# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 3 Layered Double Hydroxides
### Nanocomposites with Layered Double Hydroxides and Carbon-Based Materials  
Carbon-based materials (CBM) have been widely explored as adsorbents in the past two decades. They have remarkable characteristics and versatile applications in various domains, including chemistry, physics, and engineering [79]. Some of the most popular CBM are activated carbon (AC), graphene oxide (GO), carbon nanotubes (CNT), carbon nanofibers (CNF), carbon nanodots (C-dots), biochar (BC), and carbon aerogels (CAs). Many of these CBM have also been combined with LDH structures to form composite materials. These composites can remove selenium oxyanions from water samples, as discussed in this subsection. The co-sorption of cationic species, represented by Sr\({}^{2+}\), and anionic species, represented by SeO\({}_{4}\)\({}^{2-}\), was studied using two types of nanocomposites: Mg/Al(NO\({}_{3}\)\({}^{-}\))-LDH with carbon nanodots (C-dots) and Mg/Al(NO\({}_{3}\)\({}^{-}\))-LDH with graphene oxide (GO) [43; 44; 80]. The removal mechanism of SeO\({}_{4}\)\({}^{2-}\) was the same for both nanocomposites, involving ion exchange with NO\({}_{3}\)\({}^{-}\). However, the adsorption mechanism of Sr\({}^{2+}\) differed depending on the type of carbonaceous material. For the Mg/Al(NO\({}_{3}\)\({}^{-}\))-LDH/C-dots nanocomposite, Sr\({}^{2+}\) mainly coordinated with the carboxyl groups (\(-\)COO\(-\)) present on the surface of negatively charged C-dots [44]. For the Mg/Al(NO\({}_{3}\)\({}^{-}\))-LDH/GO nanocomposite, Sr\({}^{2+}\) could coordinate with both carboxyl and alkoxy groups (\(-\)COO\(-\)/\(-\)CO\(-\)) on GO [43]. The alkoxy groups were formed by the ring opening of epoxides on GO. In both nanocomposites, the interaction between LDH and Sr\({}^{2+}\) was dominated by electrostatic and ionic forces [80]. A notable observation was that both Mg/Al(NO\({}_{3}\)\({}^{-}\))-LDH/C-dots and Mg/Al(NO\({}_{3}\)\({}^{-}\))-LDH/GO composites had lower sorption capacities for SeO\({}_{4}\)\({}^{2-}\) than Mg/Al(NO\({}_{3}\)\({}^{-}\))-LDH without C-dots and GO (see Table 1). This could be attributed to the partial charge neutralization of LDH by the carbonaceous matter. Considering the higher sorption capacity of traditional LDH, one might argue that they represent a more favorable option for treating contaminated water, compared with nanocomposites. This advantage is further underscored by the fact that nanocomposites are more complicated and time-consuming to prepare, which also makes them more expensive. A more complex composite material, which is referred to as Si-rGO/LDH-Fe, was synthesized by [PERSON] et al. [81]. This sorbent was prepared from precursor materials such as reduced graphite oxide (rGO) and MgAl(NO\({}_{3}\)\({}^{-}\))-LDH. The synthesis process involved emulsion polymerization and crosslinking to form Si-rGO/LDH, followed by modification with FeCl\({}_{3}\) to enhance its sorption capacity. These steps are important as they contribute to determining the composition and structure of the final sorbent, thereby influencing the sorption mechanisms for removing the contaminants of interest. As mentioned earlier, GO has many functional groups that contain oxygen and can attract positively charged species while repelling negatively charged ones. On the other hand, rGO is more effective in capturing anionic pollutants. The presence of Fe\({}^{3+}\) also plays an important role in this composite. The pH of point of zero charge for Si-rGO/LDH-Fe was 5.6. Thismeans that below this pH, the hydroxyl groups attached to Fe become strongly protonated (=Fe\(-\)OH\({}^{2+}\)), which helps to bind the oxyanions, such as HSeO\({}_{3}\)\({}^{-}\) and H\({}_{2}\)VO\({}_{4}\)\({}^{-}\), on the surface of Si-rGO/LDH-Fe [81]. In order to modify MgAl-LDH, [PERSON] et al. [54] used carbon aerogels (CAs), which are made by treating sisal fibers with alkali, bleach, freeze-drying, and carbonization.