Functionalization of hydroxyapatite derived from cockle (Anadara granosa) shells into hydroxyapatite–nano TiO₂ for photocatalytic degradation of methyl violet

Abstract

Photocatalyst of hydroxyapatite–nano TiO₂ (HAp-nTiO₂) was prepared from phosphatation of calcined cockle (Anadara granosa) shells followed by dispersion of nano TiO₂ powder into HAp precipitate and calcination at 400 °C for 2 h. The prepared material was characterized using X-ray diffraction, scanning electron microscope–energy dispersive X-ray spectrophotometry, gas sorption analysis, and UV-Vis diffuse reflectance spectrophotometry. The photocatalytic activity of the material was evaluated for methyl violet degradation over photocatalysis and photooxidation mechanism. The results showed that the homogeneous dispersion of TiO₂ in the HAp-nTiO₂ composite was achieved, as seen in the X-ray diffraction analysis, diffuse reflectance UV-Vis, and gas sorption analyses. The physicochemical and photocatalytic character of the composite exhibited the positive role of HAp as TiO₂ support in enhancing the photocatalytic activity with a higher turnover number and reusability property than that of pure TiO₂. It was also noted that the HAp-nTiO₂ composite demonstrated rapid methyl violet degradation over photooxidation rather than by photocatalytic mechanism.

Introduction

Photocatalysis is a developed technology with many wastewater treatment applications, one of which is the treatment of dye-containing wastewater from industries like textiles, painting, printing, and other related industries [1]. It has been reported that approximately 10–15 wt% of the dyes used in textile industries are discharged as wastewater [2, 3]. With the presence of mainly stable organic compounds, a powerful photocatalyst is required for the photooxidation process in treating dye-containing wastewater. Among metal oxide photocatalysts, such as ZnO, ZrO₂, and Fe₂O₃, TiO₂ is the most popular photocatalyst material given its low cost, non-toxicity, and effective band gap energy that sufficiently supports an economic photocatalysis process [4]. Along with the exploration of low-cost processes for photocatalysis, some modifications have been attempted to enhance the photocatalytic activity of TiO₂ [5,6,7]. The preparation of composites containing TiO₂ has been reported as offering several advantages related to material stability [8]. The use and conversion of waste materials into functional materials has also received intensive attention. In line with this exploration of sustainable materials, the conversion of calcium-containing animal waste into valuable materials has been reported. Significant research has reported the utilization of CaO derived from snail shells, egg shells, and fish bones for the preparation of calcium-based materials, such as hydroxyapatite (HAp), perovskite, and other composites, together with other metal oxides or minerals [9,10,11,12]. The use of animal shells from scallops, snails, and cockles, as well as fish bones, has been reported for these purposes [13,14,15]. The simple conversion of these waste materials into CaO (sometimes called biogenic CaO), which requires calcination at approximately 600–1200 °C, is the challenge for the preparation of functional material.

HAp [Ca₁₀(PO₄)₆(OH)₂] is a material that can be prepared from biogenic CaO through sol-gel synthesis and precipitation, among other techniques. Some examples of HAp preparation are the conversion of oyster shells and rice field snail shells into HAp using precipitation [9, 16], a sol-gel method of converting snail shells [17], and a microwave-assisted preparation [18] of snail shells. Among its potential uses, HAp has been reported as effective in enhancing TiO₂ photocatalytic activity, and HAp prepared from waste mussel shells has been reported to be active in catalyzing dye oxidation. The combination of HAp and TiO₂ was reported to produce developed microstructures, the phases of which are useful for densification and the mechanical properties of the composite [19]. Several investigations have remarked upon the improved performance of photocatalysis profiles by the combination and/or dispersion of TiO₂ with HAp. The enhanced performance was influenced by the coating effect of HAp and the supporting adsorption capability involved in the photocatalysis mechanism [20,21,22,23]. Based on our previous research into the preparation of HAp from biogenic CaO and other investigations of the photocatalytic activity of nano TiO₂ [4, 18], this research aims to investigate the preparation of HAp–nano TiO₂ composite for dye photodegradation applications. Among CaO sources, Anadara granosa shell was interesting to be explored since it has been cultivated for its meat that highly consumed, so it produces shell as waste in large amount. The research focused on physicochemical property evolution from the biogenic CaO into the HAp–nano TiO₂ (HAp-nTiO₂) composite and the evaluation of the photocatalytic activity of the material.

Materials and methods

Cockle shells were obtained from beaches in Yogyakarta Province, Indonesia; methyl violet (MV), aquadest, ammonium phosphate [(NH)₄HPO₄], Whatman 41 filter paper, potassium hydrogen phthalate, potassium bichromate, hydrogen peroxide (H₂O₂), ethanol, nTiO₂, and ammonium hydroxide (NH₄OH) were purchased from Merck-Millipore (Germany).

Preparation of HAp and HAp-nTiO₂

Cockle shells were crushed using hammer followed by grounding. The first step was converting cockle shells into CaO by calcination at 1000 °C for 3 h, and the powder of biogenic CaO was obtained. The obtained CaO powder of 15 g was diluted into 200 mL of distilled water, and then diammonium hydrogen phosphate solution was slowly added under stirring. The molar ratio of Ca/P was set as 1.67. The mixture was stirred overnight before being filtered off and calcined at 400 °C for 2 h. The powder obtained from these steps was the prepared HAp.

Preparation of HAp-nTiO₂ began with the preparation of HAp, described above. After the mixture of CaO and diammonium hydrogen phosphate solution was precipitated, nano TiO₂ was added into the precipitate at the theoretic Ti content set up at 5 wt%. The mixture was then stirred overnight, filtered off, and dried. The dry powder was calcined at 400 °C for 2 h, and the powder sample was designated as HAp- nTiO₂.

Material characterization

Analysis of the materials was conducted using X-ray diffraction (XRD), scanning electron microscope–energy dispersive X-ray (SEM-EDX) spectrophotometry analysis, UV-Vis diffuse reflectance spectrophotometry (UV-DRS), gas sorption analysis, and Fourier transform infrared (FTIR) spectrometry. XRD patterns were recorded using a Rigaku Miniflex with Ni-filtered Cu-Kα as a radiation source. The reflections obtained were recorded in the range of 2θ of 10–70^o and a step size of 0.2^o min^− 1. Gas sorption analysis was performed using the Quantachrome NOVA 1200e instrument. The samples were degassed at 90 °C for 6 h prior to liquid N₂ adsorption at 77 K. SEM (JSM-5410, JEOL, Japan) was employed for surface image and elemental analyses, UV-DRS (JASCO, Japan) was used for diffuse reflectance spectra, and an FTIR spectrometer (Perkin Elmer, USA) was used for the functional group analysis. The spectra obtained were in the range of 4000–400 cm^− 1.

Photocatalytic activity test of HAP-nTiO₂ composite

The photocatalytic activity of HAp-nTiO₂ was evaluated in MV photocatalysis and photooxidation by using a photocatalytic reactor equipped with a UVB Lamp (20 W, Philips, USA) placed in the inner part of the batch reactor, as presented in Fig. 1.

Fig. 1

Schematic representation of photocatalytic reactor

For each experiment, the mixture of 1 L of MV solution and 0.25 g photocatalyst was placed in the reactor and was exposed by UV light at the pH of 7. Two varied treatments were prepared: photocatalysis and photooxidation; the difference is the absence or presence of H₂O₂ as an oxidant, respectively. The degradation efficiency was calculated based on the change of MV before and after treatment using the following Eq. (1):

$$ Degradation\ efficiency=\frac{C_o-{C}_t}{C_o}x100\% $$

(1)

C_o and C_t are MV concentrations at initial and time of t, respectively. Those concentrations were spectrophotometric determined using UV-Vis spectrophotometric analysis was performed with the wavelength of 583 nm. Spectrophotometer HITACHI U-2010 was utilized. Identification of MV degradation was performed by high performance liquid chromatography (HPLC) analysis. For HPLC, Waters Alliance e2695 (Milford, MA, USA) was employed. Kinetex C18 100A column was used for analysis using 0.1% formic acid in acetonitrile and water as mobile phase (1:1), and UV detector at 214 nm.

Results and discussion

Physicochemical character of materials

Figure 2 shows the XRD patterns for biogenic CaO derived from calcination of cockle shells, HAp, and HAp-nTiO₂ samples. The pattern of CaO shows a major phase of calcite, confirmed by reference to JCPDS card no. 5–586, with some peaks associated with the presence of aragonite in small intensities as an indication of the incomplete conversion of CaCO₃ into CaO. The pattern is similar to biogenic CaO derived from other animal shells, such as mussels, cockles, and scallops [19, 24].

Fig. 2

XRD pattern of (a) HAp-nTiO₂, (b) HAp, and (c) CaO derived from Cockle shell (d) nano-TiO₂

From the XRD pattern of the HAp sample, it can be concluded that HAp was successfully prepared as confirmed by the fitness with the pattern of JCPDS no. 09–0432 (pure HAp). However, small intensity reflections indicated the presence of calcite at 52.9^o and 64^o, implying incomplete conversion of Ca to HAp. This is similar to a pattern that was reported in the preparation of HAp from snail (Achatina achatina) shells [25]. Referring to the reflection of nano TiO₂, it can be seen that there was no significant difference detected in the HAp-nTiO₂ reflection with respect to the HAp. There is no new reflection associated with the presence of TiO₂ in any phase, which was likely related to the homogeneous dispersion of nano TiO₂ in the porous structure of HAp [26]. The absence of an nTiO₂ reflection in the composite is an indication of the absence of TiO₂ aggregation in the composite formation, meaning that TiO₂ was homogeneously dispersed in the composite form. The data can be confirmed by reference to previous similar work on TiO₂ dispersion onto HAp, which reported the relatively low intensity peak of TiO₂ along with decreasing TiO₂ content in the composition [20, 27].

The presence of Ti was identified by EDX analysis. The surface profile depicted by SEM analysis (Fig. 3) exhibits the increased open surface found in HAp-nTiO₂ compared to HAp, CaO, and raw cockle shells. The change in chemical composition of the material was also seen, changing from the Ca, C, and O, the major components of cockle shells, to Ca and O for derived CaO, and to Ca and P for HAp. The ratio of Ca/P in HAp is 1.68, slightly higher than the stoichiometric ratio of 1.67, which was likely caused by the excess Ca content of CaO converted by cockle shell calcination. Ti content in HAp-nTiO₂ was found at 4.43 at%, or equal to about 7.4 wt% of TiO₂, which is also slightly lower than the targeted content of 5 at%. The TiO₂ attachment in the composite showed the effect on the surface profile, identifiable via the gas sorption analysis with the adsorption-desorption isotherm depicted in Fig. 4; the calculated parameters of specific surface area, pore volume, and pore radius, along with the EDX analysis results, are presented in Table 1. The composite has the same pattern of isotherm as HAp, but with higher adsorbed volume at a range of P/Po. From the isotherm, it was found that the specific surface area and pore radius of HAp-nTiO₂ were higher than those of HAp.

Fig. 3

SEM profile and EDX spectra of (a) Cockle shell, (b) CaO, (c) HAp, and (d) HAp-nTiO₂

Fig. 4

Adsorption-desorption profile of HAp and HAp-nTiO₂

Table 1 Surface parameters and elemental analysis result of materials

The attached TiO₂ in the HAp-nTiO₂ contributes to the band gap energy as shown by UV-DRS spectra in Fig. 5. HAp-nTiO₂ shows the band gap energy of 3.21 eV, which is the same value as the band gap energy of nTiO₂. The slight shift of the absorption in the range of 300–400 nm revealed the interaction between HAp and TiO₂ [20]. The same value is in line with the XRD data, which indicates that nTiO₂ is homogeneously dispersed in the HAp matrix.

Fig. 5

DRUV- Visible spectra of HAp-nTiO₂ and TiO₂

Kinetics of photodegradation

Figure 6 shows the treatment-varying plot of the percentage of degradation to the photodegradation process of MV using HAp-nTiO₂, HAp, and TiO₂. Three treatments—adsorption, photocatalysis, and photooxidation—were examined to evaluate the role of the photocatalyst. Adsorption is the treatment of photocatalyst addition without H₂O₂ under dark conditions, while photocatalysis and photooxidation are treatments under UV exposure, distinguished by the addition of H₂O₂ as an oxidant for the photooxidation treatment. From the curve, it can be seen that, in general, the photocatalytic and photooxidation treatments showed higher photodegradation efficiency with respect to adsorption. Significant degradation was demonstrated in photooxidation and photocatalysis using HAp-nTiO₂, TiO₂, and HAp.

Fig. 6

Kinetics of MV degradation by using (a) HAp-nTiO₂, (b) TiO₂, and (c) HAp [Initial MV concentration = 25 mg L^− 1, H₂O₂ = 10^− 4 M, catalyst dosage = 0.25 g L^− 1]

A comparison of kinetics data revealed that TiO₂ is the superior photocatalyst, and affected by the H₂O₂ concentration. The total photoactive surface of TiO₂ plays an intensive role in the MV degradation.

In contrast, both photocatalysis and photooxidation using HAp showed lower efficiency compared with the use of HAp-nTiO₂ andTiO₂ during the monitored time of treatment. The absence of photoactive material is the main point of significance for the data, which is in line with previously reported research that revealed the band gap energy of HAp is 4.85 eV, exceeding the exciting limit of the used UV lamp [28]. The MV degradation is likely caused by the photolysis effect forced by surface activation in the presence of UV exposure [29, 30]. The kinetics data (Table 2) confirmed the effect of the addition of H₂O₂ to the increasing MV degradation by using all photocatalysts.

Table 2 Kinetics equation and parameters of MV photocatalysis and photooxidation using varied photocatalysts

The accelerating rate of degradation is revealed by the increasing initial rate and kinetics constant of reactions. All reactions obey pseudo–first order kinetics. From the data, kinetics constants of photooxidation using all photocatalysts are shown to be higher compared to the values from photocatalysis, meaning the faster reaction is obtained by photooxidation. In general, for all photocatalysts, photooxidation occurred more quickly than photocatalysis. This is in line with the theoretical approach and some previous research on dye degradation enhanced by the presence of oxidant. The presence of H₂O₂ enhances the oxidation mechanism due to its cleavage to form •OH, which poses as a strong oxidant referring to the following mechanisms (2–6): [5].

$$ {\mathrm{TiO}}_2+\mathrm{h}v\to {{\mathrm{e}}^{-}}_{cb}+{{\mathrm{h}}^{+}}_{vb} $$

(2)

$$ {\mathrm{e}}^{-}+{\mathrm{O}}_{2\left(\mathrm{ads}\right)}\to {\mathrm{O}}_{2\left(\mathrm{ads}\right)}\bullet $$

(3)

$$ {\mathrm{e}}^{-}+{{\mathrm{H}}^{+}}_{\mathrm{ads}}\to \mathrm{H}\bullet {}_{\mathrm{ads}} $$

(4)

$$ {\mathrm{O}}_{2\left(\mathrm{ads}\right)}\bullet +{{\mathrm{H}}^{+}}_{\mathrm{ads}}\to \mathrm{HOO}\bullet $$

(5)

$$ {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_{2\left(\mathrm{ads}\right)}\bullet \to \bullet \mathrm{OH} +^{-}\mathrm{OH}+{\mathrm{O}}_2 $$

(6)

The interaction between photoactive semiconductor (TiO₂) and photon (UV light) generates hydroxyl radicals at the surface of the photocatalyst and the conduction band (e⁻), which potentially negatively reduces molecular oxygen. The generation of hydroxyl radicals and oxidant agents will be rapidly enhanced in the presence of H₂O₂ [1, 2].

The increasing rate was confirmed by the UV-Vis spectra of the treated solution (Fig. 7). The molecular structure degradation of MV is revealed not only by the decreasing absorbance of the maximum wavelength in the visible region spectrum (593 nm), but also by the red shift of the spectrum, which is higher with increasing time of treatment, indicating de-methylation of the MV structure [31]. It was also noted that the degradation via de-methylation by photooxidation was the fastest using TiO₂ as the photocatalyst, while the slowest occurred from photocatalysis using HAp. Degradation efficiency of almost 100% was seen in photooxidation under TiO₂ after 45 min of treatment.

Fig. 7

UV-Visible spectra of treated solution by (a) photocatalysis using HAp, (b) photooxidation using HAp, (c) photocatalysis using HAp-nTiO₂, (d) photooxidation using HAp-nTiO₂, (e) photocatalysis using TiO₂, and (f) photooxidation using TiO₂ [Initial MV concentration = 25 mg L^− 1, H₂O₂ = 10^− 4 M, catalyst dosage = 0.25 g L^− 1]

The extent of the degradation was proven from HPLC chromatograms depicted in Fig. 8. The initial solution showed a single peak at the retention time of 6.0 min as an indication of MV. The peak area was reduced along time of treatment, together with the presence of other peaks at 4.9 and 6.4 min, indicating the oxidation intermediate compounds. These changes are in line with the reduction of COD values from 22 mg L^− 1 at initial to 4.2 mg L^− 1 by photooxidation and to approximately 8.2 mg L^− 1 by photocatalysis after 60 min of treatment.

Fig. 8

Chromatogram from HPLC analysis of MV photooxidation treatment [Initial MV concentration = 25 mg L^− 1, H₂O₂ = 10^− 4 M, catalyst dosage = 0.25 g L^− 1]

In terms of catalyst efficiency, the turnover number (TON) is an important parameter. The TON is the quantity of reactant molecules that are converted into products in the presence of a certain weight, specific surface area, or specified active species in the catalyst [32]. Referring to the elemental analysis results tabulated in Table 3, which show the Ti content of 4.43 at%, meaning that TiO₂ content is 7.4 wt%, the TON of reaction using HAp-TiO₂ was calculated based on 7.4 wt%. TiO₂ with the 100 wt% value used for the pure nano TiO₂. The degradation efficiency and TON obtained from 30 min of photocatalysis and photooxidation treatment using HAp-nTiO₂ and TiO₂ are presented in Fig. 9. The degradation efficiency and TON were evaluated at varied catalyst dosages: 0.25, 0.50, and 1.0 g L^− 1. The TON values of the photocatalysis increased as increasing catalyst dosage within the range of 77–84%, while for the photooxidation, the values are ranging from 81 to 97%. These TON values are comparable with the TON values in MV photodegradation by using TiSiW₁₂O₄₀/TiO₂ [33]. Refer to the TiO₂ content of 1.35 wt% and the degradation yield at 30 min by using photocatalyst dosage of 0.15–0.30 g L^− 1, the TON values are ranging from 77 to 88%. Comparability is also found due to the photocatalytic activity of the same composites of TiO₂/HAp for methyl orange and methylene blue photocatalytic degradation [34, 35]. With the degradation efficiency of 33% at 30 min by TiO₂ content of 8.06 wt% in HAp/TiO₂ composite and the catalyst dosage of 20 mg L^− 1, photocatalytic degradation of methyl orange gives the TON value of 82% [34]. Moreover, the photocatalytic degradation of methylene blue by TiO₂/HAp with TiO₂ content of 40 wt% gives the TON of 113% [35], and from other similar research with the TiO₂ content of 5.01, 10.02, and 18.37 wt%, the TON are ranging from 60 to 80% [30].

Table 3 Langmuir-Hinshelwood parameters of MV photocatalysis and photooxidation

Fig. 9

Degradation efficiency and TON at varied photocatalyst dosage of (a) photooxidation using TiO₂, (b) photooxidation using HAp-nTiO₂, (c) photocatalysis using TiO₂, and (d) photocatalysis using HAp-nTiO₂ [Initial MV concentration = 25 mg L^− 1, H₂O₂ = 10^− 4 M, TON values are calculated from 30 min of treatment]

From the above discussion, the use of HAp-nTiO₂ instead of TiO₂ should be made due to the higher TON as the indication of more efficient photocatalytic material. Moreover, the use of Cockle shells in this research is an advantage related to the sustainability of raw material for HAp synthesis.

Effects of the initial MV concentration on the photocatalysis and photooxidation rates using HAp-nTiO₂ as a photocatalyst can be seen by the kinetic curve in Fig. 10a–b. Both treatments obey pseudo–first order kinetics in all initial concentration ranges (Fig. 10b). Given numerous investigations on dye photodegradation, the dependence of the reaction rates on the concentration of the dye can be well described by the Langmuir–Hinshelwood (L–H) kinetic model [30, 33, 34] as:

$$ \frac{1}{{\mathrm{r}}_{\mathrm{o}}}=\frac{1}{{\mathrm{k}}_{\mathrm{c}}{\mathrm{K}}_{\mathrm{LH}}\left[{\mathrm{C}}_0\right]}+\frac{1}{{\mathrm{k}}_{\mathrm{c}}} $$

(7)

Fig. 10

a Pseudo-first order kinetics plot of photocatalysis using HAp-nTiO₂, (b) Pseudo-first order kinetics plot of photooxidation using HAp-nTiO₂, and (c) Langmuir-Hinshelwood plot photocatalysis and photooxidation treatments using HAp-nTiO₂ and TiO₂

Here, r_o is the initial rate of degradation, C₀ is the initial concentration of MV (mg L^− 1), K_LH is the L-H adsorption constant (mg^− 1 L), and k_c is the rate constant of surface reaction (mg L^− 1 min^− 1). The values of k_c and K_LH describe the limiting rate constants of reaction at maximum coverage of the photocatalyst surface under the given experimental conditions and the equilibrium constant for adsorption of MV onto the photocatalyst, respectively. The L–H plots are presented in Fig. 10c.

The plot of 1/r₀ versus 1/C₀ shows a linear variation for all treatments using HAp-nTiO₂ and TiO₂ photocatalysts, confirming the fitness of the L-H relationship for the initial rates of degradation. The higher the initial concentration of MV, the faster the degradation, which fits into the general theory of reaction rates. The values of k_c and K_LH calculated from the equation are listed in Table 3. The photooxidation process gives higher k_c values, which describe the higher apparent kinetics constants. The data are consistent with the possible oxidation mechanism being accelerated in the presence of H₂O₂ as an oxidant.

Reusability test of the HAp-nTiO₂ photocatalyst

HAp-nTiO₂ reusability was observed by the kinetics profile and degradation efficiency as presented in Fig. 11. From these patterns, it was concluded that there was no significant change in kinetics profile between the first and the third uses. In contrast, significant reduction of activity appeared in TiO₂ utilization at the second and third uses. The reduction in degradation efficiency was most likely due to the loss of the photocatalyst surface active sites after the first use. Similar patterns have also been reported by the use of nanoparticles as photocatalysts [36,37,38]. From this comparison, it can also be confirmed that HAp plays a role in stabilizing TiO₂ in the photocatalysis mechanism.

Fig. 11

a Comparison on MV photooxidation kinetics by fresh and recycled photocatalysts by HAp-nTiO₂, and (b) Comparison on MV photooxidation kinetics by fresh and recycled photocatalysts by TiO₂ (c) Comparison on degradation efficiency of MV photooxidation by fresh and recycled photocatalysts [Initial MV concentration = 100 mg L^− 1, Time of sampling = 60 min, H₂O₂ = 10^− 4 M, catalyst dosage = 0.25 g L^− 1]

Conclusions

HAp-nTiO₂ composite was successfully prepared using HAp powders derived from cockle (A. granosa) shells. The results indicated that HAp was completely formed, as shown by the XRD pattern, and, furthermore, that the nTiO₂ was homogeneously dispersed in the composite. The dispersion of nTiO₂ in the composite resulted in increasing specific surface area, pore volume, and in the band gap energy having the same value as nTiO_2. The composite exhibited photocatalytic activity for photocatalysis and photooxidation of MV. The photocatalytic processes under HAp-nTiO₂ and nTiO₂ fit well with the L-H model. The use of HAp-nTiO₂ gave a higher TON for the processes, which was attributed to the more effective photocatalysis compared to the use of pure nTiO₂ for the same reactions.