# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 3 Layered Double Hydroxides
### Iron-Based Layered Double Hydroxides  
The substitution of a potential risk element, such as aluminum, in LDH structures intended for drinking water treatment, has led to the development of iron-based LDH. The effectiveness of various MgFe-LDH structures with different interlayer anions (nitrate, chloride, sulfate, or carbonate) and different specific surface areas in separating selenite [46; 52; 59], selenate [45; 46; 59], and selenocyanate [59] was assessed.  
Three MgFe-LDH variants with different interlayer anions (Cl\({}^{-}\), CO\({}_{4}\)\({}^{2-}\), and CO\({}_{3}\)\({}^{2-}\)) underwent a kinetic evaluation to examine their ability to adsorb dissolved selenium species (selenate, SeO\({}_{4}\)\({}^{2-}\), and selenite, HSeO\({}_{3}\)\({}^{-}\)/SeO\({}_{3}\)\({}^{2-}\)) [46]. The extent of selenium species uptake by MgFe-LDH depended on the identity of the interlayer anion, following the order Cl\({}^{-}\) > SO\({}_{4}\)\({}^{2-}\) > CO\({}_{3}\)\({}^{2-}\). This order reflects the varying strength of anion attachment within the interlayer, ranging from least strongly held to most strongly held. The interlayer anion exchange was confirmed as an important uptake mechanism for the studied selenium species using MgFe(Cl\({}^{-}\))-LDH and MgFe(SO\({}_{4}\)\({}^{2-}\))-LDH. The kinetics of selenium species uptake typically involved a rapid initial anion exchange phase, followed by a slower release back into the solution due to competition with displaced interlayer anions. This reverse exchange of selenium species was intensified by excess dissolved interlayer anions and high pH values.  
Even though the crucial role of pH in selenite removal through adsorption on MgFe(CO\({}_{3}\)\({}^{2-}\))-LDH was also acknowledged by [PERSON] et al. [52], the authors confirmed that the adsorption is not solely achieved due to electrostatic forces of attraction between the negatively charged selenite species and the positively charged LDH surfaces. This is because a significant amount of selenite adsorption occurs even at pH above the pH\({}_{\text{pzc}}\) (where the sorbent surface becomes negatively charged). This suggests that another type of sorption mechanism, most likely involving the formation of inner-sphere complexes, is responsible for selenite adsorption.  
A comparison of different LDH, specifically MgAl-LDH and MgFe-LDH with intercalated sulfate and carbonate ions, revealed that LDH containing interlayer SO\({}_{4}\)\({}^{2-}\) and CO\({}_{3}\)\({}^{2-}\) anions along with Fe\({}^{3+}\) cation exhibit superior scavenging properties for selenate, molybdate, and arsenate [45] compared with LDH with CO\({}_{3}\)\({}^{2-}\) and Al\({}^{3+}\). In all cases, a higher adsorption rate was observed with an excess of softestes, but the overall percentage of sorbate uptake decreased once the surface of LDH reached saturation. The sorbate uptake was also negatively affected by increasing pH levels and the presence of competing anions. For selenate and molybdate, outer-sphere complexation on surfaces and edges appears to be the primary sorption mechanism. In contrast, strong chemical bonds form between arsenate and MgFe-LDH, indicating a distinct mode of interaction.  
[PERSON] and [PERSON] [59] recently examined the utility of MgFe(NO\({}_{3}\)\({}^{-}\))-LDH for removing selenite, selenate, and selenocyanate. They explored a range of conditions, including LDH dosage (0.5-1.5 g/L), LDH calcination temperature (0-500 \({}^{\circ}\)C), and selenium species concentration (2.5-7.5 mg/L). The highest removal efficiency for selenite and selenate was achieved using MgFe-LDH calcined at 500 \({}^{\circ}\)C with a dosage of 1.5 g/L. In contrast, the highest removal efficiency for selenocyanate was observed for MgFe-LDH calcined at 250 \({}^{\circ}\)C. Interestingly, the kinetic and thermodynamic models for the studied selenium species did not show universal consistency.