# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 3 Layered Double Hydroxides
Energy-dispersive X-ray spectroscopy (EDX) reveals the LDH chemical structure, while elemental mapping determines its distribution. X-ray photoelectron spectroscopy (XPS) investigates chemical structure, valence states, and binding energies. X-ray fluorescence spectroscopy (XFS) detects cation ratios and trace elements. Microscopy techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HR-TEM) reveal LDH shape, size, and orientation. The Brunauer-Emmett-Teller (BET) method determines specific surface area, pore volume, and diameters. Barrett-Joyner  
Figure 2: Schematic illustration of layered double hydroxide (LDH) structure with intercalated anions. This figure depicts brucite-like LDH sheets, with intercalated anions occupying the interlayer region. The light blue octahedra represent the positively charged hydroxide layers, featuring octahedrally coordinated M\({}^{2+}\) and M\({}^{3+}\) cations (marked in purple). The exchangeable anions are indicated in yellow, and the associated water molecules are shown in dark blue. The diagram highlights the orderly arrangement of hydroxide layers and anions within the structure. Halenda (BJH) analysis provides information on pore size and distribution. Additionally, differential thermal analysis (DTA), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) are valuable tools for understanding material transformations in response to temperature changes. In practice, achieving a comprehensive LDH characterization often involves a combination of these techniques, as relying solely on one method may not yield all the necessary information. Research papers commonly utilize various combinations of these methods to thoroughly describe laboratory-synthesized LDH structures, compositions, and properties. The Application of Layered Double Hydroxides for the Effective Removal of Selenium Species from Contaminated Environmental Waters  
Selenium can be introduced into aquatic environments through both natural and anthropogenic processes. Worldwide, incidents of selenium contamination in water bodies are not uncommon, and such occurrences have been reported in several places, including China, the United States of America, Australia, New Zealand, and Finland [23]. Selenium primarily enters the aquatic environment in the form of inorganic species, specifically selenite (Se(IV)) and selenate (Se(VI)). Among these, selenite is recognized as more toxic and more easily absorbed by organisms as supported by available data [24; 25]. Conversely, selenate is considered the more stable species [26]. Furthermore, selenate and selenite have varying adsorption mechanisms on inorganic anion exchangers, which results in selenate being more challenging to remove than selenite [27; 28; 29]. Despite these distinctions, both species have roughly the same number of published studies on their removal. In the context of achieving drinking water quality standards through cleaning procedures, the following information may be of interest. Since the 1970s, selenium's presence in drinking water has been a subject of debate [30] due to its potential to make the water toxic and harmful to both humans and the environment. As a result of these controversial discussions, various selenium limits have been adopted. The upper limit for selenium concentration in drinking water varies across different countries and regions. Many countries like Canada, Australia, Japan, Thailand, and New Zealand, along with the European Union, follow the same limit of 10 \(\upmu\)g/L. On the other hand, the U.S. Environmental Protection Agency (USEPA) has a considerably higher limit of 50 \(\upmu\)g/L, while the California Environmental Protection Agency has a limit of 30 \(\upmu\)g/L. The World Health Organization (WHO) recommends a guideline value of 40 \(\upmu\)g/L for selenium in drinking water. These differences reflect the uncertainty and controversy over the health effects of selenium exposure.