Abstract:
A composite material for water or wastewater treatment is described. The composite material has a buoyant substrate, an adsorbant for adsorbing organic compounds, a photocatalyst for degrading organic compounds, and an enhancer for facilitating mass transfer between the adsorbent and the photocatalyst, increasing the selectivity of the composite material, or for proving the photocatalytic efficiency is described. The adsorbent, photocatalyst, and enhancer are immobilized on the substrate.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/300,514, filed on Feb. 2, 2010. The entire teachings of the above application are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Conventionally, the bulk amount of organic compounds in wastewater is usually removed through various biological processes. For relatively low levels of organic compounds in effluents from wastewater treatment plants for reclamation or in raw water for a water supply, adsorption has usually been used as the removal method in common industrial practices. However, many organic compounds in industrial effluents, such as dyes, phenolic and synthetic matters, or in natural water, such as humic matters, are not practically biodegradable. Thus, conventional biological processes have often failed to achieve the desired treatment goals. On the other hand, the removal of organic compounds by adsorption is largely dependent upon the capacity and property of the adsorbents used. The adsorbents usually require frequent regeneration to restore their function, which in many cases has proven difficult, if not impossible, to achieve. Furthermore, frequent regeneration incurs high capital and operational costs. 
         [0003]    Advanced oxidation processes (AOPs), such as ozone oxidation, Fenton reaction or photocatalysis, that can degrade organic compounds, including toxic ones, ultimately into minerals (e.g., carbon dioxide and water) have attracted increasingly greater interest in recent years for the decontamination of water and wastewater. Among these processes, photocatalysis has been an area of focus because it does not require additional chemicals in the treatment reaction. In a photocatalytic process, photocatalysts under light irradiation produce active radicals that can attack the organic compounds in water or wastewater and degrade them into simpler or nontoxic ones. However, these radicals can easily lose their activity within the time scale of less than 10 −5  of a second through either a reaction with the organic pollutants or by recombining with other radicals or carriers. When the targeted organic pollutants are present at low concentrations or when the mass transfer of the organic pollutants from water to the photocatalysts is a limiting factor, most radicals can quickly lose their activity before they have a chance to encounter a pollutant compound and participate in the degradation reaction. In order to overcome this problem, some studies have combined an adsorbent with a photocatalyst, either through immobilizing photocatalyst particles onto an adsorbent powder or mixing an adsorbent powder with the photocatalyst particles. Y. Li et al., Water Res. 40 (2006) 1119-1125; X. Wang et al., J. Hazard. Mater. 169 (2009) 1061-1067. Prior studies have found that these approaches improved the kinetics of pollutant removal, i.e., pollutants more rapidly photo-decomposed in a combined adsorbent and photocatalyst system as compared to a single photocatalyst system. 
         [0004]    The accelerated reaction rate was explained as resulting from the adsorption of the pollutants onto the adsorbent followed by rapid migration of the pollutants to the surface of the photocatalyst. However, there have been various issues that need to be resolved. First, both photocatalysts and adsorbents were in very small sized powders (often in the nanometer or micrometer range) and hence were very difficult and costly to separate from the treated water. Second, the photocatalysts in powder or small particle forms were usually applied in the water to be treated in a slurry manner. Light provided either from UV lamps installed in the water or above the water surface or from natural sunlight must travel through the water to reach the surface of the photocatalyst particles. Unfortunately, light attenuates more significantly with distance in water, as compared to attenuation with distance through air. As a result, the provided light often has a very low utilization efficiency in these conventional photocatalytic processes. Third, the porous adsorbent used in the combined adsorption/photocatalysis system was fouled by the photocatalyst particles in the pores, and the performance of both the adsorbent and photocatalyst was greatly reduced. Therefore, there is a need for materials and methods that achieve the synergetic effect of adsorption, photocatalysis and light utilization efficiency for the effective and low cost removal and mineralization of organic compounds in water and wastewater. Also, there is a need for developments that address the selectivity of such treatment systems for specific organic contaminants. 
       SUMMARY OF THE INVENTION 
       [0005]    In this invention, two to three types of component materials with different functions are immobilized on a thermoplastic substrate through a melting-binding method under controlled temperatures to obtain a buoyant multifunctional composite material. The component materials include a photocatalyst, an adsorbent and a synergetic enhancer. The substrate is selected so that it not only serves as the carrier for the component materials, but it also provides the bulk density for buoyancy of the final product. 
         [0006]    The final buoyant multifunctional composite material can easily be suspended in water, but it will naturally float to the water surface. Therefore, the body of water in the treatment process can be separated into a top layer with the composite material and a bottom zone of water only. Thus, it is easier to separate the composite material from the treated water. Since the composite material floats at the water surface, the photocatalyst on the substrate can use light, either from UV lamps or natural sunlight, at a higher efficiency because light does not attenuate as significantly when it travels through the air as compared to when it travels through water. 
         [0007]    The adsorbent on the substrate can quickly concentrate organic compounds from the bulk water when they are suspended in water and provide the photocatalyst with organic compounds for degradation at an enhanced mass transfer rate. This overcomes the problem in conventional photocatalytic degradation technology where the supply of organic compounds from water to photocatalyst is often limited by slow mass transfer. The photocatalyst on the substrate can degrade organic compounds into simple ones (ultimately into minerals) from the adsorbent and continuously regenerate the adsorbent. This eliminates the additional regeneration process, which is necessary in conventional adsorption technology. 
         [0008]    An enhancer can be added in the components and immobilized on the substrate to provide a synergetic effect to the combination of adsorption and photocatalysis and can add selectivity to the composite material. For example, the enhancer may function as a bridge or passage for mass transfer of organic compounds between the adsorbent and the photocatalyst. As another example, the enhancer can prevent the recombination of electrons and holes generated on the photocatalyst during light irradiation, thereby improving the activity of photocatalyst by increasing photocatalytic degradation efficiency for organic compounds. 
         [0009]    The buoyant composite material is prepared from a thermoplastic with a specific gravity of less than 1. The thermoplastic functions as a substrate on which the adsorbent, enhancer and photocatalyst components are disposed. The thermoplastic can be, but is not limited to, polypropylene, polyethylene, polystyrene, nylon, etc. and their blends or alloys. 
         [0010]    The composite material includes adsorbents and photocatalysts and thus combines adsorption and photocatalysis functions together. The adsorbent concentrates organic compounds in water and provides faster mass transfer of organic compounds to the photocatalyst. The adsorbent can be, but is not limited to, activated carbon, zeolite, any synthetic or natural adsorbents, and combinations thereof. The photocatalyst degrades organic compounds from the adsorbent and regenerates/recovers the adsorbent continuously. The photocatalyst can be, but is not limited to, titanium dioxide (TiO 2 ), zinc oxide (ZnO), cadmium sulfide (CdS), tungsten (VI) trioxide (WO 3 ), silicon carbide (SiC), metal oxides doped with inorganic elements, or any combination thereof. 
         [0011]    Compared to conventional adsorption technologies, which require an additional process to frequently regenerate the adsorbent to recover its capacity, and thus are very costly, the present invention does not require an additional process to regenerate the adsorbent. Compared to conventional photocatalytic technologies that often suffer from the problem of slow mass transfer of organic compounds from bulk water to the photocatalyst, the present invention provides higher mass transfer rates to the photocatalyst because the adsorbent can quickly concentrate organic compounds from the bulk water. 
         [0012]    The composite material can contain an enhancer that provides a synergetic effect between the adsorbent and photocatalyst, such as facilitating mass transfer from the adsorbent to the photocatalyst, increasing the selectivity for organic compounds to be separated and degraded, or entrapping electrons to prevent electron-hole recombination, which can improve photocatalytic reaction efficiency. There have so far not been any such developments in preparing the composite material. 
         [0013]    The photocatalytic reaction of the composite material can take place at the water surface and can fully utilize the light provided in the air medium. This solves the problem of low light utilization efficiency encountered in conventional photocatalytic technologies that often use light in water, which results in high installation cost as well as significant attenuation of the light provided. 
         [0014]    The buoyant multifunctional composite material can be used in any water and wastewater treatment where removal of organic compounds is needed. The material provides competitive solutions especially where toxic and non-biodegradable organic compounds are involved, including most industrial effluents. It also has the advantage of providing a simple treatment system that potentially requires lower capital and operational costs. 
         [0015]    In accordance with the present invention, the buoyant multifunctional composite material can be prepared from the following processes: 
         [0016]    (a) The selected adsorbents, photocatalysts and enhancers in proper particle or molecular sizes and weight or volume ratios are mixed together. These components can be in the form of particles, small tubes, fibers, powders, etc. and should be chemically stable at temperatures up to 30° C. above the melting point of the substrate. 
         [0017]    (b) The selected thermoplastic substrate in the form of granules, fibers, sheets or other shapes with a much larger size than the component materials in (a) is cleaned with water, alcohol or other solvent as needed and then dried. 
         [0018]    (c) In a reactor, the mixture of the adsorbent, photocatalyst and enhancer is heated, under, stirring, to and then maintained at a specific temperature in the range of 10° C. below to 30° C. above the melting point of the substrate, depending on the final shape of the composite material to be prepared. The substrate is then added into the mixture and the mixing continues for a time in the range of 1 min to 15 min until the surface of the substrate is fully covered with the component mixture. 
         [0019]    (d) The composite material is separated from the remaining component mixture through a sieve and is cooled down to room temperature. 
         [0020]    (e) The prepared material is washed with water or a water/alcohol mixture and dried to obtain the final product. 
         [0021]    The present invention provides several advantages. The composite material is buoyant and thus can be used at the water surface. Since light does not attenuate as significantly while traveling through air as compared to water, the light provided to the photocatalyst can be more fully utilized. In addition, natural sunlight can be used as the light source for the photocatalytic processes. 
         [0022]    The composite material also has good adsorption performance to quickly concentrate organic compounds in water or wastewater, and thus improves or enhances the mass transfer rate of organic compounds in water to the photocatalytic reaction site on the surface of the material. 
         [0023]    The composite material has good photocatalytic degradation performance for organic compounds under the irradiation of UV light, visible light or both, which will not only degrade the organic compounds on the material into harmless simpler ones, but will also simultaneously regenerate the material and recover its adsorptive performance to organic compounds in water. 
         [0024]    The material can contain one or more enhancers that enhance the synergetic effects between adsorption and photocatalysis and increase or improve the selectivity of the material to specific organic compounds or the chemical stability of the prepared composite material. 
         [0025]    Thus, the current invention provides a simple solution that is cost-effective and can achieve multiple functions in one process, which conventional technologies may not be able to achieve or may need multiple stages to achieve. The buoyant feature of the material solves the separation problem that has been encountered for the commonly used slurry systems of photocatalysts or adsorbents. In conventional technologies, photocatalysts and adsorbents are often used in the form of nano or micro particles. The conventional technologies have presented a significant problem in separation after water is treated, and separation usually incurs very high operational costs. The buoyant material can float to the surface and hence can be easily handled and separated when needed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0027]      FIG. 1  is a schematic illustration of a configuration of the multifunctional composite material described in this invention. The small-sized components immobilized on the granular substrate include the adsorbent, photocatalyst and enhancer. 
           [0028]      FIG. 2  is a schematic illustration of the mechanisms by which enhancers can improve the synergetic effect between the adsorbent and the photocatalyst. Organic compounds are adsorbed by the adsorbent and transferred to the photocatalyst where active groups such as hydroxyl radicals (OH.) are produced. The enhancer can improve the selectivity of the composite material, the activity of the photocatalyst, or the mass transfer between adsorbent and photocatalyst. 
           [0029]      FIG. 3  is a schematic illustration of a water treatment system that uses the composite material in a simple and cost-effective manner. The light source can be either natural sunlight or UV lamps installed above the water surface. The composite material is mixed with the water to be treated and it floats to the surface of the water. The treated water can be collected from the bottom of the reactor without any practical difficulties. 
           [0030]      FIG. 4  is a graph of phenol concentration (ppm) versus time (hours) for a photocatalytic reaction under a xenon lamp in a beaker with water, phenol, and a buoyant multifunctional composite. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    A description of example embodiments of the invention follows. 
         [0032]    The present invention is concerned with a buoyant multifunctional composite material, its preparation method and its application process. 
         [0033]    a) The photocatalyst component can be any active photocatalysts, typically TiO 2 , in powder, tube or fiber form with an effective size of approximately 1 nm to approximately 50,000 nm, typically approximately 10 nm to approximately 100 nm. The adsorbent component can be any adsorbent, such as inorganic or organic, and can be a single type of adsorbent or a mixture of adsorbents. Typically, the adsorbent can be activated carbon or zeolite or both, in powder or tube form, with effective sizes in the range of approximately 1 nm to approximately 100,000 nm. Both the adsorbent and photocatalyst components are stable at temperatures ranging from below the melting point of the substrate to 30° C. above the melting point of the substrate. For example, the lower limit of the range of temperatures below the melting point may include, but is not limited to, 0° C. The enhancers can be any compounds that improve the selectivity and activity of the prepared composite material. Typically, the enhancers can be a carbon nanotube, a precious metal salt; an inorganic such as SiO 2  or a functional polymer. The adsorbent, photocatalyst or both may be pretreated with the selected enhancer or enhancers. The enhancer or enhancers may also be directly mixed with the adsorbent and the photocatalyst components. The mass ratio of the adsorbent to the photocatalyst in the mixture can vary from approximately 0.1 to approximately 10, typically approximately 0.2 to approximately 6. The mass of the enhancer can be approximately 0.001% to approximately 5% of the mass of the adsorbent, the mass of the photocatalyst or the combined mass of the adsorbent and the photocatalyst, typically approximately 0.01% to approximately 0.2% of the mass of the adsorbent, the mass of the photocatalyst, or the combined mass of the adsorbent and the photocatalyst. The substrate can be any thermoplastics or their blends or alloys with a specific gravity of approximately 0.8 to approximately 1, typically approximately 0.9 to approximately 0.95. 
         [0034]    b) The mixture of adsorbent, photocatalyst and enhancer is well mixed and then pre-heated to and maintained at a temperature in the range of approximately 10° C. below to approximately 30° C. above the melting point of the substrate. Then, the substrate having a volume of approximately 10% to approximately 60% of the mixture, typically approximately 30% to approximately 50%, is added into the pre-heated mixture with stirring for approximately 0.5 min to approximately 30 min, typically approximately 2 min to approximately 10 min, until all the substrate surfaces are fusion-bonded and fully covered with the photocatalyst/adsorbent/enhancer mixture. The prepared composite material is then separated from the mixture by a sieve. The thermoplastic substrate may have a melting point in the range of approximately 80° C. to approximately 300° C., typically approximately 100° C. to approximately 180° C. The prepared composite material can be in, but is not limited to, the shape of a fiber, a fabric, a sheet or granules. 
         [0035]    In an alternative route, the substrate is heated to approximately 10° C. to approximately 25° C. above its melting point. Then, the liquid substrate can be extruded through a mould and cut into granules, tubes, fibers etc. and mixed with the adsorbent, photocatalyst and enhancer mixture. After cooling, the prepared composite material is separated from the mixture by a sieve. 
         [0036]    c) The prepared multifunctional composite material can be used in a water or wastewater treatment reactor with UV lamps or sunlight as the light source. The multifunctional composite material can be put into a reactor with a minimum quantity that just covers the water surface, or with a maximum quantity filled up to 70% of the reactor volume. The light source is designed to be provided to the reactor from the top of the reactor at a wavelength determined by the photo sensitivity of the photocatalyst and with a light intensity of at least 30 W/m 2 . The mass transfer of organic compounds in water may be enhanced by stirring, such as, but not limited to, stirring by air bubbling or mechanical mixing. 
       Example 1 
       [0037]    A 5 gram amount of TiO 2  particles with a size of 25 nm is treated in a 2 g/L salicylic acid solution for 30 min, and dried in an oven at 100° C. for 2 h. Then, the treated TiO 2  particles are mixed with 0.05 grams of multiwall carbon nanotubes (110˜170 nm diameter at 5˜9 μm length), and heated at 200° C. for 2 h in an oven. Then, a 10 gram amount of 100 mesh activated carbon particles is mixed with the TiO 2  and carbon nanotube mixture, and all of the components are then placed into a 250 mL reactor. The mixture in the reactor is preheated to and maintained at 200° C. with a hot-plate heater and stirred with a mechanical mixer. Then, a 30 gram amount of polypropylene (PP) granules with a diameter of approximately 4 mm is added into the reactor. The mixture in the reactor is further heated with stirring for the temperature to increase to and be maintained at 160° C. The process continues for another 3 min. Then, the PP granules are fully immobilized with small-sized powder mixture and are separated from the remaining powder through a sieve and cooled down to room temperature to obtain the composite material to be prepared. For the demonstration of an application, a 3 gram amount of the buoyant multifunctional composite material is added into a 100 mL beaker filled with 50 mL of a 50 ppm phenol solution with air bubbling. The content in the beaker is put under a xenon lamp with a UV light of 48 W/m 2  power (Newport). The phenol in the solution was found to be completely removed within 4 h. 
       Example 2 
       [0038]    A multifunctional buoyant photocatalyst was prepared from 50 grams P25 TiO 2  (AEROXIDE, Degussa) mixed with 50 grams of 100 mesh activated carbon particles in an 800 mL reactor. The mixture was preheated to and maintained at 185° C. with a hot-plate heater and stirred with a mechanical mixer. Next, 50 polypropylene (PP) granules having a diameter of about 4 mm were added into the reactor. The mixture was further heated with stirring for 10 min. The PP granules were coated with TiO 2  and activated carbon particles. The treated PP granules were then collected and washed with ethanol and water. The washed granules were added to a 300 mL glass beaker along with 300 mL of a 10 ppm phenol solution. The glass beaker was irradiated by a 150 W xenon lamp having a 3″ diameter light beam. One and a half liters per minute of air was introduced to the phenol solution with an air diffuser. The phenol concentration was analyzed by HPLC equipped with a C18 column. As shown in  FIG. 4 , the concentration of phenol approached 0 ppm after five hours. 
         [0039]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.