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Removal of micro-pollutant using an indigenous photo membrane reactor
Journal Pre-proof Removal of micro-pollutant using an indigenous photo membrane reactor Santanu Sarkar (Conceptualization) (Methodology) (Data curation) (Writing - original draft), Chiranjib Bhattacharjee (Visualization) (Investigation) (Writing - review and editing)
PII:
S2213-3437(20)30021-X
DOI:
https://doi.org/10.1016/j.jece.2020.103673
Reference:
JECE 103673
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
14 November 2019
Revised Date:
16 December 2019
Accepted Date:
6 January 2020
Please cite this article as: Sarkar S, Bhattacharjee C, Removal of micro-pollutant using an indigenous photo membrane reactor, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2020.103673
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Removal of micro-pollutant using an indigenous photo membrane reactor Santanu Sarkara* Chiranjib Bhattacharjeeb, a
R&D Department, Tata Steel Limited, Jamshedpur, Jharkhand, India Chemical Engineering Department, Jadavpur University, Kolkata, India
Corresponding Author’s Email: [email protected], Phone: +91 7033094998, +91-657 6648938
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b
Abstract
Pharmaceutical wastewater treatment using heterogeneous photocatalysis in presence of TiO2
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nanoparticles has become a lucrative method because the convectional wastewater and biological treatments usually fail to remove the detrimental effect of different drugs and
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hormones on the environment. The nanoparticles recovery and reuse is the most challenging task in practice. The main goal of the present study is to recover and reuse of photocatalyst by Consequently, two types of immobilization
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immobilizing it on the membrane surface.
technique have been used here, one is surface immobilization by dipped coating, and other is impregnation of catalyst during fabrication of the membrane. To conduct photocatalysis, an
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indigenous photo membrane reactor has also been fabricated during the research work. The presence of TiO2 nanoparticles has not only improved the catalytic property of the membrane, but also the contact angle analysis has confirmed the enhanced hydrophilic as well as
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antifouling property of the surface modified membrane. The fabricated membranes were characterized by scanning electron microscopy (SEM). To analyses the performance of
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photo membrane reactor as well as TiO2 coated or impregnated membranes, the photocatalytic degradation of aqueous solution of chlorhexidine digluconate (CHD), an antibacterial drug, was performed using the developed system and the removal percentage of CHD obtained from the system was found to be quite appreciable. The results show that the recovery and reuse of the catalyst by the present system is very much significant. Meanwhile, the developed reactor can be operated in cross flow configuration under batch, semi-batch and continuous mode.
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Keywords: Membrane; titanium dioxide; surface modification; photo membrane reactor; pharmaceutical waste; advanced photocatalysis.
1. Introduction Environmental pollution due to several pharmaceutical wastes has become a very significant global threat in very recent years. Both pharmaceutical companies and municipal corporations are the major contributors of such pollution. Moreover, the unscientific use of
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different drugs in medical science influences to increase the level of pollution, results in gradual growth of such compounds in the environment. The presences of those micro pollutants in different water bodies and their detrimental effects have been confirmed by several researchers [1-6]. The limitations over the pharmaceutical wastewater treatment are the pollutants i.e. antibiotics, hormones, steroids, etc. cannot be easily separated through
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conventional wastewater treatment and not even degraded in biological treatment [7, 8]. From last two decades heterogeneous photocatalysis in presence of TiO2 semiconductor
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nanoparticles has been used by several research groups for pharmaceutical wastewater treatment [9, 10, 25, 26], though most of the researchers carried out their treatment process in
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batch suspension mode operation. The major problem related to such processes is the recovery and reuse of TiO2 nano particle at the end of photocatalysis.
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The recycle and reuse of photocatalyst particles can be done with the help of different immobilization techniques [10, 26]. Very recently nanoparticles are trapped using the membrane technology, which has become promising pathway for the surface modification of
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the membrane [11-16]. The immobilization technique can be classified in two different ways such as membrane surface coating with TiO2 nanoparticles, and other is the preparation of
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nanoparticles impregnated membrane [11-16]. According to S. Mozia et al. [17] the incorporation of membrane in a photo-reactor serves both the purposes of catalyst recovery, and also acts as a separating barrier between by-products generated during photocatalysis of the targeted pollutants. Hence, not only preparation of photocatalytic membrane is enough for pharmaceutical wastewater treatment, but it is also essential to developed membrane photo reactor (PMR) to carry out photocatalytic reaction.
Both the fabrication of catalytic
membranes and subsequent development of indigenous PMR are very important for effective implementation of the heterogeneous photocatalysis. 2
In the present study, bare and nanoparticles impregnated poly ether sulfone (PES) membranes with different concentration TiO2 nanoparticles have been fabricated by phase inversion method. The bare PES membrane then surface coated with TiO2 nanoparticles under UV irradiation. In house prepared membranes have been characterized with the help of SEM and contact angle analysis. To use those modified membrane in field of pharmaceutical wastewater treatment, an indigenous PMR has also been fabricated having some novelties. To study the performance of PMR, the photocatalytic degradation of chlorhexidine digluconate (CHD), a pharmaceutical drug in presence of catalytic membrane has been carried out here. The same research group has already established that photocatalysis is very much effective in
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removal of CHD using TiO2 suspension in batch mode and the details of impact of CHD has been reported elsewhere [18, 19].
The novelty of the present research work is clear and straight. According to the literature survey the nanoparticles modified membrane was used first in the field of pharmaceutical
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wastewater treatment, as well as in the photocatalytic degradation of CHD. The successful incorporation of an indigenous PMR in the field of membrane science to carry out
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photocatalysis is another innovation of this work. The developed PMR can be operated in under batch, semi-batch, as well as continuous mode of operations. Moreover, in presence of
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nanoparticles membranes become more hydrophilic in nature, results in better anti-fouling property. The cross flow configuration of the PMR ensures better contact between target molecules and catalyst particles on the membrane surface, which results in better degradation
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efficiency. Hence, the developed system is more economic by alleviating the problems related to recovery and reuse of the catalyst. In subsequent sections, the development of whole process and its performance will be described elaborately. Thus, this article may
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contribute some scientific aspects in the field of membrane science, heterogeneous photocatalysis and pharmaceutical wastewater treatment as well.
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2. Experimental 2.1.
Materials
The catalyst system used was Aeroxide P25 (mixture of rutile and anatase, 718467) of particle size 21 nm with surface area (BET) 35−65 m2g−1 from Sigma-Aldrich, and were used as received. Chlorhexidine digluconate solution (20% w/v) was purchased from SigmaAldrich to prepare the simulated solutions for experimental purpose. Polyether sulfone (PES) (Solvay Specialities India Pvt. Ltd., Veradel 3000P) with a molecular weight (Mw) of 62,000–64,000 g/mol was used for PES membrane synthesis. The solvent was n-methyl-23
pyrrolidinone (NMP, anhydrous 99.5%, Sigma–Aldrich), and polyvinylpyrrolidone (PVP, Mw: 10,000 g/mol, Sigma–Aldrich) was used as the hydrophilic and pore formation additive. All experiments were carried out with ultrapure water from Arium Pro VF (Sartorius Stedim Biotech) of 18.2 MΩ-cm resistivity. All other chemicals which were used during experimentation were purchased from Sigma Aldrich Chemical Co., USA. 2.2. Preparation of modified membrane with TiO2 2.2.1. Membrane fabrication and surface coating The polyether sulfone membrane was fabricated using a casting knife, made of laser precision machined iron, and was used to obtain a constant thickness and surface for the membranes.
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In the current study, the membrane was fabricated using the casting solution containing 20 wt.% PES polymer by phase inversion via immersion precipitation, and the polymeric solution was casted over thin porous sheet of polyethyleneterephthalate, which provide better mechanical strength to the membrane. The composition of PES membrane thus prepared
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(Membrane I) has been shown in the table 1. Henceforth, the prepared PES membrane was cured by deionized water and coated with TiO2 nanoparticles by dipping the membrane into
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TiO2 colloidal suspension and radiated under UV irradiation. During the dipped coating process, concentration of TiO2, dipping time and irradiation time was varied. After all trial
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experimental runs and analysis, the dipping time and irradiation time was fixed at 30 min. The concentration of TiO2 was varied in three different ways such as 0.05 wt.%, 0.1 wt.% and 0.15 wt.%. The formation of TiO2 coated layer could be single or multiple. During the
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single layer coating process, after 30 min of dipping into TiO2 suspension, the coated membrane was UV irradiated for 30 min. In case of double and triple layer coating, the same process was followed for two and three times, respectively. The nano-particles were
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completely stirred with water to form stable suspension before coating operation. Finally, the membranes were washed with distilled water to remove excess TiO2 nanoparticles from the
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membrane surface. Then the membrane was stored in 40C for future use. 2.2.2. TiO2 nanoparticles impregnated membrane fabrication The casting solution for the preparation of TiO2 impregnated membrane has been prepared with various composition of PES, NMP, PVP and TiO2 as mentioned in table -1. In casting solution, the weight percentage of PES and PVP was fixed at 20.0 and 1.0 respectively. NMP was varied from 79 to 73 wt.% and corresponding TiO2 was 0-6 wt.%. According to mentioned compositions, the casting solution was prepared by continuous shaking at 250C for 4
48 hours in shaker incubator. After that to ensure the complete dispersion of TiO2 nanoparticles, the casting solution was well stirred with the help of magnetic stirrer at 400C for 4 hours. Furthermore, sonication and degasing operation was performed over the same solution to achieve homogeneous mixture of casting solution. Then only, the membrane was fabricated by phase inversion via immersion precipitation using a casting knife. The nonwoven fabric of polyethyleneterephthalate was used as the bottom layer for such type of membrane. The membrane was cured by dipping it in deionized water for overnight. 2.3. Simulated pharmaceutical wastewater preparation
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The simulated pharmaceutical wastewater was prepared using purchased 20% (w/v) CHD solution as per the desired concentration of CHD. Deionised water was used to prepare the simulated solution matrix and water has been collected from Sartorius arium® pro VF water system which was supplied by Sartorius Stedim, Germany.
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2.4. Characterisation of fabricated membrane
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The surface characteristics of fabricated and modified membranes were investigated using SEM (S-4700, Hitachi, Japan). Contact angles (Phoenix 300, Surface electro optics, South
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Korea) were measured to observe and compare the hydrophilicity between normal and modified membranes. All of the membranes were fully dried before the SEM and the contact angle analysis, to minimize the error as much as possible.
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2.5. Experimental set up
A novel indigenous membrane photo reactor (PMR) was used to carry out photocatalytic
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degradation of CHD. The PMR was fabricated by a local fabricator according to the design provided by the present research group. The reactor was operated in batch, semi-batch and
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continuous mode under cross flow configuration. The working principle of experimental set up of the batch mode has been shown schematically in the figure 1(A). During the batch experiment permeate stream was send back to the feed tank. In case of semi-batch mode of operation permeate stream was continually withdrawn from the system as shown in the figure 1(B). During continuous mode of operation fresh feed was added to the intermediate or mixed tank and at the same time, permeates was continually withdrawn from the system which has been 5
shown in the figure 1(C). However, the volumetric flow rate of added fresh feed was equal to
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the permeate flow rate under such condition.
Fig. 1. Schematic representation of experimental set up for (A) batch mode, (B) semi-batch mode and (C) continuous mode of operations. Both TiO2 coated and impregnated membranes were used during photocatalysis in the PMR operated under different modes. The trans membrane pressure (TMP) was maintained with the help of pressure regulator valve. At constant time interval aliquot sample was collected from the permeate line to analyses the degradation of CHD using PMR. Initially, a set of
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collected sample was centrifuged to remove TiO2 nano particle, though nothing was collected as residue. Moreover, optical absorption of collected permeate solution before and after centrifuged was same. Hence, it confirmed that no nano particle was presence in permeate. After such confirmation, concentration of CHD was measured in permeate directly. During all experimental run the temperature was maintained at 250C and the pH was adjusted at 10.5 according to earlier studies by the same research group [18, 19]. The concentration of CHD was measured by using Varian Cary 50Bio UV spectrophotometer (part No. EL07113760) and validated with HPLC analysis using Zorbax SB-phenyl column. Moreover, details of analysis protocols of CHD have been provided in published literatures [18, 19]. Finally, the
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percentage of CHD removal during and after photocatalytic degradation was calculated using following equation.
C % CHD = 1 t 100 C0
(1)
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where, Ct is the substrate concentration at any time t and C0 is corresponding value at t=0.
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3. Results and Discussion 3.1. Characterization of the TiO2 coated membrane
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The SEM images of bare PES and TiO2 coated membrane have been shown in the figure 2. TiO2 suspensions of 0.05 to 0.15 wt.% was used to produce coated membrane. Figures 2 (bd) have shown the single, double and triple layer of TiO2 coated membrane using 0.05 wt.%
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TiO2 suspensions. The SEM images have confirmed well dispersed TiO2 nanoparticles coated membrane can be obtained with help of dipped coating method. A clear comparison between three types triple layer coated membrane with different concentrated TiO2 suspension has
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been observed from figures (d-f). In case of triple layer coated membrane with 0.05 wt.% TiO2 suspension (figure 2(d)) no such agglomeration of TiO2 nanoparticles has been observed
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whereas in other two cases, large formation of such agglomerations have been found on the membrane surface via SEM images (figures 2(e & f)). Figures 2(e) and 2(f) represent the SEM images of triple layer coated membrane with 0.10 and 0.15 wt.% TiO2 suspensions. The triple layer coating (figure 2(d)) has indicated better surface coverage than the single and double layer coating by 0.05 wt.% TiO2 suspensions, as well as triple layer coating don’t exhibit any agglomeration problem. Moreover, 0.05 wt.% TiO2 suspensions was the most stable and could sustain for several days [20, 21], and also coated membrane with same concentration of TiO2 in some cases showed better performances [22, 23]. Subsequently 7
triple layer coated membrane with 0.05 wt.% TiO2 suspension was used throughout
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photocatalytic degradation of CHD.
Figure 2: SEM images of (a) Bare PES; (b-d) single, double and triple layer coated
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membrane with 0.05 wt.% TiO2 suspensions respectively; (e) triple layer coated membrane with 0.10 wt.% TiO2 suspensions; (f) triple layer coated membrane with 1.5 wt.% TiO2 suspensions.
3.2. Characterization of the TiO2 Impregnated membrane The SEM images (figure 3) have confirmed the presence of nanoparticles on the surface as well as inside the porous structure of the membrane. During the fabrication of nanoparticles impregnated membrane, TiO2 binds with the polymer matrix.
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Figure 3: Surface and cross-section SEM images of (a-b) 2% TiO2, (c-d) 4% TiO2 and (e-f) 6% TiO2 impregnated membrane. SEM images both surfaces and cross-sections have indicated that with increase of TiO2 concentration the distribution of the nano particles becomes better though it is impossible to increase such concentration beyond some limiting value. It has been experimentally observed during the preparation of casting solution that after 6wt.% of TiO2 nanoparticles concentration it is very hard to dissolve PES in MNP and to form a homogeneous casting 9
solution. In the present study, as modified membrane containing 6wt.% TiO2 (figure 3(e) & 3(f)) shows the better distribution of nano particle, it had been used for the photocatalytic degradation of pharmaceutical component, CHD in PMR. 3.3.Contact angle and enhanced antifouling properties The measured values of contact angle for modified and unmodified membrane have been indicated in the figure 4(a & b), and such measurement is required for the evaluation of hydrophilicity as well as the wettability of any membrane surface. The values of contact
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angle sharply decrease in case of both TiO2 coated and impregnated membrane.
Figure 4: Contact angle of (a) pure PES and (b) modified membrane using TiO2 nanoparticles; (c) Pure water flux of different types of membranes at different TMP and (d) plot of volumetric flux vs. TMP
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The membrane performance significantly depends up on the anti-fouling property of the membrane and this property depends on the hydrophilicity of the membrane surface. There is an inverse relationship between contact angle and hydrophilicity of any surface and it refers that the lower contact angle shows the higher hydrophilicity. With increase hydrophilicity in presence of TiO2 nanoparticles, the membrane surface shows better anti-fouling property [1116]. Therefore, in the present study, according to the contact angle measurement the modified PES membrane demonstrated better hydrophilicity and hence, the fabricated TiO2 modified membranes had better anti-fouling property. Moreover, in presence on nanoparticles, not only the roughness of membrane increases [11] but the presence of number of hydroxyl group
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on the membrane surface increases also [11, 24]. Hence, the membrane surface becomes polarity in presence of TiO2 nanoparticles and this hydroxyl radial interact with water molecules via van der Waals’ attraction force and hydrogen bond [11]. This is the most significant reason for increasing hydrophilicity in presence of nano particle.
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3.4. Water flux of fabricated membranes
The pure water flux of different membrane was tested, which has been shown in the figure
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4(c). After the compaction of all membranes it was found from the figure 4(c) that the presence of nanoparticles on the membrane surface greatly affected the water flux. Moreover,
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the drop of flux was more pronounced in case triple layered surface coated membrane with 0.05 wt.% TiO2 suspension than the 6 wt.% TiO2 impregnated membrane. This was happened because during the surface coating the pores of pure PES membrane were partially blocked
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by nanoparticles, whereas in case of impregnated membrane, pore channels were partially occupied by nanoparticles, which is confirmed by SEM images (figure 2 & 3). In the current research work the water flux of the modified membrane is not the major concern and
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subsequently lower water flux provide better residence time (τ) to the substrate molecules for photocatalytic degradation. Here, the membrane works as catalyst support to recover valuable
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catalyst after the completion of the reaction. The figure 4(c & d) also indicates that for any type of membrane, the flux improves with increase of TMP which is very common in practice for any type of membrane. According to “Hagen-Poiseuille” equation the flux through the membrane pores can be calculated using equation 2.
J=
ΔP μR m
(2)
where, J, ΔP, µ and Rm represent the volumetric flux (m3/m2-s), trans membrane pressure 11
(Pa), viscosity of the working solution (kg/m-s) and flow resistance of the membrane per unit area (m-1) respectively. µ represents the viscosity of the water i.e. 9˟10-4 kg/m-s at 250C. From the slope of the plots (figure 4(d)) of J vs. ΔP one can calculate the average membrane hydraulic resistance (Rm) and the measured values are 6.98848˟1013, 1.92985˟1014 and 1.34122˟1014 m-1 for pure PES, 0.05 wt.% TiO2 coated membrane, 6.0 wt.% TiO2 impregnated membrane, respectively. Eventually, the resistance of the pure membrane is much less than the modified membranes. 3.5. Photocatalytic degradation of CHD Using PMR 3.5.1. Working principle of photo membrane reactor
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The photocatalytic reaction takes place mainly on the catalytic membrane surface under UV irradiation. The working principle of photocatalytic membrane can be represented with the
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help of figure 5.
Figure 5: Schematic representation of working principle of photocatalytic degradation of CHD.
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In the current study when CHD molecules came in contact with TiO2 nanoparticles, it got adsorbed on the catalyst surface, and then got degraded. The micro pollutants like CHD are
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very small in size; therefore those can easily pass through the pure PES membrane in the present study. But in presence of nanoparticles, those molecules face some potential barrier during permeation through the membrane and eventually, those molecules are converted to other by-products by photocatalysis. Subsequently, some of the targeted molecules can pass through the membrane without taking part in photocatalytic reaction, whereas others are degraded; the whole process has been indicated in the figure 5.
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As photocatalysis is a surface adsorption reaction phenomenon, it can be assumed that a single catalyst particle when involved in such type of reaction, it cannot uptake other target molecules during that period for further degradation. Hence, that particular catalyst particle can be available for photocatalytic reaction when it becomes free from by-products generated during degradation of target molecules. Therefore, when all the nanoparticles on the membrane surface are engaged in photocatalysis, the free molecules of CHD in the working solution are easily passed through the membrane under the action of TMP. Moreover, crossflow action somehow helpful to regenerate the catalytic membrane surface by removing the by-products as well as it helps to nullify the effect of concentration polarization of target
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molecules and by-products on the membrane surface. This explaination may be helpful to understand the performance analysis of any type of photo membrane reactor. 3.5.2. Photocatalysis of CHD using TiO2 modified membrane
The degradation behavior of CHD in PMR using both TiO2 coated and impregnated
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membrane was carried out in three different mode of operation i.e. batch, semi-batch and continuous mode and the observed results have been plotted in the figure 6. Also, a trial was
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conducted with bare PES membrane to ensure that the bare membrane had no effect on degradation or removal of CHD. CHD immediately passed through the bare membrane to the
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permeate side. The experimental details have already been described earlier. The concentration of CHD in the permeate stream was measured accordingly the analysis protocol of CHD [18, 19] and the removal percentage of CHD was calculated using equation 1.
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During the experiment, the concentration of CHD in feed solution was maintained at as much as lower value was possible. so that the simulated solution could replicate the real-life environmental condition and for the present case the value was 0.1 g/L (100 ppm).
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Furthermore, lower initial concentration of CHD refers less population of target molecules on the catalytic membrane surface which increases the photocatalytic efficiency of the system
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[18, 19]. The pH and temperature was maintained at 10.5 and 250C respectively to ascertain the maximum removal of CHD [18, 19]. In all three modes of operations within 5-10 min of photocatalysis, the removal percentage of CHD reached its maximum value and then it started to fall for all nanoparticles coated and impregnated membranes. Again after 30-45 min removal percentage started to improve and after approximately 180 min it reached almost steady state value. It was observed the, the steady state value is less than the maximum value, attained at initial stage (5-10 min) of photocatalysis for each mode of operation.
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The above observation can be well explained with the help of section 3.5.1. At the starting of the reaction (within 5 to 10 min) the catalyst particles on the membrane surface were highly active and thus, when CHD molecules came in contact with the nanoparticles they were readily adsorbed and initiate photocatalysis. Henceforth, free catalyst particles were not available on the membrane surface for further adsorption as well as photocatalysis thus, free CHD molecules from the bulk solution readily passed the membrane with permeate stream. As a result after 5-10 min of reaction, the decrement in rate of photocatalysis prevailed in the PMR. Again after some time duration, the catalytic surface became regenerated as byproducts detached from catalyst surface and then only it can be involved in further
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photocatalysis. Consequently, after about 45 min the improvement in removal efficiency of pharmaceutical component using the PMR has been observed (figure 6(a-c) & 6(e-g)) so far and finally it has reached its steady state value.
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3.5.2.1.Performance analysis with variation of TMP
The figures 6(a-c) & 6(e-g) is also very helpful to understand the effect of TMP on the
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process efficiency. It has been already mentioned that with increase of TMP the flux of the membrane increases.
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In the present study the modified membrane is not working as a selective barrier for CHD molecules. The membrane basically provides support to the catalyst particles therefore, with increase of TMP most of the CHD molecules pass the catalytic surface before reaction (figure
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5). Moreover, higher rate of flux reduces the residence time for the target molecule and hence poor degradation encompasses in the system operated under any mode of operation, which has been clearly indicated in the figures 6(a-c) & 6(e-g). In semi-batch or continuous mode of
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operation unreacted CHD molecules leaves the system along with the permeate stream and with increase of TMP the permeate flux as well as number of CHD molecules in the permeate
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increases. Hence, poor percentage removal of CHD has been obtained at higher TMP, which has been indicated in the figures 6(b & c) and 6(f & g). However, the effect of TMP on batch mode has been shown in the figures 6(a) and 6(e), in such cases unreacted CHD molecules along with the permeate stream is recycled back. As targeted molecules again reenter to system the removal efficiency has been improved to some extent but at higher TMP they immediately leave the system causing lower photocatalytic
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degradation. Thus, for any system TMP should be maintained in such a way so that targeted molecules get enough time for photocatalytic degradation on the membrane surface. 3.5.2.2.Performance analysis under different mode of operations The comparison of between three modes using two different types of membrane (figure 6(d) & 6(h)) shows that in the batch mode, CHD can be removed significantly than other modes of operation using PMR. In the figures figure 6(d) & 6(h), the batch steady state percentage removal of CHD in PMR at 196.133 kPa TMP has been shown. In the batch mode operation, the permeate stream is recycled back to the reactor thus, non-degraded CHD molecules may
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reenter in PMR for further degradation. On the other hand in both semi-batch and continuous mode, the permeate stream not only leaves the system with unreacted target molecules but also in continuous mode fresh feed is introduced to the system. The fresh feed always increases the population of pollutant molecules on the membrane surface thus less removal
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percentage can be obtained in continuous mode operation than any other mode of operation.
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3.5.3. Comparison of photocatalytic efficiency between different types of membranes The steady state removal percentage of CHD using both types of membrane under different
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modes of operation of PMR has been indicated in the figure 7(a). In all modes of operation TiO2 impregnated membrane has shown better performance than the coated membrane with respect photocatalytic degradation of CHD at TMP of 196.133 kPa. This observation can be
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explained with the help of SEM images (figure 2(d) & 3(e)). The figure 3(e) of TiO2 impregnated membrane has better distribution as well as population of nanoparticles on the membrane surface than the TiO2 coated membrane and thus, first type of membrane shown
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better performance. Moreover, the figure 3(f) also confirms the presence of nanoparticles in the tortuous path of pores of the membrane and those nanoparticles can adsorb or degrade
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CHD molecules which may enhance the performance of such type of membrane. From the quantitative point of view, the above findings can be explained in another way. During the fabrication of impregnated membrane 6 wt.% TiO2 was used among that the maximum percentage TiO2 incorporated in the membrane structure resulting huge population of nanoparticles per unit area on the membrane surface. On the other hand during dipped coating only 0.05 wt.% TiO2 suspension was used and that quantity was very insignificant compare to 6 wt.%. Moreover, from 0.05 wt.% TiO2 suspension, a very small amount nanoparticles are deposited over the membrane surface. Subsequently, poor population of 16
catalyst particles per unit area of membrane surface was achieved by dipped coating technique. Therefore, dipped coated membrane always shows poor photocatalytic
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performance as well as surface adsorption than TiO2 impregnated membrane.
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Figure 7: (a) Steady state comparison between TiO2 coated and impregnated membranes under different modes of operation in PMR; (b) Flux recovery after photocatalysis and (c)
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catalyst recovery and reuse for both TiO2 coated and impregnated membranes The performance of the catalytic membrane can be explained with the help of the nature of attachment of nanoparticles with the membranes. In the impregnated membrane the photo catalysts particles are entrapped strongly inside the cross linked polymeric network and those particles cannot be easily removed from the membrane structure, whereas TiO2 nanoparticles are loosely bonded with the help of adhesive forces on membrane surface in case of catalyst coated membrane. Therefore, there is huge possibility of wash out of nanoparticles from the 17
catalyst coated membrane surface under cross flow action during photocatalysis in PMR, resulting ineffective photocatalysis of CHD. The above discussion revels that the coated membrane lags behind in different ways from the catalyst impregnated membrane. Thus, TiO2 nanoparticles coted membrane showed poor photocatalytic performance using the PMR under same operating condition. 3.6. Flux and catalyst recovery In the present study to measure the flux recovery for both types of membranes, pure water flux was measured after five consecutive batch photocatalysis using same membrane in PMR operated under 196.133 kPa TMP. After photocatalytic degradation all membranes were
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washed with deionized water for 4 hrs. The obtained result has been shown in the figure 7(b) and it has been evaluated that for any type of membrane the flux recovery up to 95% which very significant in deed. Therefore, such type of membrane may be very effective for photocatalytic degradation of pharmaceutical wastes without altering the pure water flux of
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the membrane. The antifouling property of TiO2 modified membrane may be responsible for such observation.
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The main objective of the current study is the recycle and recovery of catalyst particle. To measure such effectiveness of the present system, the steady state percentage removal of
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CHD was measured for five consecutive batch cycle photocatalysis using same membrane in PMR operated under 196.133 kPa TMP which has been shown in the figure 7(c). In all degradation processes the initial CHD concentration were maintained at 0.1 g/L. It has been
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observed from the experimental data that the removal percentage of CHD has been reduced by only 5% for impregnated and 14% for coated membrane after consecutive five batch cycles. The reduction of photocatalytic efficiency is more pronounced for the coated
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membrane than impregnated condition as TiO2 nanoparticles can be washed out from the surface coated membrane under cross flow action during photocatalysis as well as washing of
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the membrane which is not possible for impregnated catalyst particles. However, the present system can efficiently recover catalyst after photocatalysis with help of immobilization techniques for future use thus present development is economically sustainable for the pharmaceutical wastewater treatment. 4. Conclusion The present study again proved that photocatalysis is inevitable for pharmaceutical wastewater treatment. A new concept of PMR under three modes of operation has been 18
implemented
for
the
pharmaceutical
wastewater
treatment.
Moreover,
successful
immobilization of TiO2 nanoparticles has been achieved by confirming the complete reuse and recycle of TiO2 nanoparticles, which has made the whole process more economic and environmentally sustainable. The main influencing parameter is TMP for PMR, and the effects TMP has greatly acknowledged in the process efficiency when all other parameters maintained at optimum levels. SEM images confirm the distribution of nanoparticles over the membrane surface whereas contact angle analysis proves improved hydrophilicity of modified membranes. The flux recovery of the photocatalytic membrane is very much significant which also indicates the enhance antifouling property of such type of membrane.
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It has already been proved that nanoparticles impregnated membrane has shown superior performance than the coated membrane. The most significant achievement by PMR is that CHD has been removed from the system using TiO2 impregnated membrane in batch mode near to the value obtained in the batch photocatalysis process using TiO2 suspension [18, 19]. However, using same type of membrane under semi-batch and continuous mode operation
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the percentage removal of CHD is quite appreciable. The semi-batch and continuous processes are advantageous over the batch by handling large amount of wastewater.
rigorous and precision job.
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However, the development of whole process with fabrication of the membrane is very Above discussion confirms that the present system can be
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effectively implemented in the pharmaceutical wastewater treatment as well as wastewater contains organic pollutants, which are degradable under photocatalysis. Some necessary modifications should be incorporated to enhance accuracy of fabricated membrane as well as
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lower down tediousness of fabrication, and also further development is required to scale up the whole process. Then only this methodology can be viable for large scale pharmaceutical
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wastewater treatment.
Author Contribution Statement
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1.
Santanu Sarkar: Conceptualization, Methodology, Data curation, Writing- Original draft preparation.
2.
Chiranjib Bhattacharjee: Visualization, Investigation, Writing- Reviewing and Editing,
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M. Klavarioti, D. Mantzavinos and D. Kassinos, Removal of residual pharmaceuticals
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and various modes of application of TiO2 nanoparticles in heterogeneous photocatalysis of pharmaceutical wastes – a short review, RSC Adv., 2014, 4, 57250.
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S. Pourjafar, A. Rahimpour and M. Jahanshahi, Synthesis and characterization of
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fillers on the morphologies and properties of PSF UF membrane, J. Membr. Sci., 2007, 288, 231–238. 13.
J.B. Li, J.W. Zhu and M.S. Zhen, Morphologies and properties of poly (phthalazinone
ether sulfone ketone) matrix ultrafiltration membranes with entrapped TiO2 nanoparticles, J. Appl. Polym. Sci., 2007, 103, 3623-3629. J. H. Li, Y.Y. Xu, L.P. Zhu, J.H. Wang and C.H. Du, Fabrication and characterization
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of a novel TiO2 nanoparticle self-assembly membrane with improved fouling resistance, J. Membr. Sci., 2009, 326, 659-666. 15.
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nanoparticles with UV irradiation for modification of polyethersulfone ultrafiltration membranes, J. Mem. Sci., 2008, 313, 158–169. 22
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M.-L. Luo, J.-Q. Zhao, W. Tang and C.-S. Pu, Hydrophilic modification of
poly(ether sulfone) ultrafiltration membrane surface by self-assembly of TiO2 nanoparticles, Appl. Surf. Sci., 2005, 249, 76-84. 25.
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removal of pharmaceuticals from water: A review, J. Env. Man. 2018, 219, 189-207. 26. J. Wang, R. Zhuan, Degradation of antibiotics by advanced oxidation processes: An Science
of
the
Total
Environment
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https://doi.org/10.1016/j.scitotenv.2019.135023
23
(2019),
doi:
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overview,
Table 1: Composition of the casting solutions. Components
Composition (wt.%)
Amount (g)
Membrane I
PES
20
5.0
NMP
79
19.75
PVP
1
0.25
TiO2
2
0.5
PES
20
5.0
NMP
77
19.25
PVP
1
0.25
TiO2
4
1.0
PES
20
NMP
75
PVP
1
TiO2
6
PES
20
NMP
73
Membrane IV
1
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lP
PVP
5.0
18.75 0.25
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Membrane III
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Membrane II
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Types
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1.5 5.0
18.25 0.25
PII:
S2213-3437(20)30021-X
DOI:
https://doi.org/10.1016/j.jece.2020.103673
Reference:
JECE 103673
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
14 November 2019
Revised Date:
16 December 2019
Accepted Date:
6 January 2020
Please cite this article as: Sarkar S, Bhattacharjee C, Removal of micro-pollutant using an indigenous photo membrane reactor, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2020.103673
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Removal of micro-pollutant using an indigenous photo membrane reactor Santanu Sarkara* Chiranjib Bhattacharjeeb, a
R&D Department, Tata Steel Limited, Jamshedpur, Jharkhand, India Chemical Engineering Department, Jadavpur University, Kolkata, India
Corresponding Author’s Email: [email protected], Phone: +91 7033094998, +91-657 6648938
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b
Abstract
Pharmaceutical wastewater treatment using heterogeneous photocatalysis in presence of TiO2
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nanoparticles has become a lucrative method because the convectional wastewater and biological treatments usually fail to remove the detrimental effect of different drugs and
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hormones on the environment. The nanoparticles recovery and reuse is the most challenging task in practice. The main goal of the present study is to recover and reuse of photocatalyst by Consequently, two types of immobilization
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immobilizing it on the membrane surface.
technique have been used here, one is surface immobilization by dipped coating, and other is impregnation of catalyst during fabrication of the membrane. To conduct photocatalysis, an
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indigenous photo membrane reactor has also been fabricated during the research work. The presence of TiO2 nanoparticles has not only improved the catalytic property of the membrane, but also the contact angle analysis has confirmed the enhanced hydrophilic as well as
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antifouling property of the surface modified membrane. The fabricated membranes were characterized by scanning electron microscopy (SEM). To analyses the performance of
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photo membrane reactor as well as TiO2 coated or impregnated membranes, the photocatalytic degradation of aqueous solution of chlorhexidine digluconate (CHD), an antibacterial drug, was performed using the developed system and the removal percentage of CHD obtained from the system was found to be quite appreciable. The results show that the recovery and reuse of the catalyst by the present system is very much significant. Meanwhile, the developed reactor can be operated in cross flow configuration under batch, semi-batch and continuous mode.
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Keywords: Membrane; titanium dioxide; surface modification; photo membrane reactor; pharmaceutical waste; advanced photocatalysis.
1. Introduction Environmental pollution due to several pharmaceutical wastes has become a very significant global threat in very recent years. Both pharmaceutical companies and municipal corporations are the major contributors of such pollution. Moreover, the unscientific use of
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different drugs in medical science influences to increase the level of pollution, results in gradual growth of such compounds in the environment. The presences of those micro pollutants in different water bodies and their detrimental effects have been confirmed by several researchers [1-6]. The limitations over the pharmaceutical wastewater treatment are the pollutants i.e. antibiotics, hormones, steroids, etc. cannot be easily separated through
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conventional wastewater treatment and not even degraded in biological treatment [7, 8]. From last two decades heterogeneous photocatalysis in presence of TiO2 semiconductor
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nanoparticles has been used by several research groups for pharmaceutical wastewater treatment [9, 10, 25, 26], though most of the researchers carried out their treatment process in
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batch suspension mode operation. The major problem related to such processes is the recovery and reuse of TiO2 nano particle at the end of photocatalysis.
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The recycle and reuse of photocatalyst particles can be done with the help of different immobilization techniques [10, 26]. Very recently nanoparticles are trapped using the membrane technology, which has become promising pathway for the surface modification of
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the membrane [11-16]. The immobilization technique can be classified in two different ways such as membrane surface coating with TiO2 nanoparticles, and other is the preparation of
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nanoparticles impregnated membrane [11-16]. According to S. Mozia et al. [17] the incorporation of membrane in a photo-reactor serves both the purposes of catalyst recovery, and also acts as a separating barrier between by-products generated during photocatalysis of the targeted pollutants. Hence, not only preparation of photocatalytic membrane is enough for pharmaceutical wastewater treatment, but it is also essential to developed membrane photo reactor (PMR) to carry out photocatalytic reaction.
Both the fabrication of catalytic
membranes and subsequent development of indigenous PMR are very important for effective implementation of the heterogeneous photocatalysis. 2
In the present study, bare and nanoparticles impregnated poly ether sulfone (PES) membranes with different concentration TiO2 nanoparticles have been fabricated by phase inversion method. The bare PES membrane then surface coated with TiO2 nanoparticles under UV irradiation. In house prepared membranes have been characterized with the help of SEM and contact angle analysis. To use those modified membrane in field of pharmaceutical wastewater treatment, an indigenous PMR has also been fabricated having some novelties. To study the performance of PMR, the photocatalytic degradation of chlorhexidine digluconate (CHD), a pharmaceutical drug in presence of catalytic membrane has been carried out here. The same research group has already established that photocatalysis is very much effective in
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removal of CHD using TiO2 suspension in batch mode and the details of impact of CHD has been reported elsewhere [18, 19].
The novelty of the present research work is clear and straight. According to the literature survey the nanoparticles modified membrane was used first in the field of pharmaceutical
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wastewater treatment, as well as in the photocatalytic degradation of CHD. The successful incorporation of an indigenous PMR in the field of membrane science to carry out
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photocatalysis is another innovation of this work. The developed PMR can be operated in under batch, semi-batch, as well as continuous mode of operations. Moreover, in presence of
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nanoparticles membranes become more hydrophilic in nature, results in better anti-fouling property. The cross flow configuration of the PMR ensures better contact between target molecules and catalyst particles on the membrane surface, which results in better degradation
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efficiency. Hence, the developed system is more economic by alleviating the problems related to recovery and reuse of the catalyst. In subsequent sections, the development of whole process and its performance will be described elaborately. Thus, this article may
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contribute some scientific aspects in the field of membrane science, heterogeneous photocatalysis and pharmaceutical wastewater treatment as well.
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2. Experimental 2.1.
Materials
The catalyst system used was Aeroxide P25 (mixture of rutile and anatase, 718467) of particle size 21 nm with surface area (BET) 35−65 m2g−1 from Sigma-Aldrich, and were used as received. Chlorhexidine digluconate solution (20% w/v) was purchased from SigmaAldrich to prepare the simulated solutions for experimental purpose. Polyether sulfone (PES) (Solvay Specialities India Pvt. Ltd., Veradel 3000P) with a molecular weight (Mw) of 62,000–64,000 g/mol was used for PES membrane synthesis. The solvent was n-methyl-23
pyrrolidinone (NMP, anhydrous 99.5%, Sigma–Aldrich), and polyvinylpyrrolidone (PVP, Mw: 10,000 g/mol, Sigma–Aldrich) was used as the hydrophilic and pore formation additive. All experiments were carried out with ultrapure water from Arium Pro VF (Sartorius Stedim Biotech) of 18.2 MΩ-cm resistivity. All other chemicals which were used during experimentation were purchased from Sigma Aldrich Chemical Co., USA. 2.2. Preparation of modified membrane with TiO2 2.2.1. Membrane fabrication and surface coating The polyether sulfone membrane was fabricated using a casting knife, made of laser precision machined iron, and was used to obtain a constant thickness and surface for the membranes.
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In the current study, the membrane was fabricated using the casting solution containing 20 wt.% PES polymer by phase inversion via immersion precipitation, and the polymeric solution was casted over thin porous sheet of polyethyleneterephthalate, which provide better mechanical strength to the membrane. The composition of PES membrane thus prepared
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(Membrane I) has been shown in the table 1. Henceforth, the prepared PES membrane was cured by deionized water and coated with TiO2 nanoparticles by dipping the membrane into
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TiO2 colloidal suspension and radiated under UV irradiation. During the dipped coating process, concentration of TiO2, dipping time and irradiation time was varied. After all trial
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experimental runs and analysis, the dipping time and irradiation time was fixed at 30 min. The concentration of TiO2 was varied in three different ways such as 0.05 wt.%, 0.1 wt.% and 0.15 wt.%. The formation of TiO2 coated layer could be single or multiple. During the
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single layer coating process, after 30 min of dipping into TiO2 suspension, the coated membrane was UV irradiated for 30 min. In case of double and triple layer coating, the same process was followed for two and three times, respectively. The nano-particles were
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completely stirred with water to form stable suspension before coating operation. Finally, the membranes were washed with distilled water to remove excess TiO2 nanoparticles from the
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membrane surface. Then the membrane was stored in 40C for future use. 2.2.2. TiO2 nanoparticles impregnated membrane fabrication The casting solution for the preparation of TiO2 impregnated membrane has been prepared with various composition of PES, NMP, PVP and TiO2 as mentioned in table -1. In casting solution, the weight percentage of PES and PVP was fixed at 20.0 and 1.0 respectively. NMP was varied from 79 to 73 wt.% and corresponding TiO2 was 0-6 wt.%. According to mentioned compositions, the casting solution was prepared by continuous shaking at 250C for 4
48 hours in shaker incubator. After that to ensure the complete dispersion of TiO2 nanoparticles, the casting solution was well stirred with the help of magnetic stirrer at 400C for 4 hours. Furthermore, sonication and degasing operation was performed over the same solution to achieve homogeneous mixture of casting solution. Then only, the membrane was fabricated by phase inversion via immersion precipitation using a casting knife. The nonwoven fabric of polyethyleneterephthalate was used as the bottom layer for such type of membrane. The membrane was cured by dipping it in deionized water for overnight. 2.3. Simulated pharmaceutical wastewater preparation
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The simulated pharmaceutical wastewater was prepared using purchased 20% (w/v) CHD solution as per the desired concentration of CHD. Deionised water was used to prepare the simulated solution matrix and water has been collected from Sartorius arium® pro VF water system which was supplied by Sartorius Stedim, Germany.
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2.4. Characterisation of fabricated membrane
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The surface characteristics of fabricated and modified membranes were investigated using SEM (S-4700, Hitachi, Japan). Contact angles (Phoenix 300, Surface electro optics, South
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Korea) were measured to observe and compare the hydrophilicity between normal and modified membranes. All of the membranes were fully dried before the SEM and the contact angle analysis, to minimize the error as much as possible.
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2.5. Experimental set up
A novel indigenous membrane photo reactor (PMR) was used to carry out photocatalytic
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degradation of CHD. The PMR was fabricated by a local fabricator according to the design provided by the present research group. The reactor was operated in batch, semi-batch and
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continuous mode under cross flow configuration. The working principle of experimental set up of the batch mode has been shown schematically in the figure 1(A). During the batch experiment permeate stream was send back to the feed tank. In case of semi-batch mode of operation permeate stream was continually withdrawn from the system as shown in the figure 1(B). During continuous mode of operation fresh feed was added to the intermediate or mixed tank and at the same time, permeates was continually withdrawn from the system which has been 5
shown in the figure 1(C). However, the volumetric flow rate of added fresh feed was equal to
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the permeate flow rate under such condition.
Fig. 1. Schematic representation of experimental set up for (A) batch mode, (B) semi-batch mode and (C) continuous mode of operations. Both TiO2 coated and impregnated membranes were used during photocatalysis in the PMR operated under different modes. The trans membrane pressure (TMP) was maintained with the help of pressure regulator valve. At constant time interval aliquot sample was collected from the permeate line to analyses the degradation of CHD using PMR. Initially, a set of
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collected sample was centrifuged to remove TiO2 nano particle, though nothing was collected as residue. Moreover, optical absorption of collected permeate solution before and after centrifuged was same. Hence, it confirmed that no nano particle was presence in permeate. After such confirmation, concentration of CHD was measured in permeate directly. During all experimental run the temperature was maintained at 250C and the pH was adjusted at 10.5 according to earlier studies by the same research group [18, 19]. The concentration of CHD was measured by using Varian Cary 50Bio UV spectrophotometer (part No. EL07113760) and validated with HPLC analysis using Zorbax SB-phenyl column. Moreover, details of analysis protocols of CHD have been provided in published literatures [18, 19]. Finally, the
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percentage of CHD removal during and after photocatalytic degradation was calculated using following equation.
C % CHD = 1 t 100 C0
(1)
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where, Ct is the substrate concentration at any time t and C0 is corresponding value at t=0.
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3. Results and Discussion 3.1. Characterization of the TiO2 coated membrane
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The SEM images of bare PES and TiO2 coated membrane have been shown in the figure 2. TiO2 suspensions of 0.05 to 0.15 wt.% was used to produce coated membrane. Figures 2 (bd) have shown the single, double and triple layer of TiO2 coated membrane using 0.05 wt.%
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TiO2 suspensions. The SEM images have confirmed well dispersed TiO2 nanoparticles coated membrane can be obtained with help of dipped coating method. A clear comparison between three types triple layer coated membrane with different concentrated TiO2 suspension has
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been observed from figures (d-f). In case of triple layer coated membrane with 0.05 wt.% TiO2 suspension (figure 2(d)) no such agglomeration of TiO2 nanoparticles has been observed
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whereas in other two cases, large formation of such agglomerations have been found on the membrane surface via SEM images (figures 2(e & f)). Figures 2(e) and 2(f) represent the SEM images of triple layer coated membrane with 0.10 and 0.15 wt.% TiO2 suspensions. The triple layer coating (figure 2(d)) has indicated better surface coverage than the single and double layer coating by 0.05 wt.% TiO2 suspensions, as well as triple layer coating don’t exhibit any agglomeration problem. Moreover, 0.05 wt.% TiO2 suspensions was the most stable and could sustain for several days [20, 21], and also coated membrane with same concentration of TiO2 in some cases showed better performances [22, 23]. Subsequently 7
triple layer coated membrane with 0.05 wt.% TiO2 suspension was used throughout
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photocatalytic degradation of CHD.
Figure 2: SEM images of (a) Bare PES; (b-d) single, double and triple layer coated
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membrane with 0.05 wt.% TiO2 suspensions respectively; (e) triple layer coated membrane with 0.10 wt.% TiO2 suspensions; (f) triple layer coated membrane with 1.5 wt.% TiO2 suspensions.
3.2. Characterization of the TiO2 Impregnated membrane The SEM images (figure 3) have confirmed the presence of nanoparticles on the surface as well as inside the porous structure of the membrane. During the fabrication of nanoparticles impregnated membrane, TiO2 binds with the polymer matrix.
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Figure 3: Surface and cross-section SEM images of (a-b) 2% TiO2, (c-d) 4% TiO2 and (e-f) 6% TiO2 impregnated membrane. SEM images both surfaces and cross-sections have indicated that with increase of TiO2 concentration the distribution of the nano particles becomes better though it is impossible to increase such concentration beyond some limiting value. It has been experimentally observed during the preparation of casting solution that after 6wt.% of TiO2 nanoparticles concentration it is very hard to dissolve PES in MNP and to form a homogeneous casting 9
solution. In the present study, as modified membrane containing 6wt.% TiO2 (figure 3(e) & 3(f)) shows the better distribution of nano particle, it had been used for the photocatalytic degradation of pharmaceutical component, CHD in PMR. 3.3.Contact angle and enhanced antifouling properties The measured values of contact angle for modified and unmodified membrane have been indicated in the figure 4(a & b), and such measurement is required for the evaluation of hydrophilicity as well as the wettability of any membrane surface. The values of contact
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angle sharply decrease in case of both TiO2 coated and impregnated membrane.
Figure 4: Contact angle of (a) pure PES and (b) modified membrane using TiO2 nanoparticles; (c) Pure water flux of different types of membranes at different TMP and (d) plot of volumetric flux vs. TMP
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The membrane performance significantly depends up on the anti-fouling property of the membrane and this property depends on the hydrophilicity of the membrane surface. There is an inverse relationship between contact angle and hydrophilicity of any surface and it refers that the lower contact angle shows the higher hydrophilicity. With increase hydrophilicity in presence of TiO2 nanoparticles, the membrane surface shows better anti-fouling property [1116]. Therefore, in the present study, according to the contact angle measurement the modified PES membrane demonstrated better hydrophilicity and hence, the fabricated TiO2 modified membranes had better anti-fouling property. Moreover, in presence on nanoparticles, not only the roughness of membrane increases [11] but the presence of number of hydroxyl group
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on the membrane surface increases also [11, 24]. Hence, the membrane surface becomes polarity in presence of TiO2 nanoparticles and this hydroxyl radial interact with water molecules via van der Waals’ attraction force and hydrogen bond [11]. This is the most significant reason for increasing hydrophilicity in presence of nano particle.
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3.4. Water flux of fabricated membranes
The pure water flux of different membrane was tested, which has been shown in the figure
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4(c). After the compaction of all membranes it was found from the figure 4(c) that the presence of nanoparticles on the membrane surface greatly affected the water flux. Moreover,
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the drop of flux was more pronounced in case triple layered surface coated membrane with 0.05 wt.% TiO2 suspension than the 6 wt.% TiO2 impregnated membrane. This was happened because during the surface coating the pores of pure PES membrane were partially blocked
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by nanoparticles, whereas in case of impregnated membrane, pore channels were partially occupied by nanoparticles, which is confirmed by SEM images (figure 2 & 3). In the current research work the water flux of the modified membrane is not the major concern and
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subsequently lower water flux provide better residence time (τ) to the substrate molecules for photocatalytic degradation. Here, the membrane works as catalyst support to recover valuable
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catalyst after the completion of the reaction. The figure 4(c & d) also indicates that for any type of membrane, the flux improves with increase of TMP which is very common in practice for any type of membrane. According to “Hagen-Poiseuille” equation the flux through the membrane pores can be calculated using equation 2.
J=
ΔP μR m
(2)
where, J, ΔP, µ and Rm represent the volumetric flux (m3/m2-s), trans membrane pressure 11
(Pa), viscosity of the working solution (kg/m-s) and flow resistance of the membrane per unit area (m-1) respectively. µ represents the viscosity of the water i.e. 9˟10-4 kg/m-s at 250C. From the slope of the plots (figure 4(d)) of J vs. ΔP one can calculate the average membrane hydraulic resistance (Rm) and the measured values are 6.98848˟1013, 1.92985˟1014 and 1.34122˟1014 m-1 for pure PES, 0.05 wt.% TiO2 coated membrane, 6.0 wt.% TiO2 impregnated membrane, respectively. Eventually, the resistance of the pure membrane is much less than the modified membranes. 3.5. Photocatalytic degradation of CHD Using PMR 3.5.1. Working principle of photo membrane reactor
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The photocatalytic reaction takes place mainly on the catalytic membrane surface under UV irradiation. The working principle of photocatalytic membrane can be represented with the
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help of figure 5.
Figure 5: Schematic representation of working principle of photocatalytic degradation of CHD.
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In the current study when CHD molecules came in contact with TiO2 nanoparticles, it got adsorbed on the catalyst surface, and then got degraded. The micro pollutants like CHD are
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very small in size; therefore those can easily pass through the pure PES membrane in the present study. But in presence of nanoparticles, those molecules face some potential barrier during permeation through the membrane and eventually, those molecules are converted to other by-products by photocatalysis. Subsequently, some of the targeted molecules can pass through the membrane without taking part in photocatalytic reaction, whereas others are degraded; the whole process has been indicated in the figure 5.
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As photocatalysis is a surface adsorption reaction phenomenon, it can be assumed that a single catalyst particle when involved in such type of reaction, it cannot uptake other target molecules during that period for further degradation. Hence, that particular catalyst particle can be available for photocatalytic reaction when it becomes free from by-products generated during degradation of target molecules. Therefore, when all the nanoparticles on the membrane surface are engaged in photocatalysis, the free molecules of CHD in the working solution are easily passed through the membrane under the action of TMP. Moreover, crossflow action somehow helpful to regenerate the catalytic membrane surface by removing the by-products as well as it helps to nullify the effect of concentration polarization of target
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molecules and by-products on the membrane surface. This explaination may be helpful to understand the performance analysis of any type of photo membrane reactor. 3.5.2. Photocatalysis of CHD using TiO2 modified membrane
The degradation behavior of CHD in PMR using both TiO2 coated and impregnated
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membrane was carried out in three different mode of operation i.e. batch, semi-batch and continuous mode and the observed results have been plotted in the figure 6. Also, a trial was
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conducted with bare PES membrane to ensure that the bare membrane had no effect on degradation or removal of CHD. CHD immediately passed through the bare membrane to the
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permeate side. The experimental details have already been described earlier. The concentration of CHD in the permeate stream was measured accordingly the analysis protocol of CHD [18, 19] and the removal percentage of CHD was calculated using equation 1.
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During the experiment, the concentration of CHD in feed solution was maintained at as much as lower value was possible. so that the simulated solution could replicate the real-life environmental condition and for the present case the value was 0.1 g/L (100 ppm).
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Furthermore, lower initial concentration of CHD refers less population of target molecules on the catalytic membrane surface which increases the photocatalytic efficiency of the system
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[18, 19]. The pH and temperature was maintained at 10.5 and 250C respectively to ascertain the maximum removal of CHD [18, 19]. In all three modes of operations within 5-10 min of photocatalysis, the removal percentage of CHD reached its maximum value and then it started to fall for all nanoparticles coated and impregnated membranes. Again after 30-45 min removal percentage started to improve and after approximately 180 min it reached almost steady state value. It was observed the, the steady state value is less than the maximum value, attained at initial stage (5-10 min) of photocatalysis for each mode of operation.
13
ro of -p re lP na ur Jo Figure 6: (a-c) Removal percentage of CHD under different modes of operation of PMR and (d) Steady state comparison using 0.05 wt.%TiO2 coated membrane; (e-g) Removal percentage of CHD under different modes of operation of PMR and (h) Steady state comparison using 6.0 wt.% TiO2 impregnated membrane. 14
The above observation can be well explained with the help of section 3.5.1. At the starting of the reaction (within 5 to 10 min) the catalyst particles on the membrane surface were highly active and thus, when CHD molecules came in contact with the nanoparticles they were readily adsorbed and initiate photocatalysis. Henceforth, free catalyst particles were not available on the membrane surface for further adsorption as well as photocatalysis thus, free CHD molecules from the bulk solution readily passed the membrane with permeate stream. As a result after 5-10 min of reaction, the decrement in rate of photocatalysis prevailed in the PMR. Again after some time duration, the catalytic surface became regenerated as byproducts detached from catalyst surface and then only it can be involved in further
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photocatalysis. Consequently, after about 45 min the improvement in removal efficiency of pharmaceutical component using the PMR has been observed (figure 6(a-c) & 6(e-g)) so far and finally it has reached its steady state value.
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3.5.2.1.Performance analysis with variation of TMP
The figures 6(a-c) & 6(e-g) is also very helpful to understand the effect of TMP on the
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process efficiency. It has been already mentioned that with increase of TMP the flux of the membrane increases.
lP
In the present study the modified membrane is not working as a selective barrier for CHD molecules. The membrane basically provides support to the catalyst particles therefore, with increase of TMP most of the CHD molecules pass the catalytic surface before reaction (figure
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5). Moreover, higher rate of flux reduces the residence time for the target molecule and hence poor degradation encompasses in the system operated under any mode of operation, which has been clearly indicated in the figures 6(a-c) & 6(e-g). In semi-batch or continuous mode of
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operation unreacted CHD molecules leaves the system along with the permeate stream and with increase of TMP the permeate flux as well as number of CHD molecules in the permeate
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increases. Hence, poor percentage removal of CHD has been obtained at higher TMP, which has been indicated in the figures 6(b & c) and 6(f & g). However, the effect of TMP on batch mode has been shown in the figures 6(a) and 6(e), in such cases unreacted CHD molecules along with the permeate stream is recycled back. As targeted molecules again reenter to system the removal efficiency has been improved to some extent but at higher TMP they immediately leave the system causing lower photocatalytic
15
degradation. Thus, for any system TMP should be maintained in such a way so that targeted molecules get enough time for photocatalytic degradation on the membrane surface. 3.5.2.2.Performance analysis under different mode of operations The comparison of between three modes using two different types of membrane (figure 6(d) & 6(h)) shows that in the batch mode, CHD can be removed significantly than other modes of operation using PMR. In the figures figure 6(d) & 6(h), the batch steady state percentage removal of CHD in PMR at 196.133 kPa TMP has been shown. In the batch mode operation, the permeate stream is recycled back to the reactor thus, non-degraded CHD molecules may
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reenter in PMR for further degradation. On the other hand in both semi-batch and continuous mode, the permeate stream not only leaves the system with unreacted target molecules but also in continuous mode fresh feed is introduced to the system. The fresh feed always increases the population of pollutant molecules on the membrane surface thus less removal
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percentage can be obtained in continuous mode operation than any other mode of operation.
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3.5.3. Comparison of photocatalytic efficiency between different types of membranes The steady state removal percentage of CHD using both types of membrane under different
lP
modes of operation of PMR has been indicated in the figure 7(a). In all modes of operation TiO2 impregnated membrane has shown better performance than the coated membrane with respect photocatalytic degradation of CHD at TMP of 196.133 kPa. This observation can be
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explained with the help of SEM images (figure 2(d) & 3(e)). The figure 3(e) of TiO2 impregnated membrane has better distribution as well as population of nanoparticles on the membrane surface than the TiO2 coated membrane and thus, first type of membrane shown
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better performance. Moreover, the figure 3(f) also confirms the presence of nanoparticles in the tortuous path of pores of the membrane and those nanoparticles can adsorb or degrade
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CHD molecules which may enhance the performance of such type of membrane. From the quantitative point of view, the above findings can be explained in another way. During the fabrication of impregnated membrane 6 wt.% TiO2 was used among that the maximum percentage TiO2 incorporated in the membrane structure resulting huge population of nanoparticles per unit area on the membrane surface. On the other hand during dipped coating only 0.05 wt.% TiO2 suspension was used and that quantity was very insignificant compare to 6 wt.%. Moreover, from 0.05 wt.% TiO2 suspension, a very small amount nanoparticles are deposited over the membrane surface. Subsequently, poor population of 16
catalyst particles per unit area of membrane surface was achieved by dipped coating technique. Therefore, dipped coated membrane always shows poor photocatalytic
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lP
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-p
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performance as well as surface adsorption than TiO2 impregnated membrane.
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Figure 7: (a) Steady state comparison between TiO2 coated and impregnated membranes under different modes of operation in PMR; (b) Flux recovery after photocatalysis and (c)
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catalyst recovery and reuse for both TiO2 coated and impregnated membranes The performance of the catalytic membrane can be explained with the help of the nature of attachment of nanoparticles with the membranes. In the impregnated membrane the photo catalysts particles are entrapped strongly inside the cross linked polymeric network and those particles cannot be easily removed from the membrane structure, whereas TiO2 nanoparticles are loosely bonded with the help of adhesive forces on membrane surface in case of catalyst coated membrane. Therefore, there is huge possibility of wash out of nanoparticles from the 17
catalyst coated membrane surface under cross flow action during photocatalysis in PMR, resulting ineffective photocatalysis of CHD. The above discussion revels that the coated membrane lags behind in different ways from the catalyst impregnated membrane. Thus, TiO2 nanoparticles coted membrane showed poor photocatalytic performance using the PMR under same operating condition. 3.6. Flux and catalyst recovery In the present study to measure the flux recovery for both types of membranes, pure water flux was measured after five consecutive batch photocatalysis using same membrane in PMR operated under 196.133 kPa TMP. After photocatalytic degradation all membranes were
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washed with deionized water for 4 hrs. The obtained result has been shown in the figure 7(b) and it has been evaluated that for any type of membrane the flux recovery up to 95% which very significant in deed. Therefore, such type of membrane may be very effective for photocatalytic degradation of pharmaceutical wastes without altering the pure water flux of
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the membrane. The antifouling property of TiO2 modified membrane may be responsible for such observation.
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The main objective of the current study is the recycle and recovery of catalyst particle. To measure such effectiveness of the present system, the steady state percentage removal of
lP
CHD was measured for five consecutive batch cycle photocatalysis using same membrane in PMR operated under 196.133 kPa TMP which has been shown in the figure 7(c). In all degradation processes the initial CHD concentration were maintained at 0.1 g/L. It has been
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observed from the experimental data that the removal percentage of CHD has been reduced by only 5% for impregnated and 14% for coated membrane after consecutive five batch cycles. The reduction of photocatalytic efficiency is more pronounced for the coated
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membrane than impregnated condition as TiO2 nanoparticles can be washed out from the surface coated membrane under cross flow action during photocatalysis as well as washing of
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the membrane which is not possible for impregnated catalyst particles. However, the present system can efficiently recover catalyst after photocatalysis with help of immobilization techniques for future use thus present development is economically sustainable for the pharmaceutical wastewater treatment. 4. Conclusion The present study again proved that photocatalysis is inevitable for pharmaceutical wastewater treatment. A new concept of PMR under three modes of operation has been 18
implemented
for
the
pharmaceutical
wastewater
treatment.
Moreover,
successful
immobilization of TiO2 nanoparticles has been achieved by confirming the complete reuse and recycle of TiO2 nanoparticles, which has made the whole process more economic and environmentally sustainable. The main influencing parameter is TMP for PMR, and the effects TMP has greatly acknowledged in the process efficiency when all other parameters maintained at optimum levels. SEM images confirm the distribution of nanoparticles over the membrane surface whereas contact angle analysis proves improved hydrophilicity of modified membranes. The flux recovery of the photocatalytic membrane is very much significant which also indicates the enhance antifouling property of such type of membrane.
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It has already been proved that nanoparticles impregnated membrane has shown superior performance than the coated membrane. The most significant achievement by PMR is that CHD has been removed from the system using TiO2 impregnated membrane in batch mode near to the value obtained in the batch photocatalysis process using TiO2 suspension [18, 19]. However, using same type of membrane under semi-batch and continuous mode operation
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the percentage removal of CHD is quite appreciable. The semi-batch and continuous processes are advantageous over the batch by handling large amount of wastewater.
rigorous and precision job.
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However, the development of whole process with fabrication of the membrane is very Above discussion confirms that the present system can be
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effectively implemented in the pharmaceutical wastewater treatment as well as wastewater contains organic pollutants, which are degradable under photocatalysis. Some necessary modifications should be incorporated to enhance accuracy of fabricated membrane as well as
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lower down tediousness of fabrication, and also further development is required to scale up the whole process. Then only this methodology can be viable for large scale pharmaceutical
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wastewater treatment.
Author Contribution Statement
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1.
Santanu Sarkar: Conceptualization, Methodology, Data curation, Writing- Original draft preparation.
2.
Chiranjib Bhattacharjee: Visualization, Investigation, Writing- Reviewing and Editing,
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-p
re
lP
na
ur
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of
the
Total
Environment
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https://doi.org/10.1016/j.scitotenv.2019.135023
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(2019),
doi:
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overview,
Table 1: Composition of the casting solutions. Components
Composition (wt.%)
Amount (g)
Membrane I
PES
20
5.0
NMP
79
19.75
PVP
1
0.25
TiO2
2
0.5
PES
20
5.0
NMP
77
19.25
PVP
1
0.25
TiO2
4
1.0
PES
20
NMP
75
PVP
1
TiO2
6
PES
20
NMP
73
Membrane IV
1
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na
lP
PVP
5.0
18.75 0.25
-p
Membrane III
re
Membrane II
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Types
24
1.5 5.0
18.25 0.25