Introduction
In recent decades, significant research has been done to create diverse bioceramics for biomedical uses [1]. Among the various classes of bioceramics, the most emerging is Hydroxyapatite (HAp). The formula of HAp is Ca10(PO4)6(OH)2, containing 18.5% P, 39% Ca, and 3.38% of OH by weight [2]. The best feature of HAp is its ability to take a lot of cationic and anionic substituents, making it easier to use in various applications [3,4]. The efficiency of HAp is determined by its microstructure, which includes shape and distribution, crystallinity and porosity, grain size, and distribution [5]. HAp is being investigated for various applications, including the adsorption of industrial pollutants, fuel cell material, and fluorescent lamps. To avoid phase impurities in HAp, the chemical composition and crystal structure must be regulated, which may be accomplished by developing new methods. Additionally, regulating microstructure, agglomeration, and stoichiometry during HAp synthesis is complex [6].
HAp and Bone tissue have the same physical properties and chemical composition [7]. Bones have fantastic properties like osteoconductive and osteointegration because 50% of the weight of bones is made up of HAp. It is the primary component of the bones that contributes to their hardness. Sometimes a hard tissue component, like tooth enamel, is mostly carbonate-apatite [8]. Additionally, HAp is biocompatible with the body and has bioactive properties. Hence research has been done to characterize, adopt and develop biomedical applications primarily for bone tissue engineering (BTE). HAp can be derived naturally or synthesized using a variety of precursors. Naturally, HAp can be extracted from plants, minerals, and animals, including algae and organisms. HAp is obtained from the bones, scales, and shells of both land and marine animals. Furthermore, leaves, wood, flowers, fruit peels, and stalks are usually appropriate forms from which HAp can be extracted [9]. Limestones are the major sources from which HAp is obtained [10,11]. Extracting is performed by hydrothermal treatments or via calcination. Alternatively, it can be synthesized chemically by different processes, like aqueous phase synthesis, molten salts, solid state synthesis, hydrothermal method, etc. [12].
Synthetic HAp and natural HAp are the same in terms of chemical and crystallographic studies [13]. Synthetic HAp is broadly used in replacing hard tissue and construction applications like bone substitutes, implant coating, etc. [14]. It is because synthetic HAp is stable thermodynamically at physiological pH and osteoconductive. Its porous character proposes high binding affinity for various pharmacological substances like antibody fragments, enzymes, hormones, antibiotics, steroids, etc. [[15], [16], [17], [18]]. These have opened up the possibility of using synthetically prepared HAp for delivering pharmacological substances, often in clinical applications for the treatment of osseous cancers, osteoporosis, osteomyelitis, etc., in which it is used for filling defects in skeletons.
HAp can be used as a drug carrier. Due to its properties, like its ability to adsorb various chemical species and biocompatibility, HAp constantly releases drugs and various biomolecules [19]. It does not exhibit side effects due to its greater effectiveness, non-toxicity, inevitable therapeutic response, and prolonged release [20,21].
Over 50% of HAp is used in orthopaedic applications, followed by dental applications and cosmetics. HAp's price might vary depending on the sources, form, and purity. Even though the structure of HAp does not change depending on processing conditions or sources, the structure and properties of HAp vary greatly. Since the properties and the structure of HAp are affected by conditions and sources, it is essential to know the suitable process and realizable properties [22].
Rapid industrialization and technological growth have attributed to an increase in the number of pollutants in water bodies. Organic contaminants, like dyes, and heavy metals, significantly contribute to water pollution [23]. Nano-sized HAp is gaining greater attention as a potential adsorbent due to its unique structure, which confers ionic exchange characteristics and adsorption affinities towards numerous pollutants like emerging pollutants, heavy metals, and azo dyes [24]. The crystal surface of HAp is abundant in positively charged calcium ions and negatively charged phosphate ions that can attach to other atoms, providing a great capacity for adsorption. The adsorption technique eliminates pollutants and collects contaminants found in wastewater (through desorption) that may be reused [25].
HAp is a naturally occurring wide-bandgap semiconductor [26]. Its surface has a high affinity for adsorbing various macromolecules that involve nucleic acid and proteins and possesses a hydrophilic character. Under UV irradiation, the PO4− group is modified on the surface of the HAp, which also causes the production of highly reactive OH− and O2− species and trapped electrons [[27], [28], [29]]. As a result, the materials have photoinduced activity and the potential to decompose non-degradable azo dyes (e.g., calmagite) [30]. In addition, it also provides the potential to decompose very toxic organic substances, such as dimethyl sulfide [31], and gaseous organic substances with nasty smells, such as methyl mercaptan [32]. Because of its ability to store electrons and extremely adsorptive nature, HAp has also been employed to create composites with TiO2 to increase its capacity to trigger the breakdown of organics under UV irradiation [[33], [34], [35]].
Further, HAp also creates composites with various metal oxides like CuO, ZnO, CeO2, etc. [36]. The presence of Ca-channels and the PO4− sub-lattice makes them extremely acceptable for different ion exchanges. It has been observed that doping HAp with different metal ions, such as Cr3+, Zn2+, and Fe3+, causes the adsorption band of HAp to widen from the UV to the visible region [37]. Compared with the undoped HAp, doped HAp will reduce the band gap energy because of the formation of the hybridized band inside the band gap, which will permit the decay of acetaldehyde by using near-ultraviolet light of lower photon energy [38].
The present state-of-the-art review article for the first time discussed the structure and properties of HAp along with several synthesis methods, such as the hydrothermal process, microwave-assisted synthesis, co-precipitation technique, sol-gel approach, and solid-state strategy. Moreover, numerous applications of HAp for environmental remediation such as photocatalytic activity, and the adsorption of dyes, heavy metals, and emerging pollutants are discussed in this article. Additionally, the application of HAp in drug carriers and bone treatment along with the scope of future research are also discussed which could help the researchers to develop new and innovative nanostructured HAp with enhanced properties.
Section snippets
Scope of review
Recently, many articles were published that studied different synthesizing methods of HAp and their applications (Fig. 1). Some review papers discussed the uses of HAp in photocatalytic activity. Another study looked at the adsorptive removal of dyes, heavy metals, and other pollutants from wastewater. Furthermore, several papers have discussed the uses of HAp for various applications (Table 1). However, to the best of our knowledge, there is no review article available covering synthesis
Structure and properties of HAp
HAp having formula Ca10(PO4)6(OH)2 has two types of structures, namely hexagonal or monoclinic (place group p63/m and p21/b, respectively). The hexagonal form is the most common, having cell parameters a=b=9.418Å and c=6.884Å with a reflection plane and hexagonal rotational symmetry [42].
The structure comprises an array of PO43− tetrahedra that are kept together by Ca2+ ions scattered among them. Ca2+ ions appear in two distinct locations, in precisely aligned columns (Ca(I)) and in the
Synthesis of HAp
HAp can be fabricated from inorganic compounds or natural organic-based materials like bones, eggshells, bovine, etc. Naturally occurring HAp contains a tiny amount of other ions; hence it is non-stoichiometric. The demand to synthesize HAp in a simple, efficient, cost-effective, and ecologically friendly manner is growing day by day. Following the progression, the preparation methods and strategies of HAp are gradually expanded, and outstanding clinical application improvements have been made.
Application of HAp
HAp is commonly used to treat alveolar ridge [73], bone and periodontal defects [74], tissue engineering systems [75], bioactive coating on metallic osseous implants[76], middle ear implants [77], and as a dental material [78] because of its similar composition to teeth and bone material and appropriate mechanical properties. According to recent research, HAp particles hinder the growth of several cancer cells [79,80]. The increasing use of HAp, as well as its derivatives, has resulted in a
Future perspectives
The creation of novel heterogeneous photocatalysts that are alternatives to TiO2 is a growing topic. Because of the unique properties of this molecule, research into HAp-based photocatalysts has received much attention. HAp is a popular material due to its biocompatibility and ability to adsorb molecules on its surface. Both of these properties make it suited for use in heterogeneous photocatalysis. Furthermore, HAp can be utilized for the removal of heavy metals. As a result, photocatalytic
Conclusion
The present state-of-the-art review article critically analyses the structure and physicochemical properties of HAp along with the synthesis techniques and their applications for environmental remediation and biomedical applications. Various synthesis techniques including the hydrothermal method, microwave-assisted method, sol-gel approach, co-precipitation method, and solid-state approach for fabricating HAp and its nanocomposites have been reviewed and the hydrothermal and microwave synthesis
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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