Activated Carbon Loaded N, S co-doped TiO 2 Nanomaterial and Its Dye Wastewater Treatment

Activated carbon (AC) loaded nitrogen and sulfur (N, S) co-doped TiO 2 nanomaterial (AC-N-S-TiO 2 ) was prepared by precipitation method. AC-N-S-TiO 2 material was characterized by Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray analysis (EDX), X-ray diffraction (XRD), Fourier Transform - Infrared (FT-IR), Photoluminescence (PL) and atomic force microscopy (AFM) analysis. Photodegradation and decolorization of Malachite green (MAG) and Methyl green (MEG) by using TiO 2, N-S-TiO 2 and AC-N-S-TiO 2 under UV-light irradiation has been carried out. The photocatalytic activity of the AC-N-S-TiO 2 was higher than that of the undoped and N, S-doped TiO 2 , The hydroxyl radical analysis - Fluorescence technique with coumarin has been discussed. The photodegradation of MEG was well described by analyzed to be pseudo-first order according to the Langmuir-Hinshelwood representation, the high quantum yield set up to be calculated, stability and reustability of nanomaterial. AC-N-S-TiO 2 act as a promising photocatalyst for dye wastewater treatment under UV-light irradiation.


INTRODUCTION
Metal oxides are the mainly excellent choice for the heterogeneous photocatalysis to eliminate various pollutants and hazardous dyes [1][2][3]. The use of TiO 2 as a semiconductor photocatalyst is an efficient method for elimination of environmental pollutants especially for the degradation of organic contaminants from water has been reported and researched under various contexts [4].
Malachite green (MAG) is an extensively used biocide dye in worldwide aquaculture production. It is also used as a medical disinfectant, food colouring agent, food additive and anthelminthic. However, MAG has now become a highly controversial compound due to the  LTD, 2015 associated risks in reproductive system, immune system, carcinogenic and genotoxic properties [17]. Methyl green (MEG) is used extensively in the fabric industry for dyeing nylon, silk, wool and cotton as well as for colouring of oil, waxes, fats and varnish [18]. The binding of MEG to DNA is possibly ionic, as opposed to intercalative, and it remains so stably bound to double-stranded DNA that, with its modification to the colorless carbinol from, it has been used to evaluate the binding of other molecules to DNA [19,20].
Activated carbon (AC) is having greater adsorption property; it does not degrade the dye.
But AC loaded on semiconductor oxide improves the degradation efficiency of semiconductor oxide by its synergetic effect. The increase in photocatalytic activity of semiconductor oxide by the loading of AC has been well established [21][22][23]. This phenomena having promising solution to the water purification technology.
In this work, we report the synthesis, characterization and photocatalytic activity of AC-N-S-TiO 2 nanomaterial. The obtained material was characterized by SEM with EDX, XRD, FT-IR, PL and AFM analysis. Photocatalytic activity of this material towards MAG and MEG in aqueous solution under UV-light irradiation was studied. AC-N-S-TiO 2 reveals enhanced photocatalytic activity when compared to TiO 2 and N-S-TiO 2 on the degradation and decolorization of MAG and MEG dyes under UV-light irradiation.

Materials
Tetra isopropyl orthotitanate (C 12 H 28 O 4 Ti), activated carbon, ammonium sulphate ((NH 4 ) 2 SO 4 ), nitric acid (HNO 3 ), malachite green (C 23 H 25 ClN 2 ), methyl green (C 26 H 33 N 3 Cl 2 ) and ethanol (C 2 H 5 OH) were the guaranteed reagents of Sigma Aldrich and used as such. The aqueous solutions were prepared by using double distilled water. Chemical structure of MAG and MEG is shown in Fig. 1.

Synthesis of AC-loaded N, S co-doped TiO 2 nanomaterial
AC-N-S-TiO 2 material was synthesized by the prec+ipitation method (Scheme 1). Briefly, a constant amount of ammonium sulphate was dissolved in anhydrous ethanol. The resulting solution was added dropwise into tetra isopropyl orthotitanate in ethanol solution at room temperature, further 0.1M charcoal was added to the above solution under vigorous stirring for 6 h than 2-3 drops of conc. nitric acid were added. The obtained solution was stirred for 1 h than ultra-sonication for 40 min, until precipitate formed. The obtained precipitate was washed with water and ethanol. Then the precipitate was collected and dried in oven at 100 0 C for 8 h in air. The resulting material was finally calcined at 300 o C for 2 h, to obtain AC-N-S-TiO 2 nanomaterial.

Photocatalysis
The photocatalytic activities of the photocatalysts (TiO 2, N-S-TiO 2 and AC-N-S-TiO 2 ) were evaluated by the photodegradation of MAG and MEG. The light source was a UV lamp (365 nm). The reaction was maintained at ambient temperature (303 K). In a typical experiment, aqueous suspensions of dye (50 mL, 1×10 −4 M) and 0.16 g of the photocatalyst powder were placed in reaction tubes. Prior to irradiation, the suspension was magnetically stirred in the dark to ensure the establishment of an adsorption/desorption equilibrium. The suspension was kept under constant air-equilibrated condition. At the intervals of given irradiation time. The suspension was measured spectrophotometrically MAG (614 nm) and MEG (630 nm) within the Beer-Lambert law limit.
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Characterization methods
X-ray diffraction (XRD) spectra was recorded on the X'PERT PRO model X-ray diffractometer from Pan Analytical instruments operated at a voltage of 40 kV and a current of 30 mA with Cu Kα radiation.
Scanning electron microscopy (SEM) with elementary dispersive X-ray analysis (EDX) was carried out on a FEI Quanta FEG 200 instrument with EDX analyzer facility at 25 °C. The sample was prepared by placing a small quantity of prepared material on a carbon coated copper grid and allowing the solvent to evaporate.
Fourier transform-infrared spectrum (FT-IR) was recorded using SHIMADZU FT-IR spectrometer in KBr pellet.
Photoluminescence (PL) spectra at room temperature were recorded using a Perkin-Elmer LS 55 fluorescence spectrometer. Nanoparticles were dispersed in chloroform and excited using light of wavelength 300 nm.
Atomic force microscopic (AFM) image was taken by using AGILENT-N9410A series 5500 microscope operating in the tapping mode with standard silicon nitride tips. This instrument uses a silicon nitride tip a radius of 20 mm and a low resonance frequency cantilever that has a manufactures spring, constant at 30-45 nm. Scan of 5 x 5 µm obtained for sample. The images were recorded in condition to produce 2D and 3D images.
Ultraviolet and visible (UV-vis) absorbance spectra were measured over a range of 800-200 nm with a Shimadzu UV-1650PC recording spectrometer using a quartz cell with 10 mm of optical path length.

XRD analysis
The obtained XRD of the TiO 2 and AC-N-S-TiO 2 nanomaterial is shown in The average crystalline size (L) of the AC-N-S-TiO 2 material can be calculated from the Debye-Scherrer formula [24]. L=0.89λ/βCOSθ where L is the crystalline size (in nm), λ is the wavelength (in nm), β is the full width at half maximum intensity (FWHM-in radian), and θ is the Bragg diffraction angle ( o ). The average crystalline size of the AC-N-S-TiO 2 synthesized was figured out to be about 38 nm.

SEM with EDX analysis
The surface morphology and elemental studies of the TiO 2 , N-S-TiO 2 and AC-N-S-TiO 2 nanomaterial were carried out using SEM with EDX analysis (Figs. 3 and 4). The SEM images revealed that individual nano aggregates particle were composed by a collection of elongated particles of various size and shapes. The elongated shape of the AC-N-S-TiO 2 material revealed by SEM is in perfect agreement with the results observed by XRD (Fig. 2). AC-N-S-TiO 2 shows the average particle size of 38 nm (Fig. 3c). EDX analysis confirms Titanium and Oxygen are present in pure TiO 2 (Fig. 4a), Nitrogen, Sulfur, Titanium and oxygen are present in N-S-TiO 2 material (Fig. 4b), where as Carbon, Nitrogen, Sulfur, Titanium and oxygen are present in AC-N-S-TiO 2 nanomaterial (Fig. 4c).

FT-IR analysis
The FT-IR spectra of the TiO 2 , N,S-TiO 2 and AC-N-S-TiO 2 materials were shown in

PL analysis
Photoluminescence spectra of TiO 2 , N-S-TiO 2 and AC-N-S-TiO 2 are shown in Fig. 6a, 6b and 6c respectively. As the photoluminescence occurs due to electron−hole recombination, its intensity is directly proportional to the rate of electron−hole recombination [26].

AFM analysis
The morphology of the synthesized AC-N-S-TiO 2 nanomaterial has been studied by AFM. Fig. 7a depicts typical AFM image of AC-N-S-TiO 2 . The corresponding cross-sectional view indicates that the average thickness of AC-N-S-TiO 2 is about 32.5 nm. (Fig. 7b). The existence of nanosized particles in the AC-N-S-TiO 2 is more clearly reflected in threedimensional (3D) AFM image. This image shown wave type projection in the Z direction from the fine surface and smaller size of AC-N-S-TiO 2 was clearly noticed. Thus the above results confirm the nano texture in the surface of as-synthesized AC-N-S-TiO 2 material.

Photodegradation and decolorization of MAG and MEG under UV-light irradiation
We have chosen photodegradation and decolorization of MAG and MEG dyes under UV light irradiation to evaluate the photocatalytic activity of the proposed photocatalyst.  Fig. 9a, 9b and 9c. It can be seen from UVvisible spectra changes, the strong adsorption peak of MAG and MEG solutions at 614 nm and 630 nm steadily decreased with increasing the light irradiation time. AC-N-S-TiO 2 exhibited excellent photocatalytic activity for MAG and MEG under UV-light irradiation when compared to that of TiO 2 and N-S-TiO 2 nanomaterials. Fig. 10 shows the time course of percentage degradation of MAG (Fig. 10a) and MEG (Fig. 10b) under UV-light irradiation. After 75 min irradiation time AC-N-S-TiO 2 degrade MAG aqueous solution up to 52.8% compared to that of TiO 2 and N-S-TiO 2 (42.6 % and 46.8 %). After 60 min irradiation time AC-N-S-TiO 2 can degrade the MEG aqueous solution up to 56.1 % compared to that of TiO 2 and N-S-TiO 2 (45.9 % and 38.2 %). Fig. 11 shows the time course of percentage decolourization of MAG (Fig. 11a) and MEG (Fig. 11b)

Different catalyst loading and Different concentration of MEG dye
The effect of catalyst loading AC-N-S-TiO 2 on MEG has been fast degradadation carried out in the range of 0.100g, 0.160g and 0.250g the total volume of solution to uses 50 International Letters of Chemistry, Physics and Astronomy Vol. 47 mL. The various amount of catalyst loading in the reaction medium of degradation rate. The rate is linearly related to the amount of catalyst loadings. Among the catalysts in the AC-N-S-TiO2 (0.200g) showed higher activity when compared to low catalyst loading (0.100g) shows in Fig. 12a. The effect of various dye concentration of MEG on AC-N-S-TiO2 under UV light represents. Different concentrations of (1x10-4 and 1x10-5) MEG the increase of illumination with material were prepared and it was used for the photodegradation process by using UV light. Photodegradation of MEG was higher in lower concentration when compared to of it higher concentration shows in Fig. 12b.   Fig.12(a). Different catalyst loading AC-N-S-T (0.100g, 0.160g and 0.200g) +MEG (1X10-4) (b) Different concentration of MEG dye (1x10-4 and 1x10-5).

Reusability of the catalyst
The reusability of AC-N-S-TiO 2 was tested for the degradation of dyes MEG and MAG under identical reaction conditions. After full degradation, the catalyst was alienated and washed with a bulky amount of deionized water. The improved catalyst was dried in a hot air oven at 100 °C for 90 min and used for a second run. Figure 13 shows the results of MEG degradation for fivth runs. AC-N-S-T exhibits amazing photostability as the MEG and MAG degradation percentages are 100, 98, 97, 97 and 97% and 100, 98, 97, 95 and 95 for 75 min in the 1 st , 2 nd , 3 d , 4 th , and 5 th runs, respectively. There is no significant change of reused AC-N-S-TiO 2 photocatalyst did not change during the reaction, representing the stability of photocatalyst. After conclusion of the degradation reaction, the solution was tested for Ti 3+ leaching with sodium sulfide. There is no precipitation of titanium sulfide (black color) [27]. As there is no leaching of Ti 3+ , this catalyst is non-hazardous for wastewater behavior.

Degradation of kinetic study
A solution of (1x10 -4 ) MEG was prepared in doubly distilled water and 0.160 g of catalyst was loaded to it. It was observed that there was a decrease in absorbance of MEG solution with increasing time of revelation of UV light. A linear plot between 1 + logA Vs time was obtained, which indicates that the photocatalytic degradation of MEG follows pseudo-first order kinetics. The rate constant for this reaction was measured with the facilitate of following equation [28] k= 2.303 × slope (1) Prepared nanomaterial for TiO 2 and AC-N-S-TiO 2 degradation rate calculation in precence of the rate is linearly related to the amount of catalysts. Between the two catalysts only doped showed higher the rate when compared to undoped nanomaterial. (Fig. 14).

Quantum yield
The quantum yield of a photocatalytic reaction is defined as the number of MEG dye molecules individual decaying (degraded) per photon absorbed (eq 2). Φ = Number of molecules decaying / Number of photons of light absorbed (2) The photodegradation rate constants (k′) of MEG dye under the monochromatic light source can also be used for calculation of its reaction quantum yield [29] using eq. 3 k Φ = (3) 2.303 I 0 , λ ε Dλ l where Φ is quantum yield (dimensionless), Io is the light intensity of the occurrence light range at 200−800 nm range (1.381× 10 -6 ), ε Dλ is the molar absorptivity of MEG at 630 nm (1.0 × 10 5 cm −1 M −1 ), and l is the path length (1 cm) of the reaction and is for 50 mL of irradiation solution. The consequences of degradation quantum yields by, un doped TiO 2 and AC-N-S-TiO 2 are 0.06703 and 0.5478 mole/Einstein [30], respectively. These results indicate that the quantum yield of the AC-N-S-TiO 2 process is high when compared to other processes.

Mechanism
On the basis of these observations, a tentative mechanism for photocatalytic degradation methyle green dye is future as follows: + hν →e -(CB) + h + (VB) ...
Methyl green absorbs radiations of preferred wavelength and it is excited giving its first excited. Singletstate. more, it undergoes intersystem crossing (ISC) to give its other stable triplet state. Along with this, the semiconducting AC-N-S -TiO 2 (SC) also utilizes this energy to motivate its electron from valence band to the conduction band. An electron can be abstracted from hydroxyl ion by hole (h + ) present in the valence band of semiconductor generate • OH radical. This hydroxyl radical willpower oxidizes methyl green to its leuco form, which may finally degrade to products. It was specific that the • OH radical participates as an active oxidizing AC-N-S-TiO 2 (SC)

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OH radical analysis
Definite by the detection of • OH shows the changes of the fluorescence spectra of coumarin solution in UV-light irradiation as a role of irradiation time (AC-N-S-TiO 2 -30 and 60 min). This result suggests that the fluorescence intensity arises [31]. The emission spectra excited at 320 nm in coumarin solution added with AC-N-S-TiO 2 samples were measured Fig.  15 Displays that fluorescence signal at 391.5 nm for different time of addition with AC-N-S-TiO 2 samples. The highest fluorescence intensity was found for AC-N-S-TiO 2 at 60 min. This suggests that the fluorescence is caused by chemical reactions of coumarin with hydroxyl radicals created in photocatalytic reactions [32]. Moreover at 60 mins AC-N-S-TiO 2 sample with maximal photocatalytic activity produced much more reactive hydroxyl radicals than other samples, which is also consistent with the results of photocatalytic decay. The hydroxyl radical analysis further Definite that the hydroxyl radicals are active species using photocatalytic reactions. On the other hand, the arrangement rate of • OH is directly linked to the photocatalytic activity of AC-N-S-TiO 2 nanomaterial.

CONCLUSIONS
AC-N-S-TiO 2 nanomaterial was synthesized by precipitation method. This material was characterized by XRD, SEM with EDX, FT-IR, PL and AFM analysis. XRD, SEM and AFM analysis of AC-N-S-TiO 2 showed the average particle size of as 38 nm. FT-IR and EDX shows the C, N, S, Ti and O present in the AC-N-S-TiO 2 material. The intensity of AC-N-S-TiO 2 PL emission is less when compared to that of TiO 2 and N-S-TiO 2 materials, due to the suppression of recombination of electron−hole pairs by AC-N-S-TiO 2 . The photocatalytic activity of the AC-N-S-TiO 2 material is higher than that of the TiO 2 and N-S-TiO 2 materials. The mechanism of photocatalytic defnids of MEG is due to formation of hydroxyl radical analysis. The OH radical analysis further confirms that the OH radicals are active species in the photocatalytic reactions. The formation rate of OH radicals is directly related to the photocatalytic activity of AC-N-S-TiO 2 . nanomaterial, indicated which was confirmed by the Fluorescence analysis. That the photocatalysis process in the presence of AC-N-S-TiO 2 , obtainable the maximum quantum yield, It is act as a promising photocatalyst for removal of dye containing wastewater behavior under UV-light irradiation.