Synthesis and Characterization of Titania-Coated Hollow Mesoporous Hydroxyapatite Composites for Photocatalytic Degradation of Methyl Red Dye in Water

Abstract

Hollow mesoporous hydroxyapatite (HM-HAP) composites coated with titania are prepared to increase the stability and catalytic performance of titania for azo dyes present in the wastewater system. In this work, HM-HAP particles were first synthesized by a hydrothermal method utilizing the CaCO₃ core as a template and then coated with titania to form TiO₂/HM-HAP composites. Utilizing SEM, XRD, XPS, BET, FTIR, EDS, UV–vis DRS spectroscopy, and point of zero charge (PZC) analysis, the coating morphological and physicochemical parameters of the produced samples were analyzed. The photocatalytic efficiency of the synthesized coated composites was assessed by the degradation of methyl red (MR) dye in water. The results indicated that TiO₂/HM-HAP particles could efficiently photodegrade MR dye in water under UV irradiation. The 20% TiO₂/HM-HAP coating exhibited high catalytic performance, and the degradation process was followed by the pseudo-first-order (PFO) kinetic model with a rate constant of 0.033. The effect of pH on the degradation process was also evaluated, and the maximum degradation was observed at pH 6. The analysis of degraded MR dye products was investigated using LC-MS and FTIR analysis. Finally, a good support material, HM-HAP for TiO₂ coatings, which provides a large number of active adsorption sites and has catalytic degradation performance for MR dye, was revealed.

1. Introduction

The majority of dyes used in textiles, leather, cosmetics, food processing, ink, and paper are azo dyes, which are categorized by the existence of one or more azo groups in their chemical structure. Furthermore, 15% of the world’s manufactured dyes are lost during synthesis and application as wastes that pose a threat to human health and the environment because of their toxic nature [1]. A high concentration of color, suspended particles, and COD also characterize the polluted water discharged by various industries. Even in minute quantities, the effluents of the dyes are conspicuous and hazardous. These contaminants severely impact water bodies and the plants and animals that depend on them. It is commonly known that methyl red dye is used in textile industries, paper printing, and as a pH indicator in the laboratory. However, it may create problems with the eyes, skin, and digestive tract when swallowed or inhaled [2]. Thus, owing to the health risks posed by dyes to human health, it is of the utmost importance to develop effective methods for their effective removal from wastewater.

Numerous methods have been introduced for the treatment of waste, which include chemicals such as ozonation and chlorination [3,4], physical such as adsorption [5,6] and membrane filtration [7,8], photocatalysis [9,10], and biological treatment methods [11]. Among them, photocatalytic technology provides a simple and inexpensive way to eliminate organic and inorganic contaminants from wastewater, as photocatalytic degradation may break down or mineralize the majority of organic pollutants. Among the many photocatalysts, titanium dioxide (TiO₂) is the most widely used because of its non-toxicity, low cost, and high photochemical stability, but the disadvantages of limited pollutant adsorption capacity [12,13,14,15] and significant recombination of photogenerated electron–hole pairs hinder its photocatalytic efficiency [16]. Several approaches have been implemented to improve photocatalytic performance in an effort to address these problems.

Recent research has demonstrated the capability of TiO₂-based composites to improve photocatalytic activity. For example, Sukhadeve et al. (2023) revealed the effective degradation of methylene blue dye using Ag-Fe co-doped TiO₂ nanoparticles [17]. Furthermore, Ali et al. (2023) reviewed the catalytic activity of Ag- and Zn-doped TiO₂ nano-catalysts for the removal of methylene blue and methyl orange dyes [18]. Liza et al. (2024) conducted an additional study that examined the impact of Ag-doping on the morphology, band gap, and photocatalytic activity of TiO₂ nanoparticles in the context of textile dye degradation [19]. Moreover, TiO₂ has been implemented in a variety of applications beyond wastewater treatment, including antibacterial remedies [20], protective coatings for metal prostheses [21], and self-cleaning surfaces for air purification [22].

Recent publications have indicated that TiO₂-supported materials, such as silica, hydroxyapatite (HAP), zeolite, pure natural diatomite, and activated carbon, can provide a large number of active adsorption sites, resulting in rapid mass transport and catalytic processes [23,24,25,26]. Among all these supporting materials, hydroxyapatite (HAP) has been extensively researched because of its mechanical stability, high biocompatibility, non-toxicity, and inexpensive cost [27,28,29,30]. Hydroxyapatite (HAP) is also extensively recognized for its applications in a variety of areas, such as the environmental, biomedical, and industrial sectors [31,32]. HAP’s ecological friendliness is one of its most intriguing features, as it can be produced from waste materials, rendering it a sustainable and environmentally responsible option. Recent research has investigated the potential of HAP, which is derived from sources of waste, such as egg shells, fly ash, and fish bones, to reduce environmental impacts and promote sustainable practices [33,34,35].

Hydroxyapatite (HAP) possesses hydroxyl groups (OH) and adsorbed H₂O molecules on its surface that can interact with h⁺ to create hydroxyl radicals (˙OH), hence enhancing photocatalytic efficiency. Moreover, during the photocatalytic process, the electron phase shift of the PO₄³⁻ groups on the HAP surface can also result in the formation of ˙O₂ radicals [36]. Consequently, it is anticipated that integrating the advantages of HAP and TiO₂ will not only increase the capability of TiO₂ to absorb contaminants but also reduce the combination of photogenerated electron–hole pairs. Therefore, the use of TiO₂ in conjunction with HAP may prove to be an effective wastewater treatment method.

Prior research on TiO₂/HAP composites has mainly concentrated on using rod-shaped or quasi-spherical hydroxyapatite (HAP) structures. However, these structures often face difficulties such as particle agglomeration and ineffective TiO₂ loading [37,38,39]. These problems might result in a decrease in the number of active sites on the surface, which in turn reduces the effectiveness of the composite in interacting with contaminants. Our research presents a novel method that utilizes a template technique to create hollow mesoporous HAP structures, which are then coated with TiO₂. This innovative approach effectively solves the issues of aggregation that are typically seen with traditional HAP forms while also greatly improving the surface area and ease of access to the active areas inside the composite. By employing hollow mesoporous hydroxyapatite (HAP) in this distinct morphology, the effectiveness and durability of the TiO₂/HAP composite for photocatalytic and environmental purposes are significantly enhanced. This study is the first known case of employing hollow mesoporous HAP templates for TiO₂ coatings. This opens new prospects for improving the efficiency and adaptability of composite materials in sustainable technologies. The unique structure of spherical hollow hydroxyapatite particles not only prevents aggregation but also allows for a greater surface area owing to the hollow interior. This makes them very appropriate for coating TiO₂ and conducting investigations on photocatalysis.

In this research work, we prepared HM-HAP particles through a hydrothermal technique using the CaCO₃ core as a template. The synthesized HM-HAP was then coated with different amounts of TiO₂ to form a TiO₂-coated HM-HAP composite. Figure 1 shows a schematic diagram of the TiO₂/HM-HAP particle synthesis. The synthesized TiO₂/HM-HAP composites exhibited good photocatalytic activity for MR dye removal under UV light. XRD, XPS, BET, SEM, and UV–vis DRS were used to examine the effect of TiO₂ coatings on the structural behavior of HM-HAP. The photocatalytic degradation of methyl red (MR) dye under UV irradiation was investigated over the TiO₂/HM-HAP composites. Hence, this research paper presents results on a good support material, HM-HAP, for TiO₂ coatings, which provides a large number of adsorption sites, resulting in rapid mass transport and catalytic processes, and finally studies the photocatalytic degradation of MR dye.

2. Experimental Methods

2.1. Materials

Sodium carbonate (Na₂CO₃), calcium nitrate (Ca(NO₃)₂), disodium hydrogen phosphate dodecahydrate (Na₂HPO₄·12H₂O), potassium hydrogen phthalate (KHP), potassium chloride (KCl), potassium dihydrogen phosphate (KH₂PO₄), sodium hydroxide (NaOH), and hydrochloric acid (HCl) were purchased from DAMAO. Sodium poly (styrene sulfonate) (PSS, Mw = 70,000), methyl red (MR), and Titanium dioxide (TiO₂) were purchased from MACKLIN.

2.2. Synthesis of HM-HAP Particles

A 0.1 M solution of Na₂CO₃ (20 mL) was quickly mixed with a 0.1 M Ca (NO₃)₂ solution (20 mL) containing 150 mg of PSS under magnetic stirring (600 rpm). The solution temperature was maintained at 30 °C for about 30 min. The obtained CaCO₃ particles, resulting from the quick precipitation procedure, were rinsed three times with deionized water before being collected by centrifugation (7000 rpm, 5 min). The obtained CaCO₃ particles were then added to a Na₂HPO₄ solution (30 mL, 0.8 M). The resultant mixture was then placed in an autoclave of 100 mL capacity and heated at 140 °C for 4 h. After the reaction was complete, the CaCO₃ core was removed by adding a few drops of acid to the solution and stirring it for 2 h.

2.3. Synthesis of TiO₂/HM-HAP Composite

The obtained materials, HM-HAP (150 mg) and titanium dioxide (TiO₂), were mixed in various amounts with butanol and agitated for one hour at 45 °C. The butanol was then evaporated at 60 °C using a rotary evaporator. The sample was then heated for 4 h at 450 °C. The TiO₂ was coated onto the HM-HAP at 10%, 20%, 30%, 40%, and 50%.

2.4. Characterization of TiO₂/HM-HAP Composite

The material’s surface shape and size were determined using a scanning electron microscope HITACHI-SU8220 (High-Tech corporation, Tokyo, Japan). The X-ray diffraction (XRD) patterns were acquired using a smart lab X-ray diffractometer (XRD, Rigaku corporation, Tokyo, Japan) with Cu Kα (λ = 0.15406 nm) radiation. On a ThermoFisher—6700 (Waltham, MA, USA) FTIR spectrometer, the material’s FTIR spectrum was obtained using the KBr pellet technique. The pore structure and specific surface area of the coated particles were determined using a BET surface area analyzer (BSD Instrument Technology (Beijing, China) Co., Ltd. (BSD-PS (M)) at 77 K, utilizing helium as the carrier gas at liquid nitrogen temperature. The formation of TiO₂ coatings on the HM-HAP particles was confirmed via an energy-dispersive X-ray spectrometer-QUANTA 450 (Thermo Fisher Scientific, Waltham, MA, USA). X-ray photoelectron spectroscopy (XPS) data were collected on a Thermo Scientific photoelectron spectrometer—ESCALAB250Xi (Thermo Fisher Scientific, Winsford, UK). UV-vis diffuse reflectance spectra (DRS) of the synthesized materials were measured by a UV-vis spectrophotometer (Lambda 1050+, Perkin Elmer, Shanghai, China) utilizing BaSO₄ as a reference material. A spectroscopic study utilizing a UV-vis spectrophotometer carry-100 (Agilent, Kuala Lumpur, Malaysia) was used to evaluate the absorbance of methyl red dye throughout its photocatalytic degradation. The absorbance spectra were recorded between 350 and 750 nm wavelength. The pH measurement was performed using a PHS-3C pH meter (Shanghai Yoke Instrument Co., Ltd., Shanghai, China) with an integrated temperature sensor for temperature regulation. The examination of degradation products was conducted using liquid chromatography–mass spectrometry (LC-MS, G6230, Agilent, Santa Clara, CA, USA). The TOC of before and after decolorization of the dye solution was quantified by an organic element analyzer (UNICUBE, Elementar Analysensysteme GmbH, Langenselbold, Germany).

2.5. Point of Zero Charge (PZC)

Measuring PZC aids in predicting the ionic character of a catalyst, which clarifies the interaction process between a dye and the catalyst. The salt addition method was used to measure the PZC point of the formed TiO₂-coated HM-HAP composites. Typically, a 0.1 M NaCl solution was prepared, and for each experiment, 50 mL of the solution was taken in a 100 mL beaker. To each beaker, 30 mg of TiO₂-coated HM-HAP was added and then stirred for about 3 h. Each solution’s pH was maintained using a mixture of buffers (pH 2 to 12).

2.6. Photocatalytic Studies of MR

The photocatalytic activity of the samples was evaluated by observing the degradation of MR dye as the pollutant target under ultraviolet irradiation. The chemical structure and other characteristics of the employed dye are detailed in Table S1. Initially, 25 mL of MR solution (10 mg/L) and 30 mg of catalyst were vigorously agitated for 30 min in the dark to produce an adsorption/desorption equilibrium. The solution was then agitated at room temperature under UV light irradiation (λ = 254 nm) for several minutes. Periodically, 3 mL of the solution was collected and filtered to eliminate the solid phase. Although this decreased the initial volume, parallel experiments were carried out to ensure uniformity and eliminate potential mistakes resulting from volume reduction. More precisely, 25 mL samples were prepared and subjected to the same circumstances. This enabled the extraction of small portions from various samples at each specific time interval. This method guaranteed that the decrease in volume did not impact the accuracy of the outcomes. After that, the absorbance of the filtrates was measured spectroscopically at 437 nm for MR dye. In order to measure the extent of degradation of the MR dye, a calibration curve was employed, which can be seen in Figure S1 in the Supplementary Information. The calibration curve was generated by measuring the absorbance of a range of MR dye solutions with predetermined concentrations, enabling us to establish a precise relationship between absorbance and dye concentration. Applying this calibration curve, the concentration of MR dye at various irradiation times was established, facilitating the calculation of the photodegradation rate and efficiency.

Photolysis and adsorption experiments were also conducted under the same conditions as the photocatalytic experiment.

The effect of various parameters, such as pH and contact time, on the degradation of MR by the catalysts was also analyzed. The solution pH (2, 4, 6, 8, and 10) was monitored by the addition of sodium hydroxide (NaOH), sodium bicarbonate (NaHCO₃), potassium hydrogen phthalate (KHP), potassium chloride (KCl), hydrochloric acid (HCl), and potassium dihydrogen phosphate (KH₂PO₄).

The percentage degradation of the MR dye was determined using the following relationship [40]:

$$\% Degradation={\displaystyle \frac{{\left[MR\right]}_0-{\left[MR\right]}_t}{{\left[MR\right]}_0}}\times 100$$

(1)

where [MR]₀ represents the initial concentration and [MR]_t represents the concentration at time t of the dye MR.

2.7. Kinetics of MR Degradation

The obtained experimental data were assessed further using the (PFO) pseudo-first-order kinetic model, which is represented by the given expression [41]:

$$ln{\displaystyle \frac{{\left[MR\right]}_0}{{\left[MR\right]}_t}}=k_{1}t$$

(2)

where t is the given time, [MR]₀ symbolizes the initial concentration, and [MR]_t symbolizes the concentration at any time (t) of MR dye. k₁ is the rate constant, and its values can be derived from the slope of the graph ln[MR]₀/[MR]_tvs. reaction time.

3. Results and Discussion

3.1. Synthesis of TiO₂/HM-HAP Composite

In this research study, TiO₂-coated HM-HAP composites were produced and employed in a photocatalytic study. First, HM-HAP particles were synthesized using a hydrothermal method with PSS-doped vaterite CaCO₃ as a hard template. The presence of PSS, a polyelectrolyte with a substantial negative charge, produces negatively charged HM-HAP particles. In addition, PSS (sodium poly (styrene sulfonate)) was used as a crystal growth additive to accelerate the transformation of CaCO₃ from calcite to vaterite during the CaCO₃ manufacturing process. The reagents Na₂CO₃ and Ca (NO₃)₂ were utilized first for the synthesis of CaCO₃ by the formation of precipitates. The obtained CaCO₃ particles were then added to Na₂HPO₄, and after the completion of the reaction, the core was removed by an etching process carried out with acetic acid. The synthesized HM-HAP particles were then coated with a range of TiO₂ concentrations (10% to 50%) to create a TiO₂-coated HM-HAP composite. FTIR and XRD analysis confirmed the synthesis of HM-HAP and TiO₂-coated HM-HAP composites.

The FTIR spectra of pure TiO₂, HM-HAP, and TiO₂-coated HM-HAP composites are shown in Figure 2a. The functional peaks at 3432 cm⁻¹ and 1632 cm⁻¹ are the results of lattice water in the samples [42]. At 634 cm⁻¹, the distinctive broad absorption bands of TiO₂ are detected, which can be ascribed to the stretching vibration of Ti–O–Ti bonds [43]. The distinctive bands at 563 and 602 cm⁻¹ for the HM-HAP sample correspond to the bending vibration of PO₄³⁻, whereas the band at 1031 cm⁻¹ corresponds to the stretching vibration of PO₄³⁻. The characteristic CO₃²⁻ bands are situated at 1466, 1410, and 874 cm⁻¹ [44,45,46]. The spectra of TiO₂-coated HM-HAP (10%–50%) composites show that the characteristic peaks of TiO₂ and HM-HAP are well preserved, indicating that the structure transformation of TiO₂ and HM-HAP did not change during composite production. This demonstrates the effective synthesis of TiO₂-coated HM-HAP composites, which is compatible with the XRD data.

The XRD patterns of TiO₂, HM-HAP, and TiO₂-coated HM-HAP composites are shown in Figure 2b. The diffraction peaks at 25.2°, 37.1°, 53.9°, 55.8°, 62.7°, 68.5°, and 70.3° correspond to the (101), (004), (105), (211), (204), (116), and (220) planes of anatase-TiO₂, respectively (JCPDS no. 21-1272) [47]. In the case of pure HM-HAP, all diffraction peaks and their relative intensities correspond to the standard diffraction data of pure hexagonal phase HAP (JCPDS, 09-0432) [48,49]. Specifically, the peaks at two values of 31.9°, 32.1°, and 33.1° correspond to the (211), (112), and (300) planes of HAP, respectively. These indices confirm the successful synthesis of hydroxyapatite with a well-defined crystalline structure. In addition, the diffraction patterns of the TiO₂-coated HM-HAP composites (10%–50%) demonstrate the existence of both anatase TiO₂ and hexagonal phase HAP. Thus, the TiO₂-coated HM-HAP composites were successfully synthesized using the hydrothermal technique.

3.2. Characterization of the TiO₂/HM-HAP Composite

After confirming that the material was synthesized, we further characterized a series of samples with different Ti contents. Figure 2c depicts the UV–vis DRS spectra of pure TiO₂, HM-HAP, and TiO₂-coated HM-HAP composites. The band gap energy values for the as-synthesized samples are illustrated in Table S2, and their respective graphs are shown in Figure S2. It can be seen that the absorption edge of pure TiO₂ is evidently located around 429 nm, whereas the absorption edge of pure HM-HAP is less than 350 nm. Compared with pure HM-HAP, the TiO₂-coated HM-HAP composites exhibit a considerable red shift in the absorption edge. In addition, the absorption edge of the TiO₂-coated HM-HAP composites exhibit a little blue shift relative to that of TiO₂, indicating that the TiO₂-coated HM-HAP composites have a greater oxidation capability than TiO₂.

The size and morphology of HM-HAP and TiO₂-coated HM-HAP composites were visualized using SEM (Figure 3a–f). The morphology of the particles revealed a spherical shape with a hollow interior. The micrographs indicated that the particle size of HM-HAP was around 1–2 μm. All six formulations of HM-HAP had the same morphology and shape.

Energy-dispersive X-ray spectroscopy (EDS) was used to examine the surface elemental analysis of TiO₂-coated HM-HAP composites. The levels of three elements, i.e., Ca, P, and Ti, in the samples, are summarized in Figure 4. The mapping profile illustrates the homogenous surface distribution of the three components. All TiO₂-coated HM-HAP composites exhibited a close interaction between these elements. The corresponding EDS spectra in Figure 5a confirmed the existence of the Ti element coming from TiO₂, demonstrating the successful formation of a TiO₂-coated HM-HAP composite. Hence, the incorporation of TiO₂ into HM-HAP particles may therefore be validated.

The pore volume, pore size, and Brunauer–Emmett–Teller (BET) specific surface area acquired from N₂ adsorption–desorption for all the TiO₂-coated HM-HAP composites are presented in

Table 1. BET-specific surface area and pore diameter analysis of the synthesized composites.

Sample	BET Specific Surface Area (m2/g)	Pore Size (nm)
HM-HAP	44	12.47
10% TiO2/HM-HAP	56	27.82
20% TiO2/HM-HAP	57	28.19
30% TiO2/HM-HAP	47	29.28
40% TiO2/HM-HAP	46	31.94
50% TiO2/HM-HAP	38	33.43

. Figure 5b depicts the isotherms and corresponding pore size distribution histograms, indicating the mesoporous structure of the TiO₂-coated HM-HAP composites. According to the IUPAC system, all the synthesized samples have type IV isotherms, which have an H3 hysteresis loop. This is because particle aggregation makes slit-shaped pores [50]. The BET-specific surface area of 10% TiO₂-coated HM-HAP was 56 m²/g and declined as the TiO₂ concentration increased, while the corresponding pore diameter increased gradually. A possible explanation for this result is that when the number of TiO₂ molecules increases, aggregation may occur, resulting in no greater pore occupancy. Thus, a decreasing trend in the specific surface area occurred. Moreover, the coating of TiO₂ onto HAP greatly increases the surface area of the resulting composite material (Table 1). The augmentation in surface area is essential as it offers a greater number of active sites for photocatalytic reactions, hence enhancing the efficacy of the photodegradation process. More precisely, the TiO₂-coated HM-HAP composite has a greater surface area in comparison with pure HAP, as specified in Table 1. The improvement can be attributed to the synergistic interplay between TiO₂ and HAP, with HAP serving as a supporting framework that hinders the clumping of TiO₂ nanoparticles, hence ensuring a large surface area.

The surface chemical composition and chemical states of pure TiO₂, HM-HAP, and TiO₂-coated HM-HAP composites were investigated using XPS. The XPS survey spectra of TiO₂, HM-HAP, and 20% TiO₂/HM-HAP coatings are depicted in Figure 6. In Figure 6a, the survey spectra of TiO₂-coated HM-HAP reveal the presence of the four elements including Ti, Ca, P, and O. As indicated in the figure, the additional C element peak is mostly produced from adventitious carbon. The XPS peaks in the Ti 2p of TiO₂ in Figure 6b, situated at a binding energy of 458.6 eV and 464.2 eV, are ascribed to the Ti-O bonds [51] but in the 20% TiO₂/HM-HAP coating, the Ti 2p peaks shift to lower energy levels, 458.1 eV and 463.9 eV, respectively, which may be due to the presence of Ti-O-Ca bonds in the TiO₂/HM-HAP coated composite. Likewise, the Ca 2p peaks of HAP shown in Figure 6c are situated at 346.8 eV and 350.4 eV, whereas in the 20% TiO₂/HAP coating, the Ca 2p peaks slightly shift towards the higher binding energy, i.e., 347.1 eV and 350.7 eV [52]. Figure 6d displays the high-resolution O 1s spectra that correspond to TiO₂, HM-HAP, and TiO₂/HM-HAP coated composites, respectively. Ti-O bonds and -OH groups are responsible for the two peaks in TiO₂ that are located at 528.9 and 530.4 eV, respectively. In the HM-HAP spectra, the phosphate group (PO₄³⁻) and adsorbed water are responsible for the two peaks that are positioned at 530.8 eV and 532.1 eV, respectively [53,54]. In the case of the 20% TiO₂/HM-HAP coated composite, the three peaks at 529.1 eV, 529.4 eV, and 531.1 eV are ascribed to the lattice oxygen species Ti-O bonds (TiO₂), PO₄³⁻ (phosphate group), and O-C bond of the CO₃⁻², respectively [54]. Hence, the analysis indicated that TiO₂ particles existed in the formed composition.

Moreover, the charge transfer between HAP and TiO₂ was also confirmed by the XPS analysis of the samples. By comparing the XPS data of individual atoms in pure TiO₂ and HM-HAP with those in the composite material, significant shifts in binding energy peaks were observed. These shifts in binding energy are compelling evidence of alterations in the chemical environment, suggesting the occurrence of charge transfer between HAP and TiO₂. Hence, the shift in peaks not only signifies the formation of the composite material but also provides clear evidence of the charge transfer process taking place between the two components.

Furthermore, the stability of the synthesized photocatalyst can be supported by the fact that hydroxyapatite (HAP) exhibits exceptional stability up to 1000 °C, as supported by rigorous testing through Fourier Transform Infrared (FTIR) analysis and Thermogravimetric Analysis (TGA) [55,56]. Therefore, in the formation of the TiO₂ composite with HAP, the inherent stability of HAP was utilized, which inherently imparts durability to the resulting photocatalyst. This stability is not merely asserted but substantiated through meticulous examination, as evidenced in various papers. The FTIR and TGA analysis presented in the literature elucidates the stability of HAP at different temperatures, thereby confirming the stable nature of the synthesized photocatalyst.

PZC is regarded as one of the several methods for determining the kind of surface charge. It is the value at which the surface charge density of a material is equal to zero. Consequently, the PZC points of the TiO₂/HM-HAP coated composites were determined. According to Figure 6e, the PZC point of the TiO₂/HM-HAP composites (10%–50%) is between 7.5 and 8.1. The detailed PZC values of each composite are shown in Table S3. This signifies that the synthesized composites have a negative charge above this number and a positive charge below it.

In conclusion, the findings of several characterizations revealed that the TiO₂/HM-HAP coated composite was successfully formed. The SEM micrographs indicated the spherical morphology of the particles having mesopores, as confirmed by the analysis of N₂ adsorption–desorption. The EDS elemental mapping and the corresponding spectra clearly showed the existence of Ti in the composite. Furthermore, the XPS analysis validated the peak corresponding to the lattice oxygen species of TiO₂ in the composite of TiO₂-coated HM-HAP.

3.3. Evaluation of the Photocatalytic Performance of the Synthesized Composites for the Degradation of MR

The photocatalytic activity of the synthesized TiO₂-coated composites was assessed by the photodegradation of MR under UV light irradiation. First, the linearity of MR’s spectroscopic response was measured, and a graph of absorbance against concentration was plotted to demonstrate the conformity of the dye to the Lambert–Beer law (Figure S1). As illustrated in Figure 7a, when catalyzed by a TiO₂-coated HM-HAP composite with UV irradiation (60 min), the degradation efficiency increased significantly, and in the presence of the 20% TiO₂/HM-HAP coating, up to almost 88% degradation efficiency was achieved. In contrast, pure HM-HAP and TiO₂ demonstrated degradation efficiencies of 20% and 42%, respectively (Figure 7a). This enhanced performance can be attributed to the unique properties of HAP, specifically its hydroxyl groups (⁻OH) and adsorbed H₂O molecules on the surface. These hydroxyl groups are instrumental in interacting with h⁺ to generate hydroxyl radicals (˙OH), a highly reactive species known for its effectiveness in photocatalysis. Furthermore, the photocatalytic process involves intriguing electron phase shifts within the PO₄³⁻ groups on the HAP surface. This phenomenon leads to the formation of ˙O₂ radicals, further augmenting the catalytic efficiency of the composite material [57]. Although bare TiO₂ exhibits photocatalytic activity, its efficiency is restricted by the rapid recombination of photogenerated electron–hole pairs. Conversely, HM-HAP alone offers adsorption sites but does not possess the catalytic efficiency of TiO₂. The TiO₂/HM-HAP composite optimizes charge separation and increases the number of active sites for the degradation reaction by integrating the benefits of both materials. Hence, integrating the advantages of HAP and TiO₂ not only results in an enhancement in TiO₂’s capability to absorb contaminants but also mitigates the recombination of photogenerated electron–hole pairs. This synergy between HAP and TiO₂ not only improves the material’s adsorption capacity but also amplifies its overall photocatalytic performance.

However, it can also be seen that as the concentration of TiO₂ increased by over 30%, the degradation efficiency declined gradually, demonstrating a decrease in activity. Generally, adsorption and degradation capacities should increase as the TiO₂ level rises. However, this was not the situation observed, as HM-HAP with a higher TiO₂ concentration exhibited only a little increase in adsorption in comparison with the one with a lower TiO₂ concentration. The existence of a large number of catalysts can prevent light from penetrating pores and promote intermolecular collision, thereby reducing photodegradation. In addition, the large number of TiO₂ particles could have covered the active sites and inhibited the generation of the active compounds [58]. These disadvantages counteract the enhanced photocatalysis provided by additional TiO₂.

3.3.1. Photocatalytic Degradation Kinetics

Figure 7b shows the MR percent degradation versus irradiation time graphs of the synthesized TiO₂/HM-HAP coated composites, demonstrating the gradual decomposition of MR. Figure S3 shows the UV spectra of MR at different time intervals. The rates of MR photodegradation were evaluated using a pseudo-first-order kinetic model in order to establish quantitative comparisons. In Figure 7c, it is evident that the pseudo-first-order kinetic model suited the experimental data well, and the R² values were all above 0.90, indicating that the reactions were compatible with the first-order kinetic.

Table 2. The rate constant values of the photocatalytic degradation of MR.

Sample	Rate Constant k1 (min−1)	R2
10% TiO2/HM-HAP	0.030	0.97
20% TiO2/HM-HAP	0.033	0.99
30% TiO2/HM-HAP	0.024	0.97
40% TiO2/HM-HAP	0.018	0.98
50% TiO2/HM-HAP	0.011	0.96

displays the values of the rate constant (k_MR) and regression coefficient (R²) for the TiO₂/HM-HAP coated composites for MR removal. Among the samples, the composite containing the 20% TiO₂/HM-HAP coating exhibited the highest degradation rate constant, i.e., 3.3 × 10⁻² min⁻¹. The k_MR followed the order of 20% TiO₂/HM-HAP > 10%, TiO₂/HM-HAP > 30% TiO₂/HM-HAP > 40% TiO₂/HM-HAP > 50% TiO₂/HM-HAP. As previously indicated, this could be because composites with low amounts of TiO₂ have many active sites on their surface.

These above observations clearly indicate that 10% TiO₂/HM-HAP has remarkable photocatalytic efficiency, even though it has a lower TiO₂ concentration compared with 20% TiO₂/HM-HAP. This finding indicates that 10% TiO₂/HM-HAP has high efficiency as a photocatalyst, attaining a degradation degree (DD) that is almost equivalent to 20% TiO₂/HM-HAP while containing half the amount of TiO₂. When comparing 10% TiO₂/HM-HAP to pure TiO₂, it is evident that 10% TiO₂/HM-HAP exhibits a substantially greater efficiency per unit of TiO₂. Furthermore, the degradation degree (DD) per unit of TiO₂ was calculated for each sample and is presented in Supplementary Information Section S1.

The efficiencies of different synthesized composites were compared to identify the most effective and cost-efficient photocatalyst. According to the findings, 20% TiO₂/HM-HAP exhibits the highest overall degrading efficiency. However, 10% TiO₂/HM-HAP provides a comparable performance with less TiO₂. The lower TiO₂ content of 10% TiO₂/HM-HAP is notably advantageous from a cost and application perspective, as it reduces material costs and increases the potential for large-scale applications.

3.3.2. Effect of pH on the Degradation of MR

The pH of a solution is a crucial variable in the photocatalytic degradation process. The pH affects both the properties of a dye (hydrophobicity, speciation behavior, and water solubility) and the surface charge of a catalyst. Below pH 5.3 (pKa), MR is mostly cationic (i.e., protonated), and above pH 5.3, it is anionic (i.e., unprotonated). It was previously stated that the electrostatic interaction among the material surface, solvent molecules, and dye during photocatalytic degradation is pH-dependent [59]. The influence of solution pH was studied to find the optimal pH range for maximal photocatalytic degradation. While examining the pH range (2, 4, 6, 8, and 10), a blue shift in λmax for MR was observed when the pH of the solution was changed from 4 to 6. No other shifts in the wavelength were observed when the pH was increased further. The UV spectra of MR dye at different pHs are presented in Figure S4.

Figure 7d shows the photocatalytic behavior of the synthesized composite towards the degradation of a solution of MR dye at different pH levels carried at room temperature. In Figure 7d, it is clear that the maximum photocatalytic degradation happened at pH 6. The explanation for this behavior is that the PZC of the TiO₂/HM-HAP coated composites is in the range of 7.5–8.1. Thus, the material’s surface is positively charged when the pH is <7.5 and negatively charged when the pH is >8.1. Therefore, the dye was in anionic form, and the surface of the material at pH 6 was positively charged [46,60,61]. Hence, in this condition, the interaction between the two species preferred photocatalytic degradation.

3.3.3. Post-Photodegradation FTIR Analysis and Mineralization Study of MR Dye

The FTIR spectrum of the MR dye is depicted in Figure 8a. The spectrum exhibited prominent peaks at 2919 and 2857 cm⁻¹, which correspond to the stretching vibrations of the C-H for –CH₃ groups. The O-H stretch is represented by the band at 3430 cm⁻¹. The apparent band at 1716 cm⁻¹ is the result of the C=O stretching of carbonyl groups. The C–C stretching vibration and N=N vibration of benzene are observed at 1602 cm⁻¹ and 1528 cm⁻¹, respectively. At 1368 and 1152 cm⁻¹, strong bands are ascribed to C–N stretching vibrations. The minor bands detected at 1115, 818, and 764 cm⁻¹ are indicative of C–H stretching vibrations [62,63].

The FTIR analysis of the product obtained subsequent to the degradation of MR is illustrated in Figure 8b. The disappearance of the peak at 1528 cm⁻¹, which was caused by the stretching vibration of N=N, indicates the cleavage of the azo group. The disappearance of the band at 1159 cm⁻¹ signifies a breakdown of C–N stretching vibrations [63,64]. The peak observed at 1067 cm⁻¹ is the result of stretching variations in the C–O bond. Significant variations can be observed by comparing the FTIR spectra of the original dye with those of the degraded dye or its metabolites. Certain methyl red dye peaks were observed to have vanished after degradation, while others re-emerged subsequent to degradation; this observation suggested that the dye had transformed into new compounds or metabolites. FTIR analysis confirmed and contributed to the reduction and elimination of the methyl red dye’s azo linkage. Moreover, an LC-MS [65] analysis of decolorized MR solution was also carried out. The resultant spectra are provided in Figure S5, and the identified degraded products of MR are shown in Figure S6.

As per the results of FTIR, the degradation of MR dye was confirmed. Additionally, the mineralization of MR dye was assessed by monitoring the reduction in total organic carbon (TOC) content following the treatment with the TiO₂/HM-HAP coated composite. At a pH of 6, a TOC removal of 35.64% was observed for 10 mg/L MR using the TiO₂/HM-HAP coated composite following a 1 h reaction time. Consequently, the TiO₂/HM-HAP coated composite was capable of facilitating dye decolorization and mineralization.

Moreover, the results of the photolysis experiment (Figure 9) demonstrated that there was minimal degradation of the MR dye when exposed just to direct UV light. This suggests that direct photolysis has a small impact on the overall degradation process. Furthermore, the findings from the adsorption experiment, presented in Figure 9, demonstrate that the catalyst was responsible for 20% of the overall removal of MR by adsorption. This demonstrates that although adsorption plays a role in the overall process, the main mechanism for removing dye under UV irradiation is definitely photodegradation. HAP possesses a significant number of hydroxyl groups, which contribute to its surface having a primarily negative charge. Methyl red (MR) is a negatively charged dye, and because of the like charges, there is a repulsive force that decreases its ability to stick to the HAP surface. While HAP does have some sites with a positive charge due to calcium ions, the overall negative charge from the many hydroxyl groups is more prominent. The structure of HAP and an explanation of the interaction between HAP and TiO₂ in TiO₂/HM-HAP composites are further presented in Supplementary Information Section S2.

In addition, an examination was conducted on the impact of an OH scavenger on photocatalysis. Isopropanol was employed as an *OH scavenger for the experiment. The catalyst’s catalytic efficiency decreases when a scavenger is present. This is illustrated using the graph presented in Figure 9. The predominant catalyst in this process is now clearly identified as the *OH radical [66].

In comparison with other recently researched sorbents, the adsorption capability of the Sa-modified hydroxyapatite as synthesized demonstrated promising findings. Zenefar et al. observed that the photodegradation potential of a HAp-TiO₂-ZnO photocatalyst for methylene blue (MB) and methyl orange (MO) dye was 95% and 45% after 2 h, respectively [67]. A Hap-TiO₂ nanocomposite demonstrated a degradation efficiency of 80% for methyl orange (5 mg/mL), as stated by Sharifat et al. [38]. Furthermore, Anmin et al. reported the degradation efficiency of titanium-substituted HAp for methylene blue (MB) to be 17%–37% under visible light and 39%–50% under UV light [68]. This literature review shows that the TiO₂-coated HM-HAP composite is an effective catalyst for the removal of dyes from water.

Combining the outcomes of SEM, DRS, FTIR, N₂ adsorption–desorption, and XRD analysis, the enhanced photocatalytic degradation performance of the TiO₂/HM-HAP coated composites in comparison with pure TiO₂ on the photodegradation of methyl red is associated with the creation of a uniform layer of TiO₂ particles on the highly porous structure of HM-HAP containing a large number of hydroxyl (OH) groups. These OH groups interact with the generated holes as Lewis bases, which results in the generation of hydroxyl radicals. The generated OH radicals will then oxidize the adsorbed molecules, hence improving the material’s photocatalytic efficiency. Moreover, the hollow mesoporous structure of HM-HAP promotes the degradation performance of the composite by adsorbing the dye particles on its surface, as observed in Figure 7a. The relatively better catalytic performance of the 20% TiO₂/HM-HAP coated composites compared with those with a high concentration of TiO₂ is mostly determined by the existence of active sites, surface area, and availability of active substances.

3.4. Photocatalytic Degradation Mechanism of TiO₂/HM-HAP Composites on MR Dye

The possible mechanism of TiO₂/HM-HAP coated composites for the degradation of MR is shown in Figure 10. Generally, the UV irradiation of TiO₂-coated HM-HAP alters the electronic condition of the PO₄⁻³ group on the surface and generates a vacancy on HM-HAP. In the same way, when UV light is applied to TiO₂, electrons in the VB (valance band) move to the CB (conduction band), making an equal number of holes in the VB. The reaction is given as:

$${\mathrm{TiO}}_2+\mathrm{h}\mathsf{\nu }\to {\mathrm{e}}^{-}+{\mathrm{h}}^{+}$$

(3)

The reaction of a CB electron with O₂ produces superoxide radicals (O₂^.−), which then oxidizes the organic compounds. The reaction is given as follows:

$${\mathrm{e}}^{-}+{\mathrm{O}}_2\to {\mathrm{O}}_2^{.-}$$

(4)

The VB hole interacts with the hydroxyl anions or the water to generate hydrogen peroxide.

$${\mathrm{h}}^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{OH}}^{.}+{\mathrm{H}}^{+}$$

(5)

$${\mathrm{h}}^{+}+{\mathrm{OH}}^{-}\to {\mathrm{OH}}^{.}$$

(6)

The hydrogen peroxide then breaks apart and releases hydroxyl radicals (OH·), which are powerful oxidizing agents that attack organic molecules that have adsorbed to the composite [37].

4. Conclusions

The present research demonstrated the effective synthesis of TiO₂/HM-HAP coated composites by loading the TiO₂ at various concentrations, comprising 10%, 20%, 30%, 40%, and 50%, to enhance the degradation efficiency of hazardous dyes present in aqueous solutions and address water pollution issues. Several analysis approaches, including FTIR, XPS, XRD, SEM, and BET, were utilized to analyze the physical properties of all the synthesized TiO₂/HM-HAP coated composites. TiO₂ was evenly incorporated into the hollow mesoporous HAP particles, with an insignificant effect on the overall structure. The catalytic performance was examined via the degradation of MR in the presence of UV light. We observed that the MR removal ratio of the 20% TiO₂/HM-HAP coating was 88%, which was found to be the maximum among the HM-HAP, pure TiO₂, and TiO₂/HM-HAP coated composites. It was noted that the photocatalytic degradation process was compatible with the pseudo-first-order (PFO) kinetic model. Moreover, it was found that MR could be more easily degraded at pH 6 than in strongly acidic or alkaline environments. This work provides insight into the development of TiO₂/HM-HAP coated composites as potential materials for removing organic contaminants from wastewater. This will help with both water treatment and waste management.