Layered Double Hydroxides is the name of a group of substances, which consists of natural minerals as well as synthetic compounds providing the typical layered structure. As synthetic Layered Double Hydroxides you call compounds showing the same structure and properties as their natural analogues and which are yet undiscovered or not existent in the nature. Layered Double Hydroxides are structural very similar to clay minerals and due to their ability to exchange the intercalated anions they are called as anionic clays.
In accordance with Mills et al.  natural Layered Double Hydroxides are compiled within the so called Hydrotalcite-supergroup which consists of eight subgroups. In 1842 Professor K.J.A. Scheerer discovered the first LDH in the Solvverkets Pyrite mine in Snarum, Buskerud, Norway . Due to its resemblance to talc and its high water content this natural mineral has been named as Hydrotalcite by Hochstetter . Since then every newly discovered natural Layered Double Hydroxide has been added to the Hydrotalcite-supergroup and subdivided into one of the eight subgroups. Based on the structural specification and lattice parameters, resulting in the specific structure, each Layered Double Hydroxide can be assigned to one subgroup. The subgroups are: (1) hydrotalcite group, (2) quintinite group, (3) fougèrite group, (4) woodwardite group, (5) cualtibite group, (6) glaucocerinite group, (7) wermlandite group, (8) hydrocalumite group. Currently the hydrotalcite-supergroup comprises 44 natural Layered Double Hydroxides.
Within the Metal-Aid project the focus is laid on Layered Double Hydroxides related to the hydrotalcite subgroup, in which the main layer is build up by magnesium and aluminium in a defined ratio of 3:1, hydrocalumite subgroup, build up by calcium and aluminium in the defined ratio of 2:1, and fougèrite subgroup, characterised by a main layer constructed by divalent and trivalent iron ions in variable ratios. Minerals related to the fougèrite subgroup are commonly known as Green Rust.
The structure of the Layered Double Hydroxides can basically be derived from the mineral brucite [Mg(OH)2]. There, each magnesium atom is surrounded by six oxygen atoms spanned as octahedrons (figure 1, upper left). Consequently, every magnesium is six-fold coordinated by oxygen . Additionally, every oxygen is accompanied by one hydrogen atom. As it can be seen in the lower left-hand side of figure 1, the oxygen-octahedrons are edge-linked to each other. In a brucite layer the edge-linked oxygen-octahedrons span two hexagonally closed-packed layers with magnesium in the resulting spaces (figure 1, right).
Figure 1: Schematic illustration of a magnesium atom six-fold coordinated by oxygen (left) in the structure of brucite (right). Magnesium as green, oxygen as red and hydrogen as white balls, black lines mark the unit cell of a brucite crystal. Schematic illustration created with VESTA (version 3.4.0, ) structural data based on Mills et al. .
Brucite-layers extend their selves in two spatial directions which are crystallographic called as a- and b- direction. The distance between two neighboured metal cations laying in the same plane is called a. If various brucite layers are stacked above each other’s the stacking direction is crystallographic called as c-direction and the distance between two layers is called c. Commonly, the octahedral positions do not show an ideal symmetry but are flattened in the stacking-direction (c-direction) . Consequently, the thickness of the main layer is decreased by increasing the distance a between two metal cations .
The structure of Layered Double Hydroxides is based on the partial exchange of divalent by trivalent metal cations in the main layer [3,5]. The excess of positive charges is then compensated by the intercalation of negative charged anions in the interlayer space between two main layers. Commonly, the interlayer anions are linked to the main layer via weak bonds like hydrogen bridge bonds. The most common divalent cations within the main layers of LDHs are Ca, Mg, Mn, Fe, Ni, Cu, Zn. As typical trivalent cations serve Al, Mn, Fe, Co, Ni, Cr, Ga . By intercalating cations providing a huge ion radius, e.g. Ca, Cd, Sc, La the octahedron-structure of the main layer can be broken up. The main layer is then opened to the interlayer space which results in one additional coordination of the metal cation to the water molecules in the interlayer . This phenomenon can be seen in figure 2, which shows the structure of kuzelite. Kuzelite is a CaAl-LDH related to the hydrocalumite-subgroup.
The most prominent interlayer anions are OH-, Cl-, CO32-, SO42- . Additionally, organic compounds like carboxylates, sulfonates or alkylsulfates [6,7] as well as neutral molecules, e.g. H2O or MgSO4 , can be intercalated in the interlayer of a LDH.
Corresponding to de Roy et al. the general formula of Layered Double Hydroxides can be written as :
Figure 2: Schematic illustration of the structure of hydrotalcite. Magnesium as green, aluminium as light grey, carbon as black, oxygen as red and hydrogen as white balls, black lines mark the unit cell of the hydrotalcite crystal. Structural data based on Allmann & Jepsen .
Due to their ability to intercalate or absorb anions, to exchange interlayer species and to reduce environmental relevant substances, natural as well as synthetic Layered Double Hydroxides offer a wide range of applications.
In the medical sector, they are for instance used as supporting materials for amino acids, pharmaceuticals or enzymes [9,10]. One of the best-known examples should be the usage of Hydrotalcite as an antacid binding and neutralizing the excess of acid in stomach and healing heartburn. Additionally, LDHs are widely applied within the metal, paper or agricultural industry, as catalysators , flame retardants for polymers  and short- or long-time fertilizers . Furthermore, Layered Double Hydroxides of the hydrocalumite-group are especially applied in the cement industry. There they are used as additives in the Ordinary Portland Cement, as flame retardants with a high chemical resistance, as hardening accelerators or as set retarders . Because of their layered structure and their capability to exchange the intercalated anions LDHs can be used as storage minerals [15,16].
On the one hand, heavy metals, e.g. iron, zinc, copper, cobalt, arsenic and fertilizers like phosphate or nitrate, organic compounds as well as radioactive elements are immobilized by Layered Double Hydroxides in consequence of sorption or intercalation processes. On the other hand, contaminants can be rendered harmless to the environment by reduction processes using natural and synthetic (Fe,Mn),Fe-LDHs related to the fougèrite-group.
For instance, the natural Fe2+/Fe3+-Layered Double Hydroxides, which are commonly known as Green Rust, provide the ability to reduce hexavalent chromium, which is highly toxic to single organisms or entire biocoenoses, to the less toxic and less mobile trivalent chromium. Thereby, the Layered Double Hydroxide itself is oxidized. .
Karen Maria Dietmann
University of Salamanca
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