Current World Environment

Introduction

Environmental pollution and energy shortages have become the major sectors restricting progress and economical development of the country.^1–4 Some researchers have explored nowadays various solar applications for simultaneous solar fuel cells and photocatalysis using iron-graphene oxide-titanium phosphate,⁵ photocatalysis with cadmium sulphide,⁶ transition metals doped CuO for heterojunction solar cells,⁷ carbon nanotubes on cobalt-iron-silica electrocatalysis.⁸ Active pharmaceutical ingredients (API) are the pollutants those come out from many pharmaceutical industries and pose major threats when disposed on to either land or in water bodies, as many of these refractory organics are toxic.⁹ Conventional wastewater treatment has two drawbacks: 1) it is unable to biodegrade these refractory organics in activated sludge process unit; 2) it will just transfer pollutants from wastewater to solid phase as sludge, again there will be a disposal problem leading to solid pollution. Hence, there is a strong need to develop certain alternative treatment technologies which will completely oxidize these organics. Heterogeneous photocatalysis treatment using titanium dioxide (TiO₂) nanoparticles, generate hydroxyl radicals (OH^*) which has the strong oxidizing potential of 2.8 eV,¹⁰ and is a promising technology to degrade organics present in effluent competently.¹¹ The limitation of the treatments are: 1) it absorbs only UV radiations due to higher band gap of TiO₂ ¹² and 2) feasibility for reuse of the photocatalyst after treatment; which has limited the practical application of this treatment on filed.¹³ Solar photocatalysis imbibing sustainable use of resources with economic and environmental benefits. One most important aspect to be considered for photocatalysis is recyclability which remains untouched by researchers.^12,14 For economic application of the photocatalyst, it should be recycled after use without sacrificing COD removal efficiency. It is generally observed that photocatalysts deactivation take place mainly due to adsorption of organics on the surface of a catalyst which cause poisoning catalysts. In development of a photocatalyst, its surface should be such that less poison occurs which makes it reusable for many cycles efficiently.^6-7 Many researchers have worked on synthesis, morphology and optical properties improvement by doping.^17-19 Some researchers have worked on photocatalysis using immobilized TiO₂ such as nanotube,²⁰ nanofilm^21-22 and nanowire²³ which can be less poisoned compared to nanopowders from treated water; but these structures will provide less surface area compared to suspended catalysts. At present, only few research focused on the recyclability of photocatalysts^8-10 which is one of the desired properties. The objective of this research was to develop novel Ag-Fe codoped TiO₂ nanoparticles that would facilitate a quick degradation of DFTA in solar radiation and simultaneously shows good stability over the number of recycling runs. This research also addressed kinetics of COD removal for solar and UV photocatalysis. There has been found less literature on doping with silver^11-12 and iron^11-12 in TiO₂ to enhance its activity under solar irradiations for pharmaceutical effluent treatment with high COD (75000 mg/L). In the present investigation TiO₂, Fe doped TiO₂ and Ag-Fe Codoped TiO₂ nanoparticles were synthesized using sol-gel method²⁹ and used for the photodegradation of DFTA from aqueous solution under optimized conditions of pH, catalyst dose and Ti/Ag molar ratio by prior experiments. The rate of photocatalysis was studied in terms of COD removal efficiency using TiO₂, Fe DT and Ag-Fe CT 30 which was finally compared with commercially available TiO₂ under solar and UV radiations Recyclability of novel photocatalyst has been determined. The applicability of novel Ag-Fe CT 30 was checked for treatment of high strength industrial pharmaceutical effluent at optimum conditions to remove COD and NH₃-N.

Experimental

Materials

Titanium (IV) tetraisopropoxide (TTIP)-97% and TiO₂ (Degussa P25)-99.5% were purchased from Sigma–Aldrich. Nitric acid-100%, isopropyl alcohol (IPA)-97%, ferric nitrate-99.95% and silver nitrate-99% were purchased from the Merck. Caustic soda-97% and sulphuric acid-99.99% were used to maintain the pH of the solution during experiments. DFTA was provided by Endoc lifecare private limited. All these chemicals were used without further purification. Distilled water was used to prepare all the solutions.

Synthesis of TiO₂, Fe-doped TiO₂ and Ag-Fe co-doped TiO₂ nanocomposites

Nanoparticles can be prepared by different methods.¹⁴ Amongst all, sol-gel method is simple, provides uniform size distribution and economical.²⁶ Nanoparticles were synthesized using a sol-gel synthesis method with little modification as this method provides anatase phase more after calcinations step which has more photocatalytic activity compared to amorphous and rutile structure.^30-32 Fe content in nanoparticles were kept 0.5% by weight.^14,17,18 Fig. 1 represents steps for the synthesis of Ag-Fe co-doped TiO₂ nanoparticles. For the synthesis of Fe doped TiO₂, the AgNO₃ solution addition step was skipped. For the synthesis of TiO₂ nanoparticles, dopants were not added.

Figure 1: Steps for the synthesis of nanoparticles. Click here to view Figure

Characterization of Photocatalysts

Transmission Electron Microscopy (TEM) characterization was carried out using a JEOL JEM 2100 microscope operated at an acceleration potential of 200 kV.³⁶ Optical properties of the nanocatalysts were determined using the UV–Vis absorption spectroscopy with a Varian Cary 500, Shimadzu UV 3600. X-ray diffraction (XRD) analysis of the prepared photocatalysts was carried out at room temperature with a Philips: PW3040/60 XPERT Panalytical Pro using Cu K-alpha radiation (λ = 1.54 A) and a graphite monochromator, operated at 30 mA and 40 kV. The size and structure of synthesized photocatalysts were investigated using XRD and particle size analysis (Microtrack). From this study, considering the peak at 2θ degrees, average particle size was calculated by using equation (1) (Debye-Scherrer formula)^35-36:

d: crystallite size (nm)

λ: wavelength of X-ray (0.154 nm)

β: FWHM (full width at half minimum)

θ: angle of diffraction (degrees)

A scanning electron microscope (SEM) was used to capture images to study the structure and surface of photocatalysts. The specific surface area was determined using BET Micromeritics, ASAP 2010. Elemental analysis was carried out with energy dispersive x-ray analysis (EDX) to verify the Ti/Ag molar ratio of the Ag-Fe CT 30.

Experimental setup for degradation of DFTA under solar and UV light irradiation

Fig. 2 represents solar photocatalysis experimental setup with a glass beaker as a reactor which was placed on magnetic stirrer to keep catalyst in suspension. The experiments were performed on terrace from 9 am to 2 pm and atmospheric air has worked as a natural oxidant for solar photocatalysis. Fig. 3 represents UV photocatalysis experimental set up with one closed chamber of 40 cm x 40 cm x 70 cm dimensions with a photocatalytic reactor of 800 ml capacity with a quartz tube to place UV light surrounded by glass jacket for circulation of cooling water was used as shown in Fig. 3. UV radiations were provided using a 125 W UV lamp (wavelength, λ = 200-400 nm, Philips). The reaction mixture was magnetically stirred.

Figure 2: Solar photocatalysis experimental setup Click here to view Figure

Figure 3: UV photocatalysis experimental setup Click here to view Figure

Experimentation

A study on kinetics helps in the designing of photocatalytic reactors for the treatment of pharmaceutical wastewater. The reduction in the COD reflects the extent of mineralization of organic species.³⁹ Catalyst dose and pH were optimized by prior experiments for TiO₂, Fe TiO₂ and Ag-Fe CT 30 and were used for all the experiments. Amongst all commercial TiO₂, Digussa P25 TiO₂ has shown good photocatalytic activity and selected here for the study.^{31, 40, 41} Photocatalysis for the degradation of DFTA were conducted batch wise using TiO₂ C, synthesized TiO₂ np, Fe DT and Ag-Fe CT 30 under solar and UV light irradiation. The synthetic wastewater has an initial DFTA concentration of 8 g/L (COD₀ = 75520 mg/L) and the pH was maintained to 5 for Ag-Fe CT 30, 4 for Fe TiO₂ and 3 for TiO₂ using HNO₃. 3 g/L of Ag-Fe CT 30, 4 g/L of Fe TiO₂ and 5 g/L of TiO₂ as photocatalyst was added for photocatalysis. For UV photocatalysis externally oxygen is provided via air circulation through bubbler. 0.5 hr of irradiation time in dark was provided for adsorption of organics on catalyst surface before photocatalysis. Samples were extracted at an interval of 1 hr for a period of 6 hr for COD determination using the open reflux method. The samples were centrifuged at 5500 rpm to separate the catalyst from the solution before COD analysis for 5 min. The photocatalytic activity of TiO₂ C, TiO₂ np, Fe DT and Ag-Fe CT 30 photocatalysts in terms of COD removal efficiency were compared under UV and Solar light. All the experiments were carried out thrice to check reproducibility of the results and average values were considered. COD removal efficiency is calculated using equation (2).

COD₀: initial chemical oxygen demand (mg/L)

CODt: chemical oxygen demand at time t (mg/L)

The recyclability of a photocatalyst is extremely important for practical applications.²⁵ Experiments were performed at optimized conditions repeatedly for six number of recycle runs to check the reusability of synthesized nanophotocatalysts under solar and UV radiations. At the end of first cycle, the used catalyst is separated from the solution and reused for the next consecutive six cycles hence total seven cycles have been performed. For regeneration of the photocatalyst, it was centrifuged at 5500 rpm for 15 min to recover it and washed with ethanol three times to remove traces of organics adsorbed on the surface. Finally, the separated wet particles were dried at 80-90^oC in an oven overnight. The remaining all six runs of photocatalysis were then performed with the same steps used in the first cycle.

Results and Discussions

Nanoparticles Characterization

X-ray diffractograms of the synthesized nanoparticles are shown in Fig. 4. These results indicate that there is no impurity found and for all plots, the major peak is found at diffraction angle 2θ = 25.3, which represents the anatase phase (JCPDS card no. 21-1272) without any indication of the rutile phase. Anatase structure of TiO₂ has been focused many times by researchers for photocatalysis as it has shown high photocatalytic activity.^{42–44, 45} The X-ray diffraction patterns of Ag-Fe codoped TiO₂ photocatalysts almost coincide with that of undoped TiO₂, which indicates the dispersion of Ag and Fe on the TiO₂ surface. The results of particle size analysis (Fig. 5) show that the size of all the synthesized photocatalysts was between 10-30 nm. Fig. 6 represents UV-Vis spectra of different photocatalysts. It can be seen that Fe DT and Ag-Fe CT 30 can absorb visible irradiation due to the presence of Ag and Fe dopant.

Figure 4: XRD pattern of nanophotocataysts: A. TiO2 np; B. Fe-doped TiO2; C. Ag-Fe CT 30. Click here to view Figure

Figure 5: Particle size distribution in nanophotocataysts: A. TiO2 np; B. Fe-doped TiO2; C. Ag-Fe CT 30. Click here to view Figure

Figure 6: UV-Vis spectra nanophotocatalysts. Click here to view Figure

Morphology of different nanophotocatalysts

Fig. 7 represents SEM images of TiO₂ np, Fe DT and Ag-Fe CT 30. Results show that the TiO₂ nanophotocatalysts have a spherical shape with even surface, while the Fe-doped TiO₂, as well as Ag-Fe co-doped TiO₂, have spherical shape with irregular surface, indicating deposition of metal dopants on the surface of the TiO₂. TEM images of all synthesized photocatalysts are shown in Fig. 8. BET analysis indicated the maximum surface area of 760 m²/g for Ag-Fe CT 30. Band gap was calculated from XRD peak data. Table 1 and Table 2 represent EDX elemental analysis and summary of characterization results respectively.

Figure 7: SEM results of photocatalysts: A. TiO2; B. Fe-doped TiO2; C. Ag-Fe CT 30. Click here to view Figure

Figure 8: TEM images of different photocatalysts: A. TiO2 np; B. Fe-doped TiO2; C. Ag-Fe CT 30. Click here to view Figure

Table 1: Calculation of Ti/Ag molar ratio for different nanophotocatalysts from EDX

Table 2: Summary of characterization results.

Photocatalytic degradation of DFTA using different photocatalysts

The COD removal efficiency with Ag-Fe CT 30 as a photocatalyst was compared with TiO₂ C, TiO₂ np and Fe doped TiO₂ based on the COD reduction efficiency under solar and UV light at initial concentration of DFTA, 8 g/L, and at pH 5. In all the experiments, Ag-Fe CT 30 dose of 3 g/L for solar photocatalysis and UV photocatalysis was used. Ag-Fe CT has shown maximum COD removal efficiency under solar radiations due to following reasons: 1) OH radicals generation in large quantity by the combination of photocatalysis and fenton reactions due to presence of Ag and Fe dopants; 2) inhibition of electron and hole recombination as electrons might get trapped in doped metals results in separation of electron and holes thereby enhancing photocatalytic activity; 3) absorption of visible light due to presence of Ag and Fe dopants. The formation of reactive species such as H₂O₂, O₂^-, and OH^* during photocatalysis will also oxidize DFTA. After 5 hr of irradiation time the COD removal efficiency was 37.77%, 63.09%, 52.79% and 85.54% under UV irradiations; 27.9%, 59.75%, 63.83% and 76.39% under solar irradiations respectively as shown in Fig. 9.

Figure 9: Comparison of COD reduction efficiency under solar and UV radiations at optimum conditions. Click here to view Figure

Kinetics of COD removal for solar and UV photocatalysis using Ag-Fe CT 30

Evaluation of parameters involved in the kinetic equation of COD removal is very important to study the effect of time on COD removal and thereby determination of volume requirement for the design of photocatalytic reactors. So, the kinetics of COD removal was calculated at initial DFTA concentration of 8 g/L, catalyst dose of 3 g/L and pH of 5 under solar and UV radiations. To evaluate the heterogeneous photocatalytic reaction successfully, the effect of COD remaining on the rate of COD removal rate is given in the form of equation (3).

-r = k * Cⁿ ……. (3)

log (-r) = n log C + log k

where (-r) is the COD removal rate, C is the COD at time ‘t’, k is the rate constant, n is order of degradation reaction. To determine the parameters of equation (3), differential method of analysis was used. The rates of COD removal with COD remaining were obtained from plots of COD versus time data. COD versus time is plotted (Fig. 10 (a) and (c)) and values of n and k were calculated from the slope and intercept (Fig. 10 (b) and (d)) respectively. The rate constant and order obtained for solar and UV photocatalysis are shown in Table 3.

Figure 10: Plot for kinetics of COD reduction efficiency using Ag-Fe CT as catalyst at conditions: pH = 5 and catalyst dose = 3 g/L (a) and (b) solar light; (c) and (d) UV light. Click here to view Figure

Table 3: Kinetic parameters for COD removal efficiency

The rate of COD removal with the catalyst under solar light was higher than UV light, whereas maximum COD reduction of 87% was achievable during the fifth hour of UV photocatalysis which was higher than solar photocatalysis (76.74%) due to higher surface area and generation of OH radicals in abundance due to oxygen defects. The results higher rate under solar photocatalysis for such a high strength COD wastewater has proved its effectiveness for practical applications with economic and environmental benefits to the pharmaceutical industries.

Effect of recycling of Ag-Fe CT 30

Fig. 11 shows % COD removal efficiency for 1 to 7 runs after 5 hr of irradiation under solar irradiations. After four cycles of reuse, decrease in photocatalytic activity of 5.88%, 8.7%, 6.62% and 4.86% for TiO₂ C, TiO₂ np, Fe DT, Ag-Fe CT 30 respectively were observed under solar light irradiation. With consecutive runs, the photocatalytic treatment efficiency dropped due to poisoning of catalyst and blockage of effective surface available for adsorption and oxidation reactions. The catalyst can be reused the four recycle runs without much decline in COD removal efficiency (less than 5%). 63.25% COD removal was even achievable by the novel Ag-Fe CT 30 during the sixth recycle run. The reuse efficiency was found to decrease in the following order for different photocatalysts: Ag-Fe CT 30>Fe DT>TiO₂ np>TiO₂ C. Ag-Fe CT 30 has proved its recyclability under solar radiations which makes effluent treatment economical and environmentally free as the treated effluent will not contain slight catalyst traces in discharged effluent. Recyclability of doped TiO₂ has been studied by various researchers and obtained nearly similar results.^43-44 Fe³⁺ doped TiO₂ was studied for dye degradation and there has been 9% reduction in degradation efficiency observed at the end of sixth cycle.¹⁵ Ag/Fe,N-TiO₂/Fe₃O₄@SiO₂ and graphene oxide supported Ag-Fe TiO₂ has also shown good stability for visible light photocatalytic degradation of dyes for 5 and 3 recycle runs respectively.^45-46

Figure 11: Effect of catalyst recycling on percentage COD reduction efficiency. Click here to view Figu

Industrial wastewater treatment at optimized conditions by solar photocatalysis

Batch photocatalysis experiments were performed with pharmaceutical industrial effluent having initial COD of 88660 mg/L and ammonical nitrogen (NH₃-N) of 3287 mg/L as specified in Table 4, under 5 hr (from 10 a.m. to 3 p.m) solar light irradiation and at the optimized conditions of pH and catalyst dose obtained for TiO₂, Fe DT and Ag-Fe CT 30. The optimum conditions for DFTA degradation by solar photocatalysis were: pH=3, catalyst dose= 3 g/L for TiO₂; pH=4, catalyst dose= 4 g/L for Fe DT; pH=5, catalyst dose= 3 g/L for Ag-Fe CT 30 and adsorption time in dark= 30 min under solar light. In all the experiments 500 mL volume of the industrial effluent was used. Fig. 12 shows the percentage removal of COD and NH₃-N from the effluent, obtained at the end of 5 hr at optimum conditions. Results of COD reduction showed that the synthesized catalyst worked efficiently for actual industrial wastewater treatment for COD reduction. Ag-Fe CT 30 has removed COD of effluent from 88660 mg/L to 31310 mg/L, 64.69% COD reduction. Since acidic conditions favor Organic oxidation and alkaline conditions favor NH₃-N reduction^{50, 51} simultaneous removal is not possible. Only 16.05% of NH₃-N could be removed during photocatalysis.

Table 4: Industrial wastewater treatment for COD and NH₃-N removal using solar photocatalysis

Figure 12: COD and NH3-N removal from industrial wastewater using solar photocatalysis. Click here to view Figure

Conclusion

A novel and efficient photocatalyst, Ag-Fe codoped TiO₂ with Ti/Ag molar ratio 30 was synthesized by the sol-gel method and its photocatalytic activity was compared with undoped TiO₂ for the photocatalytic degradation of DFTA, a drug intermediate. For economical operation, an attempt has been made to treat industrial pharmaceutical effluent with very high initial COD of 88660 mg/L under solar radiations using optimum conditions of pH and catalyst dose. Kinetics of COD removal favors photocatalysis under solar radiation compared to UV photocatalysis. Rate of solar photocatalysis is higher than UV photocatalysis during the first four hours with Ag-Fe CT 30 nanoparticles.

Acknowledgment

Authors thank VVP Engineering College, Gujarat Technological University and Gujarat Council of Science and Technology, Government of India, Gujarat for providing the financial support to carry out this research work.

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List of Abbreviations