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

Faculty of Chemical Engineering, Industrial University of Ho Chi Minh City, Ho Chi Minh City, Vietnam Center for Advanced Chemistry, Institute of Research & Development, Duy Tan University, Danang, Vietnam Division of Computational Physics, Institute for Computational Science, Ton Duc'ang University, Ho Chi Minh City, Vietnam Faculty of Electrical & Electronics Engineering, Ton Duc 'ang University, Ho Chi Minh City, Vietnam


Introduction
e 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][2][3][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].us, 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][7][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 Fentonphotocatalysis 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 Fentonphotocatalysts 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][15][16].Moreover, most of the methods used to modify band gaps for improving photocatalytic activity of catalysts are quite expensive and complicated [17][18][19][20].
Recently, several studies have been made on using hydroxyapatite (Ca 10 (PO 4 ) 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][22][23][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, ecofriendliness, simple preparation, and reusability are huge advantages of this material.Especially, the structure of HA can be modified because its cations Ca 2+ and functional groups of PO 4 3− and OH − are easily replaced by other ions, and therefore, its photocatalytic potential can be intervened.For example, Fe 3 O 4 -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 Fe 3 O 4 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 Fe 3 O 4 -hydroxyapatite structure, but not on its surface due to the substitution of Ca 2+ by metals ions took place mainly at lattice points [26,27].As a result, Fe 3 O 4 -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 singlephase Zn-HA as a heterogenous photo-Fenton-like catalyst for MB degradation.For this material, Zn is integrated into HA via anion substitution of PO 4 3− by Zn(OH) 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.

Preparation of Zn-HA Catalyst.
e Zn-HA samples were synthesized by the chemical precipitation method from solutions of Na 2 Zn(OH) 4 , Ca(OH) 2 , and H 3 PO 4 .e solution of Na 2 Zn(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.e solutions of 0.02 M Ca(OH) 2 and H 3 PO 4 10% were prepared from pure solid of Ca(OH) 2 and concentrated solution of H 3 PO 4 85%, respectively.For the synthesis of Zn-HA, the solution of H 3 PO 4 10% was slowly added drop by drop into a reactor containing Na 2 Zn(OH) 4 and Ca(OH) 2 with vigorous stirring for 1 h.e 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.e pH of obtained solutions was kept at pH ≥ 10 using NH 4 OH.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.

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.e 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.e 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.

Experiments for MB Degradation.
e 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.en, a calculated amount of H 2 O 2 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 10minute 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.
e influence of factors as initial pH, catalyst dosage, initial 2 Advances in Materials Science and Engineering hydrogen peroxide dosage, MB concentration, and zinc content on the MB degradation was carefully examined.e efficiency for degradation of MB using Zn-HA is calculated by the following formula: where R e (%) is the removal efficiency and C 0 (mg/L) and C 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, H 2 O 2 dosage of 0.05 M) for the next round.

Characterization of the Catalyst.
e results for study on crystalline structures of HA and Zn-HA with different Zn content are shown in Figure 1.
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][29][30].However, as compared to the HA, the peaks obtained for Zn-HA are broadened.is 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 CaCO 3 detected in Zn-HA samples at a zinc substitution degree higher than 0.4 (Figure 1). is anomaly can be explained as follows.
e increase in the amount of Zn(OH) 4 2− (which was made by the dissolution of ZnO in NaOH as described above) in the reaction mixture of Ca(OH) 2 and H 3 PO 4 led to the shortage of H 3 PO 4 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 CO 2 in atmosphere to form CaCO 3 .
e 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)).e difference of size and shape between HA and Zn-HA might be associated with the increased number of crystallization centers when Zn(OH) 4 2− was added into the reaction mixture of Ca(OH) 2 and H 3 PO 4 .Consequently, more crystals might be formed, leading to the decrease in their size.is 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.
is 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.e 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 PO 4 3− groups in HA were partially replaced by Zn(OH) 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.
e 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 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 (PO 4 3− groups) decreased in comparison with those for HA, confirming the successful integration of Zn(OH) 4 2− into HA structure.the MB solution.As seen in Figure 5, when H 2 O 2 was absent, the removal efficiencies obtained for both Zn-HA and HA were quite low as compared to those with the participation of H 2 O 2 .Besides, the maximum degradation efficiency of Zn-HA was much higher than that of HA for both cases with presence of H 2 O 2 and absence of H 2 O 2 (Figure 5).Obviously, the removal process in the absence of H 2 O 2 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 H 2 O 2 , 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.

Photo-Fenton Catalytic
e photodegradation mechanism can be formulated as follows.Under visible light irradiation, electron/hole pairs were photogenerated in the catalyst (equation ( 2)).
en, photogenerated electrons could be trapped by H 2 O 2 , leading to the formation of • OH (equation ( 3)) [31].At the same time, Zn 2+ ions after occupying electrons (equations ( 4)) were transformed into Zn which reacted with H 2 O 2 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 H 2 O 2 to release • OH 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 • OH when reacting with H 2 O and OH − (equations ( 8) and ( 9)).Finally, • OH and • O 2 − will degrade MB molecules according to equations ( 10) and ( 11) [33].

Advances in Materials Science and Engineering
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 di erent zinc substitution degrees under the same degradation conditions as mentioned.
e results showed that the zinc substitution degree of 0.4 gave the highest degradation e ciency 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.

E ect of pH. E ect of pH on degradation e ciency 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 e obtained results are presented in Figure 7(a).It is obviously seen that the degradation eciency signi cantly increased with increasing solution pH.
is can be explained by the increased amount of OH − groups, leading to rise in the amount of • OH radicals generated through Fenton-oxidation mechanism, and therefore the degradation process was improved [13].e fact that increasing pH greater than 10 does not make sense for practical applications since there is no economic bene t 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.H 2 O 2 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 eciency was observed.e 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].

E ect of H 2 O 2 Concentration.
e study was carried out using di erent initial H 2 O 2 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 H 2 O 2 concentration of 0.05 M is optimal for degradation of MB using Zn-HA.e deviation of initial H 2 O 2 concentration from 0.05 M did not improve degradation process.It is obviously seen that at H 2 O 2 concentrations lower 0.05 M, the reduction of degradation e ciency could be associated with the decrease in the number of generated • OH radicals.Meanwhile, at H 2 O 2 concentrations greater than 0.05 M, the degradation e ciency did not increase (Figure 7(c)), probably, due to the generation of perhydroxyl radicals caused by the combination of extra H 2 O 2 dosage with hydroxyl radicals [35].

Stability and Reusability of Catalyst.
e 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 ve continuous cycles.e 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 e ciency from 100% to 96.5% after ve continuous cycles.Besides, the XRD pattern of Zn-HA after the fth degradation cycle contained almost all characteristic peaks (Figure 8(b)), demonstrating high stability of composite structure during degradation process.

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, H 2 O 2 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.
e 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.Advances in Materials Science and Engineering 7

Figure 1 :
Figure 1: XRD patterns of HA and Zn-HA samples with different Zn content.

Figure 2 :
Figure 2: TEM and SEM images for HA (a, c) and Zn-HA (b, d).

Figure 6 : 1 HAFigure 4 :
Figure 6: Comparative study on catalytic activity of Zn-HA samples with di erent zinc substitution degrees.

Figure 5 :Figure 7 :Figure 8 :
Figure 5: Comparative study on catalytic activity of the HA and Zn-HA for degradation of MB.
Initial MB Concentrations.Di erent initial MB concentrations of 20, 30, 50, 80, and 100 mg/L were used to evaluate their e ect on degradation e ciency of Zn-HA catalyst under pH of 10, catalyst dosage of 0.1 g, and