Nanosized Zincated Hydroxyapatite as a Promising Heterogeneous Photo-Fenton-Like Catalyst for Methylene Blue Degradation

Abstract

This study is devoted to synthesis of nanosized zincated hydroxyapatite (Zn-HA) and its utilization as a heterogeneous photo-Fenton-like catalyst for degradation of methylene blue (MB) in aqueous solution. The prepared catalyst was characterized by various techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray, and Fourier transform infrared spectroscopy. The catalytic activity of Zn-HA towards MB and the effects of various experimental factors such as pH, zinc substitution degrees, initial MB concentration, and H₂O₂ dosage were studied in detail. The results showed that the zinc substitution degree of 0.4 is optimal to get the highest degradation efficiency under conditions of pH = 10, H₂O₂ dosage of 0.05 M, and MB concentration of 30 mg/L for a contact time of 120 min. The degradation mechanism was proposed and discussed thoroughly. Besides, the ability of long-term use for the synthesized catalyst was also evaluated.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The rapid industrialization with developing various kinds of chemical-based industries leads to several major environmental issues caused by a huge amount of toxic substances discharged into ecosystems. In this context, persistent organic pollutants as dyes must be taken into account due to their high toxicity and nonbiodegradability under normal conditions [1–4]. Recently, an examination conducted in Singapore in 2017 reported that essential everyday foods such as vegetables, canned meat, fruits, and cheese in local supermarkets contained at least one type of azo dyes which might cause several health problems as lethal, genotoxic, mutagenic, and carcinogenic effects [5]. Thus, removal of toxic dyes from contaminated sources is extremely important and attracts a great attention from the global scientific community.

Up to now, there have been several methods used for removal of organic pollutants such as flocculation, ion-exchange, reverse osmosis, adsorption, etc. [6–8]. A major drawback of these methods is related to the secondary polluted compounds that can be generated since the separated pollutants are not destroyed after detoxification [9, 10]. To overcome this barrier, heterogeneous Fenton-photocatalysis has been utilized to remove organic pollutants by degrading them into eco-friendly biodegradable substances [11, 12]. In this photocatalysis, properties of photocatalysts play a leading role in ensuring success of a treatment process. In this regard, many kinds of Fenton-photocatalysts have been developed. Among them, heterogeneous photocatalysts containing transition metals such as Fe, Cu, Zn, Mn, Co, Mn, and Ti are widely used due to their high photocatalytic activity and low cost [11, 13]. However, these materials exhibit a good performance mostly in UV region, but not under a wide spectrum of visible light due to the limits of their band gaps [14–16]. Moreover, most of the methods used to modify band gaps for improving photocatalytic activity of catalysts are quite expensive and complicated [17–20].

Recently, several studies have been made on using hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂-HA), a main mineral constituent of mammalian hard tissues, as a supporting material for improving photocatalytic activity of catalysts because of its biocompatibility and excellent adsorption capacity and photocatalytic activity for removal of both organic compounds and toxic heavy metals [21–24]. With regards to photocatalytic activity, Valizadeh et al. have successfully synthesized magnetite-hydroxyapatite for photocatalytic degradation of acid blue 25 [24]. Besides, another research group [25] used waste mussel shells to develop HA as a greener and renewable photocatalyst for the degradation of methylene blue (MB) [25]. Although the photocatalytic capacity of HA is not higher than those of metal-oxide photocatalysts, cost benefits, eco-friendliness, simple preparation, and reusability are huge advantages of this material. Especially, the structure of HA can be modified because its cations Ca²⁺ and functional groups of ${{\text{PO}}_4}^{3-}$ and OH⁻ are easily replaced by other ions, and therefore, its photocatalytic potential can be intervened. For example, Fe₃O₄-hydroxyapatite modified by Mn, Fe, Co, Ni, Cu, and Zn as reported in [26] exhibited very high photocatalytic capability towards dyes in water under UV irradiation. However, there have been several drawbacks that are needed to overcome. Firstly, the Fe₃O₄ and hydroxyapatite components were physically linked to each other, leading to the low stability of material structure during a long-term treatment process. Secondly, the used metals were distributed mainly inside Fe₃O₄-hydroxyapatite structure, but not on its surface due to the substitution of Ca²⁺ by metals ions took place mainly at lattice points [26, 27]. As a result, Fe₃O₄-hydroxyapatite could not be able to exhibit its full photocatalytic potential.

Based on the above arguments, this study is devoted to using chemical precipitation method to develop a single-phase Zn-HA as a heterogenous photo-Fenton-like catalyst for MB degradation. For this material, Zn is integrated into HA via anion substitution of ${{\text{PO}}_4}^{3-}$ by ${\text{Zn}{\left(\text{OH}\right)}_4}^{2-}$. Crystalline structure, morphology, features of functional groups, and element analysis of Zn-HA were carefully characterized using XRD, SEM, TEM, FTIR, and EDX techniques. Besides, the optimal conditions for degradation of MB were also determined. Finally, the reusability was also evaluated for the long-term of the catalyst.

2. Materials and Methods

2.1. Reagents and Materials

Calcium hydroxide (Ca(OH)₂, ≥96%), phosphoric acid (H₃PO₄, 85%), sodium hydroxide (NaOH, 99%), zinc oxide (ZnO, 99%), MB (C₁₆H₁₈ClN₃S, ≥82%), hydrogen peroxide (H₂O₂, 30%), and ammonium hydroxide (NH₄OH, 30%) were reagent-grade, supplied by Merck chemical company (Germany), and used without further purification. Distilled water was used to prepare all necessary solutions for the synthesis of Zn-HA.

2.2. Preparation of Zn-HA Catalyst

The Zn-HA samples were synthesized by the chemical precipitation method from solutions of Na₂Zn(OH)₄, Ca(OH)₂, and H₃PO₄. The solution of Na₂Zn(OH)₄ was prepared by adding gradually 0.2 M NaOH solution into a suspension of 0.05 M ZnO until a clear solution was obtained. The solutions of 0.02 M Ca(OH)₂ and H₃PO₄ 10% were prepared from pure solid of Ca(OH)₂ and concentrated solution of H₃PO₄ 85%, respectively. For the synthesis of Zn-HA, the solution of H₃PO₄ 10% was slowly added drop by drop into a reactor containing Na₂Zn(OH)₄ and Ca(OH)₂ with vigorous stirring for 1 h. The solutions were taken out at different Ca : P : Zn ratios of 9 : 6 : 0; 9 : 5.9 : 0.1; 9 : 5.75 : 0.25; 9 : 5.6 : 0.4; 9 : 5.4 : 0.55; and 9 : 5.3 : 0.65 named as HA, Zn-HA-0.1, Zn-HA-0.25, Zn-HA-0.4, Zn-HA-0.55, and Zn-HA-0.65, respectively. The pH of obtained solutions was kept at pH ≥ 10 using NH₄OH. After the reactions finished, the mixtures were left for aging at room temperature for 24 h and then filtered, washed, dried, crushed, and pulverized through a 100-mesh sieve.

2.3. Characterization of Catalysts

X-ray powder diffraction (XRD) on a Shimadzu 6100 diffractometer (Japan) with Cu target (λ = 1.5406 Å), accelerating voltage of 40 kV, and current stream of 30 mA in a scanning range from 5 to 90° at a speed of 0.05°/s with a step size of 0.02° was used to explore structural properties of materials. The features of functional groups were analyzed by Fourier transform infrared spectroscopy (FTIR) technique on a Tensor 27 spectrophotometer (Germany) in a scanning range from 500–4000 cm⁻¹ using KBr pellet method. The morphology of materials was tested by the methods of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) on Jeol JSM-6480 LV and Jeol 1400 (Japan) microscopes, respectively. Finally, energy-dispersive X-ray (EDX) and element mapping techniques were utilized on a Horiba 7593 dispersive X-ray microanalyzer (Japan) to analyze the chemical elemental characteristics of the obtained materials.

2.4. Experiments for MB Degradation

The photo-Fenton catalytic potential of the as-prepared Zn-HA composite was evaluated through the degradation of MB as a model azo dye. A certain amount of catalyst was dispersed into 100 mL of MB solution (20, 30, 50, 80, 100 mg/L) and stirred in the dark for 30 min to reach adsorption-desorption equilibrium between MB and catalyst. Then, a calculated amount of H₂O₂ was added into the suspension to initiate the reaction under light irradiation turned on at the same time from 250 W halogen lamp (HLX 64653; Osram, Germany, wavelength range of 300–800 nm) equipped with a 420 nm cut-off filter. Besides, a thermostatic water-bath was employed to keep the reaction temperature at a desired value. In addition, the initial pH of MB solutions was adjusted to a desired value using 0.1 M NaOH or 0.1 M HCl solutions. After each 10-minute contact time, 2 mL of the mixture was taken out, the catalyst was then eliminated by centrifugation, and the remaining MB concentration was analyzed using an Evolution 600 UV-visible spectrophotometer at λ_max = 664 nm. The influence of factors as initial pH, catalyst dosage, initial hydrogen peroxide dosage, MB concentration, and zinc content on the MB degradation was carefully examined. The efficiency for degradation of MB using Zn-HA is calculated by the following formula:$$\begin{array}{}\text{(1)}& R_{\text{e}}=\frac{C_0-C_{\text{f}}}{C_0}\cdot 100,\end{array}$$where $R_{\text{e}}$ (%) is the removal efficiency and $C_0$(mg/L) and $C_{\text{f}}$(mg/L) are the initial and final concentrations of MB solution before and after degradation, respectively.

Photocatalytic stability and reusability of the catalysts were evaluated by testing the photocatalytic degradation efficiency of MB in five consecutive cycles, each cycle lasted 60 min. After each cycle, the photocatalyst was centrifuged and then added again into fresh MB solution (100 mL, 50 mg/L, pH of 10, H₂O₂ dosage of 0.05 M) for the next round.

3. Results and Discussion

3.1. Characterization of the Catalyst

The results for study on crystalline structures of HA and Zn-HA with different Zn content are shown in Figure 1.

[figure omitted; refer to PDF]

It is obviously seen that the XRD patterns of Zn-HA samples contain all characteristic peaks for HA at 2θ of 10.8, 25.7, 28.2, 29.1, 31.7, 32.8, 39.7, 46.9, 48, 49.5, and 53.1° (ICD PDF 00-024-0033). In other words, the zinc doping did not affect crystalline structure of HA. Similar results were also reported in previous studies [28–30]. However, as compared to the HA, the peaks obtained for Zn-HA are broadened. This might be due to the decrease in the crystallinity and particle size of the Zn-HA sample [30]. Besides, the XRD patterns also indicated that there were additional phases of Ca(OH)₂ and CaCO₃ detected in Zn-HA samples at a zinc substitution degree higher than 0.4 (Figure 1). This anomaly can be explained as follows. The increase in the amount of ${\text{Zn}{\left(\text{OH}\right)}_4}^{2-}$ (which was made by the dissolution of ZnO in NaOH as described above) in the reaction mixture of Ca(OH)₂ and H₃PO₄ led to the shortage of H₃PO₄ due to its neutralization by NaOH and therefore resulting in the superabundance of Ca(OH)₂. At the same time, a part of extra Ca(OH)₂ could react with CO₂ in atmosphere to form CaCO₃.

The morphology of HA and Zn-HA at Zn-substitution degree of 0.4 is presented in Figure 2. It can be seen that the crystals of HA were grown in needle shape with 150–200 nm length and 30–50 nm width (Figure 2(a)) while Zn-HA crystals have a spherical shape with a diameter of about 20 nm (Figure 2(b)). The difference of size and shape between HA and Zn-HA might be associated with the increased number of crystallization centers when ${\text{Zn}{\left(\text{OH}\right)}_4}^{2-}$ was added into the reaction mixture of Ca(OH)₂ and H₃PO₄. Consequently, more crystals might be formed, leading to the decrease in their size. This is also in good agreement with the above XRD patterns. It should be noticed that the smaller the size of crystals, the larger the specific surface area obtained. This feature is an advantage of Zn-HA over HA towards photocatalytic ability. On a larger-scale view as shown in SEM images (Figures 2(c) and 2(d)), both HA and Zn-HA show a similar morphology.

[figures omitted; refer to PDF]

The doping of Zn into HA was also confirmed by EDX spectra and element mapping as shown in Figure 3. According to the EDX spectrum for Zn-HA (Figure 3(b)), along with the main elements characteristic for HA such as Ca, P, and O (Figure 3(a)), Zn was detected. Besides, the contents of Ca, P, and O elements in HA and Zn-HA are different. Comparing with HA, the content of P and O in Zn-HA was found to decrease from 13.77 to 2.41 w% and 59.97 to 50.01 w%, respectively. Meanwhile, the Ca content increased from 23.81 to 44% equal to the Ca content in stoichiometric HA. It obviously indicated that ${{\text{PO}}_4}^{3-}$ groups in HA were partially replaced by ${\text{Zn}{\left(\text{OH}\right)}_4}^{2-}$. Moreover, the element mapping shows also the appearance of 4 elements of Ca, Zn, O and P on Zn-HA (Figures 3(c)–3(f)). In addition, the even distribution of Zn ions on Zn-HA surface (Figure 3(d)) is expected to improve photocatalytic activity of the synthesized catalyst as compared to those of HA.

[figures omitted; refer to PDF]

The results for study on features of functional groups for HA and Zn-HA with different Zn content are shown in Figure 4. It is obviously seen that FTIR spectra obtained for HA and Zn-HA have a similar shape and most of characteristic peaks as 1030, 1085, 956, and 874 cm⁻¹ referred to OH⁻ were detected at the same positions. Besides, the presence of ${{\text{CO}}_3}^{2-}$ as mentioned above was also detected at 1650 and 1423 cm⁻¹. However, a difference of intensity between some peaks in HA and Zn-HA was observed. Specifically, for Zn-HA samples, the intensity of a peak at 3640 cm⁻¹ (OH⁻ groups) was found to be increased and at 1030 cm⁻¹ (${{\text{PO}}_4}^{3-}$ groups) decreased in comparison with those for HA, confirming the successful integration of ${\text{Zn}{\left(\text{OH}\right)}_4}^{2-}$ into HA structure.

[figure omitted; refer to PDF]

3.2. Photo-Fenton Catalytic Activity of the Catalysts

In order to evaluate the photocatalytic advantage of Zn-HA over HA, a comparative study was conducted under the same experimental conditions of light irradiation, pH = 10, and catalyst loading of 0.1 g added into 100 mL of 50 mg·L⁻¹ MB solution at room temperature. The Fenton-photocatalytic activity of catalysts was tested by adding 0.05 M H₂O₂ into the MB solution. As seen in Figure 5, when H₂O₂ was absent, the removal efficiencies obtained for both Zn-HA and HA were quite low as compared to those with the participation of H₂O₂. Besides, the maximum degradation efficiency of Zn-HA was much higher than that of HA for both cases with presence of H₂O₂ and absence of H₂O₂ (Figure 5). Obviously, the removal process in the absence of H₂O₂ was controlled mainly by adsorption mechanism, and therefore, the smaller size of Zn-HA helps it to take advantage over HA. In the presence of H₂O₂, along with adsorption mechanism, the Fenton-photocatalytic reactions could be activated and improved the degradation process. In this regard, the higher degradation efficiency of Zn-HA than that of HA indicated its stronger Fenton-photocatalytic potential. In other words, the presence of Zn could significantly improve the Fenton-photocatalytic ability of HA.

[figure omitted; refer to PDF]

The photodegradation mechanism can be formulated as follows. Under visible light irradiation, electron/hole pairs were photogenerated in the catalyst (equation (2)). Then, photogenerated electrons could be trapped by H₂O₂, leading to the formation of ${}^{\cdot }\text{O}\text{H}$ (equation (3)) [31]. At the same time, Zn²⁺ ions after occupying electrons (equations (4)) were transformed into Zn which reacted with H₂O₂ to form ZnO, a strong photocatalyst (equation (5)) [32]. During light irradiation, ZnO was capable of creating more electrons and holes (equation (6)), and therefore, the decomposition process of H₂O₂ to release ${}^{\cdot }\text{O}\text{H}$ will be accelerated due to the support of electrons (equation (8)). In addition, the generated electrons could also stimulate the formation of superoxide radicals through reactions with dissolved oxygen in the solution (equation (7)). Meanwhile, the holes could also contribute to the formation of ${}^{\cdot }\text{O}\text{H}$ when reacting with H₂O and OH⁻ (equations (8) and (9)). Finally, ${}^{\cdot }\text{O}\text{H}$ and ${{{}^{\cdot }\text{O}}_2}^{-}$ will degrade MB molecules according to equations (10) and (11) [33].$$\begin{array}{}\text{(2)}& \text{Zn}-\text{HA}+h\nu ⟶\text{Zn}-\text{HA}+e^{-}+h^{+}\text{(3)}& {\text{H}}_2{\text{O}}_2+e^{-}⟶{}^{\cdot }\text{O}\text{H}+{\text{OH}}^{-}\text{(4)}& {\text{Zn}}^{2+}+2e^{-}⟶\text{Zn}\text{(5)}& \text{Zn}+{\text{H}}_2{\text{O}}_2⟶\text{ZnO}+{\text{H}}_2\text{O}\text{(6)}& \text{ZnO}+h\nu ⟶e^{-}+h^{+}\text{(7)}& e^{-}+{\text{O}}_2⟶{{{}^{\cdot }\text{O}}_2}^{-}\text{(8)}& h^{+}+{\text{H}}_2\text{O}⟶{\text{H}}^{+}+{}^{\cdot }\text{O}\text{H}\text{(9)}& h^{+}+{\text{OH}}^{-}⟶{}^{\cdot }\text{O}\text{H}\text{(10)}& {}^{\cdot }\text{O}\text{H}+\text{MB}⟶\text{degradation\hspace{0.17em}products}\text{(11)}& {{{}^{\cdot }\text{O}}_2}^{-}+\text{MB}⟶\text{degradation\hspace{0.17em}products}\end{array}$$

Although the above results indicated the advantage of Zn in enhancing Fenton-photocatalytic activity of Zn-HA over HA, the optimum content of Zn is needed to be determined. For this purpose, a comparative study on catalytic activity of Zn-HA samples with different zinc substitution degrees under the same degradation conditions as mentioned. The results showed that the zinc substitution degree of 0.4 gave the highest degradation efficiency of 95% after a contact time of 30 min as shown in Figure 6. In this regard, the obtained value of 0.4 is reasonable and was chosen for further experiments.

[figure omitted; refer to PDF]

3.3. Influence of Experimental Conditions on MB Degradation

3.3.1. Effect of pH

Effect of pH on degradation efficiency of Zn-HA was studied in pH range from 4 to 10 using 0.1 g of Zn-HA added into 100 mL solution of 50 mg/L MB and 0.05 M H₂O₂. The obtained results are presented in Figure 7(a). It is obviously seen that the degradation efficiency significantly increased with increasing solution pH. This can be explained by the increased amount of ${\text{OH}}^{-}$ groups, leading to rise in the amount of ${}^{\cdot }\text{O}\text{H}$ radicals generated through Fenton-oxidation mechanism, and therefore the degradation process was improved [13]. The fact that increasing pH greater than 10 does not make sense for practical applications since there is no economic benefit and the secondary pollutants could be generated at high pH. In this regard, pH = 10 was chosen as an optimal pH for all catalytic experiments in this work.

[figures omitted; refer to PDF]

3.3.2. Effect of Initial MB Concentrations

Different initial MB concentrations of 20, 30, 50, 80, and 100 mg/L were used to evaluate their effect on degradation efficiency of Zn-HA catalyst under pH of 10, catalyst dosage of 0.1 g, and H₂O₂ dosage of 0.05 M at room temperature. Based on the obtained results, the optimal initial MB concentration was determined as 30 mg/L (Figure 7(b)). At concentrations higher than 30 mg/L, the reduction of degradation efficiency was observed. The anomaly could be related to the formation of a layer of MB molecules accumulated on Zn-HA surface at high MB concentrations, inhibiting light from entering Zn sites and therefore holding back the decolorization process [34].

3.3.3. Effect of H₂O₂ Concentration

The study was carried out using different initial H₂O₂ concentrations of 0.01, 0.03, 0.05, and 0.10 M under the established above optimal conditions. According to the results shown in Figure 7(c), the initial H₂O₂ concentration of 0.05 M is optimal for degradation of MB using Zn-HA. The deviation of initial H₂O₂ concentration from 0.05 M did not improve degradation process. It is obviously seen that at H₂O₂ concentrations lower 0.05 M, the reduction of degradation efficiency could be associated with the decrease in the number of generated ${}^{\cdot }\text{O}\text{H}$ radicals. Meanwhile, at H₂O₂ concentrations greater than 0.05 M, the degradation efficiency did not increase (Figure 7(c)), probably, due to the generation of perhydroxyl radicals caused by the combination of extra H₂O₂ dosage with hydroxyl radicals [35].

3.4. Stability and Reusability of Catalyst

The stability and reusability of a catalyst are essential for its practical applications. For Zn-HA catalyst, the recycling tests were carried out under optimal conditions in five continuous cycles. The results shown in Figure 8(a) indicated that the catalytic potential of Zn-HA towards MB was well maintained with a slight reduction of degradation efficiency from 100% to 96.5% after five continuous cycles. Besides, the XRD pattern of Zn-HA after the fifth degradation cycle contained almost all characteristic peaks (Figure 8(b)), demonstrating high stability of composite structure during degradation process.

[figures omitted; refer to PDF]

4. Conclusions

In this study, the nanosized Zn-HA was successfully synthesized and applied as a heterogeneous photo-Fenton-like catalyst for MB degradation under optimal conditions of pH = 10, H₂O₂ dosage of 0.05 M, and MB concentration of 30 mg/L for a contact time of 120 min. Besides, the zinc substitution degree of 0.4 is optimal to improve the photocatalytic activity of the catalyst. The recycling study demonstrated a good maintenance of degradation ability and high stability of catalyst structure in a long-term degradation process. Overall, the positive results suggested that the synthesized catalyst can be expected as a promising heterogeneous photocatalyst for degradation of MB in industrial wastewater.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors acknowledge the Industrial University of Ho Chi Minh City, Vietnam, for supporting this work (Funding No. 184.HHO5).

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