# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 3 Layered Double Hydroxides  
Layered double hydroxides (LDH), also known as hydrotalcite-like compounds or anionic clays, are an intriguing class of inorganic materials with unique structural properties. They consist of positively charged layers of metal hydroxides with charge-balancing anions and water molecules located between the layers. Their chemical composition is often represented by the following formula: [(M\({}^{2+}\))\({}_{1-x}\)(M\({}^{3+}\))\({}_{x}\)(OH)\({}_{2}\)]\({}^{x+}\)[(A\({}^{n-}\))\({}_{x/n}\)\({}_{m}\)H\({}_{2}\)O]\({}^{x-}\) where M\({}^{2+}\) and M\({}^{3+}\) represent a divalent and a trivalent cation, respectively, which are octahedrally coordinated in hydroxide layers; A\({}^{n-}\) represents an interlayer \(n\)-valent anion; and \(x\) represents the molar ratio of M\({}^{3+}\)/(M\({}^{2+}\) + M\({}^{3+}\)), determining the layer charge density [15]. The pure hrodtalcite phase can be obtained when \(x\) is in the range of 0.20-0.33, resulting in the M\({}^{2+}\)/M\({}^{3+}\) ratios of 2-4 [16]. For \(x>0.33\), an increased number of neighboring M\({}^{3+}\) containing octahedra leads to the formation of M(OH)\({}_{3}\), and for \(x<0.20\), a high density of M\({}^{2+}\) containing octahedra in the hydroxide layer results in the precipitation of M(OH)\({}_{2}\)[17]. This information emphasizes the importance of strictly adhering to the specified value of \(x\)[18]. Positively charged hydroxide layers contain cations with similar effective ionic radii. Some examples of M\({}^{2+}\) cations include Mg (86 pm), Zn (88 pm), Fe (92 pm), Co (88 pm), Ni (83 pm), Cu (87 pm), and Mn (97 pm); some examples of M\({}^{3+}\) cations include Al (67 pm), Fe (78 pm), Cr (75 pm), V (93 pm), Ga (76 pm), In (84 pm), and Rh (80 pm) [19]. Charge-balancing anions (A\({}^{n-}\)) can vary and may include common inorganic ones, such as NO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\), CO\({}_{3}\)\({}^{2-}\), SO\({}_{4}\)\({}^{2-}\), and ClO\({}_{4}\)\({}^{-}\), as well as various organic anions and some complex ions [20]. In the interlayer region, anions and a variable amount (\(m\)) of water molecules are present. These, along with the charge, size, shape, and orientation of the hydrated anions, affect the interlayer space. Figure 2 provides a schematic structural representation of LDH. From the above, it is evident that the composition of LDH can vary significantly. This variation is attributed not only to the combination of different octahedrally coordinated M\({}^{2+}\) and M\({}^{3+}\) cations and the presence of diverse A\({}^{n-}\) anions but also to the intercalation of various molecules in the interlayer space. Some intercalates can form when water molecules are replaced with other polar molecules, such as polyols or amines, between the layers of a crystalline material. There are many ways to synthesize LDH, some of which have been extensively covered in recent reviews [14; 21]. Hence, we will not go into details here. In summary, commonly utilized techniques include co-precipitation, anion exchange, the memory effect method, and their modifications, each with its own set of advantages and disadvantages. [PERSON] et al. [21] summarized the pros and cons of the most common synthesis methods in a well-organized table. It is worth highlighting that the choice of synthesis method can significantly impact the ultimate properties of the prepared LDH, thus influencing their suitability for the intended application [22]. Following the synthesis, it is essential to characterize the prepared LDH in order to assess their structure, composition, and physical and chemical properties. A recent paper by [PERSON] and [PERSON] [14] provides an overview of LDH characterization methods, allowing for a brief summary here. Crystalline solid structure analysis primarily relies on X-ray diffraction (XRD). Fourier transform infrared spectroscopy (FTIR) identifies molecular structures. Combining Raman spectroscopy with FTIR provides comprehensive molecular information.