# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
###### Abstract  
This research paper provides a comprehensive overview of the use of layered double hydroxides (LDH) in the removal of selenium species from contaminated water sources. Key studies on sorption mechanisms and the impact of competing ions on selenium removal are presented, and the effectiveness of LDH is compared across different structures and compositions. Scholarly sources extensively document the application of conventional LDH for effective selenium removal, with notable advancements achieved through innovative synthesis approaches. Comparative studies between LDH synthesized through various methods reveal the potential of tailored LDH for enhanced selenium adsorption. The paper further explores the influence of competing anions on LDH efficacy, emphasizing the impact of sulfate on selenium removal. Additionally, investigations into calcined LDH and commercially available variants underscore the potential for industrial applications. Beyond conventional LDH, the paper delves into iron-based LDH, LDH with intercalated thiomolybdate anions, and layered rare earth hydroxides, exploring their effectiveness in separating different selenium species. The role of pH in the removal of selenium species and the impact of three-metal cation LDH are also discussed. The study extends to nanocomposites, combining LDH with zero-valent iron, carbon-based materials, and organic compounds, illustrating their potential for selenium species immobilization. The presented findings offer valuable insights for researchers and practitioners in environmental science, addressing the growing demand for efficient selenium remediation strategies. s 2024 14 8513 15 September 2024 202  
species. Selenate, representing the fully oxidized form of selenium, can exist in solution as hydrogen selenate (HSeO\({}_{4}\)\({}^{-}\)) or selenate (SeO\({}_{4}\)\({}^{2-}\)), with an estimated pKa value of 1.80 \(\pm\) 0.10 [5]. Under oxidizing conditions, selenate predominates, displaying high solubility and minimal retardation due to its low adsorption and/or precipitation capacities. On the other hand, selenite is commonly found in environments with moderate redox potential and neutral pH. In an aqueous solution, Se(IV) exists as a weak acid in the forms of selenious acid (H\({}_{2}\)SeO\({}_{3}\)), hydrogen selenite (HSeO\({}_{3}\)\({}^{-}\)), or selenite (SeO\({}_{3}\)\({}^{2-}\)), with corresponding pKa values of 2.70 \(\pm\) 0.06 (H\({}_{2}\)SeO\({}_{3}\)/HSeO\({}_{3}\)\({}^{-}\)) and 8.54 \(\pm\) 0.04 (HSeO\({}_{3}\)\({}^{-}\)/SeO\({}_{3}\)\({}^{2-}\)) [5]. In reducing environments, water-insoluble elemental selenium can persist across a wide pH range. Under strongly reducing conditions, selenide remains thermodynamically stable and can exist as insoluble metal selenides or as H\({}_{2}\)Se(g) produced through microbial processes [5,6]. A diagram illustrating the various stable forms of inorganic selenium under different pH and redox potential conditions is available in the paper published by [PERSON] et al. [6]. The fate and transport of selenium species in the environment are governed by a complex interplay of biogeochemical processes, including redox reactions, sorption, solubility, volatilization, and biotransformation. These processes can alter the speciation, mobility, bioavailability, and toxicity of Se across various environmental compartments. Therefore, it is important to monitor and control the Se levels and forms in various matrices, such as water, soil, sediment, air, and biota. Addressing the complexities of selenium remediation, this study evaluates the efficacy of layered double hydroxides (LDH) as sorbents. We explore a range of LDH variants, including traditional forms, iron-enriched versions, those combined with rare earth elements, multiple metal cations, thiomolybdate anions, as well as nanocomposites incorporating zero-valent iron, carbon substances, or organic compounds.