# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 2 Separation of Selenium from Aqueous Media  
While some populations exhibit alarmingly low selenium status, there are areas where selenium levels in the environment have far exceeded the norm. These circumstances give rise to two distinct research avenues: (1) the accurate measurement of (ultra)trace selenium concentrations in water, soil, and food samples within regions characterized by low selenium content, and (2) the efficient removal of selenium from contaminated environmental compartments in regions where elevated levels of this element are prevalent. In this  
Figure 1: Study aims and objectives.  
literature review, our focus will be on the latter objective. Given that the majority of studies are centered around aqueous environments, our paper will primarily address this medium and the procedures employed in decontaminating water samples containing selenium.  
Various methods have been explored for selenium removal from water. There have been comprehensive reviews published on this topic. For instance, [PERSON] et al. [7] reviewed wastewater treatment technologies, covering options based on zero-valent iron, iron oxyhydroxides, supported materials, nanofiltration, reverse osmosis, chitosan-enhanced ultrafiltration, electrodialysis, and activated granular sludge. Similarly, [PERSON] et al. [4] offered a critical review of the applications, characteristics, and latest developments in current selenium removal methods, systematically analyzing and comparing their overall efficiencies, applicability, advantages, and drawbacks.  
The extraction procedures using suitable solid sorbents achieved remarkable results. The selection of a sorbent mainly depends on factors such as the specific form of selenium to be removed and the concentration of selenium in the water. There are various sorbents that can be utilized for the effective separation of this element, including activated carbon [8], zero-valent iron [7], iron oxyhydroxides [7; 9; 10], mono-metal oxides and hydroxides such as alumina (Al2O3), titania (TiO2), zirconium hydroxide (Zr(OH)4) [8], bimetallic composites such as iron-manganese oxides [11; 12], iron-zirconium oxides [13], and more. Among them, LDH stand out as highly efficient and versatile sorbents.  
Depending on the desired outcome and specific requirements, LDH can be applied in different modes of operation. The two main modes are (1) column arrangement and (2) dispersive arrangement. Each mode has its benefits and drawbacks. The column arrangement may encounter some problems, such as sorbent leaching, cartridge channeling, and cartridge clogging, which can make the procedure longer than desired [14]. The dispersive arrangement involves adding an appropriate amount of the sorbent to the liquid sample and stirring it for a few minutes. Then, the phases are separated, usually by centrifugation [14]. However, this mode poses a challenge of separating the sorbent quantitatively after extraction, especially when the sorbent is in its nanoform. Some of the sorbent may still remain in the solution even after centrifugation. Therefore, an additional separation step, such as filtration, is required.