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Nanostructured bioceramics and applications
Nanostructured bioceramics and applications
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R.S. Khairnar⁎, V.N. Narwade⁎,†, V. Kokol† ⁎ Swami Ramanand Teerth Marathwada University, Nanded, India, †University of Maribor, Maribor, Slovenia
Abstract Ceramics materials are inherent part of our life, and can be classified as inorganic and nonmetallic materials. The ceramics is an inorganic, nonmetallic solid material comprising metal, nonmetal, or metalloid atoms principally held in ionic and covalent bonds. This chapter is about the special class of ceramic material called hydroxyapatite which is generally well known for the biomedical applications. Here we have shown the promising applications of hydroxyapatite in wastewater treatment, that is, heavy metal and dye adsorption with special emphasis on synthesis methodologies. Keywords: Ceramic materials, Hydroxyapatite, Hydrothermal, Precipitation, Irradiation, Adsorption.
10.1 Introduction Ceramics materials are inherent part of our life, and can be classified as inorganic and nonmetallic materials. The ceramics is an inorganic, nonmetallic solid material comprising metal, nonmetal, or metalloid atoms principally held in ionic and covalent bonds. Ceramics are incredibly diverse family of materials whose members span conventional ceramics (such as pottery and refractories) to the modern-day engineering ceramics (such as alumina and silicon nitride) found in electronic devices, aerospace components, and cutting tools [1–8]. Ceramics exhibit very strong ionic and/or covalent bonding (stronger than the metallic bond) and this confirms their unique properties such as high hardness, high compression strength, low thermal and electrical conductivity, and chemical inertness [9–13]. Ceramic materials, specifically developed for their usage in medical and dental implants, are termed bioceramics [13–15]. They include various compounds of alumina and zirconia, bioactive glasses, glass- ceramics, coatings and composites, hydroxyapatite (HAp), and resorbable calcium phosphates [16–20]. Bioceramics are ceramics materials that are biocompatible, its important subclass of biomaterials. These ceramics products or components are mainly used in bone implants and replacements. Bioceramics materials are commonly subdivided by their bioactivity. Bioinert materials (such as oxide ceramics, silica ceramics, and carbon fiber) Fundamental Biomaterials: Ceramics. https://doi.org/10.1016/B978-0-08-102203-0.00010-X Copyright © 2018 Elsevier Ltd. All rights reserved.
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Table 10.1 Various calcium-phosphate compounds, including hydroxyapatite, showing the importance of variations in Ca/P ratio in their structural aspect S. no. 1 2 3 4 5 6 7 8
Name
Symbol
Formula
Ca/P
References
Monocalcium phosphate monohydrate Dicalcium phosphate dehydrate Dicalcium phosphate anhydrous Octacalcium phosphate
(MCPM) and (MCPH) (DCPD)
Ca(H2PO4)2·H2O
0.5
[22]
CaHPO4·2H2O
1.0
[23]
CaHPO4
1.0
[23]
Ca8(HPO4)2 (PO4)4·5H2O Ca3(PO4)2 Ca3(PO4)2 Cax(PO4)y·nH2O
1.33
[23]
1.5 1.5 1.2– 2.2 1.66
[23] [23] [23]
α-Tricalcium phosphate β-Tricalcium phosphate Amorphous calcium phosphate Hydroxyapatite
(DCPA) and (DCP) (OCP) (α-TCP) (β-TCP) (ACP) (HA) and (HAp)
Ca10(PO4)6(OH)2
[23]
are nontoxic and noninflammatory. These materials must be long lasting, structural failures resistant, and corrosion resistant. Bioceramics, additionally, must have a low Young’s modulus to help prevent cracking of the material [21]. Table 10.1 shows the most important CaP salts, indicating variations in Ca/P ratio. This chapter is dedicated to a special class of ceramics, particularly termed as nanostructured bioceramics, with emphasis on its applications in waste water treatment. The main object is to utilize HAp nanoceramics, in particular, calcium apatite family, for its application in removal of impurities dissolved in water.
10.2 Hydroxyapatite (HAp) HAp is a mineral bioceramic, comprising complex calcium phosphates, which is also a chief structural element of vertebrate bone. Hence, it is popular material for biomedical applications such as a bone substitute material in orthopedics and dentistry due to its excellent biocompatibility, bioactivity, and osteoconduction properties [24–28].
10.2.1 Structure of HAp HAp crystallizes in a hexagonal system, although with some exception in a monoclinic system (Fig. 10.1) [29]. The system belongs to hexagonal space group P6 3/m, with hexagonal rotational symmetry and reflection plane. The lattice parameters of unit cell of HAp are a-axis = 0.9422 nm and c-axis = 0.688 nm [30]. HAp structure is formed by tetrahedral arrangement of phosphate (PO43−), which constitutes the s keleton of unit cell.
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Calcium
Oxygen
Hydrogen
Phosphorous
Fig. 10.1 Unit cell of Hydroxyapatite, view along c-axis (http://molview.org/).
Unit cell of HAp with 44 atoms per unit cell is made of Ca2+, PO43−, and OH− ionic groups arranged together in hexagonal form.
10.2.2 Synthesis of HAp During past few decades, considerable research efforts have been directed toward the synthesis of HAp bioceramic by utilizing several methods. The popular methods of synthesis of HAp include wet chemical precipitation, sol-gel method, hydrothermal method, microwave irradiation method, etc.
10.2.2.1 Wet chemical precipitation method The wet chemical method precipitation process is the most convenient and commonly used process. This process is simple and easy to use. The preparative reaction and the character of reaction product can be regulated. Wet chemical reactions have advantages to control the morphology and the mean size of crystallites, due to which it is the most promising technique for the fabrication of nanosized HAp. Mahabole et al. [31] performed wet chemical method to synthesize HAp, in calcium nitrate, diammonium hydrogen phosphate, and ammonium hydroxide were used as precursors. The stoichiometric ratio of the calcium nitrate and diammonium hydrogen phosphate solutions was adjusted so as to get theoretical (Ca/P) molar ratio close to 1.66 [23]. The precipitation was performed by slow addition of diammonium phosphate solution (0.6 M) to calcium nitrate solution (1 M) under continuous and gentle stirring, while maintaining the pH of reaction mixture by the addition of NH4OH. As a result of reaction, milky precipitation was obtained. The precipitate was continuously stirred for 24 h using magnetic stirrer. The resulting white precipitate was washed thoroughly several times with double distilled water, oven dried at about 100 °C, and crushed into powder form. The grain size of HAp, in the range of 0.032 μm, can be produced by this technique.
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(NH4)2HPO4 (0.6 M)
NH4 OH
Ca(NO3)2.4 H2O (1 M)
80°C
Fig. 10.2 Schematic representation of wet chemical precipitation method.
This method was modified [32] by carrying out chemical reaction at elevated temperature of 80°C during precipitation formation for considerable amount of time followed by gelation for 24 h. The resulting white precipitate, after thoroughly rinsing with deionized water, was dried by heating in air 100°C for brief time and was later sintered at 1000°C for 2 h in PID-controlled furnace. The average crystallite size obtained was in the range of 15 nm for synthesized HAp. This process gives hexagonal structure for synthesized HAp. The following simplified equation shows chemical reaction adopted for HAp synthesis by means of wet chemical precipitation method (Fig. 10.2). 10Ca ( NO3 )2 + 6 ( NH 4 )2 HPO 4 + 8NH 4 OH ® Ca10 ( PO 4 )6 ( OH )2 + 20 NH 4 NO3 + 6H 2 O
(10.1)
10.2.2.2 Sol-gel method Sol-gel technique has attracted much attention recently due to its well-known inherent advantages to generate glass, glass ceramics, and ceramic powders. The sol-gel process is easily applicable to surface coating and it allows the preparation of high-quality HAp thin films on metal substrates. The sol-gel process can be utilized to synthesize both HAp thin films and HAp powder under significantly mild conditions. Agrawal et al. [33], synthesized HAp powder by sol-gel method, in which two different chemical reagent were used. At first, phosphoric pentoxide (P2O5) was dissolved in absolute ethanol to form 0.5 mol/L solution and second calcium nitrate tetrahydrate Ca(NO3)2·4H2O was dissolved in ethanol to form 1.67 mol/L solution.
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Both the solutions were mixed to obtain the desired Ca/P molar ratio of 1.67. The solution was stirred slowly for 10–15 h until the formation of gel. Further the gel was dried in an electric oven at 80°C in air for 20 h, followed by two-stage heat treatment in air starting from 400°C to 750°C for 8 h. Authors reported that this study presents an alternative method to form pure, stable, crystalline nanosized HAp powder at low temperature as compared to other existing methods where temperature of treatment is more than 800°C to achieve all HAp characteristics. Liu et al. [34] showed another procedure for HAp synthesis by sol-gel method. Triethyl phosphite sol was diluted in anhydrous ethanol and then a small amount of distilled water was added for hydrolysis. The molar ratio of water to the phosphorus precursor was kept at 3. The mixture was sealed in a glass beaker immediately after solvent addition and then stirred vigorously. The mixture initially appears opaque, due to the immiscibility among phosphate and water. However, after approximately 30 min of mixing, the emulsion transformed into a clear solution, suggesting that the phosphite was completely hydrolyzed. This was confirmed by the loss of phosphite odor of the mixture. A stoichiometric amount (i.e., to maintain Ca/P = 1.67) of 3 M calcium nitrate dissolved in anhydrous ethanol was subsequently added dropwise into the hydrolyzed phosphorus sol. Vigorous stirring was continued for an additional 10 min after the titration. As a result of this process, a clear solution was obtained and which was later aged at room temperature for 16 h before drying. The solvents were then dried at 60°C until a viscous liquid was obtained. The corresponding HAp concentration changed from 3.6 vol% (in the solution) to 13.6 vol% (in viscous liquid). Further drying of the viscous liquid at 60°C resulted in a white gel. The gel was ground with a mortar and pestle into fine powder and subjected to different calcination temperatures, from 300°C to 800°C with 25–50°C intervals, for 2 h. This study demonstrated a new way for synthesis of HAp ceramic via a novel low-temperature sol-gel process. HAp phase can be synthesized at a temperature as low as 350°C, which is lower than those reported in literature for alternative sol-gel routes to HAp (200–300°C).
10.2.2.3 Hydrothermal method The hydrothermal method is often used to prepare HAp with good crystallinity and homogeneous size and shape. Hydrothermal synthesis can also simply be considered as a chemical precipitation in which the aging step is conducted at a high temperature, typically above the boiling point of water, inside an autoclave or pressure vessel (Fig. 10.3). Jingbing Liu et al. [35] carried out hydrothermal synthesis process for HAp whiskers and crystal synthesis. The initial reaction substances of CaHPO4·2H2O (0.5162 g) and Ca(OH)2 (0.1482 g) were dissolved in 40-mL deionized water. To investigate the influence of pH value, a series of experiments were devised. The pH of initial solution was varied by means of CH3COOH. For higher pH values KOH is used. The ready-adjusted solutions with certain pH value were poured in a teflon vessel and heated at temperature 140°C for 24 h at its self-generated pressure, and then cooled to room temperature. The precipitates were filtered and washed with deionized water till the pH turned to 7. Finally, the filtrates were dried in vacuum at 60–100°C for 2 h. It is observed that the
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Conditions Spherical Ca2+ Rod like
Dandelion like PO43− Chemicals
Teflon liner
Autoclave reactor
Hexagonal prism Plate like
Fig. 10.3 Schematic representation of hydrothermal method used for synthesis of different HAp nanostructures.
pH value of the starting reaction solution and the temperature of hydrothermal treatment are the most significant variables in altering the HAp structure and morphology. Whiskers with high aspect ratio were obtained from the starting reaction solution having 9 pH, at the relatively low temperature of 120°C. Zhu et al. [36] synthesized HAp using EDTA as chelate. First, 1 mL of NH4OH, 0.169 g of Ca(NO3)2, 0.209 g of EDTA, and 0.065 g of KH2PO4 were dissolved into 22 mL of water, so as to get the mole ratio of EDTA/Ca as 1. Aqueous solution of KOH was used to adjust the pH value of the solution to 12.9, 11.4, and 9.7 separately. After that solutions were sealed in Teflon-lined stainless autoclave and heated at different temperature for predetermined time and then cooled to room temperature. The products were washed and filtrated, and dried at 60°C in air. HAp crystals with prismlike morphology have been successfully prepared by using EDTA as chelate under hydrothermal condition. The results showed that the products were pure HAp with preferential orientation along c-axis.
10.2.2.4 Microwave irradiation method The synthesis of HAp by microwave irradiation method is also reported [37]. In such synthesis process of HAp, high-purity (CaNO3)4H2O and Na2HPO4 were used as the starting materials. EDTA served as the complex reagent with molar ratio of (CaNO3)4H2O:EDTA:Na2HPO4 as 5:5:3. In a typical procedure, 50 mL of a mixed solution of (CaNO3)4H2O (0.1 M) and EDTA (0.1 M) was introduced into 50 mL of Na2HPO4 (0.06 M) solution. Since the initial pH of such prepared was 4, it was adjusted to higher pH in the range of 9–13 by adding NaOH solution (0.1 M). After stirring for 10 min, these ready-adjusted aqueous solutions, with certain pH, were
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put into a household-type microwave oven of 700-W powers with a refluxing system and the reaction was performed under ambient air for 30 min. Typically, microwave oven followed a working cycle of 6 s on and 10 s off (37% power), to irradiate the solutions by microwave radiations. After cooling to room temperature, the precipitate was centrifuged, washed with deionized water, and dried in vacuum at 70°C for 2 h. The synthesized HAp showed to exhibit nanorods, bow-knot-like and flower-like nanostructures.
10.2.3 Properties of HAp These are some noteworthy properties of HAp, which leads to HAp as useful ceramics. HAp has various meritorious properties, viz. biocompatibility [38], nontoxicity [39], chemical inertness [40], and ion exchange capacity [41]. Carbonate is easily sorbed by synthetic HAp and participates in the structure of biological apatite with substituting either PO43− and OH−. This substitution causes important changes in the apatite unit-cell volume, crystal morphology, solubility, thermal stability, and bioactivity [42]. The ionic conductivity of calcium HAp increases with addition of fluoride up to 50% replacement of the OH– by F− ions. Besides these, high porosity is the key property for HAp for its application in various fields [43,44].
10.3 Applications of HAp Due to useful efficient properties as mentioned earlier, HAp is widely used in the diverse fields. The potential applications include fuel cells because HAp can be anticipated as an electrolyte for high temperature in fuel cell [45,46]. HAp is used for many practical applications, to name a few are (i) catalyst [47], (ii) biomaterial [48], (iii) gas sensors [49–54], and (iv) bioceramic coatings [55]. Coatings of HAp are often applied to metallic implants (most commonly titanium/titanium alloys and stainless steels) to alter the surface properties. In this manner, the body sees HAp-type material, which is readily acceptable in human body plasma. Also HAp is used for water purification due to presence of pores and extraordinary ion-exchange capacity. HAp seems to be suitable substrate for adsorption of heavy metals and organic dyes.
10.4 Applications of HAp in heavy metal/dyes/organic pollutant adsorption One of the recent and interesting applications of HAp is its utilization in water purification as well as removal of impurities from industrially discharges, typically, heavy metal and organic/dyes dissolved liquid solutions. Heavy metals are naturally occurring elements that have a high atomic weight and a density at least five times greater than that of water. Their multiple industrial, domestic, agricultural, medical, and technological applications have led to their wide
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distribution in the environment, raising concerns over their potential effects on human health and the environment. Their toxicity depends on several factors including the dose, route of exposure, and chemical species, as well as the age, gender, genetics, and nutritional status of exposed individuals. Because of their high degree of toxicity, arsenic, cadmium, chromium, lead, and mercury rank among the priority metals that are of significance from the point of view of public health. These metallic elements are considered systemic toxicants that are known to induce multiple organ damage, even at lower levels of exposure. They are also classified as human carcinogens (known or probable) according to the U.S. Environmental Protection Agency, and the International Agency for Research on Cancer. Many studies have acknowledged the ability of HAp [Ca10(PO4)6(OH)2] to combine with divalent heavy metal ions. Earlier studies have shown that synthetic HAp has a high removal capacity for Pb, Zn, Cu, Cd, Co, and Sb from aqueous solutions [56–66]. Mechanisms such as ion exchange [41], surface complexation [30], dissolution of HA and precipitation of metal phosphates [30], substitution of Ca in HAp by metals during recrystallization (coprecipitation) have been proposed in order to describe the uptake of heavy metals from aqueous solutions by synthetic HAp. However, controversy connected with the relative contribution of each process, in removing heavy metals, still exists in the literature. Synthetic dyestuffs widely exist in the effluents of industries such as textiles, printing, paper, plastics, and leather. Many dyes and pigments contain aromatic rings in their structures, which make them toxic, nonbiodegradable, carcinogenic, and mutagenic for aquatic systems and human health [67–69]. Because of their accumulative effects in biota, the presence of dyes even at very low concentrations in water is not allowed. Moreover, dyes cannot be easily removed with the conventional wastewater treatment methods since they are difficult to be biodegraded or photodegraded. Therefore, removing dyes from aqueous solution has become an important and challenging area in wastewater treatment. HAp is used efficiently for dyes adsorption and is known as a prominent ceramic adsorbents. The reported literature showed that the various types of dyes can be adsorbed using HAp, such as Congo red, blue SBL dye, Reactive yellow 84, Methylene blue, etc. Moreover, HAp also demonstrated excellent adsorption capabilities for phenol and its derivatives [70–74]. Additionally, HAp is also proved to be a desirable candidate as a photocatalyst, for degradation of dyes and organic contaminants [75–79]. To improve the adsorption capacities of HAp, various types are dopants are utilized, which increases the functionality of adsorbent material. TiO2 is widely used dopant for enhancing photocatalytic activity of HAp. Various researchers worked on TiO2-HAp as a composite material for dyes and organic contaminants degradation [80,81]. Gold-HAp system is used as photocatalyst for methylene blue dye degradation under visible light [82]. Various polymer-supported HAp composites are also adopted to utilize for heavy metal and dye adsorption. Polymer-HAp have also shown to provide a new platform for their applicability in adsorption. Cellulose-HAp [62,83,84], chitosan-HAp [61,85], gelation-HAp [86], alginate-HAp are some of the examples of HAp composite, which are reported as future composite adsorbents.
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10.5 Conclusions HAp bioceramic is important class of ceramic materials. HAp can be successfully synthesized by various methods and these techniques are found useful in different applications depending on their morphology. HAp has the important role in heavy metal adsorption/dye adsorption due to their ion exchange, surface complexation, and dissolution properties. HAp is also well known for biomedical application due to chemical similarities with bone chemistry. Thus HAp has shown its applicability as multipurpose ceramic material.
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[33] K. Agrawal, G. Singh, D. Puri, S. Prakash, Synthesis and characterization of hydroxyapatite powder by sol-gel method for biomedical application, J. Miner. Mater. Charact. Eng. 10 (08) (2011) 727–734. [34] D.M. Liu, T. Troczynski, W.J. Tseng, Water-based sol–gel synthesis of hydroxyapatite: process development, Biomaterials 22 (13) (2001) 1721–1730. [35] J. Liu, X. Ye, H. Wang, M. Zhu, B. Wang, H. Yan, The influence of pH and temperature on the morphology of hydroxyapatite synthesized by hydrothermal method, Ceram. Int. 29 (6) (2003) 629–633. [36] R. Zhu, R. Yu, J. Yao, D. Wang, J. Ke, Morphology control of hydroxyapatite through hydrothermal process, J. Alloys Compd. 457 (1) (2008) 555–559. [37] J. Liu, K. Li, H. Wang, M. Zhu, H. Yan, Rapid formation of hydroxyapatite nanostructures by microwave irradiation, Chem. Phys. Lett. 396 (4) (2004) 429–432. [38] J. Sun, X. Zheng, H. Li, D. Fan, Z. Song, H. Ma, J. Hui, Monodisperse selenium-substituted hydroxyapatite: controllable synthesis and biocompatibility, Mater. Sci. Eng. C 73 (2017) 596–602. [39] H. Jiao, K. Zhao, T. Bian, Y. Tang, Hydrothermal synthesis and properties characterization of barium titanate/hydroxyapatite spherical nanocomposite materials, J. Alloys Compd. 715 (2017) 73–82. [40] J. Cui, C. Ma, Z. Li, L. Wu, W. Wei, M. Chen, Z. Deng, Polydopamine-functionalized polymer particles as templates for mineralization of hydroxyapatite: biomimetic and in vitro bioactivity, RSC Adv. 6 (8) (2016) 6747–6755. [41] V.N. Narwade, M.P. Mahabole, K.A. Bogle, R.S. Khairnar, Waste water treatment by nanoceramics: removal of lead particles, Int. J. Eng. Sci. Innov. Technol. 3 (3) (2014) 324–329. [42] N.Y. Mostafa, Characterization, thermal stability and sintering of hydroxyapatite powders prepared by different routes, Mater. Chem. Phys. 94 (2) (2005) 333–341. [43] A.K. Gosain, L. Song, P. Riordan, M.T. Amarante, P.G. Nagy, C.R. Wilson, J.L. Ricci, A 1-year study of osteoinduction in hydroxyapatite-derived biomaterials in an adult sheep model: part I, Plast. Reconstr. Surg. 109 (2) (2002) 619–630. [44] N. Specchia, A. Pagnotta, M. Cappella, A. Tampieri, F. Greco, Effect of hydroxyapatite porosity on growth and differentiation of human osteoblast-like cells, J. Mater. Sci. 37 (3) (2002) 577–584. [45] K. Yamashita, H. Owada, T. Umegaki, T. Kanazawa, K. Katayama, Protonic conduction in yttrium-substituted hydroxyapatite ceramics and their applicability to H2-O2 fuel cell, Solid State Ionics 40 (1990) 918–921. [46] C.C. Yang, S.J. Chiu, C.T. Lin, Electrochemical performance of an air-breathing direct methanol fuel cell using poly (vinyl alcohol)/hydroxyapatite composite polymer membrane, J. Power Sources 177 (1) (2008) 40–49. [47] S. Wuyts, D.E. de Vos, F. Verpoort, D. Depla, R. de Gryse, P.A. Jacobs, A heterogeneous Ru–hydroxyapatite catalyst for mild racemization of alcohols, J. Catal. 219 (2) (2003) 417–424. [48] M.A. Surmeneva, R.A. Surmenev, Y.A. Nikonova, I.I. Selezneva, A.A. Ivanova, V.I. Putlyaev, M. Epple, Fabrication, ultra-structure characterization and in vitro studies of RF magnetron sputter deposited nano-hydroxyapatite thin films for biomedical applications, Appl. Surf. Sci. 317 (2014) 172–180. [49] R.U. Mene, M.P. Mahabole, K.C. Mohite, R.S. Khairnar, Fe doped hydroxyapatite thick films modified via swift heavy ion irradiation for CO and CO2 gas sensing application, J. Alloys Compd. 584 (2014) 487–493. [50] R.U. Mene, M.P. Mahabole, K.C. Mohite, R.S. Khairnar, Improved gas sensing and dielectric properties of Fe doped hydroxyapatite thick films: effect of molar concentrations, Mater. Res. Bull. 50 (2014) 227–234.
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[51] M.P. Mahabole, R.U. Mene, R.S. Khairnar, Gas sensing and dielectric studies on cobalt doped hydroxyapatite thick films, Gas 900 (2013) 1. [52] S.R. Anjum, R.S. Khairnar, Integrated sensing of alcohols by CNT blended HAp Nano ceramics, Sens. Transducers 206 (11) (2016) 1. [53] S.R. Anjum, R.S. Khairnar, in: Development of methanol sensors: carbon nanotubes blended hydroxyapatite nano-ceramics, Proceedings of the 7th International Conference on Sensor Device Technology and Applications (SENSORDEVICES’2016), 2016, pp. 24–28. [54] S.R. Anjum, R.S. Khairnar, Carbon nanotubes blended hydroxyapatite ethanol sensor, Sens. Imaging 17 (1) (2016) 18. [55] L. Sun, C.C. Berndt, K.A. Gross, A. Kucuk, Material fundamentals and clinical performance of plasma-sprayed hydroxyapatite coatings: a review, J. Biomed. Mater. Res. 58 (5) (2001) 570–592. [56] P.K. Chaturvedi, C.S. Seth, V. Misra, Sorption kinetics and leachability of heavy metal from the contaminated soil amended with immobilizing agent (humus soil and hydroxyapatite), Chemosphere 64 (7) (2006) 1109–1114. [57] Y. Feng, J.L. Gong, G.M. Zeng, Q.Y. Niu, H.Y. Zhang, C.G. Niu, M. Yan, Adsorption of cd (II) and Zn (II) from aqueous solutions using magnetic hydroxyapatite nanoparticles as adsorbents, Chem. Eng. J. 162 (2) (2010) 487–494. [58] J. Reichert, J.G.P. Binner, An evaluation of hydroxyapatite-based filters for removal of heavy metal ions from aqueous solutions, J. Mater. Sci. 31 (5) (1996) 1231–1241. [59] X.Y. Zhao, Y.J. Zhu, J. Zhao, B.Q. Lu, F. Chen, C. Qi, J. Wu, Hydroxyapatite nanosheet- assembled microspheres: hemoglobin-templated synthesis and adsorption for heavy metal ions, J. Colloid Interface Sci. 416 (2014) 11–18. [60] A. Corami, S. Mignardi, V. Ferrini, Cadmium removal from single-and multi-metal (Cd+ Pb+ Zn+ Cu) solutions by sorption on hydroxyapatite, J. Colloid Interface Sci. 317 (2) (2008) 402–408. [61] M. Aliabadi, M. Irani, J. Ismaeili, S. Najafzadeh, Design and evaluation of chitosan/ hydroxyapatite composite nanofiber membrane for the removal of heavy metal ions from aqueous solution, J. Taiwan Inst. Chem. Eng. 45 (2) (2014) 518–526. [62] S. Choi, Y. Jeong, The removal of heavy metals in aqueous solution by hydroxyapatite/ cellulose composite, Fibers Polym. 9 (3) (2008) 267–270. [63] A. Corami, S. Mignardi, V. Ferrini, Copper and zinc decontamination from single-and binary-metal solutions using hydroxyapatite, J. Hazard. Mater. 146 (1) (2007) 164–170. [64] I. Mobasherpour, E. Salahi, M. Pazouki, Comparative of the removal of Pb2+, Cd2+ and Ni2+ by nano crystallite hydroxyapatite from aqueous solutions: adsorption isotherm study, Arab. J. Chem. 5 (4) (2012) 439–446. [65] J.G. del Rıo, P.J. Morando, D.S. Cicerone, Natural materials for treatment of industrial effluents: comparative study of the retention of cd, Zn and co by calcite and hydroxyapatite. Part I: batch experiments, J. Environ. Manag. 71 (2) (2004) 169–177. [66] C. Piccirillo, S.I.A. Pereira, A.P. Marques, R.C. Pullar, D.M. Tobaldi, M.E. Pintado, P.M. Castro, Bacteria immobilisation on hydroxyapatite surface for heavy metals removal, J. Environ. Manag. 121 (2013) 87–95. [67] S. Agrawal, D. Tipre, B. Patel, S. Dave, Bacterial decolourization, degradation and detoxification of azo dyes: an eco-friendly approach, in: Microbial Applications, vol. 1, Springer International Publishing, 2017, pp. 91–124. [68] M. Saad, H. Tahir, Synthesis of carbon loaded γ-Fe2O3 nanocomposite and their applicability for the selective removal of binary mixture of dyes by ultrasonic adsorption based on response surface methodology, Ultrason. Sonochem. 36 (2017) 393–408. [69] D. Banerjee, P. Bhowmick, D. Pahari, S. Santra, S. Sarkar, B. Das, K.K. Chattopadhyay, Pseudo first ordered adsorption of noxious textile dyes by low-temperature synthesized amorphous carbon nanotubes, Physica E 87 (2017) 68–76.
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[70] G. Vasugi, A. Thamizhavel, E.K. Girija, Surface modification of hydroxyapatite by yttrium ion substitution for adsorption of reactive red dye, JMES 8 (2) (2017) 714–723. [71] G. Wang, J. Qi, S. Wang, Z. Wei, S. Li, J. Cui, W. Wei, Surface-bound humic acid increased rhodamine B adsorption on nanosized hydroxyapatite, J. Dispers. Sci. Technol. 38 (5) (2017) 632–641. [72] Y. Wang, L. Hu, G. Zhang, T. Yan, L. Yan, Q. Wei, B. Du, Removal of Pb (II) and methylene blue from aqueous solution by magnetic hydroxyapatite-immobilized oxidized multi-walled carbon nanotubes, J. Colloid Interface Sci. 494 (2017) 380–388. [73] N. Barka, S. Qourzal, A. Assabbane, A. Nounah, A.I. Yhya, Adsorption of disperse blue SBL dye by synthesized poorly crystalline hydroxyapatite, J. Environ. Sci. 20 (10) (2008) 1268–1272. [74] H. Hou, R. Zhou, P. Wu, L. Wu, Removal of Congo red dye from aqueous solution with hydroxyapatite/chitosan composite, Chem. Eng. J. 211 (2012) 336–342. [75] A. Hu, M. Li, C. Chang, D. Mao, Preparation and characterization of a titanium- substituted hydroxyapatite photocatalyst, J. Mol. Catal. A Chem. 267 (1) (2007) 79–85. [76] M.P. Reddy, A. Venugopal, M. Subrahmanyam, Hydroxyapatite-supported Ag–TiO2 as Escherichia coli disinfection photocatalyst, Water Res. 41 (2) (2007) 379–386. [77] M.P. Reddy, A. Venugopal, M. Subrahmanyam, Hydroxyapatite photocatalytic degradation of calmagite (an azo dye) in aqueous suspension, Appl. Catal. B Environ. 69 (3) (2007) 164–170. [78] H. Nishikawa, K. Omamiuda, Photocatalytic activity of hydroxyapatite for methyl mercaptane, J. Mol. Catal. A Chem. 179 (1) (2002) 193–200. [79] Z.P. Yang, X.Y. Gong, C.J. Zhang, Recyclable Fe3O4/hydroxyapatite composite nanoparticles for photocatalytic applications, Chem. Eng. J. 165 (1) (2010) 117–121. [80] G. Sheng, L. Qiao, Y. Mou, Preparation of TiO2/hydroxyapatite composite and its photocatalytic degradation of methyl orange, J. Environ. Eng. 137 (7) (2011) 611–616. [81] R.M. Mohamed, E.S. Baeissa, Preparation and characterisation of Pd–TiO2–hydroxyapatite nanoparticles for the photocatalytic degradation of cyanide under visible light, Appl. Catal. A Gen. 464 (2013) 218–224. [82] S. Mondal, E.D.A.R. Ma, U. Pal, Plasmon induced enhanced photocatalytic activity of gold loaded hydroxyapatite nanoparticles for methylene blue degradation under visible light, RSC Adv. 7 (14) (2017) 8633–8645. [83] S. Saber-Samandari, S. Saber-Samandari, M. Gazi, Cellulose-graft-polyacrylamide/ hydroxyapatite composite hydrogel with possible application in removal of cu (II) ions, React. Funct. Polym. 73 (11) (2013) 1523–1530. [84] S. Hokkanen, A. Bhatnagar, E. Repo, S. Lou, M. Sillanpää, Calcium hydroxyapatite microfibrillated cellulose composite as a potential adsorbent for the removal of Cr (VI) from aqueous solution, Chem. Eng. J. 283 (2016) 445–452. [85] N. Gupta, A.K. Kushwaha, M.C. Chattopadhyaya, Adsorptive removal of Pb2+, Co2+ and Ni2+ by hydroxyapatite/chitosan composite from aqueous solution, J. Taiwan Inst. Chem. Eng. 43 (1) (2012) 125–131. [86] K. Sangeetha, G. Vasugi, E.K. Girija, Removal of lead ions from aqueous solution by novel hydroxyapatite/alginate/gelatin composites, Int. J. ChemTech Res. 8 (5) (2015) 117–125.
Further reading [1] F. Googerdchian, A. Moheb, R. Emadi, Lead sorption properties of nanohydroxyapatite– alginate composite adsorbents, Chem. Eng. J. 200 (2012) 471–479.
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R.S. Khairnar⁎, V.N. Narwade⁎,†, V. Kokol† ⁎ Swami Ramanand Teerth Marathwada University, Nanded, India, †University of Maribor, Maribor, Slovenia
Abstract Ceramics materials are inherent part of our life, and can be classified as inorganic and nonmetallic materials. The ceramics is an inorganic, nonmetallic solid material comprising metal, nonmetal, or metalloid atoms principally held in ionic and covalent bonds. This chapter is about the special class of ceramic material called hydroxyapatite which is generally well known for the biomedical applications. Here we have shown the promising applications of hydroxyapatite in wastewater treatment, that is, heavy metal and dye adsorption with special emphasis on synthesis methodologies. Keywords: Ceramic materials, Hydroxyapatite, Hydrothermal, Precipitation, Irradiation, Adsorption.
10.1 Introduction Ceramics materials are inherent part of our life, and can be classified as inorganic and nonmetallic materials. The ceramics is an inorganic, nonmetallic solid material comprising metal, nonmetal, or metalloid atoms principally held in ionic and covalent bonds. Ceramics are incredibly diverse family of materials whose members span conventional ceramics (such as pottery and refractories) to the modern-day engineering ceramics (such as alumina and silicon nitride) found in electronic devices, aerospace components, and cutting tools [1–8]. Ceramics exhibit very strong ionic and/or covalent bonding (stronger than the metallic bond) and this confirms their unique properties such as high hardness, high compression strength, low thermal and electrical conductivity, and chemical inertness [9–13]. Ceramic materials, specifically developed for their usage in medical and dental implants, are termed bioceramics [13–15]. They include various compounds of alumina and zirconia, bioactive glasses, glass- ceramics, coatings and composites, hydroxyapatite (HAp), and resorbable calcium phosphates [16–20]. Bioceramics are ceramics materials that are biocompatible, its important subclass of biomaterials. These ceramics products or components are mainly used in bone implants and replacements. Bioceramics materials are commonly subdivided by their bioactivity. Bioinert materials (such as oxide ceramics, silica ceramics, and carbon fiber) Fundamental Biomaterials: Ceramics. https://doi.org/10.1016/B978-0-08-102203-0.00010-X Copyright © 2018 Elsevier Ltd. All rights reserved.
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Table 10.1 Various calcium-phosphate compounds, including hydroxyapatite, showing the importance of variations in Ca/P ratio in their structural aspect S. no. 1 2 3 4 5 6 7 8
Name
Symbol
Formula
Ca/P
References
Monocalcium phosphate monohydrate Dicalcium phosphate dehydrate Dicalcium phosphate anhydrous Octacalcium phosphate
(MCPM) and (MCPH) (DCPD)
Ca(H2PO4)2·H2O
0.5
[22]
CaHPO4·2H2O
1.0
[23]
CaHPO4
1.0
[23]
Ca8(HPO4)2 (PO4)4·5H2O Ca3(PO4)2 Ca3(PO4)2 Cax(PO4)y·nH2O
1.33
[23]
1.5 1.5 1.2– 2.2 1.66
[23] [23] [23]
α-Tricalcium phosphate β-Tricalcium phosphate Amorphous calcium phosphate Hydroxyapatite
(DCPA) and (DCP) (OCP) (α-TCP) (β-TCP) (ACP) (HA) and (HAp)
Ca10(PO4)6(OH)2
[23]
are nontoxic and noninflammatory. These materials must be long lasting, structural failures resistant, and corrosion resistant. Bioceramics, additionally, must have a low Young’s modulus to help prevent cracking of the material [21]. Table 10.1 shows the most important CaP salts, indicating variations in Ca/P ratio. This chapter is dedicated to a special class of ceramics, particularly termed as nanostructured bioceramics, with emphasis on its applications in waste water treatment. The main object is to utilize HAp nanoceramics, in particular, calcium apatite family, for its application in removal of impurities dissolved in water.
10.2 Hydroxyapatite (HAp) HAp is a mineral bioceramic, comprising complex calcium phosphates, which is also a chief structural element of vertebrate bone. Hence, it is popular material for biomedical applications such as a bone substitute material in orthopedics and dentistry due to its excellent biocompatibility, bioactivity, and osteoconduction properties [24–28].
10.2.1 Structure of HAp HAp crystallizes in a hexagonal system, although with some exception in a monoclinic system (Fig. 10.1) [29]. The system belongs to hexagonal space group P6 3/m, with hexagonal rotational symmetry and reflection plane. The lattice parameters of unit cell of HAp are a-axis = 0.9422 nm and c-axis = 0.688 nm [30]. HAp structure is formed by tetrahedral arrangement of phosphate (PO43−), which constitutes the s keleton of unit cell.
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Calcium
Oxygen
Hydrogen
Phosphorous
Fig. 10.1 Unit cell of Hydroxyapatite, view along c-axis (http://molview.org/).
Unit cell of HAp with 44 atoms per unit cell is made of Ca2+, PO43−, and OH− ionic groups arranged together in hexagonal form.
10.2.2 Synthesis of HAp During past few decades, considerable research efforts have been directed toward the synthesis of HAp bioceramic by utilizing several methods. The popular methods of synthesis of HAp include wet chemical precipitation, sol-gel method, hydrothermal method, microwave irradiation method, etc.
10.2.2.1 Wet chemical precipitation method The wet chemical method precipitation process is the most convenient and commonly used process. This process is simple and easy to use. The preparative reaction and the character of reaction product can be regulated. Wet chemical reactions have advantages to control the morphology and the mean size of crystallites, due to which it is the most promising technique for the fabrication of nanosized HAp. Mahabole et al. [31] performed wet chemical method to synthesize HAp, in calcium nitrate, diammonium hydrogen phosphate, and ammonium hydroxide were used as precursors. The stoichiometric ratio of the calcium nitrate and diammonium hydrogen phosphate solutions was adjusted so as to get theoretical (Ca/P) molar ratio close to 1.66 [23]. The precipitation was performed by slow addition of diammonium phosphate solution (0.6 M) to calcium nitrate solution (1 M) under continuous and gentle stirring, while maintaining the pH of reaction mixture by the addition of NH4OH. As a result of reaction, milky precipitation was obtained. The precipitate was continuously stirred for 24 h using magnetic stirrer. The resulting white precipitate was washed thoroughly several times with double distilled water, oven dried at about 100 °C, and crushed into powder form. The grain size of HAp, in the range of 0.032 μm, can be produced by this technique.
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(NH4)2HPO4 (0.6 M)
NH4 OH
Ca(NO3)2.4 H2O (1 M)
80°C
Fig. 10.2 Schematic representation of wet chemical precipitation method.
This method was modified [32] by carrying out chemical reaction at elevated temperature of 80°C during precipitation formation for considerable amount of time followed by gelation for 24 h. The resulting white precipitate, after thoroughly rinsing with deionized water, was dried by heating in air 100°C for brief time and was later sintered at 1000°C for 2 h in PID-controlled furnace. The average crystallite size obtained was in the range of 15 nm for synthesized HAp. This process gives hexagonal structure for synthesized HAp. The following simplified equation shows chemical reaction adopted for HAp synthesis by means of wet chemical precipitation method (Fig. 10.2). 10Ca ( NO3 )2 + 6 ( NH 4 )2 HPO 4 + 8NH 4 OH ® Ca10 ( PO 4 )6 ( OH )2 + 20 NH 4 NO3 + 6H 2 O
(10.1)
10.2.2.2 Sol-gel method Sol-gel technique has attracted much attention recently due to its well-known inherent advantages to generate glass, glass ceramics, and ceramic powders. The sol-gel process is easily applicable to surface coating and it allows the preparation of high-quality HAp thin films on metal substrates. The sol-gel process can be utilized to synthesize both HAp thin films and HAp powder under significantly mild conditions. Agrawal et al. [33], synthesized HAp powder by sol-gel method, in which two different chemical reagent were used. At first, phosphoric pentoxide (P2O5) was dissolved in absolute ethanol to form 0.5 mol/L solution and second calcium nitrate tetrahydrate Ca(NO3)2·4H2O was dissolved in ethanol to form 1.67 mol/L solution.
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Both the solutions were mixed to obtain the desired Ca/P molar ratio of 1.67. The solution was stirred slowly for 10–15 h until the formation of gel. Further the gel was dried in an electric oven at 80°C in air for 20 h, followed by two-stage heat treatment in air starting from 400°C to 750°C for 8 h. Authors reported that this study presents an alternative method to form pure, stable, crystalline nanosized HAp powder at low temperature as compared to other existing methods where temperature of treatment is more than 800°C to achieve all HAp characteristics. Liu et al. [34] showed another procedure for HAp synthesis by sol-gel method. Triethyl phosphite sol was diluted in anhydrous ethanol and then a small amount of distilled water was added for hydrolysis. The molar ratio of water to the phosphorus precursor was kept at 3. The mixture was sealed in a glass beaker immediately after solvent addition and then stirred vigorously. The mixture initially appears opaque, due to the immiscibility among phosphate and water. However, after approximately 30 min of mixing, the emulsion transformed into a clear solution, suggesting that the phosphite was completely hydrolyzed. This was confirmed by the loss of phosphite odor of the mixture. A stoichiometric amount (i.e., to maintain Ca/P = 1.67) of 3 M calcium nitrate dissolved in anhydrous ethanol was subsequently added dropwise into the hydrolyzed phosphorus sol. Vigorous stirring was continued for an additional 10 min after the titration. As a result of this process, a clear solution was obtained and which was later aged at room temperature for 16 h before drying. The solvents were then dried at 60°C until a viscous liquid was obtained. The corresponding HAp concentration changed from 3.6 vol% (in the solution) to 13.6 vol% (in viscous liquid). Further drying of the viscous liquid at 60°C resulted in a white gel. The gel was ground with a mortar and pestle into fine powder and subjected to different calcination temperatures, from 300°C to 800°C with 25–50°C intervals, for 2 h. This study demonstrated a new way for synthesis of HAp ceramic via a novel low-temperature sol-gel process. HAp phase can be synthesized at a temperature as low as 350°C, which is lower than those reported in literature for alternative sol-gel routes to HAp (200–300°C).
10.2.2.3 Hydrothermal method The hydrothermal method is often used to prepare HAp with good crystallinity and homogeneous size and shape. Hydrothermal synthesis can also simply be considered as a chemical precipitation in which the aging step is conducted at a high temperature, typically above the boiling point of water, inside an autoclave or pressure vessel (Fig. 10.3). Jingbing Liu et al. [35] carried out hydrothermal synthesis process for HAp whiskers and crystal synthesis. The initial reaction substances of CaHPO4·2H2O (0.5162 g) and Ca(OH)2 (0.1482 g) were dissolved in 40-mL deionized water. To investigate the influence of pH value, a series of experiments were devised. The pH of initial solution was varied by means of CH3COOH. For higher pH values KOH is used. The ready-adjusted solutions with certain pH value were poured in a teflon vessel and heated at temperature 140°C for 24 h at its self-generated pressure, and then cooled to room temperature. The precipitates were filtered and washed with deionized water till the pH turned to 7. Finally, the filtrates were dried in vacuum at 60–100°C for 2 h. It is observed that the
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Conditions Spherical Ca2+ Rod like
Dandelion like PO43− Chemicals
Teflon liner
Autoclave reactor
Hexagonal prism Plate like
Fig. 10.3 Schematic representation of hydrothermal method used for synthesis of different HAp nanostructures.
pH value of the starting reaction solution and the temperature of hydrothermal treatment are the most significant variables in altering the HAp structure and morphology. Whiskers with high aspect ratio were obtained from the starting reaction solution having 9 pH, at the relatively low temperature of 120°C. Zhu et al. [36] synthesized HAp using EDTA as chelate. First, 1 mL of NH4OH, 0.169 g of Ca(NO3)2, 0.209 g of EDTA, and 0.065 g of KH2PO4 were dissolved into 22 mL of water, so as to get the mole ratio of EDTA/Ca as 1. Aqueous solution of KOH was used to adjust the pH value of the solution to 12.9, 11.4, and 9.7 separately. After that solutions were sealed in Teflon-lined stainless autoclave and heated at different temperature for predetermined time and then cooled to room temperature. The products were washed and filtrated, and dried at 60°C in air. HAp crystals with prismlike morphology have been successfully prepared by using EDTA as chelate under hydrothermal condition. The results showed that the products were pure HAp with preferential orientation along c-axis.
10.2.2.4 Microwave irradiation method The synthesis of HAp by microwave irradiation method is also reported [37]. In such synthesis process of HAp, high-purity (CaNO3)4H2O and Na2HPO4 were used as the starting materials. EDTA served as the complex reagent with molar ratio of (CaNO3)4H2O:EDTA:Na2HPO4 as 5:5:3. In a typical procedure, 50 mL of a mixed solution of (CaNO3)4H2O (0.1 M) and EDTA (0.1 M) was introduced into 50 mL of Na2HPO4 (0.06 M) solution. Since the initial pH of such prepared was 4, it was adjusted to higher pH in the range of 9–13 by adding NaOH solution (0.1 M). After stirring for 10 min, these ready-adjusted aqueous solutions, with certain pH, were
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put into a household-type microwave oven of 700-W powers with a refluxing system and the reaction was performed under ambient air for 30 min. Typically, microwave oven followed a working cycle of 6 s on and 10 s off (37% power), to irradiate the solutions by microwave radiations. After cooling to room temperature, the precipitate was centrifuged, washed with deionized water, and dried in vacuum at 70°C for 2 h. The synthesized HAp showed to exhibit nanorods, bow-knot-like and flower-like nanostructures.
10.2.3 Properties of HAp These are some noteworthy properties of HAp, which leads to HAp as useful ceramics. HAp has various meritorious properties, viz. biocompatibility [38], nontoxicity [39], chemical inertness [40], and ion exchange capacity [41]. Carbonate is easily sorbed by synthetic HAp and participates in the structure of biological apatite with substituting either PO43− and OH−. This substitution causes important changes in the apatite unit-cell volume, crystal morphology, solubility, thermal stability, and bioactivity [42]. The ionic conductivity of calcium HAp increases with addition of fluoride up to 50% replacement of the OH– by F− ions. Besides these, high porosity is the key property for HAp for its application in various fields [43,44].
10.3 Applications of HAp Due to useful efficient properties as mentioned earlier, HAp is widely used in the diverse fields. The potential applications include fuel cells because HAp can be anticipated as an electrolyte for high temperature in fuel cell [45,46]. HAp is used for many practical applications, to name a few are (i) catalyst [47], (ii) biomaterial [48], (iii) gas sensors [49–54], and (iv) bioceramic coatings [55]. Coatings of HAp are often applied to metallic implants (most commonly titanium/titanium alloys and stainless steels) to alter the surface properties. In this manner, the body sees HAp-type material, which is readily acceptable in human body plasma. Also HAp is used for water purification due to presence of pores and extraordinary ion-exchange capacity. HAp seems to be suitable substrate for adsorption of heavy metals and organic dyes.
10.4 Applications of HAp in heavy metal/dyes/organic pollutant adsorption One of the recent and interesting applications of HAp is its utilization in water purification as well as removal of impurities from industrially discharges, typically, heavy metal and organic/dyes dissolved liquid solutions. Heavy metals are naturally occurring elements that have a high atomic weight and a density at least five times greater than that of water. Their multiple industrial, domestic, agricultural, medical, and technological applications have led to their wide
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distribution in the environment, raising concerns over their potential effects on human health and the environment. Their toxicity depends on several factors including the dose, route of exposure, and chemical species, as well as the age, gender, genetics, and nutritional status of exposed individuals. Because of their high degree of toxicity, arsenic, cadmium, chromium, lead, and mercury rank among the priority metals that are of significance from the point of view of public health. These metallic elements are considered systemic toxicants that are known to induce multiple organ damage, even at lower levels of exposure. They are also classified as human carcinogens (known or probable) according to the U.S. Environmental Protection Agency, and the International Agency for Research on Cancer. Many studies have acknowledged the ability of HAp [Ca10(PO4)6(OH)2] to combine with divalent heavy metal ions. Earlier studies have shown that synthetic HAp has a high removal capacity for Pb, Zn, Cu, Cd, Co, and Sb from aqueous solutions [56–66]. Mechanisms such as ion exchange [41], surface complexation [30], dissolution of HA and precipitation of metal phosphates [30], substitution of Ca in HAp by metals during recrystallization (coprecipitation) have been proposed in order to describe the uptake of heavy metals from aqueous solutions by synthetic HAp. However, controversy connected with the relative contribution of each process, in removing heavy metals, still exists in the literature. Synthetic dyestuffs widely exist in the effluents of industries such as textiles, printing, paper, plastics, and leather. Many dyes and pigments contain aromatic rings in their structures, which make them toxic, nonbiodegradable, carcinogenic, and mutagenic for aquatic systems and human health [67–69]. Because of their accumulative effects in biota, the presence of dyes even at very low concentrations in water is not allowed. Moreover, dyes cannot be easily removed with the conventional wastewater treatment methods since they are difficult to be biodegraded or photodegraded. Therefore, removing dyes from aqueous solution has become an important and challenging area in wastewater treatment. HAp is used efficiently for dyes adsorption and is known as a prominent ceramic adsorbents. The reported literature showed that the various types of dyes can be adsorbed using HAp, such as Congo red, blue SBL dye, Reactive yellow 84, Methylene blue, etc. Moreover, HAp also demonstrated excellent adsorption capabilities for phenol and its derivatives [70–74]. Additionally, HAp is also proved to be a desirable candidate as a photocatalyst, for degradation of dyes and organic contaminants [75–79]. To improve the adsorption capacities of HAp, various types are dopants are utilized, which increases the functionality of adsorbent material. TiO2 is widely used dopant for enhancing photocatalytic activity of HAp. Various researchers worked on TiO2-HAp as a composite material for dyes and organic contaminants degradation [80,81]. Gold-HAp system is used as photocatalyst for methylene blue dye degradation under visible light [82]. Various polymer-supported HAp composites are also adopted to utilize for heavy metal and dye adsorption. Polymer-HAp have also shown to provide a new platform for their applicability in adsorption. Cellulose-HAp [62,83,84], chitosan-HAp [61,85], gelation-HAp [86], alginate-HAp are some of the examples of HAp composite, which are reported as future composite adsorbents.
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10.5 Conclusions HAp bioceramic is important class of ceramic materials. HAp can be successfully synthesized by various methods and these techniques are found useful in different applications depending on their morphology. HAp has the important role in heavy metal adsorption/dye adsorption due to their ion exchange, surface complexation, and dissolution properties. HAp is also well known for biomedical application due to chemical similarities with bone chemistry. Thus HAp has shown its applicability as multipurpose ceramic material.
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Further reading [1] F. Googerdchian, A. Moheb, R. Emadi, Lead sorption properties of nanohydroxyapatite– alginate composite adsorbents, Chem. Eng. J. 200 (2012) 471–479.