# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 6 Conclusions  
LDH materials have garnered considerable interest from researchers due to their remarkable versatility and potential in various industrial applications. The composition of the layers and interlayer anions in LDH can be adjusted to tailor their properties for specific purposes. These materials have proven to be highly effective adsorbents and (photo)catalysts in a range of chemical processes. Their use extends to energy storage devices, where they serve as electrode materials to enhance performance, and as nanocomposites that improve the mechanical and functional properties of materials. LDH-based systems also show great potential in drug delivery, environmental remediation, and the development of highly sensitive sensors and biosensors, making them crucial components in catalysis, adsorption, and beyond. Given their versatility and effectiveness, it is particularly interesting to explore their application in specific areas. Composed of nontoxic, earth-abundant metals such as magnesium and aluminum, LDH are environmentally friendly compared with other materials. Their ability to be regenerated and reused multiple times further reduces waste and disposal concerns, contributing to a more sustainable process. Furthermore, LDH can be synthesized under relatively mild conditions, which reduces the energy consumption  
Figure 4: Layered double hydroxides (LDH): (**A**) historical timeline; (**B**) emerging research trends and applications; (**C**) future research directions for selenium removal. and environmental footprint of their production. In this article, we focus on LDH's utility in removing selenium species from contaminated aqueous matrices. Selenium primarily enters the aquatic environment in the form of inorganic species, namely, selenite and selenate. The adsorption mechanisms of these species on inorganic exchangers differ, making selenate more challenging to remove compared with selenite. Despite this fact, the published literature shows a balanced distribution of studies on the removal of both species. In regard to LDH, both commercially available and laboratory-synthesized hydrotalcite-like materials have been used effectively to remove selenium oxyanions. To understand how selenate and selenite are sorbed on calcined and uncalcined LDH structures, the researchers conducted several studies on these variants. Calcined LDH (CLDH) proved to be more efficient than LDH, achieving higher sorption capacities. This is attributed to the thermal activation and memory effect, which allows selenium oxyanions to enter the interlayer of the CLDH, forming a MgAl(selenium oxyanion)-LDH hydrolattice structure. In the case of LDH, simultaneous removal of selenite and selenate has been successful due to the different sorption mechanisms employed. Selenite adsorption by LDH involved chemisorption (formation of inner-sphere complexes) rather than ion exchange with interlayer anions. On the other hand, selenate removal occurred solely through ion exchange with surface OH\({}^{-}\) ions and interlayer anions. The different sorption mechanisms on these structures accounted for the higher sorption capacities for selenite over selenate (often more than double). Binary LDH are not the only material that can remove selenium oxyanions effectively; ternary LDH and LDH-based nanocomposites have also yielded satisfactory results. The success of the nanocomposites arises from the synergistic effects of other materials, such as the reduction capabilities of zero-valent iron, the zwitterionic nature of incorporated organic substances, or the presence of numerous oxygen-containing functional groups present in carbon-based materials. LDH containing yttrium cations in their hydroxide layers or thiomolybdate anions as counterions in the interlayer space have also been explored for their ability to remove selenium oxyanions from contaminated water samples.