AN ANTIBACTERIAL MEMBRANE AND A PRODUCTION METHOD

THEREOF Field of the Invention

The present invention relates to an antibacterial membrane and a production method thereof which is used especially in water treatment, and eliminates the fouling problem occurring as a result of accumulation of microorganisms. Background of the Invention

The intermediate surface which controls passing from one phase to another phase depending on several characteristics of the different materials between two phases is called membrane. The membrane processes are classified in 4 different processes being microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) as removal efficiency and pore diameter. Membrane filtration is one of the most efficient processes which enable to purify waste water, drinking water and industrial waste water in an advanced level. However, a decrease is seen in time in the filtrate flux and removal efficiency of the membranes used in membrane filtration. The main reason for that is membrane fouling. Membranes are designed for the removal of particular material or microorganisms, biofouling, which is an unwanted situation, occurs on the membranes as a result of the materials accumulating on the surface of membrane or membrane pores. With the improvement in nanotechnology in recent years, inorganic nanoparticles have been started to be used in order to enhance the performance of membranes. Especially the antibacterial property exhibited by the nano dimensional metallic materials is effective for solving membrane fouling problems. Generally, inorganic nanoparticles are added to the polymer as mixture and these kinds of materials are called as nanocomposite material. In the literature for the production of antibacterial or high performance membranes, nanoparticles such as titanium dioxide (Ti0 ₂ ), catalytic metal nanoparticles, zeolite, and aluminum are used. Rahimpour et al. obtained antibacterial membrane by adding Ti0 ₂ nanoparticles on the surface of membranes prepared with PVDF/sulfonated polyethersulfone in their study. Antibacterial property of the membrane was obtained with H ₂ 0 ₂ and HO radicals formed on the surface after contacting UV.

Yao et al. have achieved to provide antibacterial property to polyvinyldenfluoride (PVDF) membranes plasma modified with hexafluoropropylene by grafting their surfaces with 4-vinylpyridine. However antibacterial mechanism was not explained.

Shi et al. have prepared PVDF membranes with silver loaded zeolite. Silver nanoparticles can deform the DNA of the bacteria. This property of silver nanoparticles provides antibacterial property to the membranes. Therefore fouling caused by bacteria was diminished.

In 2010, Mater searched the antibacterial effect of gold nanoparticles and examined the effect of nanoparticle concentration. The antibacterial tests were performed on gram positive (Staphylococcus aureus) and gram negative (Escherichia coli) bacteria and it was discovered that gold has quite powerful antimicrobial activity.

Another way to provide antibacterial effect to the membranes is to use antimicrobial peptides. European Patent Document no EP2509614 discloses obtaining antibacterial effect by adding antimicrobial peptides on the surface of thin film composite membranes used in reverse osmosis systems. United States Patent documents no US20020051819, US 20030050247, US20030194445, US20050065072 and US20060166883 disclose industrial studies carried out with antimicrobial peptides. Fouling is the main problem in systems wherein membrane technologies are used. Fouling can be conventionally controlled with methods such as back washing with filtrate water, flushing the membrane surface and chemical washing. However, none of these methods is an exact solution. In addition to the backwashing, membranes are chemically washed. Because of that additional energy, time, water and chemicals are needed. Especially in order to control the biofouling, chemical washing carried out under high alkali and acidic conditions shortens the life of the membranes. Increasing hydrophilic property of the membrane cannot prevent fouling totally however it can be used to elongate fouling time.

Nanotechnological methods used for decreasing biofouling, also have disadvantages such as their high cost and leaching of metallic nanoparticles from membrane matrix.

The antibacterial membranes obtained with photocatalytic nanoparticles such as Ti0 ₂ requires continuous UV contact. This complicates system operation and limits module configurations.

In method of attaching antimicrobial peptides on the membrane surface, production is expensive and it is difficult to carry out industrial production since graft polymerization is used. Summary of the Invention

The objective of the present invention is to provide an antibacterial membrane and production method wherein bismuth chelate is used.

Another objective of the present invention is to provide an antibacterial ultrafiltration membrane. Yet another objective of the present invention is to provide an antibacterial membrane in which fouling problem is not seen.

A further objective of the present invention is to provide an antibacterial membrane which has low both operation and investment cost, easily scalable to industrial scale.

Detailed Description of the Invention

An antibacterial membrane and production method thereof was developed to fulfill the objective of the present invention is illustrated in the accompanying figures, in which;

Figure 1 is the flow chart of the inventive antibacterial membrane production method.

Figure 2 is the graphic wherein the inventive antibacterial membranes and the membranes used in the technique are compared, and which shows the amounts of outlet water obtained from both membrane filtration experiments.

The inventive production method (10) comprises the steps of;

- synthesizing bismuth chelate (11),

dissolving bismuth nitrate pentahydrate in a solvent (1 11),

mixing the prepared solution with 2,3-dimercapto-l-propanol (1 12), - obtaining membrane from the synthesized bismuth chelate (12),

adding prepared bismuth chelate into the solvent (121),

adding polymer to the solution (122),

mixing the polymer solution by heating (123),

removing the bubbles from the polymer solution (124),

forming the membranes with the prepared solution (125). In the inventive method (10), first bismuth chelate is synthesized (11). With this purpose, first bismuth nitrate pentahydrate is dissolved in propylene glycol (1 1) and the prepared solution is mixed with 2,3-dimercapto-l-propanol and is stirred until a homogenous composition is obtained (112). In mixing step (1 12), IN NaOH is added to the solution for pH adjustment. Bismuth chelate synthesis (11) is preferably carried out at room temperature at pH range of 2-12, and preferably pH 3 to 5. The molar ratio of bismuth nitrate pentahydrate:2,3-dimercapto-l-propanol in the prepared bismuth chelate can vary between 3: 1 and 1 : 1. In the preferred embodiment of the invention this ratio is 1 :1.

After bismuth chelate is prepared (1 1), the inventive antibacterial membrane is produced by using this chelate (12). For this purpose, prepared bismuth chelate in previous step is added on the dimethylacetamide solvent (121). In the preferred embodiment of the invention, bismuth chelate is added in ratio of 1 5-30 micromolar. Then polymer is added to this composition (122). Polyvinylpyrrolidone (PVP) and polyethersulfone (PES) are used as polymer. In one embodiment of the invention, besides polyethersulfone (PES), for comparison at least one of polysulfone (PSf), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN) is used. In the preferred embodiment of the invention, after polymer addition step (122), total PVP ratio was 2-15%, and the PES ratio was 5-25% in the solution. In another embodiment of the invention, this ratio was chosen as 8% for PVP and 16% for PES .

Finally, obtained the solution is heated to 60-70°C and stirred until it becomes homogenous (123). Then the bubbles are removed by waiting in ultrasonic bath (124).

Preferably water induced phase inversion technique is applied for forming membranes (125). With this technique, the liquid polymer solution is immersed into water and the solvent in the solution is vaporized, the exchange between nonsolvent (water) and solvent is occurred which leads to membrane pore formation. First liquid polymer solution is poured on the supporting layers (unwoven fabric) in thickness of 90 μηι. Then the support layer is moved at various speeds (10-150mm/second). Doctor blade is passed through the liquid polymer solution on the supporting layer. The membrane thickness on the support layer is fixed such that it will be in range of 50-500 μηι. The polymer solution passing from the doctor blade enters into the water bath after a short waiting period. Therefore the exchange between non solvent and solvent is occurred and polymer precipitates, thus the membrane structure is formed. After membrane thickness is controlled in different points of the membrane, they are taken to plastic containers comprising water in order to be stored. These containers are kept in a cold room approximately at 0°C.

For the optimization of condition in synthesis of the inventive antibacterial membranes, three different parameters are taken as basis, namely temperature, pH and bismuth nitrate pentahydrate:2,3-dimercapto-l-propanol molar ratio. Room temperature and 25 and 45°C representing high temperature conditions are selected as temperature; pH 4, 7 and 10 values are respectively selected for representing acidic, neutral and basic conditions for the optimization of pH. 3: 1, 2:1 and 1 : 1 ratios are selected in order to optimize bismuth nitrate pentahydrate:2,3-dimercapto-l-propanol molar ratio.

In accordance with these parameters, a plurality of membranes was synthesized, and analyses such as fouling and biofilm formation amount were performed. As a result of these analyses, the best results were seen in antibacterial membrane having 1 :1 bismuth nitrate pentahydrate:2,3-dimercapto-l-propanol molar ratio which was produced at 25 °C and pH 4.

When the surface views of the membranes produced with the inventive method (10), it was seen that the surfaces are smooth and defect-free. This proves that the bismuth chelate addition does not affect the morphological structure of the membrane. In order to measure the efficiency of the antibacterial membranes produced with the inventive method, bismuth chelate added membrane and pristine samples were prepared and compared. For this purpose, the active sludge was filtered from the membrane samples. After filtration experiment, it was observed that less biofilm was formed on the bismuth chelate added membrane surfaces. According to the results obtained after hydraulic washing, it was seen that fouling layer occurred on the bismuth chelate added membranes are more reversible. The microorganisms holding on their surfaces could be removed from the surface with water without requiring chemical washing. Irreversible fouling was seen in pristine membranes. The comparison of outlet water obtained during active sludge filtration is given in Figure 2. As it can be seen from the figure, the amount of outlet water obtained from bismuth chelate added membranes are much higher than the amount of outlet water obtained from pristine membranes. This proves that the antibacterial membranes solve fouling problem of ultrafiltration membranes.

The inventive antibacterial membrane is easily producible at industrial scale and it is also easy and cheap to use since it does not require extra process.