Solar Light Induced Photocatalysis for Treatment of High COD Pharmaceutical Effluent with Recyclable Ag-Fe Codoped TiO2: Kinetics of COD Removal

A wide range of active pharmaceutical ingredients (API) is found in various water streams. These synthetic non-biodegradable organics create trouble in conventional wastewater treatment due to toxicity. There is a strong need to develop substitute technology such as visible light driven photocatalysis with a reusable photocatalyst to completely oxidize these substances into carbon dioxide and water. Sol-gel method was used for synthesis of Fe doped TiO2 and Ag-Fe codoped TiO2 nanoparticles with 0.5 wt% Fe and Ti/Ag molar ratio 30 (Ag-Fe CT 30). The morphology and structure of nanoparticles were studied using various analytical techniques. Ag-Fe CT 30 photocatalyst has exhibited excellent photocatalytic activity compared to commercial TiO2, undoped TiO2 and Fe doped TiO2 nanophotocatalysts under solar and UV irradiation for removal of an antifungal drug intermediate, Difloro triazole acetophenone (DFTA) from water. COD reduction efficiency was highest with Ag-Fe CT 30 under solar and UV irradiation proves the potential of Ag-Fe CT 30 photocatalyst to absorb both UV as well as visible radiations. Ag-Fe CT 30 has shown good stability for 4 runs without much decline in the efficacy. This study provides insights on the solar application of a reusable Ag-Fe CT 30 photocatalyst for the treatment of high strength COD wastewater. Kinetics of COD reduction by photocatalysis has been determined. Current World Environment www.cwejournal.org ISSN: 0973-4929, Vol. 15, No. (1) 2020, Pg. 137-150 CONTACT Darshana Tushar Bhatti darshana333@yahoo.com Department of Chemical Engineering, VVP Engineering College affiliated to Gujarat Technological University, Rajkot, Gujarat, India. © 2020 The Author(s). Published by Enviro Research Publishers. This is an Open Access article licensed under a Creative Commons license: Attribution 4.0 International (CC-BY). Doi: http://dx.doi.org/10.12944/CWE.15.1.17 Article History Received: 16 March 2020 Accepted: 18 April 2020


Introduction
Environmental pollution and energy shortages have become the major sectors restricting progress and economical development of the country. [1][2][3][4] Some researchers have explored nowadays various solar applications for simultaneous solar fuel cells and photocatalysis using iron-graphene oxide-titanium phosphate,ss 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 (TiO 2 ) 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 ofTiO 2 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][18][19] Some researchers have worked on photocatalysis using immobilized TiO 2 such as nanotube, 20 nanofilm [21][22] and nanowire 23 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][9][10] which is one of the desired properties. The objective of this research was to develop novel Ag-Fe codoped TiO 2 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 TiO 2 to enhance its activity under solar irradiations for pharmaceutical effluent treatment with high COD (75000 mg/L). In the present investigation TiO 2 , Fe doped TiO 2 and Ag-Fe Codoped TiO 2 nanoparticles were synthesized using sol-gel method 29 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 TiO 2 , Fe DT and Ag-Fe CT 30 which was finally compared with commercially available TiO 2 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 NH 3 -N.

Experimental Materials
Titanium (IV) tetraisopropoxide (TTIP)-97% and TiO 2 (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 TiO 2 , Fe-doped TiO 2 and Ag-Fe codoped TiO 2 nanocomposites
Nanopar ticles 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 solgel 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][31][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 TiO 2 nanoparticles. For the synthesis of Fe doped TiO 2 , the AgNO 3 solution addition step was skipped. For the synthesis of TiO 2 nanoparticles, dopants were not added.

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 : ... (1) 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. 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.

Fig. 3: UV photocatalysis experimental setup 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 TiO 2 , Fe TiO 2 and Ag-Fe CT 30 and were used for all the experiments. Amongst all commercial TiO 2 , Digussa P25 TiO 2 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 TiO 2 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 (COD 0 = 75520 mg/L) and the pH was maintained to 5 for Ag-Fe CT 30, 4 for Fe TiO 2 and 3 for TiO 2 using HNO 3 . 3 g/L of Ag-Fe CT 30, 4 g/L of Fe TiO 2 and 5 g/L of TiO 2 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 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 o C 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 TiO 2 has been focused many times by researchers for photocatalysis as it has shown high photocatalytic activity. [42][43][44]45 The X-ray diffraction patterns of Ag-Fe codoped TiO 2 photocatalysts almost coincide with that of undoped TiO 2 , which indicates the dispersion of Ag and Fe on the TiO 2 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. BET analysis indicated the maximum surface area of 760 m 2 /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.   , 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.

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.
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.   Fig. 12 shows the percentage removal of COD and NH 3 -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 NH 3 -N reduction 50,51 simultaneous removal is not possible. Only 16.05% of NH 3 -N could be removed during photocatalysis.