# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 3 Layered Double Hydroxides
### Layered Hydroxides Containing Three Different Metal Cations  
If hydroxide layers contain three different metal cations, they are referred to as layered triple hydroxides (LTH), trimetallic layered double hydroxides (TLDH), ternary layered double hydroxides, or layered double hydroxides, with each metal specified (e.g., MgAlFeLDH). These terms are interchangeable in the literature, but we follow the original author's nomenclature. The variation in metal composition within the hydroxide layers of LDH leads to differences in specific surface areas and charge densities. By optimizing the composition, it is possible to enhance the efficiency of removal processes. [PERSON] et al. [63] discuss how the distribution of OH bonds among various metal oxides within ternary LDH affects the removal processes for selenate and chromate. They focus on MgCaFe(Cl\({}^{-}\))-LDH specifically. The study investigated four unique LDH structures with varying calcium and magnesium concentrations, denoted as Mg\({}_{3-x}\)Ca\({}_{x}\)Fe(Cl\({}^{-}\))-LDH, with x representing calcium content at values of 0, 0.3, 1.0, and 1.5. Special attention was given to the anion sorption process on LDH after the dissolution of Ca. The researchers found that the ternary LDH removed less selenate and chromate than Mg\({}_{3}\)Fe(Cl\({}^{-}\))-LDH. Selenate removal was three times lower. The observed reduction in selenate and chromate removal can be linked to the breakdown of the CaFe-LDH structure, which, in turn, diminished the MgFe-based LDH structure's capacity for anion adsorption. By 'Mg-Fe-based LDH', the authors likely refer to the fact that as calcium dissolved from the structure, the compound ceased to be a ternary LDH consisting of magnesium, iron, and calcium. Instead, it became a binary LDH containing only Mg and Fe. In order to understand the effect of silicate on the phase transformations of different types of calcined LDH (CLDH) materials, five CLDH with different cation ratios were tested [64]. These CLDH samples were derived from calcining five LDH with carbonate in the interlayer and different proportions of Mg, Al, and Fe: Mg\({}_{2}\)Al-LDH, Mg\({}_{2}\)Al\({}_{0.75}\)Fe\({}_{0.25}\)-LDH, Mg\({}_{2}\)Al\({}_{0.5}\)Fe\({}_{0.5}\)-LDH, Mg\({}_{2}\)Al\({}_{0.25}\)Fe\({}_{0.75}\)-LDH, and Mg\({}_{2}\)Fe-LDH. The objective was to evaluate their effectiveness in immobilizing SeO\({}_{3}\)\({}^{2-}\) and SeO\({}_{4}\)\({}^{2-}\) at two different pH values, 10 and 13. The results of this study revealed that both pH and the presence of silicate had significant impacts on the immobilization of SeO\({}_{3}\)\({}^{2-}\) and SeO\({}_{4}\)\({}^{2-}\) by CLDH materials. At higher pH values, the sorption capacities for selenium oxyanions were notably reduced. Higher pH reduced the sorption capacities due to an increased OH\({}^{-}\) concentration in the solution, which decreased the positive surface charge of the regenerated LDH and enhanced the competition between selenium oxyanions and OH\({}^{-}\) in the interlayer space. Moreover, silicate ions impaired the memory effect of LDH by reacting with MgO and Al\({}_{2}\)O\({}_{3}\) components in CLDH and forming magnesium silicate hydrate and geopolymer-like substances. These substances covered the CLDH surface, preventing the hydroxylation of metal oxides and hindering LDH regeneration. [PERSON] et al. [65] reported the synthesis and characterization of MgCuAl(CO\({}_{3}\)\({}^{2-}\))-LTH, a ternary LDH, and tested its ability to remove both inorganic selenium species simultaneously. The authors used a response surface methodology (RSM) to optimize the key process conditions, such as selenite concentration, selenate concentration, and adsorbent dosage. Their findings indicated that the adsorbent dosage was the most critical parameter affecting the removal efficiency of both selenite and selenate in the mixed streams. By using a 2.5 g/L adsorbent dosage, with 1 mg/L initial concentrations of selenite and selenate each, they achieved almost complete removal of selenium species.