Mankind's impact on the environment today has both direct and indirect implications on our quality of life. Today, significant efforts are being undertaken to raise awareness of the possible harm that anthropogenic influences may do to the ecosystem and methods that this harm may be minimized. Implementing education on eco-friendly behavior, such as recycling, cutting back on consumption and switching to renewable energy sources, is one of these attempts [1]. Sustainability is the overall expression that describes all of these strategies for minimizing the negative environmental effects of industries, eliminating hazardous wastes and not only turning it into useful materials but also setting up greener synthetic routes to recycle it. As calcium-phosphate-based bioceramic materials, hydroxyapatite (HAP, Ca10(OH)2(PO)6, Ca/P = 1.67) is the primary inorganic component of bone and teeth and provides them their high stability, hardness and function. Hence; it is extremely important, particularly in the biomedical sector and tissue engineering [2]. HAP is a naturally occurring mineral with notable characteristics, including biocompatibility, water insolubility, thermal and chemical stability and structural flexibility[3]. Its highly functionalized surface features both acidic and basic groups, making it amphoteric and enabling a wide range of applications, ranging from biomedicine to catalysis[3]. Its surface properties showed promising outcomes, signifying the potential importance of HAP in various domains [4]. These exceptional attributes have captured the attention of researchers to make extensive investigations on the synthesis of HAP from low-cost wastes. HAP may be produced using inexpensive and easily accessible mostly Ca-based minerals, e.g., gypsum [5] and biological wastes such as animal bones[6], Fish scales[7] and eggshells [8]. There are several HAP preparation techniques, which may be divided into dry and wet preparation routes. The term "dry preparation" most often refers to the solid-phase reaction preparation between raw material powders (e.g. CaHPO4 and CaO) via techniques such as thermal treatment, mechanochemical and plasma spray [9]. Contrarily, there are a variety of wet preparation techniques, including chemical precipitation [10], hydrothermal [11], microemulsion [12], calcination [13] and sol-gel [14].
To achieve sustainability and efficiency, there is a growing emphasis on using industrial by-products and bio-waste as raw materials for nano-HAP synthesis. This approach mitigates environmental impacts and introduces unique functionalities, expanding the potential applications. In this study, three different calcium-based sources of HAP from industrial and biological wastes, including Cement Kiln Dust (CKD), egg shells and Buffalo bone were employed and investigated.
The cement industry generates substantial amounts of hazardous waste known as CKD, which poses risks to ecosystems and organisms due to its alkalinity and heavy metal concentration [15]. The CEMEX cement plant near Assiut City in Upper Egypt is a substantial contributor to CKD generation. For every ton of cement clinker produced, a significant amount of CKD, ranging from 54 to 200 kg, is generated [16]. Research in this area aims to propose effective strategies and protocols for managing industrial waste, mitigating environmental pollution and achieving a balance between environmental preservation and resource utilization through waste recycling. Therefore, the current study also explores the economic potential of harnessing CKD, as a potentially hazardous byproduct, to produce valuable resources like HAP, thus mitigating the cement industry's environmental footprint [17]. For eggshell utilization, chicken eggshells mainly consist of calcium carbonate (CaCO3, 94%), calcium phosphate (1%), organic matter (4%) and magnesium carbonate (1%). The choice of eggshells as a resource is driven by their high calcium content, making them suitable for the synthesis of HAP [18]. For bone utilization, bones compose of 30% organics and 70% inorganics with a mineral phase consisting a nonstoichiometric HAP, which has a variable Ca/P molar ratio (less or more than 1.67) and additional minerals, including Fe2+, Na2+, Mg2+, Zn2+, Si2+, Ba2+ and CO3 − 2 [19]. Extraction of a stoichiometric bioactive HAP from animal bones is a crucial challenge where its bioactivity is largely influenced by the proportion of calcium, phosphate (Ca/P)[19]. In general, few methods of extracting hydroxyapatite from animal bones such as thermal decomposition, subcritical water process and alkaline hydrolysis have been studied [20]. The current work is aimed to investigate the feasible preparation of HAP nanoparticles from calcium-based local wastes that disposed in large quantities such as cement kiln dust (CKD), eggshells and buffalo bones under different methods. The following tools such as X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric (TG), N2 adsorption-desorption at -196oC and scanning electron microscope (SEM) analyses were all used to examine the final properties of synthesized HAP compared with the purchased commercial HAP.