# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 3 Layered Double Hydroxides
### Layered Double Hydroxides with Intercalated Thiomolybdate Anions  
Previous studies showed that replacing the conventional anions in the LDH interlayers with thiomolybdate anions (MoS4\({}^{2-}\)) not only functionalized the materials for capturing heavy metal cations through selective M-S bonding [66], but these materials also performed well in removing oxyanions such as HASO3\({}^{2-}\), HASO4\({}^{2-}\), and CrO4\({}^{2-}\)[67]. In this section, we will discuss both binary and ternary LDH with intercalated thiomolybdate anions and their applicability for the removal of selenium species from aqueous media [47; 48; 49].  
The investigation of simultaneous removal of selenium oxyanions, specifically SeO4\({}^{2-}\), HSeO3\({}^{-}\), and SeO3\({}^{2-}\), along with heavy metal cations, including Hg\({}^{2+}\), Cu\({}^{2+}\), and Cd\({}^{2+}\), using MoS4\({}^{2-}\)-intercalated MgAl-LDH, revealed that the presence of Hg\({}^{2+}\) ions greatly enhances the capture of SeO4\({}^{2-}\), while the three metal ions (Hg\({}^{2+}\), Cu\({}^{2+}\), Cd\({}^{2+}\)) improve the removal of SeO3\({}^{2-}\) and HSeO3\({}^{-}\)[47]. There are several processes that contribute to the removal of the studied species. In the absence of other ions, SeO4\({}^{2-}\) can react with LDH-(MoS4\({}^{2-}\)) to form mixed phases of LDH-(MoS4\({}^{2-}\)) \(\times\) (SeO4\({}^{2-}\))y. When a mixture of SeO4\({}^{2-}\) and M\({}^{2+}\) is present, the enhanced removal capacity can be attributed to reactions between the introduced metal ions and the interlayer MoS4\({}^{2-}\), allowing SeO4\({}^{2-}\) to access the interlayer space. The rapid uptake and high capacity for selenite can be attributed to the reduction of Se(IV) to Se\({}^{0}\) by the S\({}^{2-}\) sites within the MoS4\({}^{2-}\) component of the MgAl(MoS4\({}^{2-}\))-LDH.  
A three-step procedure for the FeMgAl(MoS4\({}^{2-}\))-LDH synthesis was outlined by [PERSON] et al. [49]. This ternary LDH was used for simultaneous removal of selenate anions and metal cations, particularly Cr\({}^{3+}\) and Cu\({}^{2+}\). The presence of these cations has shown synergetic effect on selenate removal. Moreover, the simultaneous removal of selenate and metal cations has minimized the leaching of Mo from the LDH, likely due to co-precipitation reactions between leached MoS4\({}^{2-}\), metal cations, and selenate anions. This process enhances selenate removal efficiency and minimizes secondary contamination caused by leached Mo.  
A more thorough investigation into the potential roles of Fe ions in the ternary LDH, specifically in FeMgAl(MoS4\({}^{2-}\))-LDH, was conducted by [PERSON] et al. [48]. Their findings can be summarized as follows. The inclusion of Fe within the layers alters the layer charge density, allowing for the incorporation of more functional groups within the (inter)layers. Moreover, the presence of heterogeneous Fe acts as a catalyst for reductive reactions between selenium oxyanions and S\({}^{2-}\) ions (from MoS4\({}^{2-}\) groups). This catalytic effect offers distinct benefits in terms of uptake capacities and efficiency for the sequestration of selenium species. The adsorption capacities were impressive, with values reaching 483.9 mg/g for Se(IV) and 167.2 mg/g for Se(VI). For selenite, this represents the highest sorption capacity achieved using LDH (see Table 1).