Applications of Nano Hydroxyapatite as Adsorbents: A Review

Composites of hydroxyapatite and their application in adsorption, medicine and as catalysts

Composites of hydroxyapatite, recognized by its peculiar crystal architecture and distinctive attributes showcased the potential in adsorbing heavy metal ions and radioactive elements as well as selected organic substances. In this paper, the intrinsic mechanism of adsorption by composites hydroxyapatite was proved for the first time. Subsequently, selectivity and competitiveness of composites of hydroxyapatite for a variety of environments containing various interferences from cations, anions, and organic molecules are elucidated. Next, composites of hydroxyapatite were further categorized according to their morphological dimensions. Adsorption properties and intrinsic mechanisms were investigated based on different morphologies. It was shown that although composites of hydroxyapatite were characterized by excellent adsorption capacity and cost-effectiveness, their application is often challenging due to inherent fragility and agglomeration, technical problems required for their handling as well as difficulty in recycling. Finally, to address these issues, the paper discusses the tendency of hydroxyapatite composites to adsorb heavy metal ions and radioactive elements as well as the limitations of their applications. Summarizing the limitations and future directions of modification of HAP in the field of heavy metal ions and different substances contamination abatement, the paper provides insightful perspectives for its gradual improvement and rational application.

Design, characterization and implementation of cost-effective sodium alginate/water hyacinth microspheres for remediation of lead and cadmium from wastewater

The aquatic plant water hyacinth was dried then cross-linked with sodium alginate to produce ionic cross-linked microspheres. The mechanism of controlling cadmium (Cd) and lead (Pb) in wastewater was tested by DFT at B3LYP level using LANL2DZ basis set. Modeling results indicated that the hydrated metals could interact with sodium alginate (SA)/water hyacinth (WH) microspheres through hydrogen bonding. Adsorption energies showed comparable results while total dipole moment and HOMO/LUMO band gap energy showed slight selectivity towards the remediation of Pb. FTIR spectra of cross-linked microspheres indicated that WH is forming a composite with SA to change its structure into a microsphere to remove Cd and Pb from water. Raman mapping revealed that the active sites along the surface of the microspheres enable for possible adsorption of metals through its surface. This finding is supported by molecular electrostatic potential and optical confocal microscopy. Atomic absorption spectroscopy results confirmed that the microspheres are more selective for Pb than Cd. It could be concluded that WH cross-linked with SA showed the potential to remove heavy metals through its unique active surface as confirmed by both molecular modeling and experimental findings.

Ultrasound-assisted formation of composite materials from fish scale waste hydroxyapatite in the presence of gamma-irradiated chitosan for the removal of malachite green

The fish processing sector produces millions of tons of trash annually-a biologically dangerous substance that could eventually turn into a source of pathogenic contamination. This work successfully shows how to extract tilapia fish scale hydroxyapatite with ultrasonic assistance and modify it using gamma-irradiated chitosan to remove malachite green from water samples. The prepared adsorbent was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, X-ray fluorescence, scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, thermogravimetric analysis and dynamic light scattering. Isotherm modeling was employed to investigate the sorption process of malachite green. The results revealed that the adsorbent could be used to remove malachite green in aqueous media, with a maximum adsorption capacity of 285.7 mg g-1. A pseudo-second-order model was then fitted to the kinetic data. The R 2 value of 0.9851 obtained indicated that the adsorption behavior was consistent with the Langmuir model. Analysis of the computed thermodynamic parameters revealed that the adsorption of the dye was a spontaneous and exothermic process. Proper waste management practices not only ensure environmental responsibility but also contribute to positive community relations by minimizing the impact on the local environment.

Synthesis of High-Purity Hydroxyapatite and Phosphoric Acid Derived from Moroccan Natural Phosphate Rocks by Minimizing Cation Content Using Dissolution-Precipitation Technique

This study investigates, in the first part, the synthesis and purification of a poorly crystalline hydroxyapatite (HAp) using natural Moroccan phosphate (Boucraa region) as a raw material. Despite its successful preparation, the obtained HAp was contaminated by several metallic cations (mostly Cd, Pb, Sn, Ti, Mn, Mg, Fe, and Al) migrated from the natural rocks during the digestion process, inhibiting HAp application in several sectors. To minimize the existence of these elements, the dissolution-precipitation technique (DP) was investigated as a non-selective purification process. Following the initial DP cycle conducted on the precipitated HAp, the removal efficiency was approximately 60% for Al, Fe, Mg, Mn, and Ti and 90% for Cd and Pb. After three consecutive DP cycles, notable improvement in the removal efficiency was observed, reaching 66% for Fe, 69% for Mg, 73% for Mn, and 74% for Al, while Cd, Pb, and Ti were totally removed. In the second part of this study, the purified HAp was digested using sulfuric acid to produce high-quality phosphoric acid (PA) and gypsum (GP). The elemental analysis of the PA indicates a removal efficiency of approximately 89% for Fe and over 94% for all the examined cations. In addition, the generated GP was dominated by SO3 and CaO accompanied with minor impurities. Overall, this simple process proves to be practically useful, to reduce a broad spectrum of cationic impurities, and to be flexible to prepare valuable products such hydroxyapatite, phosphoric acid, and gypsum.

Meso-Macroporous Hydroxyapatite Powders Synthesized in Polyvinyl Alcohol or Polyvinylpyrrolidone Media

Mesoporous hydroxyapatite (HA) is widely used in various applications, such as the biomedical field, as a catalytic, as a sensor, and many others. The aim of this work was to obtain HA powders by means of chemical precipitation in a medium containing a polymer-polyvinyl alcohol or polyvinylpyrrolidone (PVP)-with concentrations ranging from 0 to 10%. The HA powders were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, atomic emission spectroscopy with inductively coupled plasma, electron paramagnetic resonance, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The specific surface area (SSA), pore volume, and pore size distributions were determined by low-temperature nitrogen adsorption measurements, and the zeta potential was established. The formation of macropores in powder agglomerates was determined using SEM and TEM. The synthesis in 10% PVP increased the SSA from 101.3 to 158.0 m2/g, while the ripening for 7 days led to an increase from 112.3 to 195.8 m2/g, with the total pore volume rising from 0.37 to 0.71 cm3/g. These materials could be classified as meso-macroporous HA. Such materials can serve as the basis for various applications requiring improved textural properties and may lay the foundation for the creation of bulk 3D materials using a technique that allows for the preservation of their unique pore structure.

Enhancing Bioactivity and Mechanical Properties of Nano-Hydroxyapatite Derived from Oyster Shells through Hydrothermal Synthesis

Nano-hydroxyapatite (nHA) demonstrates favorable biological activity, cell adhesion, cell proliferation, and osteoconductivity, making it highly valuable in biomedicine. It is extensively used as a bone substitute and in bone transplantation within the dental and orthopedic fields. This study employed oyster shells as a calcium source to synthesize nHA at 150 °C with various hydrothermal reaction durations (10 min, 1 h, 6 h, and 12 h). As a control, HA synthesized via a wet precipitation method for 1 h at room temperature was utilized. Subsequent material analyses, including XRD, FE-SEM, FTIR, and ICP-MS, were conducted, followed by comprehensive evaluations of the bioactivity, cell attachment, cell proliferation, and sintering properties of the synthesized nHA. The results indicated that nHA synthesized through the hydrothermal reaction produced nanoscale crystals, with the aspect ratio of nHA particles increasing with the duration of hydrothermal treatment. Notably, rod-like nHA particles became prominent with hydrothermal durations exceeding 6 h. nHA particles derived from oyster shells contained carbonate and trace elements (Na, Mg, K, and Sr), similar to constituents found in human hard tissue such as bone and teeth. The immersion of nHA synthesized at 150 °C for 1 h (HT2) in simulated body fluid (SBF) for 28 d led to the formation of a bone-like apatite layer on the surface, indicating the excellent bioactivity of the synthesized nHA. The cell culture results revealed superior cell attachment and proliferation for nHA (HT2). Following the sequential formation and sintering at 1200 °C for 4 h, HT2 ceramics exhibited enhanced microhardness (5.65 GPa) and fracture toughness (1.23 MPa·m0.5), surpassing those of human tooth enamel.

Deterioration phenomenon of Pb-contaminated aqueous solution remediation and enhancement mechanism of nano-hydroxyapatite-assisted biomineralization

Modern metallurgical and smelting activities discharge the lead-containing wastewater, causing serious threats to human health. Bacteria and urease applied to microbial-induced carbonate precipitation (MICP) and enzyme-induced carbonate precipitation (EICP) are denatured under high Pb2+ concentration. The nano-hydroxyapatite (nHAP)-assisted biomineralization technology was applied in this study for Pb immobilization. Results showed that the extracellular polymers and cell membranes failed to secure the urease activity when subjected to 60 mM Pb2+. The immobilization efficiency dropped to below 50% under MICP, whereas it due to a lack of extracellular polymers and cell membranes dropped to below 30% under EICP. nHAP prevented the attachment of Pb2+ either through competing with bacteria and urease or promoting Ca2+/Pb2+ ion exchange. Furthermore, CO32- from ureolysis replaced the hydroxyl (-OH) in hydroxylpyromorphite to encourage the formation of carbonate-bearing hydroxylpyromorphite of higher stability (Pb10(PO4)6CO3). Moreover, nHAP application overcame an inability to provide nucleation sites by urease. As a result, the immobilization efficiency, when subjected to 60 mM Pb2+, elevated to above 80% under MICP-nHAP and to some 70% under EICP-nHAP. The findings highlight the potential of applying the nHAP-assisted biomineralization technology to Pb-containing water bodies remediation.

Multicomponent synthesis via acceptorless alcohol dehydrogenation: an easy access to tri-substituted pyridines

Herein, we report palladium supported on a hydroxyapatite catalyst for synthesizing tri-substituted pyridines using ammonium acetate as the nitrogen source via acceptorless alcohol dehydrogenation strategy. The strategy offers a broad substrate scope using inexpensive and readily available alcohols as the starting material. The catalyst was prepared using a simple method and analyzed by several techniques, including FE-SEM, EDS, HR-TEM, BET, XRD, FT-IR, UV-visible spectroscopy, and XPS, demonstrating the anchoring of Pd nanoparticles on hydroxyapatite in the zero oxidation state. Moreover, several controlled experiments were carried out to understand the reaction pathway and a suitable mechanism has been proposed.

Carbon quantum dots-containing poly(β-cyclodextrin) for simultaneous removal and detection of metal ions from water

This study investigates the synthesis and characterization of supramolecular composites composed of poly(β-cyclodextrin-co-citric acid) and carbon quantum dots (QDs). These composites serve a dual purpose as adsorbents and photoluminescent probes for divalent metal ions, including Ni(II), Cu(II), Cd(II), and Pb(II), which can have detrimental effects on the environment. Various characterization techniques were employed to confirm the successful synthesis of the composites and the interaction between cyclodextrins and QDs. By using mathematical tools, optimal conditions for metal adsorption were determined, resulting in the composites exhibiting high adsorption capacities, reaching 220 mg/g, and impressive removal efficiencies exceeding 90 % for Ni(II) and Cu(II). The supramolecular composites also exhibit selective adsorption of metal ions with small ionic radio and can be reused with minimal loss of efficiency. In addition to their adsorption capabilities, these composites display luminescence quenching upon the adsorption of metal ions, which can be utilized for sensing applications. Spectroscopic evaluation reveals Stern-Volmer quenching constants for the accessible fraction of QDs in the range of 3777 to 13,359 M-1. The high stability of QDs on the composites allows for long-term storage. In summary, this original supramolecular composite shows promise for simultaneously monitoring and treating water and wastewater, making it a valuable tool in environmental applications.

S-Alkylation of dithiocarbamates via a hydrogen borrowing reaction strategy using alcohols as alkylating agents

Herein, we report an operationally simple, environmentally benign and scalable approach towards the synthesis of S-benzyl/alkyl dithiocarbamates via a hydrogen borrowing reaction between alcohols and dithiocarbamate anions catalyzed using a hydroxyapatite-supported copper nano-catalyst. This strategy has a broad substrate scope and offers high yields of products using inexpensive and readily available alcohols as starting materials. The catalyst was prepared by easy and straightforward methods and analyzed by several analytical techniques, e.g., FESEM, HR-TEM, BET, XRD, EDS, and XPS, demonstrating the anchoring of Cu nanoparticles on hydroxyapatite in the zero oxidation state.

Activated Carbon and Clay Pellets Coated with Hydroxyapatite for Heavy Metal Removal: Characterization, Adsorption, and Regeneration

In the present work, activated-carbon-containing pellets were preparedby direct chemical activation of sawdust, using clays as a binder. The obtained pellets (ACC) were coated with hydroxyapatite (HAp) nanoparticles (ACC-HAp) to improve adsorption towards Pb(II), Cu(II), Zn(II), and Ni(II). The pellets were characterized by scanning electron microscopy (SEM), by Fourier transform infrared spectroscopy (FTIR), and with a gas sorptometer. The effect of pH, contact time, and initial concentration on adsorption performance was investigated. Additionally, desorption studies were performed, and the regeneration influence on compressive strength and repeated Pb(II) adsorption was investigated. The results showed that, after coating ACC pellets with HAp nanoparticles, the adsorption capacity increased for all applied heavy metal ions. Pb(II) was adsorbed the most, and the best results were achieved at pH 6. The adsorption process followed the pseudo-second-order kinetic model. The adsorption isotherm of Pb(II) is better fitted to the Langmuir model, showing the maximum adsorption capacity of 56 and 47 mg/g by ACC-HAp and ACC pellets, respectively. The desorption efficiency of Pb(II)-loaded ACC-HAp pellets increased by lowering the pH of the acid, resulting in the dissolution of the HAp coating. The best desorption results were achieved with HCl at pH 1 and 1.5. Therefore, the regeneration procedure consisted of desorption, rinsing with distilled water, and re-coating with HAp nanoparticles. After the regeneration process, the Pb(II) adsorption was not affected. However, the desorption stage within the regeneration process decreased the compressive strength of the pellets.

The Removal of Pertechnetate from Aqueous Solution by Synthetic Hydroxyapatite: The Role of Reduction Reagents and Organic Ligands

The use of knowledge from technetium radiochemistry (even from nuclear medicine applications) allows us to select an sorbent for 99mTc radionuclide sorption, which is hydroxyapatite. Using radioisotope indication, the 99mTcO₄- sorption process on synthetic hydroxyapatite was studied by the batch method in the presence of SnCl2 and FeSO4 reducing agents. The complexing organic ligands' effect on the 99mTcO₄- sorption under reducing conditions was investigated. In the presence of Sn2+ ions without the addition of organic ligand, the sorption percentage reached above 90% independently of the environment. In the presence of Fe2+ ions without the addition of organic ligand, the sorption of 99mTcO₄- was significantly lower and was at approximately 6%, depending on the concentration of Fe2+ ions in solution. The effect of complexing organic ligands on the 99mTcO₄- sorption on hydroxyapatite from the aqueous solution, acetate buffer and phosphate buffer decreases in the following order for Sn2+: oxalic acid > ethylenediaminetetraacetic acid > ascorbic acid. In the presence of Fe2+ ions without organic ligands, the sorption reached up to 15% depending on the composition of the solution. The addition of oxalic acid and ascorbic acid increased the sorption up to 80%. The ethylenediaminetetraacetic acid had no significant effect on the sorption of technetium on hydroxyapatite.

Effects of Various Ripening Media on the Mesoporous Structure and Morphology of Hydroxyapatite Powders

Mesoporous hydroxyapatite (HA) materials demonstrate advantages as catalysts and as support systems for catalysis, as adsorbent materials for removing contamination from soil and water, and as nanocarriers of functional agents for bone-related therapies. The present research demonstrates the possibility of the enlargement of the Brunauer-Emmett-Teller specific surface area (SSA), pore volume, and average pore diameter via changing the synthesis medium and ripening the material in the mother solution after the precipitation processes have been completed. HA powders were investigated via chemical analysis, X-ray diffraction analysis, Fourier-transform IR spectroscopy, transmission electron microscopy (TEM), and scanning (SEM) electron microscopy. Their SSA, pore volume, and pore-size distributions were determined via low-temperature nitrogen adsorption measurements, the zeta potential was established, and electron paramagnetic resonance (EPR) spectroscopy was performed. When the materials were synthesized in water-ethanol and water-acetone media, the SSA and total pore volume were 52.1 m²g^-1 and 116.4 m²g^-1, and 0.231 and 0.286 cm³g^-1, respectively. After ripening for 21 days, the particle morphology changed, the length/width aspect ratio decreased, and looser and smaller powder agglomerates were obtained. These changes in their characteristics led to an increase in SSA for the water and water-ethanol samples, while pore volume demonstrated a multiplied increase for all samples, reaching 0.593 cm³g^-1 for the water-acetone sample.

Simultaneous synthesis of hydroxyapatite fibres and β-tricalcium phosphate particles via a water controlled-release solvothermal process

Fibrous hydroxyapatite, rice-like β-tricalcium phosphate and DCPA plates were synthesised by water controlled-release solvothermal process using the esterification reaction.

Development of Root Caries Prevention by Nano-Hydroxyapatite Coating and Improvement of Dentin Acid Resistance

There is no established method for optimizing the use of dentin to prevent root caries, which are increasing in the elderly population. This study aimed to develop a new approach for root caries prevention by focusing on bioapatite (BioHap), a new biomaterial, combined with fluoride. Bovine dentin was used as a sample, and an acid challenge was performed in three groups: no fluoride (control group), acidulated phosphate fluoride treatment (APF group), and BioHap + APF treatment (BioHap group). After applying the new compound, the acid resistance of dentin was compared with that of APF alone. The BioHap group had fewer defects and an increased surface hardness than the APF group. The BioHap group had the smallest lesion depth and least mineral loss among all groups. Using a scanning electron microscope in the BioHap group showed the closure of dentinal tubules and a coating on the surface. The BioHap group maintained a coating and had higher acid resistance than the APF group. The coating prevents acid penetration, and the small particle size of BioHap and its excellent reactivity with fluoride are thought to have contributed to the improvement of acid resistance in dentin. Topical fluoride application using BioHap protects against root caries.