# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 3 Layered Double Hydroxides
### Conventional Layered Double Hydroxides
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  
* n.r.: not reported;
* in the presence of Cr\({}^{3+}\) ions; *
* in the presence of Cu\({}^{2+}\) ions; LM: Langmuir model; FM: Freundlich model; P-S-O: pseudo-second-order; P-F-O: pseudo-first-order; AFSG: alkoxide-free sol-gel; HTP: hydrothermal precipitation; ASG: alkoxide sol-gel; C-dots: carbon nanodots; GO: graphene oxide; CAs: carbon aerogels; CLDH: calcined LDH; LRH: layered rare earth hydroxide.  
\end{table} 
Table 1: Comparison of sorption parameters for selenium oxyanions adsorbed by various layered double hydroxide (LDH) materials. MgAl(SeO3\({}^{2-}\))-LDH hydrotalcite [32; 40; 41]. A comparison of the sorption capacity between LDH and CLDH revealed that CLDH was a more efficient sorbent for selenite removal, with sorption values more than double [40] and even over five times larger [41]. In a number of studies, commercially available MgAl(CO3\({}^{2-}\))-LDH were tested for their capacity to remove selenate [34; 35; 37]. In each of these studies, calcined LDH were utilized. In the research conducted by [PERSON] et al. [34; 37], the characterization of commercially available granulated MgAl(CO3\({}^{2-}\))-LDH revealed that the manufacturer had removed interlayer water and carbonate anions. In another study by [PERSON] et al. [35], commercially available MgAl(CO3\({}^{2-}\))-LDH were calcined at 500 \({}^{\circ}\)C. After immersing the calcined LDH into a selenate aqueous solution, it demonstrated the ability to regenerate the layered structure. Post-reconstruction, the material exhibited proficiency in removing trace levels of selenate through surface adsorption, which is a rapid process. However, at higher concentrations, ion exchange played a more significant role due to the limited availability of surface sites [34; 37]. Many research papers primarily focus on examining the influence of other co-existing anions on selenate removal. Given the chemical and physical similarities between selenate and sulfate, extensive research is conducted on water samples with elevated sulfate levels. When the sulfate concentration is much higher than the selenate concentration, it can reduce selenate removal by competing for ion exchange sites or forming precipitates. For instance, in a study where the sulfate concentration was more than 20,000 times higher than the selenate concentration [34], pre-treatment with BaCl2 was necessary to improve selenate removal from 33% to 65% (which is approaching the rate of 74% obtained for selenate removal from deionized water). However, this also caused some selenate co-precipitate with BaSO4. Granulated LDH is a promising sorbent for removing selenate and other anions from power plant wastewater. It has high anion adsorption capacities and can function under dynamic sorption conditions. Laboratory column tests and field experiments have confirmed that granulated LDH can effectively remove selenate and other co-anions from wastewater [37]. [PERSON] et al. [39] studied the removal of both selenium species by a commercial MgAl(CO3\({}^{2-}\))-LDH and its calcined form. The calcination was carried out in the laboratory at 500 \({}^{\circ}\)C for 4 h. The material's sorption and desorption properties for selenite and selenate were thoroughly investigated in the presence of nitrate, sulfate, and phosphate ions. It was found that sulfate ions competed more with selenate, while phosphate ions competed more with selenite. A dual-mode Langmuir-Freundlich model was applied to explain how selenium species adsorbed to the material, overcoming some limitations of the Langmuir and Freundlich models, particularly in dealing with different types of sorption sites and affinities [56; 57]. The sedimentation rate is an important parameter to consider when developing effective cleaning procedures in the dispersive mode. Keeping this in mind, [PERSON] et al. [58] assessed the performance of two variants of LDH for selenate removal, MgAl-LDH and MgOAl-LDH.