# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 3 Layered Double Hydroxides
### Conventional Layered Double Hydroxides  
The application of conventional LDH, such as MgAl(CO\({}_{3}\)\({}^{2-}\))-LDH, for the removal of selenium species from contaminated water samples has been extensively documented in the existing literature [27; 32; 33; 34; 35; 36; 37; 38; 39]. Both laboratory-synthesized [27; 32; 33; 36; 38; 40; 41] and commercially accessible [34; 35; 37; 39] variants of MgAl(CO\({}_{3}\)\({}^{2-}\))-LDH have consistently demonstrated their effectiveness in these studies. Significant efforts have been directed toward improving the synthesis of MgAl(CO\({}_{3}\)\({}^{2-}\))-LDH, especially by [PERSON] in 2011 [42]. She introduced an innovative sol-gel synthesis approach, which differed from conventional methods. This technique, known as alkoxide-free sol-gel (AFSG) synthesis, eliminates the need for toxic and expensive metal alkoxides as precursor materials. In another study, [PERSON] et al. [38] compared three MgAl(CO\({}_{3}\)\({}^{2-}\))-LDH samples prepared by different methods: AFSG synthesis, hydrothermal precipitation (HTP), and alkoxide sol-gel (ASG) synthesis. They used various methods to characterize the surface, structure, and adsorptive properties of the samples, with a particular focus on the speciation of interlayer carbonate, aluminum, magnesium, and moisture location. Valuable and complementary information was obtained from the surface area, pore size, pore volume data, and the shapes of the isotherms from N\({}_{2}\) physisorption and CO\({}_{2}\) chemisorption studies. This information allowed the research team to predict the adsorptive removal properties of the three MgAl(CO\({}_{3}\)\({}^{2-}\))-LDH samples. They concluded that the alkoxide-free sol-gel synthesis route enabled the tailoring of highly reactive MgAl(CO\({}_{3}\)\({}^{2-}\))-LDH with abundant surface hydroxyl groups and mobile interlayer ionic carbonate. Therefore, the authors suggested using this method to prepare LDH for the effective removal of selenium species from aqueous matrices. They supported this suggestion by showing that this LDH has the highest sorption capacities for selenite and selenate among the tested LDH (see Table 1). The inorganic ion exchanger prepared through AFSG synthesis exhibited remarkable efficacy in simultaneously removing two selenium species [27]. This efficacy arose from different sorption mechanisms for these two species. Selenite adsorption by MgAl(CO\({}_{3}\)\({}^{2-}\))-LDH was dominated by chemisorption (inner-sphere complex formation) rather than ion exchange. Conversely, selenate removal occurred exclusively through an ion exchange mechanism, entailing the exchange of surface OH\({}^{-}\) ions and interlayer CO\({}_{3}\)\({}^{2-}\) ions. To facilitate the transition of MgAl(CO\({}_{3}\)\({}^{2-}\))-LDH into industrial applications, its efficacy in removing selenite and selenate was assessed under both static and dynamic adsorption conditions. Special attention was given to the influence of competing anions such as phosphate, sulfate, carbonate, silicate, and chloride [33]. A few years later, further investigations into the impact of sulfate on the adsorption of selenate [36] and selenite [55] onto MgAl(CO\({}_{3}\)\({}^{2-}\))-LDH prepared via the ASFG synthesis were conducted. The authors utilized XPS surface analysis to delineate the influence of different phases of this LDH on the removal of selenium species in relation to two variables: the absence/presence of sulfate and varying pH levels. In multiple studies, a simple co-precipitation method was employed to prepare MgAl(CO\({}_{3}\)\({}^{2-}\))-LDH [32; 40; 41]. Subsequent calcination steps at different temperatures, including 340 \({}^{\circ}\)C [32], 500 \({}^{\circ}\)C [40], or 550 \({}^{\circ}\)C [41], triggered various processes like dehydration, dehydroxylation, and carbonate loss. As a result of this thermal activation and the memory effect, selenite entered the interlayer of the calcined LDH (CLDH), forming