# Layered Double Hydroxides as Next-Generation Adsorbents for the Removal of Selenium from Water
## 5 Layered Double Hydroxides: Evolution and Future Research Directions  
Almost two centuries have passed since the German-Austrian mineralogist [PERSON] first identified and described hydrotalcite [87], a naturally occurring mineral now recognized as an LDH. This early documentation of hydrotalcite is one of the foundational records recognizing a material that would later be classified as part of the versatile LDH family. Despite their early discovery, LDH did not receive significant scientific attention until the early 20 th century. During that time, the focus was primarily on minerals with direct commercial applications, and LDH were not considered materials of practical importance. Some researchers even initially thought hydrotalcite was a mixture. It was not until [PERSON]'s 1915 study [88], which included optical, thermal, and analytical characterization, that hydrotalcite was widely accepted as a distinct species. The scientist derived the currently accepted formula, which was later confirmed by [PERSON] [88, 89, 90]. In the mid-20 th century (1940s-1960s), the structure of hydrotalcite and similar minerals began to be studied more systematically. The ability to prepare these materials in the lab opened up systematic investigations into their structures. This was a pivotal moment, marking the beginning of the controlled laboratory synthesis of LDH, a key step in understanding their structure and properties. The first synthesis of hydrotalcites was reported by [PERSON] and [PERSON] in 1942 [91], but their structure was not determined until 1968. The 1960s were marked by significant advancements in LDH synthesis and characterization. [PERSON] [92] and [PERSON] [93] were the first to analyze the structure using X-ray diffraction. Advancements in X-ray diffraction techniques during the 1970s enabled more detailed structural characterization, deepening the understanding of LDH and confirming their layered double hydroxide nature. LDH also began to attract attention for their potential in catalysis [94] as their anion exchange capacity and tunable chemistry made them useful as solid-state catalysts and precursors for mixed-metal oxides. This marked the first major industrial application of LDH (e.g., for water purification). Various synthetic routes, such as co-precipitation, hydrothermal synthesis, and ion exchange, were developed to create LDH with different metal combinations and interlayer anions during this period. Although still being referred to as \"hydrotalcite-like compounds,\" [PERSON] is credited with introducing the general formula of LDH in 1975 [95]. In his paper, he further formalized their structure and properties, making it one of the most frequently cited references for LDH structural formulas. This foundational work was further built upon by [PERSON] [96] in the 1980s, who advanced the understanding of synthetic LDH structures. The era of the 1980s marked substantial research into their use in catalysis, adsorption, and anion exchange applications. In the 1990s, research broadened to include environmental remediation and methods for targeting pharmaceutical compounds to their specific sites, resulting in a surge of studies exploring their practical uses. In the 21 st century, LDH began to be explored as components in nanomaterials, expanding their potential applications to areas such as energy storage in addition to their established roles in drug delivery and contaminant removal. Advancements in synthesis and functionalization techniques during the 2000s enhanced their adaptability across a wide range of fields. By the 2010s, research intensified on using LDH for water purification, waste treatment, heavy metal removal, and energy storage devices like batteries, supercapacitors, and fuel cells due to their excellent electrical conductivity and redox properties when converted to metal oxides. Studies from this period emphasized the multifunctionalcapabilities of LDH.