## 3 General Application of the Hydrotalcites  
There are various possible applications of hydrotalcites. Due to the layered structure, hydrotalcites have a large surface area. They have great ion exchange ability and also, they are insoluble in water and most of the organic solvents. Therefore, these materials, most commonly Mg-Al hydrotalcite, are suitable as basic heterogeneous catalysts for fine chemicals' synthesis. Due to their low reactivity, the as-prepared hydrotalcites must be activated before use as the catalysts. The first activation step consists of thermal decomposition of as-prepared Mg-Al hydrotalcite (see Figure 2).  
Calcination of as-prepared Mg-Al hydrotalcite below 550 \({}^{\circ}\)C results in the formation of a well-dispersed mixture of magnesium and aluminum oxides (mixed oxide, MgAlO) with the presence of Mg\({}^{2+}\)-O\({}^{2}\)- acid-base pairs (Lewis-type basicity). Mg-Al mixed oxide has a so-called \"memory effect\", which allows the reconstruction of the original layered structure in contact with water  
Figure 2: Hydrotalcite activation for catalysis.  
Therefore, the second activation step consists of rehydration of mixed oxide leading to hydrotalcite intercalated with OH' ion as compensating anions in the interlayer [44], which are Bronsted-type basic sites. These rehydrated hydrotalcites exhibit high activity in base-catalysed reactions which require Bronsted basicity, i.e., condensation reactions of aldehydes and ketones [45; 46; 47; 48; 49; 50; 51], Michael addition [52], Baeyer-Villiger oxidation [53; 54], and others.  
Hydrotalcites are also quite promising heterogeneous catalysts which enable the multistep processes, particularly when different and incompatible active sites are required [55]. Metal containing hydrotalcites are found to be very useful in many important industrial reactions, such as aldol condensation, followed by hydrogenation [56; 57].  
The complex structure of hydrotalcites, enabling complexation of intercalated anions with various charged species, can be used in wastewater cleaning applications. For example, Mg-Al hydrotalcites can be used as adsorbents for the removal of chloride ions from wastewater [58]. In addition, Fe-Mg-Al hydrotalcites with different intercalated anions were evaluated as selective adsorbents for capturing heavy metals from a complex aqueous environment [59]. Adsorbents derived from hydrotalcites can be used for the removal of acute toxic drug diclofenac in wastewater [60], while the usual treatment processes are not able to totally eliminate this compound. Moreover, Zn/Al layered double hydroxides can be used for the removal of nanoscale plastic debris from aqueous systems [61].  
A medical application of hydrotalcite materials is also common, e.g., it can be used as an antacid. Usually, it is used together with aspirin without losing the medical effectiveness of both drugs [62]. Additionally, the hydrotalcites can be synthesized with bactericidal metal ions incorporated into the structure and used in cotton fibres for medical textile supplies [63]. Moreover, the hydrotalcites can be used as a raw material for various supplies, e.g., for black ceramic pigment [64].  
Hydrotalcites also show very promising results in the ion-exchange technologies [65; 66; 67; 68], where especially the anion exchange preference can be adjusted by different synthesis methods or anion occlusions [69]. The exchange properties are mainly affected by the interlayer anions [70].  
Another very extensively studied problem, where the usage of hydrotalcites has great potential, is the CO\({}_{2}\) and H\({}_{2}\)O capture [71; 72]. In contrast to the materials which are widely used for the CO\({}_{2}\) capture, such as zeolites or activated carbon, hydrotalcites show stable adsorption capacities at high temperatures [73; 74]. This is a very important property used in the steam reforming processes [75], but also for the removal of carbon dioxide from power plant fuel gases and for the reduction of total carbon dioxide emissions [76; 77].