Abstract:
A reverse osmosis membrane having excellent antifouling property, which is advantageous in terms of fundamental prevention of fouling generated by inorganic crystalloids, organic contaminants, and microorganisms, and a method for preparing the same. The membrane can be manufactured by hydrolyzing a titanium compound in an acidic aqueous solution to produce TiO 2  particles of nano scale; dispersing the produced TiO 2  particles in an acidic aqueous solution of pH 1-6 or an alkaline aqueous solution of pH 9-13 to yield a stable TiO 2  dispersed solution; and immersing a reverse osmosis membrane prepared by a known method in the TiO 2  dispersed solution to incorporate TiO 2  into the membrane.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates to a reverse osmosis membrane having excellent anti-fouling property, and, more specifically, to a nano structure type reverse osmosis membrane, in which TiO 2 , a representative photocatalyst having not only decomposition activity against various organic contaminants, but also antimicrobial activity against microorganisms, is incorporated to the membrane surface, and a method for preparing the same. 
     2. Description of the Related Art 
     In general, a reverse osmosis membrane has been applied to various fields, such as desalination of sea water, wastewater treatment, production of ultra pure water, treatment of purified water for households, and the like. With a view of increasing water permeability of the membrane or improving removal of salt, research for such reverse osmosis has been carried out. In this regard, U.S. Pat. No. 4,872,984 to Tomashke and U.S. Pat. No. 4,983,291 to Chau disclose a novel polyamide reverse osmosis membrane, capable of increasing water permeability and improving the removal of salt. 
     Meanwhile, water treatment processes using the reverse osmosis membrane suffer from fouling. Fouling is the deposition of material, referred to as foulant, on the membrane surface or in its pores, leading to a change in membrane behavior or even complete plugging of the membrane. These phenomena manifest themselves over time by increased operating pressure whereby water permeability properties, such as water permeability or removal of salt, are decreased. Hence, most of operation cost of water treatment facilities is used to repair damage caused by such fouling and in prevention of the fouling. There is thus a widely recognized need for fundamental preventive measures. 
     Examples of the foulant classified by form include inorganic crystalloids, organic contaminants, particulate matters and colloids, and microorganisms. In the case of the polyamide reverse osmosis membrane, the fouling attributed to microorganisms, in which microorganisms present in water are absorbed into the surface of the membrane and thus a thin bio-film is formed, is known to be the most serious one. 
     In order to decrease the fouling, pretreatment of water, modification of electrical properties of the membrane surface, modification of module process condition, and periodical cleaning are widely utilized. In particular, it is reported that the fouling by microorganisms, which is seriously generated in the reverse osmosis membrane, is drastically decreased by treatment with sterilizing agents, such as chlorine. However, chlorine creates carcinogenic by-products. So, when chlorine is applied as is to the process of producing potable water, it is known to cause many problems. Recently, research to produce membranes with antifouling properties is focused on change of electrical charge characteristics of the membrane surface. But, the fact is that a novel membrane, capable of drastically decreasing the fouling, is not developed yet. 
     Meanwhile, TiO 2  which is a highly effective photocatalyst, kills microorganisms under ultraviolet or solar radiation and decomposes organic contaminants. By use of such antimicrobial properties, many cases applied to packaging films, storing boxes, wall paper, seats, interior materials of automobiles, lamps, eye shields, air conditioners, electronic ranges, bottom materials, refrigerators and so on, are described in patents and papers. 
     In addition, techniques for decomposing organic contaminants and killing microorganisms by directly dispersing TiO 2  powder into water, in water treatment process, are known. Conventional water treatment processes using TiO 2  can decompose organic contaminants and kill microorganisms present in water, without harmful by-products, so that much research into applicability for water treatment processes has been continuously performed. However, the water treatment process for photodecomposition by adding TiO 2  particles to water suffers from the disadvantages of requiring a secondary process for recovering TiO 2  particles and difficultly in recycling the particles. Therefore, such process cannot be completely applied for water treatment. 
     BRIEF SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to overcome the above problems encountered in the prior arts and to provide a novel reverse osmosis membrane having excellent antifouling property, by incorporating TiO 2  particles of nano scale with photocatalytic properties to the membrane in a fixed form in order to fundamentally prevent fouling generated during the membrane use. 
     It is another object of the present invention to provide a method for manufacturing the reverse osmosis membrane having excellent antifouling property. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the present invention, TiO 2  particles of nano scale are obtained by adding a titanium compound, such as Ti[OCH(CH 3 ) 2 ] 4 , TiCl 4 , TiOSO 4 , or TiOCl 2 , to an acidic aqueous solution, to perform hydrolysis. As such, use of aqueous solution of strong acid with a pH range of 0.5-3 results in favorable generation of nano particles. Soon after, a pH range of thusly obtained aqueous titanic acid solution is regulated to 1-6 or 9-13, thereby yielding a stable TiO 2  dispersed solution. However, it is more preferable that, because of concern about adulteration caused by impurities in said condition, TiO 2  particles are separated from the aqueous titanic acid solution obtained by hydrolyzing the titanium compound, and dried, to yield TiO 2  as a white powder, which is then dispersed into an acidic solution of pH 1-6 or an alkaline aqueous solution of pH 9-13, thereby obtaining a stable TiO 2  dispersed solution. In the case of using the aqueous solution less than 1 or exceeding 13 in pH, the membrane may be degraded by strong acid or strong alkali. On the other hand, if the pH of the aqueous solution exceeds 6 and is less than 9, TiO 2  particles of nano scale are aggregated and thus precipitated. Hence, stability of dispersed solution becomes poor. 
     The methods for separating TiO 2  particles from aqueous titanic acid solution and for drying the TiO 2  particles are not limited in the present invention, and all separation and drying methods can be utilized. In this regard, the aqueous solution can be directly evaporated and thus dried. 
     Meanwhile, upon production of TiO 2  through hydrolysis of titanium compound, various metal oxides are added in the amount of 0.1-1.0 wt % to the aqueous solution, thereby obtaining TiO 2  particles of nano scale doped with the metal ions. When such particles are used for preparation of stable TiO 2  dispersed solution, TiO 2  photocatalytic effect can be increased. As such, the usable metal oxides comprise LiOH, Mg(ClO 4 ) 2 , AlCl 3 , VCl 3 , VOSO 4 .3H 2 O, VOCl 3 , Cr(NO 3 ) 3 , MnF 3 , F 2 (NO 3 ) 3 .9H 2 O, CoF 3 , NiCl 2 , Zn(ClO 4 ) 2 , Ga(NO 3 ) 3 , Zr[OCH(CH 3 ) 2 ] 4 , NbCl 5 , MoCl 3 , RUCl 3 , Rh(NO 3 ) 3 .2H 2 O, SnCl 4 , SbCl 3 , TaCl 5 , ReCl 5 , OsCl 3 , SbCl 5  and so on. Such metal oxide is.preferably added in the amount of about 0.1-1.0 wt %, based on the weight of the aqueous solution. 
     In the present invention, the reverse osmosis membrane incorporated with TiO 2  particles of nano scale is prepared by immersing the reverse osmosis membrane into stable TiO 2  dispersed solution. The immersing period of time varies with pH of the solution, but is not specifically limited, as long as TiO 2  particles are sufficiently incorporated to the membrane. The mechanism of incorporation of TiO 2  into the membrane is believed to be such that TiO 2  is self-assembled to functional groups present on the surface of the membrane by coordination bonds and thus fixed. In addition, as the reverse osmosis membrane of the present invention, use can be made of all kinds of membranes including a polyamide type reverse osmosis membrane. 
     In the present invention, TiO 2  having excellent photocatalytic effect is used and thus the reverse osmosis membrane with superior anti-fouling property is disclosed. Additionally, by using various photocatalytic inorganic oxides, such as CdS, WO 3 , SrTiO 3 , MoSe 2 , and GaAs, which are known to show photocatalytic effect similar to TiO 2 , the reverse osmosis membrane with excellent anti-fouling property can be manufactured. 
    
    
     Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which ate provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. 
     EXAMPLE 1 
     1 ml of Ti[OCH(CH 3 ) 2 ] 4  completely dissolved in 20 ml of ethanol was slowly added dropwise to 200 ml of a stirred aqueous solution of pH 1.5, thus performing hydrolysis, to yield transparent aqueous solution of TiO 2 . 
     The yielded aqueous solution of TiO 2  was evaporated using a rotavapor at 35° C., and dried in a vacuum oven at 60 ° C. for 1 hour, to obtain TiO 2  particles as white powders, which were then dissolved in water at a predetermined ratio (0.5 g/L) and the pH was adjusted to 2.8, to produce a stable TiO 2  dispersed solution. 
     2 wt % of aqueous MPD (m-phenylenediamine) solution was interfacially polymerized with 0.1 wt % of TMC (trimesoyl chloride) dissolved in an organic solvent, to obtain a polyamide reverse osmosis membrane, which was immersed in said stable TiO 2  dispersed solution for 1 hour, thereby preparing a TiO 2 -incorporated reverse osmosis membrane. 
     EXAMPLE 2 
     The same procedure as set forth in Example 1 was carried out, except that TiO 2  particles as white powders were dissolved in water at a predetermined ratio (0.5 g/L) and pH was adjusted to 10, to yield a TiO 2 -incorporated reverse osmosis membrane. 
     EXAMPLE 3 
     The same procedure as set forth in Example 1 was carried out, except that LiOH was added in the amount of 0.1 g upon preparation of aqueous solution of TiO 2  from hydrolysis of Ti[OCH(CH 3 ) 2 ] 4 , to yield a TiO 2 -incorporated reverse osmosis membrane. 
     TiO 2  particles synthesized in the above examples 1 to 3 were observed by a transmission electron microscopy and an ultraviolet-visible spectroscopy, whereby the particles were found to be have been synthesized in nano scale with dimensions of 10 nm or smaller. 
     The reverse osmosis membrane after incorporation with TiO 2  synthesized in the above example 1 and the membrane before incorporation with TiO 2  were measured for flux and removal of salt. The results are shown in Table 1, below. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Sample 
                 Flux (gfd) 
                 Removal of Salt (%) 
               
               
                   
               
             
             
               
                 Before incorporation with TiO 2   
                 13.2 
                 96.5 
               
               
                 After incorporation with TiO 2   
                 14.4 
                 96.6 
               
               
                   
               
             
          
         
       
     
     To assay antimicrobial property of the reverse osmosis membrane according to the present invention, each surface of the membrane incorporated with TiO 2  prepared in the above example 2 and the non-TiO 2  incorporated membrane was cultivated with  Escherichia coli  at a predetermined concentration, under ultraviolet light. Subsequently, the  E.coli  was recovered for a period of time and cultivated on agar plates, and then colonies were counted. Thereby, the survival ratio was determined, by which antimicrobial property of each membrane was assayed. The results are given in Table 2, below. As such, ultraviolet light was irradiated at 365 nm, and cultivation temperature was 37° C. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Time (hr) 
                 0 
                 0.5 
                 1 
                 2 
                 3 
                 4 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Survival ratio of membrane 
                 100 
                 78.9 
                 71.3 
                 43.6 
                 10.9 
                 0.0 
               
               
                 after incorporation with 
               
               
                 TiO 2  (%) 
               
               
                 Survival ratio of membrane 
                 100 
                 83.7 
                 78.2 
                 51.4 
                 40.3 
                 33.7 
               
               
                 before incorporation with 
               
               
                 TiO 2  (%) 
               
               
                   
               
             
          
         
       
     
     To assay the fouling effect of the reverse osmosis membrane according to the present invention, the fouling solution comprising  E.coli  of the predetermined concentration was made. Then, the membrane incorporated with TiO 2  prepared in the above example 3 and the non-TiO 2  incorporated membrane were immersed in the fouling solution for the period of time shown in the following table 3, and thus were artificially subjected to fouling. Each membrane was measured for water permeability, thereby assaying the extent of the fouling. The results are presented in Table 3, below. As such, the cultivation temperature was 30° C. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Time (day) 
                 0 
                 0.5 
                 1 
                 2 
                 3 
               
               
                   
               
             
             
               
                 Flux of membrane after 
                 76.2 
                 73.1 
                 70.4 
                 68.7 
                 66.1 
               
               
                 incorporation with TiO 2  (gfd) 
               
               
                 Flux of membrane before 
                 77.8 
                 72.3 
                 63.5 
                 60.9 
                 58.1 
               
               
                 incorporation with TiO 2  (gfd) 
               
               
                   
               
             
          
         
       
     
     As mentioned above, the reverse osmosis membrane incorporated with TiO 2  prepared according to the present invention has more excellent antimicrobial effect and decomposition activity against organic contaminants, while maintaining the flux and the removal of salt at the same level, compared with conventional membranes. Therefore, the present membrane can remove microorganisms present in water and prevent the fouling which is generated by such microorganism adsorbed on the membrane. 
     The present invention has been described in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.