate. LDHs exhibit considerably low nitrate ion storage capacities due to the co-precipitation synthesis method. Hence, a topochemical reaction is applied to control the cation arrangement in the LDHs. Structural analyses determine that a hexagonal cation arrangement is facilitate...
