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Processes 2014, 2(1), 12-23; doi:10.3390/pr2010012
Published: 27 December 2013
Abstract: The traditional use of TiO2 powder as a photocatalyst for degradation of organic compounds has several post-degradation treatment problems, such as filtration, precipitation, etc. A novel method was developed to immobilize TiO2 to minimize/eliminate such problems. Polymeric membrane was used as a base material, over which the TiO2 photocatalyst was immobilized as a thin layer. Preparation and characterization of five different types of polymeric/TiO2 film photocatalysts were elucidated. The catalysts’ films were cross-linked by physical, chemical, and combination of these two processes. The polymers used in the formulation of the catalysts membranes are nontoxic in nature (approved by the World Health Organization (WHO) and Food and Drug Administration (FDA). The morphology of the films were studied by SEM. Photocatalytic degradation of acetylsalicylic acid was carried out to study the efficacy and efficiency of the polymeric membrane based TiO2 as photocatalysts under both UV and solar light irradiation. The degradation was observed to be dependent on the catalyst loading as well as the film thickness. The effects of the types of cross-link bonds on the photocatalytic degradation were also investigated.
One of the most pressing environmental issues of present and future is the effective protection and utilization of the precious fresh water resources of the world. According to the World Health Organization (WHO) more than one billion people in the world are suffering from the lack of access to clean potable water. Waterborne infections account for 80 percent of all infectious diseases in the world. Pharmaceuticals, in general, enter the environment through different pathways, resulting in the contamination of ground/underground water, where bacteria are most likely the primarily affected organisms. Due to high intake of non-steroidal anti-inflammatory drugs (NSAIDs), their presence in drinking water has been widely reported . Aspirin belongs to this class of medications called NSAIDs. NSAIDs are the most frequently prescribed agents to treat fever, pain, arthritis, etc. In addition to its effects on pain, fever, and inflammation, aspirin also has an important inhibitory effect on platelets in blood. This antiplatelet effect is used to prevent blood clot formation inside arteries, particularly in individuals who have atherosclerosis (narrowing of the blood vessels), or are otherwise prone to develop blood clots in their arteries preventing heart attack and strokes. Molecular structure of aspirin is shown in Figure 1.
In aqueous solutions, organic compounds can undergo photochemical transformations with sunlight via direct or indirect photoreactions . Such photochemical degradation can be one of the major transformation processes and one of the factors that control the ultimate fate of organic pollutants in the environment. Various technologies are in use to purify aqueous municipal and industrial effluents containing pharmaceutical substances, before entering surface waters. Among them, advanced oxidation processes (AOPs) have been of major interest in recent years.
Among all the Advance Oxidation Processes (AOPs), Titanium Dioxide (TiO2) photocatalytic oxidation holds a great deal of promise to address this issue due to the low cost of TiO2 and its chemical stability. It is also remarkably active, and non-toxic over a wide range of pHs. In general, the goal of photocatalysis in water treatment is transformation, deactivation, and, finally, mineralization of environmentally persistent compounds. The TiO2 photocatalyst is able to utilize sunlight and air to produce many reactive species, including the powerful and non-selective oxidant hydroxyl radicals, to destroy organic compounds. One of the major drawbacks is expensive filtration technique needed to remove the slurry TiO2 from the purified water [3,4]. This problem has resulted in development of several kinds of immobilization techniques to immobilize the TiO2 powder, which may reduce the post degradation expenses and time. TiO2 in anatase, Degussa form has been immobilized by several methods. The design and development of an immobilized thin catalyst film facilitates commercial-scale applications of TiO2-based photocatalytic processes for water treatment significantly. The dimensionality associated with the structure of a TiO2 material can affect its properties and functions, including its photocatalytic performance, and also, more specifically, its surface area, adsorption, reflectance, adhesion, and carrier transportation properties . Immobilization of TiO2 photocatalyst eliminates the need for the separation of the catalyst particles from the treated liquid and enables the contaminated water to be treated continuously. The catalyst film is porous and can therefore provide a large surface area for the degradation of contaminant molecules. If a conductive material is used as the support, the catalyst film can be connected to an external potential source to remove excited electrons to reduce electron-hole recombination, thereby, significantly improving the process efficiency [6,7,8,9]. Di Paola et al.  reviewed different type of TiO2 catalysts and claimed that although many of these photocatalysts were effective for the photodecomposition of many pollutants, most of them do not allow a complete mineralization of the starting compounds.
Several researches have been carried out on immobilization of the TiO2 [11,12]. Two obvious problems arising from this arrangement: the accessibility of the catalytic surface to the photons and the reactants and a significant influence of the external mass transfer resistance particularly at low fluid flow rate, due to the increasing diffusion length of the reactant from the solution to the catalyst surface . Maedaa  presented a comprehensive review of different reactor designs for air- or water-purification with the main emphasis on the enlargement of reactive surface area and improvement of mass transfer. Several kinds of film photocatalysts have been formulated by researchers, such as TiO2 loaded carbon fiber , silicone-TiO2 polymeric composite , Polypropylene/clay with TiO2 composite , and TiO2-PVP (Polyvinyl pyrrolidone) nano-composite . Ochiai and Fujishima  fabricated a novel photocatalytic filter, titanium-mesh sheet modified with TiO2. The titanium mesh sheet was claimed to be highly flexible with a large contact area with a minimum pressure drop. Modification of the photocatalyst with a suitable cocatalyst was carried out to provide an active redox site for water slitting . Due to challenges associated with the use of UV light, several attempts have been made to use visible light instead. Pelaeza et al.  presented different strategies for the modification of TiO2 photocatalyst for the utilization of visible light along with comprehensive studies on the photocatalytic degradation of contaminants.
These immobilization techniques have been reported to reduce the cost of filtration and also in some cases increase the efficiency of TiO2 as photocatalyst, due to the synergistic effect of the substrate and TiO2. All these techniques have been proved to be successful though being expensive and time consuming their use has been limited. In this study, a polymeric film was prepared as a substrate for immobilization. The degradation of aspirin was carried out to measure and compare the photocatalytic efficiency of the films. Mass transfer limitations become a dominant factor when immobilized photocatalyst film is used, which usually lead to a lower overall degradation rate compared to the suspended catalyst systems [22,23].
The degradation kinetics of aspirin follows the Langmuir-Hinshelwood (L-H) model.
2. Experimental Section
2.1. TiO2 Immobilization Procedure
2.1.1. Preparation of Polymeric/TiO2 Membranes
9% w/v of PVA and 4% w/v Gelatin were dissolved and mixed properly in distilled water to get a transparent solution. Next, 8% w/v of PVP was dissolved in a solution of ethyl alcohol and water (in 2:1 ratio) and added to the PVA-Gelatin solution and the reaction was carried out at room temperature. The solution was stirred for 15 min, followed by dispersion of 16% w/w TiO2 Degussa P25 powder in the mixture. Thereafter, the polymeric/TiO2 solutions were cross-linked by physical, chemical, and physico-chemical methods. The polymer matrix used for immobilization may be degraded under ultraviolet light however in the context of this research and during all the experiments such a phenomenon was observed. Apparently the degradation rate of polymer matrix is much longer than the duration of the experiments.
2.1.2. Physical Cross-Linking Methods
184.108.40.206. Freeze Drying
The TiO2 dispersed polymeric solution was then physically cross-linked by storing the solution below zero degrees centigrade. The polymeric solutions were stored at different temperatures ranging from 0 to −10 °C for different time intervals of 1, 3, 5, 7, 10, 15, 24 (h). It was found that the samples stored for 5 h showed the best results under solar and UV lights. Solutions stored for less than 5 h showed very soft and flexible films, which were partially dissolved in water during the photo-degradation reaction, while the ones stored for longer than 5 h formed mechanically strong catalysts due to more cross linking, but showed lower degradation. The lower degradation rate was most probably due to blockage of the TiO2 active site by the cross-linked polymers. Therefore, the freeze-dried film was cross-linked physically at −2 °C for 5 h (optimum condition).
220.127.116.11. Heat Treatment
Cross-linking also occurs at higher temperatures. Generally, cross-linking increases with increasing temperature, therefore the prepared polymeric/TiO2 solution was placed in an oven for different times, at different temperatures, to cross-link. Due to deactivation of PVP at 120 °C and due to uneven distribution of temperature in the oven, it was found that 105 °C was the optimum temperature for cross-linking. Samples were kept at 105 °C for different period of times between 5 and 120 min. The solution, which had been kept for 10 min, showed the best results with respect to the desired mechanical properties and degradation efficiency.
2.1.3. Chemical Cross-Linking Methods
18.104.22.168. Acetaldehyde Treatment
Aldehydes are known to initiate cross-linking of PVP polymer solution instantly. Acetaldehyde chemically cross-links with PVP in the polymeric solution and instantly forms a membrane. After several experiments, it was found that 0.5 mL of 2% acetaldehyde at room temperature was sufficient to crosslink the polymer solution completely. The membrane was then dipped in distilled water and washed 3 times to remove the unreacted acetaldehyde traces.
22.214.171.124. UV Treatment
Polymeric solutions with dispersed TiO2 were exposed to UV light of 275 nm wavelength for different periods of time (2, 5, 7, 10, 20, 30, and 60 min). It was observed that exposure for 10 min provided the best results among all the samples. Samples exposed for 2–7 min did not form mechanically strong films, which in turn degraded in the aqueous solution during the photo-degradation process. The catalyst films formed by exposure of 20–60 min was mechanically strong but, due to high cross-linking effects, the TiO2 particles got entangled within polymer matrix, resulting in blockage of the active sites and lower degradation efficiency.
126.96.36.199. Freeze-Dried and UV Treated
The polymeric solution was prepared following the aforementioned freeze drying method (kept at 0 °C for 5 h) and then exposed to UV lamp of 275 nm wavelength for 10 min to cure the polymeric membrane.
3. Characterization and Degradation of Aspirin
Optical microscopy, Scanning Electron Microscopy (SEM, Hitachi F-4000) and Fourier transform Infra Red were employed to characterize the immobilized TiO2 photocatalyst film. SEM studies of the film photocatalysts revealed the specific pore volume and pore density. The measuring scale was 2.5 µm. Catalyst films were also subjected to Fourier Transform Infra Red (FTIR) Spectroscopy. The specimens were prepared as thin and homogenous films. FTIR spectra were measured in the wave number range 400–4000 cm−1.
The photodegradation reactions were carried out in batch reactors, in presence of both solar and UV lights with five different kinds of polymeric/TiO2 catalyst films. The batch reactors were 250 mL in volume, containing aspirin solutions under a UV lamp of 22 mW/cm2 intensity and the solar stimulator of light intensity 27 mW/cm2. The concentrations of the degraded aspirin solutions were analyzed using a UV-Vis Agilent spectrophotometer at 270 nm.
4. Results and Discussions
4.1. Optical Microscopy
The polymeric/TiO2 catalyst films were subjected to optical microscopy (OM) to observe their porous structures. The freeze-dried film showed a spongy porous structure. The heat-treated films were observed to have a highly porous 3D structure, which made the films remain buoyant in water. The acetaldehyde and UV treated films showed solid matrices with very few pores. The UV-Freeze-dried treated films showed a spongy structure similar to freeze-dried but with fewer number of pores. Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6 show the structures of the five different immobilized photo-catalyst films.
4.2. Scanning Electron Microscopy (SEM) Study
The polymeric/TiO2 catalyst film thickness ranged from 0.45 mm to 2 mm. The micrographs taken by scanning electron microscopy revealed a three-dimensional network structure, which was typical for a porous material. Figure 7 shows the porous structure of the freeze-dried catalyst film. Obviously, the freeze-dried membrane is rather homogeneous, interspersed with pores of various diameters, ranging from 50 to 300 µm. The average pore area was found to be 572 µm2. The pore density ranged from 7 to 10 pores per mm2 for the freeze-drying film. The pore density was 12–15 pores per mm2 for the heat-treated polymeric/TiO2 catalyst film. The acetaldehyde and UV treated samples showed almost no pores and the freeze-dried-UV treated film showed 3–5 pores per mm2. Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11 show the SEM images of the samples.
4.3. Degradation of Acetylsalicylic Acid (Aspirin)
Cross-linking affected degradation of aspirin under UV and solar light over different photocatalyst films. As shown in Figure 11, Figure 12 and Figure 13, the freeze-dried catalyst was more efficient in degrading aspirin compared to the other photocatalyst films in presence of solar light. Although, the freeze-dried catalyst showed higher degrading efficiency under UV light, the efficiency decreased rapidly with time. UV radiation activates cross-linking and, with more cross-linking, less TiO2 active site would be available, resulting in gradual decrease of the degradation rate with time. The freeze-dried catalysts were useable for a short period of time as some polymeric flakes leached out into the aqueous system after six to seven hours of use. Although these polymers are nontoxic and have been approved by the FDA and WHO, and can be removed by a simple filtration method, such a phenomenon may restrict their application and more investigation is required to improve their mechanical stability. On the other hand, the heat-treated photocatalyst film was buoyant and could be used several times for the degradation of organic model compound. The UV treated and UV-freeze-dried catalysts showed the slowest degradation rate. The aldehyde treated catalyst was found to remain in submerged condition in water during the reaction and also showed a slower degradation rate. The UV treated catalyst membrane showed morphological characteristics, the same as UV-freeze-dried catalyst, and both catalysts were observed to follow almost the same rate of degradation in presence of solar light and UV light. The UV treated catalyst showed higher tensile strength due to formation of strong cross-linking bonds but became blue after the UV treatment. This blue colorization is due to the entrapped electrons, which delay the degradation rate. Table 1 shows the reaction rate constant, k, for different TiO2 immobilized catalysts under UV and solar radiation.
|Table 1. Comparison between reaction rate constants for different catalysts.|
|Catalysts||Freeze dried||Heat treated||UV treated||Acetaldehyde treated||Freeze-dried UV treated|
|k, mg/L/min UV light||0.050||0.019||0.017||0.015||0.024|
|k, mg/L/min Solar light||0.079||0.022||0.019||0.017||0.027|
A cross-linked polymeric membrane was used to immobilize TiO2 photocatalyst to avoid costly post-treatment processes, such as filtration. Five different immobilized photocatalysts were prepared, namely heat-treated, aldehyde-treated, UV-treated, freeze-dried and UV-freeze-dried. It was observed that the freeze-dried catalyst film had a porous structure and the highest degradation efficiency. The spongy 3D structure of the catalyst also made it capable of adsorbing larger amounts of organic/inorganic pollutants on its active sites, which in turn increased the degradation rate of Aspirin. The freeze-dried film was not mechanically strong, and failed to keep its identity after six to seven hours. The UV-treated catalyst films showed very few or almost no pores, while the heat-treated photocatalyst film contained several air pockets, which made it buoyant in aqueous solutions. The buoyant nature of the heat-treated film may be exploited to keep the catalyst film afloat and exposed to solar light for the continuous degradation of pollutant in water. The toxic nature of aldehyde reduces the chance of using aldehyde-treated catalyst films for drinking water purification. The UV-treated catalysts showed higher tensile strength, however, due to strong cross-linkage effect of UV light, these films have fewer active sites of TiO2 available for interacting with the photons.
|C and Co||Concentration, ppm|
|K||Langmuir-Hinshelwood rate constant|
|P25 TiO2||Degussa P25 Titanium dioxide|
|SEM||Scanning Electron Microscopy|
|f-d/UV||Freeze dried UV treated|
Conflicts of Interest
The authors declare no conflict of interest.
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