Remarkable photocatalytic degradation of Remazol Brilliant Blue R dye using bio-photocatalyst ‘nano-hydroxyapatite’

Nanostructured bio-photocatalyst hydroxyapatite (HAp) was synthesized by a simple and productive wet chemical precipitation method to explore the photocatalytic performance of the material for degradation of Remazol Brilliant Blue R (RBBR) dye. Wet chemically synthesized nano-HAp was characterized through x-ray diffraction, Transmission electron microscope, scanning electron microscope, N2-adsorption–desorption, thermo gravimetric analyzer, Fourier transform infrared and UV–vis absorption techniques. Photocatalytic degradation of RBBR under ultraviolet irradiation was examined and ∼80% degradation was achieved in 120 min by using nano-HAp. The photocatalytic degradation followed Parabolic Diffusion kinetics. Fast degradation kinetics and removal of RBBR dye was achieved by nano-HAp due to modification of surface entities leading to suppression of photo-generated electron-hole recombination. This makes nano-HAp a remarkable bio-photocatalyst material for removing dyes from water.


Introduction
Rapid industrialization across the world raises an environmental problem like water pollution. To overcome with this, photocatalysis using various metal oxide semiconducting materials [1,2] has received a great attention across researchers [3][4][5]. During photocatalytic degradation, the photocatalyst is excited by ultraviolet (UV)/ visible photons to generate electron-hole pairs. These photo-induced charges separate and migrate to the active reaction sites and at last on the surface of the photo-catalyst the pollutant is reduced and degraded. Pollutants in water are generally in the form of dangerous organic or inorganic materials; including heavy metals, chemicals, dyes, etc [6][7][8]. Existences of such pollutants are serious threat to public health and leading to the destruction of ecosystem [9].
Photocatalytic dye degradation does not require any extra effort for treating waste water. This technology requires material with high stability, non-toxicity, photo-generating radical characteristic and wide band gap [10][11][12][13][14][15][16]. Binary metal oxides (such as ZnO, TiO 2 , ZrO 2 , etc) have wide band-gap energy, whichresults in recombination of photo-generated electron-hole pairs leading to low photocatalytic activity [5]. Hydroxyapatite (HAp) is a bio-compatible material found in human bones. HAp is thermal stable, non-toxic with ion exchange capability. Itwasutilized in bulk form (crystallite size in μm) as a photo-catalyst for dye degradation [15,17,18] and was found to be low photoactive [5].
One of the best methods to improve the photocatalytic performance of HAp is the selection of synthesis method, which provides nano-HAp crystals. A number of routes, including sol-gel, chemical precipitation, hydrothermal and plasma oxidation methods were developed so far [5,15]. These methods are costly or inefficiency or high energy consuming,therefore, it is still challenging to prepare nano-HAp. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
In the present work, nano-HAp have been synthesized, characterized and tested as photocatalyst for degradation of RBBR dye under UV light irradiation. HAp is a stable biomaterial used in heterogeneous photocatalytic degradation of pollutants and it also used as a photocatalyst under UV irradiation. The electronic state of the surface PO 4 3− group changes to create a vacancy on HAp surface and the electron is transferred to the surrounding oxygen followed by the formation of O 2°r adicals. The generated superoxide radical will oxidize the organic molecule and also may react with the water molecule. The induced OH radicals oxidize the pollutant organic molecules adsorbed on to the photo-catalyst. These phenomena on the HAp would be due to photoinduced electronic excitation. The effect of dye concentration on photocatalytic activity of nano-HAp has been studies. A kinetic study was also conducted for finding the photocatalytic degradation model.

Experimental
Nano-HAp was synthesized by a wet chemical precipitation method. ) is a synthetic dye (procured from Fluka ® Analytical) and is difficult to degrade, therefore, discharged in water streams by the industries. The RBBR is a water soluble anionic anthraquinonereactive toxic dye [18] and itsmolecular structure is shown in figure 1. The photo-catalytic activity of the nano-HAp was measured by testing the degradation of RBBR dye under UV light irradiation. Photo-catalysis experiment was carried out by preparing a stock solution of RBBR dye by mixing 0.1 g of RBBR dye in 1 liter water. RBBR dye solution of three different concentrations (10, 30 and 60 mg l −1 ) was prepared by diluting the concentrated solution and labeled as sol-A, sol-B and sol-C, respectively. The volume for each solution is kept to be 50 ml and 25 mg of nano-HAp was added to each solution. This mixture was then kept in the photochemical UV reactor for irradiation. 5 ml of suspension was withdrawn periodically (5, 10, 20, 45, 60, 120, 180 and 240 min) and filtered to remove HAp. The concentration of the residual RBBR dye was analyzed by checking the absorbanceat 595 nm with a UV-vis spectrophotometer [21]. The ratio of the nano-HAp catalyst to RBBR dye were catalytically determined to check the effect of contact time of nano-HAp with RBBR dye solution in presence of UV light and hence to check its kinetics., The percentage degradation was estimated by taking ratio of C/C 0 . Where C 0 is the initial dye concentration in the solution (mg/l) and C is the final dye concentration in the solution (mg/l) at different time periods.
The structural, thermal, morphological and optical analysis of the nano-HAp was carried out using x-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET)analysis, Fourier transform infrared spectroscopy (FTIR) and UV-vis absorption spectroscopy, respectively.  The BET analysis of nano-HAp is shown in figures 2(c) and (d). The N 2 adsorption-desorption isotherm curves for nano-HAp (figure 2(c)) reveals the type IV isotherm with H 4 hysteresis loop typically obtain for mesoporous material. The BJH mesoporous fitting model/pore distribution calculation scheme of nano-HAp, figure 2(d), indicates that the average pore diameter is ∼17.5 nm, which is in the mesoporous range (between 2-50 nm). The estimated specific surface area of nano-HAp is 118.02 m 2 g −1 . Figure 3(a) show thermal stability of the nano-HAp. A negligible weight loss of ∼5% reveals good thermal stability of wet chemically synthesized nano-HAp [21]. The FTIR spectrum of HAp (in figure 3(b)) match with the reported spectra in literature [19,22]. The strong and sharp peaks at 1031 and 567 cm −1 corresponding to the stretching and bending modes of the phosphate groups (PO 4 3− ) in HAp, respectively [21]. The P-O bending vibration mode of HAp is observed around 610 cm −1 . The diffuse-reflectance UV-Vis spectrum of nano-HAp powder is shown in figure 3(c). The band gap estimated from the spectrum is ∼3.99 eV. The band gap of nano-HAp material is found to be nearly in the same range of semiconducting material [23,24]. Hence, it is concluded that the nano-HAp material can perform the photocatalytic reactions under UV irradiation.

Results and discussion
The size and shape of nano-HAp was investigated using TEM and shown in the figure 4. TEM images reveal the agglomerates of elongated shape of nano-HAp particles with length of ∼50 nm and width of ∼10 nm. Fine holes observed across the nano-HAp particles in the magnified TEM image indicate porous nature of the nano-HAp particles. The SAED pattern also indicates the crystalline nature of nano-HAp and is found to be consistent with the XRD results shown in figure 2(a).
The detailed study of adsorption of RBBR dye on nano-HAp in dark conditions was also performed to validate physisorption of RBBR dye on nano-HAp surface (figures 5(a) and (b)). For this adsorption experiment, the dye solution (sol-A) containing 25 mg of nano-HAp was kept in dark environment. The UV-vis absorption graph of the Sol-A measured at different interval depicts almost ∼50% dye adsorption in first 5 min, which remains constant up to 180 min. After that, photocatalytic activity of nano-HAp was evaluated by monitoring the degradationof RBBR concentration (C/C 0 ) as a function of irradiation time and the obtained results are shown in figure 6. For dye solution (sol-A without nano-HAp), figure 6(a),the degradation is ∼25% in 120 min UV irradiation, whereas,after 25 mg nano-HAp addition it enhances significantly to ∼70% in first 5 min and later to ∼80% in 120 min of UV irradiation figure 6(b). This addition indicates that presence of nano-HAp in  sol-A is beneficial to enhance photocatalytic activity. Similar results were obtained for nano-HAp added sol-B and sol-C, whereas, the degradation is ∼60% in 120 min, but could not achieve 80% even in 300 min UV irradiation (figures 6(c) and (d)). It can be noted that the photocatalytic degradation rate follows the Parabolic Diffusion model kinetics ( figure 6(f)). This important result confirms that nano sized HAp crystals acts as  photo-catalysts and induces photocatalytic processes (generation of OH. and O 2 − radicals on the surface of nano-HAp) under UV irradiation [25][26][27], which are responsible for enhanced dye degradation. However, at higher dye concentration, the percentage of radicals with respect to dye concentration remains low and may be responsible for lower degradation of dye. The interaction of UV light with RBBR dye solutions containing nano-HAp catalyst follow two processes; degradation and adsorption, as discussed earlier. First, UV photon interacts with nano-HAp and imparts its energy to electron, this excited electron jumpsto the conduction band and leaving a hole in the valance band. At the same time, the RBBR also get excited and the excited dye transfers an electron into the conduction band of the nano-HAp or the dye directly reacts with the valance band's holes, as shown in figure 7. The second possibility of RBBR dye degradation in presence of nano-HAp is summarized as [28][29][30][31][32][33], i. As HAp is amphoteric in nature (i.e. existence of H + and OH − ions), its surface can capture outer water molecule. At the same time,electron from conduction band of HAp get capture by the surface oxygen (O 2 ) giving superoxide anion radicals (O°2), referred as oxygen ionosorption [25,34].
ii. On the other hand, valence band holes get neutralize with OH − ions and then it will give hydroxyl radical (OH°). Then these radicals undergo redox reactions.
iii. The radicals and/or ionic species formed can react further, to form more species, which are referred to as Reactive Oxygen Species (ROS). The generated superoxide anion radicals (O°2) again react with valence band holes which produce protonated super oxide radical (HO°2). This protonated super oxide radical (HO°2) reacts with super oxide radical (O°2) and holes. ROS can then react with molecules of organic contaminants (in our case it is RBBR dye molecule) and eventually lead to degradation of these molecules.
iv. Subsequently, above mentioned interactions give hydrogen peroxide (H 2 O 2 ) and oxygen (O 2 ) molecule which can further react and enhance the OH radicals and also inhibit the electron/hole (e − /h + ) pair recombination.
These two steps generates hydroxyl radicals which interact with reactive site of RBBR for degradation. On the other hand, UV radiation directly interact with RBBR and produced (OH) group containing by products and these by product will interact with reactive sites of HAp.
The stability and reusability of a catalyst are essential for its practical application. For nano-HAp catalyst, the recycling tests were carried out under optimal conditions in 3 continuous cycles. For this, used nano-HAp powder was cleaned ultrasonically in hot water to remove the dye on it. The results indicate that the catalytic potential of HAp towards RBBR was well maintained with a slight reduction of degradation efficiency from 80% to 70% after 3 continuous cycles.

Conclusion
In summary, nano sized bio-photocatalyst 'HAp' has been synthesized via a simple and low cost wet chemical method with an average crystallite size of ∼9 nm. A remarkable photocatalytic degradation of RBBR Dye using bio-photocatalyst 'Nano-hydroxyapatite' was observed under UV light irradiation with ∼80% degradation in 120 min, which is almost 3 times higher than the experiment performed without nano-HAp and many times higher than that of bulk HAp. According to its structure/size and photocatalytic activity, nano-HAp show an effective photocatalytic property for removing dye from water and can be used for developing high quality photocatalytic filters in dye industries.