# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 3 Layered Double Hydroxides
However, epidemiological studies investigating the effects of selenate, an inorganic selenium species commonly found in drinking water, have revealed evidence of toxicity at much lower concentrations than previously assumed. This indicates that health risks may occur at exposures even below the limit of 10 \(\upmu\)g/L. As a result, a proposal to lower this limit to 1 \(\upmu\)g/L has been put forward [30]. Nevertheless, most drinking waters have very low selenium content, only a few nanograms per liter. Therefore, changing the selenium standard to this level would not affect the use of most existing sources of drinking water. Meeting the drinking water limit for selenium can be challenging when the water is highly contaminated. At this point, it is worth noting that these limits apply to the total amount of selenium in the water. Moreover, since selenium can exist in different forms, it is important to consider not only the total selenium concentration but also its speciation when developing an effective cleaning procedure. In recent environmental science literature, numerous studies have delved into the removal of inorganic selenium species from contaminated water samples. As mentioned earlier, these species typically exist in their oxyanion forms in aqueous environments, with their presence influenced by pH and redox potential conditions. Figure 3 provides a schematic representation of the most frequently employed removal mechanisms that can be applied to any of these species using LDH structures. These mechanisms encompass surface adsorption through electrostatic attraction and outer-sphere complexation, which is mainly suitable for selenate ions, a unique type of chemisorption via inner-sphere complexation,particularly effective for selenite ions, and anion exchange mechanisms that are applicable to both selenium species. These effective approaches for selenium species removal are depicted in Figure 3A. Additionally, the reconstruction of calcined LDH structures has been extensively explored and proven to be effective for this purpose, as illustrated in Figure 3B. The initial comprehensive investigations into selenium species sorption using two specific types of LDH, MgAl(Cl\({}^{-}\))-LDH and ZnAl(Cl\({}^{-}\))-LDH, were carried out by [PERSON] et al. in 2001 [31]. Since then, numerous researchers have probed deeper into the sorption mechanisms of selenium species. They have also examined how selenium species behave in the presence of other co-existing anion forms employing various LDH materials. In the following sections, we will explore conventional LDH structures, modified variants of these layered materials (including those containing iron, rare earth elements, three different  
Figure 3: The application of layered double hydroxides (LDH) for water purification: a schematic illustration of its structure and selenium oxyanion adsorption. (**A**) Surface adsorption and anion exchange mechanisms. (**B**) Reconstruction of calcined LDH (so-called memory effect method). metal cations, and unique interlayer anions like MoS\({}_{4}\)\({}^{2-}\)), as well as nanocomposites that combine LDH with other materials (such as zero-valent iron, carbon nanodots, graphene oxide, reduced graphene oxide, carbon aerogels, chitosan, and amino acids).