# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 3 Layered Double Hydroxides
### Nanocomposites with Layered Double Hydroxides and Organic Compounds  
This subsection covers nanocomposites that have LDH and organic compounds, such as chitosan [50] and amino acids [82; 83], as components. Chitosan, a linear polyaminosaccharide, has the potential to remove various inorganic ionic species, whether used on its own [84] or when impregnated with metal oxide nanopowders [85; 86]. [PERSON] et al. [50] presented two different synthesis routes to produce a composite of LDH and chitosan, which had varying sorption performances for selenium oxyanions. The effectiveness of LDH and chitosan at removing selenium species was compared in both nanopowder and granular forms. The results showed that the nanopowder form of both materials was less effective than the granular form. The LDH within the beads were well dispersed and chitosan acted as a hydrogel, which allowed selenium to access the LDH in the polymer network. On the other hand, the in-situ LDH nanoparticles tended to aggregate or agglomerate, which hindered selenium from reaching the chemisorption sites on the LDH surface. This reduced the effectiveness of selenium removal. The sorption capacity of LDH for removing oxyanions from water samples can be enhanced by intercalating zwitterions (e.g., glycine (Gly)) into its interlayer. NiAl(NO\({}_{3}\)\({}^{-}\))-LDH and NiAl(Gly)-LDH, two variants of LDH with different interlayer anions, were compared by [PERSON] et al. [83] for their sorption capacity for arsenate, chromate, and selenate. They reported that NiAl(Gly)-LDH had a significantly higher sorption capacity for all the oxyanions than NiAl(NO\({}_{3}\)\({}^{-}\))-LDH. For instance, for selenate, the sorption capacity was 208.6 mg/g for NiAl(Gly)-LDH and only 55.0 mg/g for NiAl(NO\({}_{3}\)\({}^{-}\))-LDH. This difference can be explained by the different sorption mechanisms of these two types of LDH. Previous research mainly suggests that (NO\({}_{3}\)\({}^{-}\))-LDH remove oxyanions by exchanging them with the interlayer anions and adsorbing them on the surface. However, when NO\({}_{3}\)\({}^{-}\) ions are used as interlayer anions, anion exchange becomes challenging due to their strong electrostatic interaction with the layers, resulting in decreased sorption capacity for oxyanions. In the case of (Gly)-LDH, on the other hand, oxyanion adsorption occurs due to the strong electrostatic interaction between the positively charged protonated amine group of glycine and the negatively charged oxyanions. Moreover, (Gly)-LDH has a larger surface area than (NO\({}_{3}\)\({}^{-}\))-LDH, which contributes to its higher sorption capacity. In a related study, [PERSON] et al. [82] explored a different aspect of LDH functionality. Instead of investigating the removal of selenate with LDH intercalated with amino acids, they focused on how amino acids affect the release of SeO\({}_{4}\)\({}^{2-}\), which had been immobilized into hydrotalcite. The authors used Mg\({}_{2}\)Al-LDH as the host material and compared the effects of amino acids with different molecular sizes and structures. Smaller amino acids, such as glycine (Gly), L-cysteine (Cys), and L-aspartic acid (Asp), facilitated the release of SeO\({}_{4}\)\({}^{2-}\) mainly through intercalation, Mg\({}_{2}\)Al-LDH destabilization, and dissolution processes. The intercalation of amino acids expanded the interlayer spacing of Mg\({}_{2}\)Al(SeO\({}_{4}\)\({}^{2-}\))-LDH. Conversely, larger molecules like L-tryptophan (Trp) and L-phenylalanine (Phe) had a limited impact on SeO\({}_{4}\)\({}^{2-}\) release due to their bigger size and aromatic nature. In reactions involving Phe and Trp, the concentration of released SeO\({}_{4}\)\({}^{2-}\) was similar to or lower than that of the blank test. This indicates that there was little to no ion exchange between Phe and Trp and SeO\({}_{4}\)\({}^{2-}\), and the primary factor affecting SeO\({}_{4}\)\({}^{2-}\) concentration was dissolution.