rexresearch.com
Darren SUN
Multi-Use Titanium Oxide Nanofibers
Multi-Use Titanium Oxide Nanofibers
Simultaneous production of HHO & pure water by desalination with an anti-fouling & anti-microbial flexible forward-osmosis filtration membrane; & solar panels; & 2x battery life extension.
http://www.sciencedaily.com/releases/2013/03/130320094856.htm
Multi-Purpose Wonder Can Generate
Hydrogen, Produce Clean Water & Even Provide Energy
Assoc Prof Darren Sun holding the patented Titanium dioxide nanofibre in a test tube and the hydrogen reactor in the background. (Credit: Image courtesy of Nanyang Technological University)
Mar. 23, 2013 — A new wonder material can generate hydrogen, produce clean water and even create energy.
Science fiction? Hardly, and there's more -- It can also desalinate water, be used as flexible water filtration membranes, help recover energy from desalination waste brine, be made into flexible solar cells and can also double the lifespan of lithium ion batteries. With its superior bacteria-killing capabilities, it can also be used to develop a new type of antibacterial bandage.
Scientists at Nanyang Technological University (NTU) in Singapore, led by Associate Professor Darren Sun have succeeded in developing a single, revolutionary nanomaterial that can do all the above and at very low cost compared to existing technology.
This breakthrough which has taken Prof Sun five years to develop is dubbed the Multi-use Titanium Dioxide (TiO2). It is formed by turning titanium dioxide crystals into patented nanofibres, which can then be easily fabricated into patented flexible filter membranes which include a combination of carbon, copper, zinc or tin, depending on the specific end product needed.
Titanium dioxide is a cheap and abundant material, which has been scientifically proven to have the ability to accelerate a chemical reaction (photocatalytic) and is also able to bond easily with water (hydrophilic).
More than 70 scientific papers on Prof Sun's work in titanium dioxide has been published in the last five years, the latest being papers published in Water Research, Energy and Environmental Science, and Journal of Materials Chemistry.
Prof Sun, 52, from NTU's School of Civil and Environmental Engineering, said such a low-cost and easily produced nanomaterialis expected to have immense potential to help tackle ongoing global challenges in energy and environmental issues.
With the world's population expected to hit 8.3 billion by 2030, there will be a massive increase in the global demand for energy and food by 50 per cent and 30 per cent for drinking water (Population Institute report, titled 2030: The "Perfect Storm" Scenario).
"While there is no single silver bullet to solving two of the world's biggest challenges: cheap renewable energy and an abundant supply of clean water; our single multi-use membrane comes close, with its titanium dioxide nanoparticles being a key catalyst in discovering such solutions," Prof Sun said. "With our unique nanomaterial, we hope to be able to help convert today's waste into tomorrow's resources, such as clean water and energy."
Prof Sun's multi-use titanium dioxide can:
concurrently produce both hydrogen and clean water when exposed to sunlight
be made into a low-cost flexible filtration membrane that is anti-fouling
desalinate water as a high flux forward osmosis membrane
recover energy from waste desalination brine and wastewater
be made into a low-cost flexible solar cell to generate electricity
doubles battery life when used as anode in lithium ion battery
kill harmful microbial, leading to new antibacterial bandages
How the wonder material was found
Prof Sun had initially used titanium dioxide with iron oxide to make anti-bacterial water filtration membranes to solve biofouling -- bacterial growth which clogs up the pores of membranes, obstructing water flow.
While developing the membrane, Prof Sun's team also discovered that it could act as a photocatalyst, turning wastewater into hydrogen and oxygen under sunlight while still producing clean water. Such a water-splitting effect is usually caused by Platinum, a precious metal that is both expensive and rare.
"With such a discovery, it is possible to concurrently treat wastewater and yet have a much cheaper option of storing solar energy in the form of hydrogen so that it can be available any time, day or night, regardless of whether the sun is shining or not, which makes it truly a source of clean fuel," said Prof Sun.
"As of now, we are achieving a very high efficiency of about three times more than if we had used platinum, but at a much lower cost, allowing for cheap hydrogen production. In addition, we can concurrently produce clean water for close-to-zero energy cost, which may change our current water reclamation system over the world for future liveable cities."
Hydrogen is a clean fuel which can be used for automotive fuel-cells or in power plants to generate electricity.
Producing hydrogen and clean water
This discovery, which was published recently in the academic journal, Water Research, showed that a small amount of nanomaterial (0.5 grams of titanium dioxide nanofibres treated with copper oxide), can generate 1.53 millilitre of hydrogen in an hour when immersed in one litre of wastewater. This amount of hydrogen produced is three times more than when Platinum is used in the same situation.
Depending on the type of wastewater, the amount of hydrogen generated can be as much as 200 millilitres in an hour. Also to increase hydrogen production, more nanomaterial can be used in larger amounts of wastewater.
Producing low-cost flexible forward osmosis membranes
Not only can titanium dioxide particles help split water, it can also make water filter membranes hydrophilic -- allowing water to flow through it easily, while rejecting foreign contaminants, including those of salt, making it perfect for desalinating water using forward osmosis. Thus a new super high flux (flow rate) forward osmosis membrane is developed.
This discovery was published recently in last month's journal of Energy and Environmental Science. This is the first such report of TiO2 nanofibres and particles used in forward osmosis membrane system for clean water production and energy generation.
Producing new antibacterial bandages
With its anti-microbial properties and low cost, the membrane can also be used to make breathable anti-bacterial bandages, which would not only prevent infections and tackle infection at open wounds, but also promote healing by allowing oxygen to permeate through the plaster.
The membrane's material properties are also similar to polymers used to make plastic bandages currently sold on the market.
Producing low-cost flexible solar cells
Prof Sun's research projects have shown out that when treated with other materials or made into another form such as crystals, titanium dioxide can have other uses, such as in solar cells.
By making a black titanium dioxide polycrystalline sheet, Prof Sun's team was able to make a flexible solar-cell which can generate electricity from the sun's rays.
Producing longer lasting lithium ion batteries
Concurrently, Prof Sun has another team working on developing the black titanium dioxide nanomaterial to be used in Lithium ion batteries commonly used in electronic devices.
Preliminary results from thin coin-like lithium ion batteries, have shown that when titanium dioxide sphere-like nanoparticles modified with carbon are used as the anode (negative pole), it can double the capacity of the battery. This gives such batteries a much longer lifespan before it is fully drained. The results were featured in the Journal of Materials Chemistry on its cover page last year.
Next step -- commercialisation
Prof Sun and his team of 20, which includes 6 undergraduates, 10 PhD students and 4 researchers, are now working to further develop the material while concurrently spinning off a start-up company to commercialise the product.
They are also looking to collaborate with commercial partners to speed up the commercialisation process.
Journal References:
Lei Liu, Zhaoyang Liu, Hongwei Bai, Darren Delai Sun. Concurrent filtration and solar photocatalytic disinfection/degradation using high-performance Ag/TiO2 nanofiber membrane. Water Research, 2012; 46 (4): 1101 DOI: 10.1016/j.watres.2011.12.009
Jia Hong Pan, Xiwang Zhang, Alan J. Du, Hongwei Bai, Jiawei Ng, Darren Sun. A hierarchically assembled mesoporous ZnO hemisphere array and hollow microspheres for photocatalytic membrane water filtration. Physical Chemistry Chemical Physics, 2012; 14 (20): 7481 DOI: 10.1039/C2CP40997F
Nanyang Technological University (2013, March 23). Multi-purpose wonder can generate hydrogen, produce clean water and even provide energy. ScienceDaily. Retrieved March 23, 2013, from http://www.sciencedaily.com /releases/2013/03/130320094856.htm
Patents
US2010264097
MICROSPHERIC TiO2 PHOTOCATALYST
Inventor(s): SUN DARREN DELAI [SG]; LEE PEI FUNG [SG]; LECKIE JAMES O [US] +
Applicant(s): UNIV NANYANG [SG]; UNIV LELAND STANFORD JUNIOR [US] +
Also published as: WO2008076082 (A1) WO2008076082 (A8)
Abstract -- The present invention refers to titanium oxide microspheres having photocatalytic properties which can, for example, be used in a method for cleaning wastewater which uses a submerged membrane reactor.
FIELD OF THE INVENTION
[0002] The present invention refers to titanium oxide microspheres having photocatalyst property which can be used in a process of cleaning wastewater which uses a submerged membrane reactor.
BACKGROUND OF THE INVENTION
[0003] Contaminants present in raw water (e.g. natural organic matter (NOM) and bacteria) are detrimental to the water quality. There is a growing trend in the use of membrane technology for removal of such contaminants from raw water for the production of good quality potable water.
[0004] The "membrane" in a membrane reactor works as a filter which is based on the separation of substances depending on their size. With their micropores the membrane separates particles from the wastewater.
[0005] In recent years, membrane processes have become increasingly popular in water treatment for a variety of reasons which include prospectively more stringent water quality regulations, small footprint and reduced operation and maintenance costs due to advancements in membrane technology. One of the serious problems when utilizing filtration membrane in water treatment process is the decline of permeate flux due to membrane fouling and gel formation as learned from U.S. Pat. No. 5,505,841. In general, the membrane fouling can be defined as the accumulation of contaminated compounds on the surface of a membrane which form a solid layer. The solid layer on the surface of membrane comprises bacteria, organic and inorganic species, non-biodegradable compounds. Especially, the natural organic matter is suspected to be one of the major constituents in the solid layer causing the fouling problem in the membrane process (Sun, D. D., Li, J., et al., 2000, Civil Engineering Research Bulletin, NTU, Singapore, No. 13). Thus, the term membrane fouling comprehensively refers to a series of phenomenon which comprise of pore adsorption, pore blocking or clogging, gel formation or cake formation.
[0006] Gel formation or cake formation specifically refer to the layer formed on the surface due to concentration polarization. The layer is formed at the membrane liquid interface where larger solute molecules excluded from the permeate form a coating. The fouling caused by solids or colloids deposited on the membrane surface, or gel formation or solid layer formation, is reversible and can be overcome by periodic membrane cleaning. However, the pore adsorption or pore blocking caused by colloids trapped within the pores is usually irreversible and requires membrane replacement.
[0007] To date, significant effort has been dedicated to control membrane fouling. Several physical membrane cleaning methods such as backwashing, aeration, ultrasonic cleaning (U.S. Pat. No. 7,008,540) have been suggested to minimize membrane fouling. In backwashing, permeation through the membranes is stopped momentarily. Air or water will flow through the membranes in a reverse direction to physically push solids off of the membranes. In aeration, bubbles are produced in the tank below the membranes. The bubbles will agitate or scrub the membranes and thereby remove some solids while creating an air lift effect and circulation of the tank water to carry the solids away from the membranes. In ultrasonic cleaning, ultrasonic energy is emitted by the ultrasonic transducer in the direction of the filtration membrane. Dislodged particles cleaned by the ultrasonic energy from the filtration membrane are carried away in a cross-flow stream.
[0008] These methods or combination of these methods are effective in removing the reversible membrane fouling like gel layer or solid layer.
[0009] On the other hand, chemical cleaning is applied to reduce or eliminate the irreversible membrane fouling. The permeation is stopped and a chemical cleaner is backwashed through the membranes. In some cases, the tank is emptied during or after the cleaning event so that the amount of cleaner can be collected and disposed of. In other cases, if the tank remains filled, the amount of chemical cleaner is limited and subject to the tolerance for the application. Chemical cleaning has to be limited to a minimum frequency because repeated chemical cleaning may affect membrane life, and disposal of spent chemical reagents poses another problem. Thus the control of the irreversible membrane fouling is of importance for more efficient use of membranes.
[0010] For example, large molecular size of NOM is retained on the membrane surface while the small molecular size of NOM is trapped within the membrane pore which leads to irreversible membrane fouling that could not be cleaned by merely physical cleaning. Irreversible membrane fouling will eventually lead to higher long-term operating pressures, and thus, higher operating cost and more energy consumption.
[0011] Besides chemical cleaning of the membrane, it has been proposed to remove the contaminants from water prior to membrane filtration process to prevent irreversible membrane fouling. U.S. Pat. No 6,027,649 discloses a process capable of removing contaminants from water utilizing a coagulant in combination with a semi-permeable membrane. However this process is not effective for controlling irreversible fouling because such process does not remove trace organic matter or the smaller molecular size of NOM which will trap the membrane pores.
[0012] Powdered activated carbon (PAC) is proposed to be used in conjunction with membranes to remove organic contaminants by adsorption and allow the membrane to separate the larger PAC particles (U.S. Pat. No. 5,505,841; JP 2004 016 896). Several problems are encountered for the regeneration of PAC. PAC must be heated to high temperatures to burn off the NOM. The cost of regenerating at such high temperatures has a negative impact on the economics of the process using PAC. Further more, when PAC particles are heated to such high temperatures, a certain portion of PAC are consumed by combustion. At the end of PAC lifespan, they must be disposed of, which results in additional disposal costs. Moreover, the irregular shape of PAC may damage the surface of filtration membrane.
[0013] It has been proposed that freshly precipitated iron or aluminium oxides, common adsorbents, be used in conjunction with membranes to reduce fouling of the membrane. However iron oxide or aluminium oxide also requires heat treatment for regeneration. It is reported that the freshly precipitated particles themselves contribute to the fouling of the membrane. Heated iron oxide particles have been proposed to remove contaminants and concurrently reduce membrane fouling, the recondition process is carried out in acidic or basic condition to restore its adsorption capacity (U.S. Pat. No. 6,113,792). This method is not preferable for a continuous system.
[0014] Titanium dioxide is proposed to be used as adsorbent for the removal of contaminants due to its regenerative potential. The spent titanium dioxide can be regenerated via photocatalytic oxidation (PCO) process (Fang, H., Sun, D. D., et al., 2005, Water Science & Technology, vol. 51, no. 6-7, p. 373-380). Commercial titanium dioxide, P25 is the most commonly used photocatalyst due to its high photocatalytic activity, chemical resistance, and low costs. Irradiation with light of sufficient energy creates the formation of electrons and holes on the surface of the photoreactive catalyst. The PCO process has been reported as a possible alternative for removing organic matters from potable water. A redox environment will be created in a PCO process to mineralize the NOM's and sterilize the bacteria adsorbed on the surface of the photocatalyst into carbon dioxide and water when the semiconductor photocatalyst is illuminated by light source (usually UV light) in a PCO process. The theoretical basis for photocatalysis in general is reviewed by Hoffmann, M. R., Martin, S. T., et al. (1995, Chem. Rev., vol 95, 69-96) and by Fox, M. A. and Dulay, M. T. (1993, Chem. Rev., vol. 93, p. 341-357).
[0015] Unfortunately, recycling and reuse of such P25 titanium dioxide is an existing problem, particularly separation of P25 titanium dioxide from treated water. Moreover, P25 titanium dioxide does not present individually in aqueous system, but rather as physically unstable complex primary aggregates ranging from 25 nm to 0.1 [mu]m. These physically unstable complex aggregates would reduce the surface area/active sites and subsequently affect its photocatalytic activity (Qiao, s., Sun, D. D., et al., 2002, Water Science Technology, vol. 147, no. 1, p. 211-217).
[0016] With the ever increasing concern about the quality of drinking water, there continues to be a need for improved systems for effectively and economically removing contaminants such as natural organic matter and bacteria from water.
SUMMARY OF THE INVENTION
[0017] In a first aspect, the present invention refers to a titanium oxide microsphere having photocatalytic property and having a size of about 10 [mu]m to about 200 [mu]m and a mesoporous structure with a pore size in a range of about 2 to about 50 nm wherein the microspheres are obtained by the process comprising:
preparing a sol by mixing an organometallic titanium precursor with an alcohol without adding H2O;
aging the sol;
mixing the aged sol with titanium oxide powder;
spraying the mixture to form the titanium oxide photocatalyst microspheres;
calcining the microspheres.
[0023] In another aspect, the present invention refers to a process of cleaning wastewater in a membrane filtration reactor, wherein the process comprises:
mixing the titanium oxide microsphere of the present invention with wastewater;
feeding the mixture into a membrane filtration reactor;
sucking the mixture treated in the membrane filtration reactor through the membrane, wherein the diameter of the microspheres in the mixture is greater than the diameter of the pores of the membrane, to form a cake layer of catalyst on the surface of the membrane; and
continuing feeding the membrane filtration reactor with wastewater until the wastewater is cleaned.
[0028] In still another aspect, the present invention refers to a submerged membrane reactor in which the titanium oxide microsphere of the present invention is mixed with wastewater cleaned in the reactor or which utilizes the process for cleaning waste water in a membrane filtration reactor of the present invention.
[0029] In a further aspect, the present invention is directed to the use of titanium oxide microspheres having photocatalytic property for operation of a submerged membrane reactor.
[0030] In another aspect the present invention is directed to a method of manufacturing a titanium oxide microsphere having photocatalytic property and having a size of about 10 [mu]m to about 200 [mu]m and a mesoporous structure with a pore size in a range of about 2 to about 50 nm wherein the method comprises:
preparing a sol by mixing an organometallic titanium precursor with an alcohol without adding H2O;
aging the sol;
mixing the aged sol with titanium oxide powder;
spraying the mixture to form the titanium oxide photocatalyst microspheres;
calcining the microspheres.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:
[0037] FIGS. 1 and 2 show flow charts illustrating the separate steps for manufacturing the TiO2 microspheres of the present invention. FIG. 2 illustrates a specific example which was carried out for manufacturing the TiO2 microspheres of the present invention.
[0038] FIG. 3 shows SEM micrographs of a TiO2 microsphere of the present invention at different magnifications. While the left picture shows a single microsphere the picture on the right side shows the surface of the TiO2 microsphere of the present invention. The picture showing the surface demonstrates that the nano-size TiO2 particles are uniformly distributed over the surface of the microsphere.
[0039] FIG. 4 shows the composition of the products used for preparing the TiO2 microspheres of the present invention as well as the composition of the calcined TiO2 microspheres. The composition products have been characterized by means of powder X-ray diffraction by using a Shimadzu XRD-6000 diffractometer with CuKR radiation. The top curve shows the XRD pattern for the calcined TiO2 microspheres of the present invention while the middle and the lower curve show the XRD pattern for component A and B, respectively. The different peaks indicate crystalline phase of each product. TiO2 crystallizes in three major structures: rutile, anatase and brookite. However only rutile and anatase play the role in the TiO2 photocatalysis. Anatase phase, a stable phase of TiO2 at low temperature (400-600[deg.] C.), is an important crystalline phase of TiO2 Rutile is a stable phase of TiO2 at high temperature (600-1000[deg.] C.). Hence the product has been calcined at 450[deg.] C. to obtain TiO2 with anatase phase. The peak indicating the rutile phase in the manufactured microspheres shows the rutile phase TiO2 contributed by the TiO2 powder. The raw TiO2 powder has in general a mixture of anatase and rutile at ratio of 70:30.
[0040] FIG. 5 shows a comparison of photocatalytic activity of TiO2 microspheres of the present invention and commercial TiO2 using phenol as targeted compound for cleaning (phenol concentration: 100 mg/l; photocatalyst mass concentration: 1 g/l, pH 7). FIG. 5 shows the degradation of phenol as a function of the irradiation time at pH 7. It can be observed that the degradation of phenol followed an exponential decay form. About 40% of phenol is degraded after 60 min of UV light irradiation with the absence of the TiO2 microsphere of the present invention. The removal efficiency is greatly enhanced when the TiO2 microsphere of the present invention is added into the solutions. The removal efficiency after 60 min UV light irradiation is 60% and 70% for P25 TiO2 and nano-structured TiO2 microsphere, respectively. It can be seen from FIG. 5 that the TiO2 microsphere of the present invention possessed a better photocatalytic activity than P25 TiO2. C/C0 is the ratio of the concentration of microspheres in the samples to the initial concentration of those microspheres.
[0041] FIG. 6 shows an exemplary set up of a reactor system using the TiO2 microspheres of the present invention. Raw water is introduced into the membrane reactor tank through a valve. Within the membrane reactor tank the raw water is mixed with fresh TiO2 microspheres of the present invention and recycled TiO2 microspheres from the PCO reactor. Within the reactor tank the raw water and the TiO2 microspheres are mixed by the turbulence flow created by coarse diffuser located at the bottom of the membrane reactor. As can be seen in FIG. 6 the coarse diffusers are connected to an air supply which is also connected to the PCO reactor. After cleaning the wastewater is passed through the pores of the filtration membrane by suction force generated by a pump located outside the membrane reactor. The TiO2 microspheres settle to the bottom of the membrane reactor once the coarse diffuser stops working. From the bottom of the membrane reactor they are transferred via a pump into the PCO reactor. The PCO reactor consists of a reaction chamber and a UV lamp. The PCO chamber consists of a double glass-cooling jacket (e.g., o.d. 70 mm for the outer wall, i.d. 50 mm for the inner wall and height of 350 mm). The PCO reactor can be fitted with a gas diffuser at the bottom of the PCO chamber for diffusing the air. A medium-pressure mercury lamp (12 W) with primary emission wavelength of 253.7 nm is installed vertically in the middle of the reactor as the UV source (see also Fang, H., Sun, D. D., et al., 2005, supra).
[0042] FIG. 7 shows the graphical illustration of the effect of titanium dioxide microspheres on permeates flux through a membrane. The initial permeate flux of humic acid filtration is 3.3 l*min<-1>*m<-2 >and gradually decreased to 2.3 l*min<-1>*m<-2>. There is a flux decrease of 30% after 300 min of humic acid filtration. The initial permeate flux is increased with the presence of TiO2 microsphere in solution. The initial permeate flux is enhanced to about 5.0 l*min<-1>*m<-2 >with the presence of 0.5 g/L of TiO2 microsphere in humic acid solution. The permeate flux dropped to 4.2 l*min<-1>*m<-2>, about 16% of flux drop after 300 min of filtration. However when the concentration of TiO2 microsphere increased to 1 g/L, the initial permeate flux is reduced to 4.2 l*min<-1>*m<-2>. Hence, the increase of TiO2 concentration in solution affects the degree of flux enhancement. In general the presence of TiO2 microsphere in solution is still beneficial in enhancing the permeate flux.
[0043] FIG. 8 shows the graphical illustration of the permeate quality during the membrane filtration process. The permeate quality shows that the membrane filtration is able to remove humic acid to about 75%. The removal rate of humic acid can achieve 87% and 93% with the presence of 0.5 g/L and 1.0 g/L of TiO2 microsphere, respectively. The increase of TiO2 concentration in solution will help to achieve a better removal rate.
[0044] FIG. 9 shows the pore size distribution of TiO2 microsphere and P25 TiO2. Pore size distribution curve was calculated from the adsorption branch using the BJH (Barrett-Joyner-Halenda) method. In FIG. 9, Dv, cc/nm/g (Dv stands for pore volume, and cc stands for cubic centimeters) has been plotted against the pore diameter in nm. The total pore volumes were estimated from the amounts adsorbed at a relative pressure (P/P0) of 0.99. The first peak in FIGS. 9 of 2 to 3 nm pore size is referred to intercrystalline porosity which is the pore within the TiO2 microsphere or the P25 TiO2 agglomerates. The pore sizes obtained with the method of the present invention for manufacturing the TiO2 microspheres of the present invention lies in the mesoporous range (2-50 nm) whereas, for example, the microspheres manufactured according to Li, X. Z. and Liu, H. (2003, Environ. Sci. Technol., vol. 37, p. 3989-3994) lies in the mesoporous range as well as in the macroporous range (>50 nm). The second peak refers to the interagglomerate pore which is the pore between the TiO2 microspheres or the agglomerated P25 TiO2. A smaller pore size distribution, i.e. the smaller mesoporous structure, of about 2 to 50 nm enhances the adsorption of contaminants.
DETAILED DESCRIPTION OF THE INVENTION
[0045] Considering the continuous need for improved systems for effectively and economically removing contaminants such as natural organic matter and bacteria from water the inventors have developed titanium oxide microspheres having photocatalytic property or activity and having a size of about 10 [mu]m to about 200 [mu]m and a mesoporous structure with a pore size in a range of about 2 to about 50 nm. This microspheres are obtained by a process which comprises:
preparing a sol by mixing an organometallic titanium precursor with an alcohol without adding or using H2O;
aging the sol;
mixing the aged sol with titanium oxide powder;
spraying the mixture to form the titanium oxide photocatalyst microspheres;
calcining the microspheres.
[0051] The manufacture of those particles is based on the sol-gel process. In general, the sol-gel process is based on the phase transformation of a sol obtained from metallic alkoxides or organometallic precursors. This sol, which is a solution containing particles in suspension, is polymerized at low temperature to form a wet gel. The wet gel is going to be densified through a thermal annealing to give an inorganic product like a glass, polycrystals or a dry gel. In general, the sol-gel process consists of hydrolysis and condensation reactions, which lead to the formation of the sol.
[0052] A "sol" is a dispersion of solid particles in a liquid where only the Brownian motions suspend the particles. A "gel" is a state where both liquid and solid are dispersed in each other, which presents a solid network containing liquid components.
[0053] The terms "particle", "microparticle", "bead", "microbead", "microsphere", and grammatical equivalents refer to small discrete particles, substantially spherical in shape, having a diameter of about 10 micrometers ([mu]m) to about 200 micrometers. The average size of the titanium oxide microspheres referred to herein is about 50 [mu]m.
[0054] The sol-gel process used in the present invention can be performed according to any protocol. The titanium oxide microspheres may be formed from an organometallic titanium precursor, for example in situ during the reaction process.
[0055] In this process, at first the sol may for instance be generated by hydrolysis of such a precursor. An exemplary precursor can be a titanium alkoxide. The hydrolysis of a titanium alkoxide is thought to induce the substitution of OR groups linked to titanium by Ti-OH groups, which then lead to the formation of a titanium network via condensation polymerisation. Examples of titanium alkoxides can include, but are not limited to titanium methoxide, titanium ethoxide, titanium isopropoxide, titanium propoxide and titanium butoxide.
[0056] Typically, but not limited thereto, sol preparation by hydrolysis and condensation of a titanium alkoxide can be performed in an alcohol or an absolute alcohol. Any alcohol can be used in the present method. Examples of alcohols which can be used are ethanol, methanol, isopropanol, butanol or propanol.
[0057] In general the hydrolysis does not require the use of a catalyst. However, using a catalyst can accelerate the proceeding. Thus, in one aspect, the present invention further comprises adding a catalyst to the sol for initiating the reaction between the precursor and the alcohol. Any known acidic catalyst, such as hydrochloric acid or nitric acid, can be used. In an acid-catalyzed condensation, titanium is believed to be protonized which makes the titanium more electrophilic and thus susceptible to nucleophilic attack. In an acid-catalysed process, the pH value may for instance be in the range of about 1 to about 4, such as for example about pH 1 or 2 or 3 or 4.
[0058] The ratio of the organometallic titanium precursor to alcohol can be about 1 to between about 4 to 100 mol. In one example the ratio is about 1 to between about 40 to 60 mol.
[0059] Afterwards, the reaction mixture of titanium precursor and alcohol and optionally the catalyst is aged. With "aging" it is meant that the gel which starts to form from the sol shrinks in size by expelling fluids from the pores of the aging sol. Aging can take from 24 hours up to 2, 3 or 4 days. In the present invention the sol is aged for about 24 hours before it is used for mixing with a titanium oxide powder.
[0060] For the manufacture of the titanium microspheres of the present invention no water (H2O) is used for sol preparation because water would accelerate the gelation process and the precipitation of Ti(OH)2 in sol. "Precipitation" means that the sol already precipitates to solids. That means that the present invention uses the "sol" condition during the synthesis process so that the sol will serve like a "glue" function to give a strong binding capacity to form the microspheres during the later following spray drying process. Hence, without the use of water a much more stable sol can be obtained having a shelf life of more than 1 year. With longer "shelf life" is meant that the sol will not undergo any changes during the period of storage and can be later used for the next step in the process, namely mixing the sol with titanium oxide powder and spray drying.
[0061] Another advantage of not using water is that the spray drying process can be operated at lower temperatures when alcohol is used instead of water. Another effect is that the microspheres of the present invention show a mesoporous structure, i.e. a pore size distribution of between about 2 nm to 50 nm (FIG. 9). In general, a mesoporous pore size distribution enhances the adsorption of contaminants (Lorenc-Grabowska, E. and Gryglewicz, G., 2005, Journal of Colloid and Interface Science, vol. 284, p. 416-423). The enhanced cleaning capacity for microspheres having such a pore size distribution has also been demonstrated for the microspheres of the present invention (see FIGS. 5 and 8).
[0062] In the process of obtaining the TiO2 microspheres of the present invention illustrated in FIG. 3, the use of additive or templates should also be avoided during the preparation of the sol. In general, additives or templates will contribute to the unstability of the sol which means that the shelf life of the particles will be limited. Additives, like polyethylenglycol (PEG), polyvinyl acolhol (PVA) and carboxymethylcellulose (CMC) are normally used to create the pore structure of a product, like for example a microsphere. Higher molecular weight of an additive will lead to larger pore sizes once the additive is decomposed (Antonietti, M., 2001, Current Opinion in Colloid and interface Science, vol. 6, issue 3, p. 244-248). Another problem that was found is that those additives or templates are sometimes difficult to remove during the calcination process. In this case, this would affect the degree of crystallinity of the TiO2 microspheres which is important for their cleaning capabilities. Different additives or templates might have different decomposition temperatures and therefore might not compromise with the optimum calcination temperature for phase transformation of TiO2 into the anatase type especially if the low temperature to form anatase phase (400[deg.] C.) is used. In the process of obtaining the TiO2 particles, amphipilic three-block copolymers, additives like for example polyethyleneglycol (PEG), PVA and CMC are not added to the sol as it is done for example in the method described in CN 1443601 A which was used by Li, X. Z. and Liu, H. (2003, supra).
[0063] After aging, titanium oxide powder is mixed into the aged sol. Any titanium oxide powder can be used. In one example described herein, titanium oxide powder supplied by Degussa, Germany, has been used. This powder is characterized by comprising a surface area of about 50 m<-2>g<-1>, having a crystal size of about 30 nm and about 70% of this TiO2 crystals are of anatase type. Another TiO2 powder which could also be used is supplied by Taixing Nano-Materials Company in China. Their powder is characterized by comprising a surface area of about 56.7 m<-2>g<-1>, having a crystal size of about 9.6 nm and about 89.4% of this TiO2 crystals are of anatase type.
[0064] Different weight ratios are possible for mixing titanium oxide powder with the aged sol. The minimal ratio is about 1:3 whereas the maximal ratio is about 1:10. In one example, the weight ratio for mixing aged sol with titanium oxide powder is about 1:5. Mixing is carried out using any method for mixing known in the art, for example, under stirring conditions using a magnetic stirrer for obtaining mixed slurry.
[0065] The slurry obtained after mixing is then sprayed to form the titanium microspheres. During spray drying the particles aggregate to form semisolid microspheres.
[0066] After spraying the microspheres are calcined to form solid TiO2 microspheres. Calcination reactions usually take place at or above the thermal decomposition temperature (for decomposition and volatilization reactions) or the transition temperature (for phase transitions) of the metalloxide used. The calcination step has the effect that the TiO2 particles obtained by spraying are transformed from amorphous phase to crystallite phase of anatase type. The different composition of crystal types of the different components used for the manufacture of the TiO2 microspheres are illustrated in FIG. 4.
[0067] In general, calcination is carried out at a temperature between about 400[deg.] C. to about 600[deg.] C. Calcination can be carried out at a temperature of about 400[deg.] C. as well as at 500[deg.] C. In one example, the temperature was about 450[deg.] C. Calcination is carried out for several hours, for example 3, 4, 5 or 6 hours. Calcination is normally carried out in furnaces or reactors (sometimes referred to as kilns) of various designs including shaft furnaces, rotary kilns, multiple hearth furnaces, and fluidized bed reactors. The phase transformation might also be induced using the hydrothermal method as described by Hildago et al. (2007, Catalysis Today, vol. 129, p. 50-58). Using the hydrothermal method, the sample is placed in a Teflon recipient inside of a stainless steel autoclave. Hydrothermal treatment is performed at a low temperature, for example 120-150[deg.] C. for several hours up to 24 hours and at high working pressures, for example 198.48 to 475.72 kPa.
[0068] As an optional step, the TiO2 particles obtained after spraying can be dried before calcination to allow further condensation and passing off of remaining liquids (water, alcohol) from the gel. Depending on the liquid content in the wet-gel, the drying step can be carried out for about one night up to several days. In one example, of the present invention, the TiO2 particles have been dried overnight. The particles can either be dried at room temperature or at a temperature between about 50 to about 150[deg.] C. FIGS. 1 and 2 show a general overview of the process of obtaining the TiO2 particles of the present invention.
[0069] Table 1 illustrates the physical characteristics of TiO2 microspheres of the present invention and a pure TiO2 powder, namely P25 from Degussa, Germany.
[0000]
TABLE 1
BET Surface Area Total pore volume
Sample (m<2>/g) (cm<3>/g)
P25 TiO2 Component B 43.74 +- 5.02 0.952 +- 0.245
particle size 25 nm to 0.1 [mu]m
TiO2 microspheres 41.89 +- 3.98 0.472 +- 0.092
particle size 10-200 [mu]m
[0070] Even though the TiO2 microspheres of the present invention are much larger than commonly used TiO2 particles, like P25, the BET surface is retained due to its mesoporous structure. According to the definition of the International Union of Pure and Applied Chemistry (IUPAC) the term "mesopore/mesoporous" refers to pore size in the range of 2 to 50 nm and this range enhances the adsorption of contaminants (see FIG. 8) (Lorenc-Grabowska, E. and Gryglewicz, G., 2005, supra). According to IUPAC, a pore size below 2 nm is termed a micropore range and >50 nm is termed macropore range.
[0071] Thus, the pore size of the microspheres of the present invention falls into the mesoporous range only. For example, the pore size of the microspheres mentioned by Li, X. Z. and Liu, H. (2003, supra) ranges between the mesoporous and macroporous range and depends on the additives used. Li, X. Z. and Liu, H. (2003, supra) uses another mechanism for forming the porous structure of their microspheres. Li, X. Z. and Liu, H. (2003, supra) use the additives (i.e. PEG) to create the porous structure. In the present invention, the nanosized TiO2 powder (from the sol) is embedded within the microsphere in order to create the interconnected pore structure of the microspheres. This has the effect that the pore size distribution of 2 to 3 nm which is the intercrystalline pore within the TiO2 microspheres, is 0.3 whereas the pore size distribution of Li, X. Z. and Liu, H. (2003, supra) is 0.7.
[0072] The photocatalytic activity of the microspheres of the present invention is improved due to the quantum size effect which is triggered by the embedded nano-sized anatase crystallites as illustrated in FIG. 5. Quantum size effect is a phenomenon which occurs for semiconductor particles on the order of 1-10 nm in size. Particles which fall within this range will have increased photoefficiencies as described by Linsebigler, A. L., Lu, G. and Yates, J. T. Jr (1995, Chem. Rev., vol. 95, pp. 735-758).
[0073] As can be seen from FIG. 3 the particles of the present invention have a very regular spherical shape and thus 'surface damage of filtration membranes in a reactor is avoided. Those TiO2 microspheres can also be used to avoid irreversible membrane fouling because they inhibit, for example, that small particles dissolved in wastewater clog the pores of the filtration membranes.
[0074] Therefore, the present invention is also directed to a process of cleaning waste water in a membrane filtration reactor, wherein the process comprises:
mixing the titanium oxide microsphere of the present invention with wastewater which is to be treated in a membrane reactor;
filtering the mixture treated in the membrane filtration reactor through the filtration membrane of the membrane filtration reactor by applying a suction force at the filtration membrane of the membrane reactor, wherein the diameter of the microspheres in the mixture is greater than the diameter of the pores of the membrane, to form a cake layer of microspheres on the surface of the filtration membrane; and continuing feeding the membrane filtration reactor with wastewater until the wastewater is cleaned.
[0078] In one example the process further comprises adding more titanium oxide microspheres into the membrane reactor when further wastewater is fed into the membrane filtration reactor.
[0079] "Wastewater" "raw water" or "sewage" includes municipal, agricultural, industrial and other kinds of wastewater. In general, any kind of wastewater can be treated using the process of the present invention. In one example, the wastewater has a total organic carbon content (TOC) of about 20 mg/l. In one example, the wastewater used in the method of the present invention has already been treated to remove trace organics or soluble organics from the wastewater.
[0080] If necessary, this process can comprise further mixing of the mixture of TiO2 microspheres and wastewater to achieve a uniform distribution of the microspheres in the wastewater. FIG. 6 illustrates the possible setup of a membrane reactor which uses the microspheres of the present invention. A good mixture of microspheres with wastewater can be achieved in membrane reactor tank by turbulence flow created, for example, by coarse diffuser located at the bottom of the membrane reactor.
[0081] In general, the microspheres are used for removal of NOM and bacteria from water. The nano-structured microspheric titanium dioxide photocatalysts are combined with the incoming raw water to form a suspension. Combination of wastewater and microspheres can take place before entering the membrane reactor shown in FIG. 6 or only upon entering the membrane reactor.
[0082] After mixing, the nano-structured microspheric titanium dioxide photocatalysts remove NOM and bacteria from water by adsorption (Fang, H., Sun, D. D., et al., 2005, supra). An immersed filtration membrane is used for separating the potable water from the nano-structured microspheric titanium dioxide photocatalyst suspension. Wastewater treated with the microspheres is passed or filtered through the membrane as permeate when suction force is applied to the filtration membrane.
[0083] The composition of the membrane and the size of the pores of the membrane may vary over a wide range, depending on the particular contaminants that are to be removed from the wastewater. Such membranes are usually submerged membranes. Such membranes can include those known in the art and which are used in microfiltration, ultrafiltration and nanofiltration systems. The pore size of such membranes is can be in the range of 0.001 to 0.1 [mu]m. Such membranes can be made of ceramics or polymers.
[0084] Examples of polymers which are used for filtration membranes are cellulose acetate, polyamide, polysulphone, polypropylene, polytetrafluoroethylene (PTFE). Examples of ceramics which are used for such filtration membranes are diatom earth, aluminium oxide, titanium oxide, titanium dioxide or zinc oxide. In one non-limiting example of the present invention, the membrane is made of ceramics, namely diatom earth of the MF type (Doulton, USA).
[0085] The TiO2 microspheres can adsorb the contaminants from wastewater due to its mesoporous structure. The particle size of the nano-structured microspheric titanium dioxide photocatalyst is preferred to be larger than the membrane pore to prevent clogging or irreversible fouling. Thus, a dynamic layer of photocatalyst is formed on the membrane surface due to suction force at the membrane which prevents the remaining portion of contaminants from water to trap within the pores of the filtration membrane. Water to be treated is continuously introduced into the tank and continued until predetermined hydraulic retention time to reach the adsorption equilibrium or the reduction in the degree of NOM removal or reduction in permeate flow is observed.
[0086] At this point, the permeation can be stopped and the membrane can be backwashed to remove the layer of spent microspheres that formed on the surface of the filtration membrane in the membrane reactor. During and/or after the backwashing step, a tangential flushing can be carried out in order to flush out the inner surface of the filtration membrane and recover the microspheres.
[0087] The spent titanium dioxide microspheres can be regenerated via PCO process (Fang, H., Sun, D. D., et al., 2005, supra). Therefore, the spent adsorbent is directed to a PCO reactor as indicated in FIG. 6. The photocatalyst is activated by UV light at 254 nm, electron and hole charge carrier pairs are produced within the photocatalyst microspheres. These charge carriers then perform oxidation/reduction (redox) reactions with the adsorbed species on the surface of photocatalyst. NOM is oxidized and bacteria are sterilized by the PCO process. A hydraulic retention time is provided for the regeneration to restore the adsorption capacity of the microspheric TiO2 photocatalyst. The regenerated microspheric TiO2 photocatalyst can than be recirculated back to the membrane reactor as shown in FIG. 6.
[0088] The integrated system of membrane filtration and PCO is a very efficient and cost-effective technology for the removal of NOM and bacteria from water. An important factor that determines the economics of this process is the increase of permeate flux in both quality and quantity, and the extended membrane lifespan that could be achieved due to the use of the TiO2 microspheres which avoid membrane fouling. The permeate flux is enhanced by factors of 1.5 when the microspheric titanium dioxide photocatalyst is added to the wastewater as can be seen from FIG. 7. The permeate quality is improved as well as is illustrated by FIG. 8.
[0089] In another aspect, the present invention is also directed to a method of manufacturing a titanium oxide microsphere having photocatalytic property, having a size of about 10 [mu]m to about 200 [mu]m and a mesoporous structure with a pore size in a range of about 2 to about 50 nm. This method comprises:
preparing a sol by mixing an organometallic titanium precursor with an alcohol without adding H2O;
aging the sol;
mixing the aged sol with titanium oxide powder;
spraying the mixture to form the titanium oxide photocatalyst microspheres;
calcining the microspheres.
[0095] By "consisting of is meant including, and limited to, whatever follows the phrase "consisting of". Thus, the phrase "consisting of" indicates that the listed elements are required or mandatory, and that no other elements may be present.
[0096] In another aspect, the present invention is directed to a submerged membrane reactor in which the titanium oxide microspheres of the present invention are mixed with the wastewater cleaned in the reactor or which utilizes the process of the present invention.
[0097] In another aspect, the present invention refers to the use of titanium oxide photocatalyst microspheres of the present invention for operation of a submerged membrane reactor.
[0098] By "comprising" it is meant including, but not limited to, whatever follows the word "comprising". Thus, use of the term "comprising" indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.
[0099] The inventions illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.
[0100] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.
[0101] Other embodiments are within the following claims and non-limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.
Examples
[0102] Manufacture of TiO2 Microspheres
[0103] In a non-limiting example of the present invention, the coating sol is prepared by dissolving 6.8 mL of tetra-n-butylorthotitanate [Ti(OC4H9)4], obtained from Merck in 58.2 mL of absolute alcohol (CH3CH2OH), obtained from Fluka. A uniform TiO2 sol is formed as component A under vigourous mixing. During this process, pH is monitored and kept around 3 by the addition of 0.5 mL of concentrated hydrochloric acid (HCl). The sol is aged for at least 24 hours by leaving it alone in a sealed bottle before further use. Then the commercial TiO2 powder (anatase=70%, surface area=50 m<-2>g<-1>, and crystal size=30 nm) supplied by Degussa, Germany is used as component B and mixed with component A at a powder:sol ratio of 5:1 by weight to form a slurry. Then the slurry is put into a spray dryer (capacity 1500 ml/h) with inlet air at 30[deg.] C. and outlet air at 30[deg.] C. In the spray dryer, the TiO2 particles aggregate to form semisolid microspheres. Finally the semisolid microspheres are calcined at higher temperature of 450[deg.] C. for 3 h to form solid TiO2 microspheres No water or any additive has been used in the preparation of these TiO2 microspheres.
[0104] Characterization of TiO2 Microspheres
[0105] Humic acid (HA) used in this study is obtained from Fluka Chemical. Humic acids in water are harmful compounds with a complex nature composed of carboxylic, phenolic and carbonyl functional groups. These substances cause a brown-yellow color in water and are known to be the precursor of carcinogenic halogenated compounds formed during the chlorination disinfection of drinking water. A concentrated HA solution is first prepared by mixing 4 grams of HA and 50 ml of 1 M NaOH in approximately 1.5 litres of pure water for at least 30 minutes with a magnetic stirrer. When completely mixed, the solution was diluted to 2 l in a volumetric flask. The solution was filtered through a 0.45 [mu]m membrane filter. The HA concentrated solution was refrigerated and used as needed. The concentrations of HA in this study were obtained by diluting the concentrated solution. The pH of the solution was adjusted by HCl and NaOH with a calibrated pH meter.
[0106] Different TiO2 dosages are used together with this solution of humic acid, at fixed transmembrane pressures (0.3 Mpa). The humic acid is prepared at 20 ppm concentration, with 50 ppm of Ca<2+>, at a pH of 7. Polysulfone membrane with molecular weight cut-off 50 k is used in the membrane filtration study. The filtration study is carried out using a lab-scale membrane filtration unit supplied by Nitto Denko with a maximum operating pressure at 0.6 Mpa. The flowrate, flux (for the permeate) and pH (the permeate and feed) is recorded every 15 mins for the first hour, every 30 mins in the second hour and every one hour from the third hour till the fifth hour. FIG. 7 shows the graphical illustration of the effect of titanium dioxide particles on permeates flux through a membrane. Samples of permeate are also collected during the operation. The optical absorption spectrum on the HA is determined by a Perkin Elmer (USA) Lambda Bio 20 spectrophotometer. The absorbance at 400 nm is selected for quantitative analysis of color. FIG. 8 shows the graphical illustration of the permeate quality during the membrane filtration process.
[0107] A PCO test is carried out to compare the photocatalytic activity of nano-structured TiO2 microspheres of the present invention and commercial P25 TiO2 under the same conditions. Phenol (analytical grade) with an initial concentration of 100 ppm was chosen as the targeted compound. The choice of this compound was motivated by the fact that phenol normally exhibits weak adsorption on TiO2 particles. PCO study is carried out in a cylindrical PCO reactor. The UV lamp used is a low pressure mercury UV lamp. The major emission of the UV lamp is 253.7 nm. An air pump is used to supply oxygen and create air bubbles for suspensions. TiO2 microsphere is suspended in 1000 mL phenol solution in dark for 0.5 hours to attain well mixed condition before the PCO study set off. Samples are taken at different time intervals after UV light has been turned on. All samples are filtered through 0.45 [mu]m membrane filters prior to the analysis.
[0108] FIG. 5 shows the results of this experiment. FIG. 5 shows the degradation of phenol as a function of the irradiation time at pH 7. It can be observed that the degradation of phenol followed an exponential decay form. Blank study (absence of photocatalyst) is carried out as a background check. About 40% of phenol is degraded after 60 min of UV light irradiation with the absence of the TiO2 microsphere of the present invention. The removal efficiency is greatly enhanced when the TiO2 microsphere of the present invention is added into the solutions. The removal efficiency after 60 min UV light irradiation is 60% and 70% for P25 TiO2 and TiO2 microsphere of the present invention, respectively. It is obvious that the TiO2 microspheres possess a better photocatalytic activity than P25 TiO2. C0 shows the initial concentration in FIG. 5.
WO2011133114
METHOD OF PRODUCING PURIFIED WATER AND APPARATUS THEREFOR
Inventor(s): SUN DELAI DARREN; LIU ZHAOYANG
Applicant(s): UNIV NANYANG TECH
Abstract -- A method of producing purified water from a water source which is suspected to contain impurities is disclosed. The method comprises: (a) contacting the water source with a draw solution, wherein the water source and the draw solution are separated by a semipermeable membrane that is permeable for water but essentially impermeable for solutes contained in the water and wherein the draw solution contains a chemically precipitable water-soluble salt in a concentration that allows the draw solution to draw water from the water source by osmosis; (b) allowing water from the water source to flow through the semipermeable membrane to the draw solution, thereby diluting the draw solution and concentrating the water source; (c) producing purified water from the diluted draw solution by: (i) adding a precipitant to the diluted draw solution to precipitate the salt, wherein the precipitated salt thus formed comprises a plurality of gel particles dispersed in water; (ii) adding a coagulant to the plurality of gel particles to coagulate the plurality of gel particles; and (iii) separating the coagulated gel from the water.
Cross-Reference to Related Application
[0001] This application claims the benefit of priority of United States Provisional Application No. 61/326,909, filed 22 April 2010, the contents of which being hereby incorporated by reference it its entirety for all purposes.
Technical Field
[0002] The invention relates to a method of producing purified water from a water source which is suspected to contain impurities, and in particular, to a method of producing purified water via a forward osmosis process. An apparatus for carrying out the purification method is also provided.
Background
[0003] Water scarcity has now become one of the most pressing challenges to human civilization in the earth. Growing population and industrial activities, increasing living standard and changing climate are placing extreme pressure on the already-scarce drinking water and energy resources. Wastewater reuse and seawater desalination are few options to increase drinking water availability. However, current technologies for producing purified drinking water from such resources require substantial energy input, which at some extent worsen the vicious cycle of greenhouse gas emissions and climate change. In order to address these problems, tremendous effort has been taken to identify new technologies for producing purified drinking water at lower energy input capacity in an environmentally sustainable manner. [0004] In the field of drinking water production, reverse osmosis (RO) membrane process has been widely used globally. The process employs high hydralic pressure and internal flow circulation to force water through a semipermeable membrane from a high concentration solution to a low concentration solution. This is the 'reverse' of the water's natural osmosis tendency. Heavy consumption of electrical energy for creating high hydralic pressure, coupled with severe membrane fouling problem, has led to the investigation and development of alternative approaches to the RO process.
[0005] In contrast to the RO process, forward osmosis (FO) process makes use of the natural osmosis phenomenon for the transport of water from a feed solution with a low solute concentration to a draw solution with high solute concentration across a semipermeable membrane, with the osmotic pressure difference between the feed and the draw solutions as the driving force. After water naturally permeates into the draw solution, the diluted draw solution can then be recycled for reuse while high quality product water can be produced. Driven by an osmotic pressure gradient, the FO process does not require significant energy input, only stirring or pumping of the feed and draw solutions are involved.' Meanwhile, the FO process offers the advantages of less membrane fouling tendency and high rejection of solutes. Recently, the FO process has attracted great attention in seawater desalination, wastewater reclamation, food and pharmaceutical processing, and power generation. However, the major obstacle of adopting the FO process industrially and globally in the application for producing purified drinking water is a lack of low energy methods to separate draw solutes and the purified water from the diluted draw solutions. Summary
[0006] Various embodiments provide for a method to separate draw solutes and purified water from a draw solution without a need for considerable amount of energy such as heat and pressure. The method employs a forward osmosis process thereby using much less energy than a reverse osmosis process.
[0007] Various embodiments provide for a method of producing purified water from a water source which is suspected to contain impurities. The method may include:
(a) contacting the water source with a draw solution, wherein the water source and the draw solution are separated by a semipermeable membrane that is permeable for water but essentially impermeable for solutes contained in the water and wherein the draw solution contains a chemically precipitable water-soluble salt in a concentration that allows the draw solution to draw water from the water source by osmosis;
(b) allowing water from the water source to flow through the semipermeable membrane to the draw solution, thereby diluting the draw solution and concentrating the water source;
(c) producing purified water from the diluted draw solution by:
(i) adding a precipitant to the diluted draw solution to precipitate the salt, wherein the precipitated salt comprises a plurality of gel particles dispersed in water;
(ii) adding a coagulant to the plurality of gel particles to coagulate the plurality of gel particles; and
(iii) separating the coagulated gel from the water. [0008] Various embodiments provide for an apparatus for producing purified water from a water source which is suspected to contain impurities. The apparatus may include:
(a) a first container configured to receive a semipermeable membrane placed therein thereby forming a first portion for containing the water source and a second portion for containing a draw solution, wherein the semipermeable membrane is permeable for water but essentially impermeable for solutes contained in the water and wherein the draw solution contains a chemically precipitable water-soluble salt in a concentration that allows the draw solution to draw water from the water source by osmosis to thereby form a diluted draw solution; and
(b) a second container for containing the diluted draw solution, wherein the second container is fluidly connected to the second portion of the first container, wherein the second container is configured to:
(i) receive a precipitant feed wherein a precipitant is added to the diluted draw solution to precipitate the salt, wherein the precipitated salt thus formed comprises a plurality of gel particles dispersed in water;
(ii) receive a coagulant feed wherein a coagulant is added to the plurality of gel particles to coagulate the plurality of gel particles; and
(iii) separate the coagulated gel from the water.
[0009] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawing.
[0010] Fig. 1 shows a schematic diagram of the method of producing purified water via a forward osmosis process.
Description
[0011] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised. These embodiments are described in sufficient detail to enable those skilled in the art to practise the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.
[0012] Osmosis is defined as the net movement of water across a selectively permeable membrane driven by a difference in osmotic pressure across the membrane. A selectively permeable membrane (or semipermeable membrane) allows passage of water molecules, but rejects solute molecules or ions. The semipermeable membrane filters the impurities from a water source (feed solution) which is suspected to contain impurities, leaving purified water on the other side (permeate side) of the membrane called permeate water. The impurities left on the membrane may be washed away by a portion of the feed solution that does not pass through the membrane. The feed solution carrying the impurities washed away from the membrane is also called "reject" or "brine".
[0013] Various embodiments of the invention make use of a forward osmosis (FO) process developed as an alternative membrane technology for wastewater treatment due to the low energy requirement as a result of low or no hydraulic pressure applied, high rejection of a wide range of contaminants, and low membrane fouling propensity compared to pressure-driven membrane processes, such as reverse osmosis (RO). FO process uses the osmotic pressure differential across the membrane, rather than hydraulic pressure differential (as in RO processes) as the driving force for transport of water through the membrane. The FO process results in concentration of the feed solution and dilution of a highly concentrated stream (referred to as the draw solution). In other words, the FO process utilizes the natural osmosis phenomenon, which makes use of concentration differences between the two solutions across a semipermeable membrane. The semipermeable membrane acts as a selective barrier between the two solutions, and dominates the efficiency of freshwater transportation in the FO process. A concentrated draw solution on the permeate side of the membrane is the source of the driving force in the FO process. Different terms are used in the literature to name this solution including draw solution, osmotic agent, or osmotic media to name only a few. In a FO process the draw solution has a higher osmotic pressure than the feed solution (or reject or brine).
[0014] The semipermeable membranes used may be polymer-based. Further, the membranes may have a dense selective layer embedded onto a support layer for providing rejection of dissolved compounds and providing mechanical strength respectively.
[0015] Various embodiments provide for a method of producing purified water from a water source which is suspected to contain impurities, and an apparatus therefor. Fig. 1 illustrates a schematic flow diagram in one embodiment. The water source shown in this illustration is wastewater.
[0016] The apparatus may include a first container 10 configured to receive a semipermeable membrane 12 placed therein thereby forming a first portion 10a for containing the wastewater and a second portion 10b for containing a draw solution. Wastewater may be fed to the first portion 10a by any suitable means, such as a pipe. The draw solution may be fed to the second portion 10b by any suitable means, such as a pipe. Sufficient time is given to allow water from the water source to flow through the semipermeable membrane to the draw solution, thereby diluting the draw solution and concentrating the water source.
[0017] The semipermeable membrane 12 is permeable for water but essentially impermeable for solutes contained in the water of the draw solution. The semipermeable membrane 12 may be of any suitable type conventionally used in forward osmosis or reverse osmosis process. The membrane materials for such semipermeable membranes can be cellulose, polysulfone, polyethersulfone, and polyamide.
[0018] The draw solution contains a chemically precipitable water-soluble salt in a concentration that allows the draw solution to draw water from the wastewater by osmosis to thereby form a diluted draw solution. The diluted draw solution may be withdrawn from the second portion 10b by any suitable means, such as a pipe. The wastewater may be withdrawn from the first portion 10a by any suitable means, such as a pipe, and is now concentrated with impurities that cannot pass through the semipermeable membrane 12.
[0019] The salt is soluble in the water of the draw solution and can be precipitated upon addition of a precipitant.
[0020] In various embodiments, the chemically precipitable water-soluble salt may be selected from the group consisting of aluminium sulfate, magnesium sulfate, manganese sulfate, iron chloride, iron sulphate, and aluminium chloride.
[0021] In the embodiment shown in Fig. 1, the draw solution contains (A12(S04)3) as the chemically precipitable water-soluble salt, which is commercially available.
[0022] The apparatus may also include a second container (or separator) 14 for containing the diluted draw solution. The second container 14 may be fluidly connected via a pipe to the second portion 10b of the first container 10 so that the diluted A12(S04)3 draw solution (or A12(S04)3 recycle) withdrawn from the second portion 10b is fed to the second container 14 for separation. The second container 14 may be configured to receive a precipitant feed whereby a precipitant is added to the diluted draw solution to precipitate the salt, and whereby the precipitated salt thus formed may include a plurality of gel particles dispersed in water. The precipitant may be added to the second container 14 via a separate pipe, for example. Alternatively, the precipitant may be added to the A12(S04)3 recycle stream before entering the second container 14.
[0023] In various embodiments, the precipitant may be selected from the group consisting of calcium oxide, calcium hydroxide, sodium hydroxide, potassium hydroxide, barium oxide, and barium silicate.
[0024] In the embodiment shown in Fig. 1, the precipitant is calcium oxide (CaO), which is commercially available.
[0025] The second container 14 may be further configured to receive a coagulant feed wherein a coagulant is added to the plurality of gel particles to coagulate the plurality of gel particles. The coagulant may be fed to the second container 14 by any suitable means, such as a pipe. The second container 14 may also be configured separate the coagulated gel from the water, thereby producing purified water separated from the diluted draw solution.
[0026] In various embodiments, the coagulant may be negatively charged.
[0027] In various embodiments, the coagulant may be in the form of a particulate.
The size of the particulate coagulate may be from about 1 nm to about 100 [mu][pi][iota].
[0028] In various embodiments, the coagulant may be selected from the group consisting of activated silica, anionic polyelectrolyte, iron (III) oxide, zeolite and silicate.
[0029] In one embodiment, the coagulant may be sodium silicate.
[0030] In another embodiment, the coagulant may be anionic polyacrylamide.
[0031] In further embodiments, the coagulant may also be magnetic. The coagulant may consist of a core/shell nanoparticle. In one such embodiment, the coagulant may consist of an iron (III) oxide core and an activated silica shell.
[0032] In various embodiments where the chemically precipitable soluble salt is aluminium sulfate and the precipitant is calcium oxide, each gel particle is a mixture of aluminium hydroxide and calcium sulfate. When aluminium hydroxide is precipitated in water as the solvent, the hydrated aluminium hydroxide forms gels which are positively charged. Due to such electrical charges, each gel particle repels one another and is dispersed in water as a plurality of gel particles. A negatively charged sodium silicate (Na2Si03) is added as a coagulant to coagulate the gel particles of aluminium hydroxide and calcium sulfate. The coagulated gel mixture of aluminium hydroxide, calcium sulfate, and sodium silicate clump together and form a floe. The floe may be deposited to the bottom of the diluted draw solution and is withdrawn while the top liquid is removed as purified water from the second container 14. [0033] The second container 14 may be further configured (not shown) to receive an acid feed, wherein the acid is added to the coagulated gel to recover the chemically precipitable water-soluble salt.
[0034] In various embodiments, the acid may be selected from the group consisting of sulfuric acid, chloric acid, and nitric acid.
[0035] In one embodiment where the chemically precipitable water-soluble salt is aluminium sulfate, the acid is sulfuric acid.
[0036] The recovered chemically precipitable water-soluble salt aluminium sulfate may then be reused in the draw solution again.
[0037] Calcium sulfate present in the floe may be subsequently precipitated as a useful by-product. Calcium sulfate has wide applications in construction, fertilizer and biomedicine.
[0038] The coagulant may also be subsquently recovered from the floe and reused in the purification process again.
[0039] The concentrated wastewater withdrawn from the first portion 10a may be rich in organic matters and may be fed to an anaerobic reactor 16 to generate a biogas (methane) as a useful fuel.
[0040] In order that the invention may be readily understood and put into practical effect, particular embodiments will now be described by way of the following non- limiting examples.
Examples
[0041] The following illustrates examples of various negatively charged coagulants used to coagulate positively charged gel particles dispersed in water. The draw solution consists of aluminium sulfate as the chemically precipitable soluble salt and the precipitant is calcium oxide. The resultant gel particles are mixtures of aluminium hydroxide and calcium sulfate, and are positively charged.
[0042] Fe304@Si02 core/shell nanoparticles that are negatively charged and magnetic are added to the diluted draw solution to coagulate the gel particles. The floe is formed by ionic binding between the positively charged gel particles and the negatively charged Fe304@Si02 coagulant. The floe is then easily and fully removed under application of an external magnetic field. When placed near a permanent magnet, the draw solution becomes clear within minutes and the floe is deposited near the magnetic field due to the magnetic coagulant particles. The clear top liquid is then taken out as the final water product. The roles of the negatively charged and magnetic Fe304@Si02 core/shell nanoparticles are to both coagulate the gel particles and to increase the rate of separation via a magnetic field.
Example 2: Anionic Polyacrylamide (PAM) as Coagulant
[0043] Anionic polyacrylamide (PAM) is added to the diluted draw solution to coagulate the gel particles. The floe is formed by ionic binding between the positively charged gel particles and the negatively charged PAM coagulant. The floe is then precipitated to the bottom of the draw solution. The clear top liquid is then taken out as the final water product.
Example 3: Sodium Silicate (NaiSiOV) as Coagulant
[0044] Sodium silicate (Na2Si03) is added to the diluted draw solution to coagulate the gel particles. The floe is formed by ionic binding between the positively charged gel particles and the negatively charged Na2Si03 coagulant. The floe is then precipitated to the bottom of the draw solution. The clear top liquid is then taken out as the final water product.
[0045] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come .within the meaning and range of equivalency of the claims are therefore intended to be embraced.
WO2011133116
METHOD OF PREPARING A NANOCOMPOSITE MEMBRANE
Inventor(s): SUN DELAI DARREN [SG]; LIU ZHAOYANG [SG] +
Applicant(s): UNIV NANYANG TECH [SG] +
Abstract -- The present invention relates to a method of preparing a nanocomposite membrane, comprising: (a) providing a nanocomposite solution comprising a polymer solution and nanomaterials; (b) subjecting the nanocomposite solution to a cold water bath to produce the nanocomposite membrane in a gel-like form; and (c) subjecting the gel nanocomposite membrane to a heat treatment to solidify the nanocomposite membrane, wherein the nanomaterials are dispersed within the polymer matrix of the nanocomposite membrane.
Cross-Reference to Related Application
[0001] This application claims the benefit of priority of United States Provisional Application No. 61/326,896, filed 22 April 2010, the contents of which being hereby incorporated by reference it its entirety for all purposes.
Technical Field
[0002] The invention relates to a method of preparing a nanocomposite semipermeable membrane for forward osmosis applications in wastewater treatment, seawater desalination, food and pharmaceutical processing, and power generation.
Background
[0003] Forward osmosis (FO) process makes use of the natural osmosis phenomenon for the transport of water from a low solute concentration feed solution to a high solute concentration draw solution across a semipermeable membrane, with the osmotic pressure difference between the feed and the draw solution acting as the driving force. After the water naturally permeates into the draw solution without external energy input, the diluted draw solution can then be recycled for reuse in the FO process while high quality water product can be produced.
[0004] The FO process has recently shown great potential for wastewater reuse and seawater desalination. It has various advantages over current membrane technology, especially the reverse osmosis (RO) process. These advantages include a lower energy consumption needed for the osmosis process and a lower fouling potential of the membrane. One of the most important challenges in utilizing the FO process for practical applications is the lack of such suitable membrane. Water permeate flux obtained with current FO membrane is usually lower than expected. The low permeate flux attributes to the severe internal concentration polarization (ICP) effect occurring in the porous support layer of the membrane.
[0005] Due to the limitations posed by the severe ICP effect on water permeate flux for current forward osmosis membranes, new forward osmosis membranes with superior water permeate flux and high solute rejection rate are desired.
Summary
[0006] Various embodiments provide for methods of preparing nanocomposite semipermeable membranes that contain dispersed nanomaterials within the polymer matrix for forward osmosis applications such as wastewater treatment, seawater desalination, food and pharmaceutical processing, power generation, and the like. With the addition of nanomaterials dispersed within the polymer matrix of the porous support layer of the membranes, significant increase of water permeate flux is achieved, while high solute rejection rate is maintained, compared to membranes having no added nanomaterials.
[0007] Various embodiments provide for a method of preparing a nanocomposite membrane. The method may include:
(a) providing a nanocomposite solution comprising a polymer solution and nanomaterials;
(b) subjecting the nanocomposite solution to a cold water bath to produce the nanocomposite membrane in a gel-like form; and
(c) subjecting the gel nanocomposite membrane to a heat treatment to -solidify the nanocomposite membrane, wherein the nanomaterials are dispersed within the polymer matrix of the nanocomposite membrane.
[0008] Various embodiments provide for a nanocomposite membrane which may include:
- a dense selective layer, wherein the dense selective layer is permeable to water but is impermeable to solutes; and
- a porous support layer, wherein the porous support layer comprises nanomaterials dispersed within a polymer matrix.
Brief Description of the Drawings
[0009] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.
[0010] Fig. 1 shows a schematic diagram of the casting process for preparing a flat sheet membrane.
[0011] Fig. 2 shows a schematic diagram of the spinning process for preparing a hollow fiber membrane.
[0012] Fig. 3 shows the TEM of Ti02 nanotubes prepared in the Examples.
[0013] Fig. 4 shows the cross-sectional view of a flat sheet nanocomposite forward osmosis membrane.
[0014] Fig. 5 shows the cross-sectional view of a hollow fiber nanocomposite forward osmosis membrane. [0015] Fig. 6 shows the water permeate flux comparisons of different flat sheet forward osmosis membranes. HTI(TM) denotes a commercial forward osmosis membrane from Hydration Technologies Inc.; CA denotes a cellulose acetate forward osmosis membrane prepared without nanomaterials; Nanocomposite-Cellulose acetate denotes a nanocomposite forward osmosis membrane prepared with Ti02 nanotubes.
[0016] Fig. 7 shows the water permeate flux comparisons of different hollow fiber forward osmosis membranes.
Description
[0017] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised. These embodiments are described in sufficient detail to enable those skilled in the art to practise the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.
[0018] Osmosis is defined as the net movement of water across a selectively permeable membrane driven by a difference in osmotic pressure across the membrane. A selectively permeable membrane (or semipermeable membrane) allows passage of water molecules, but rejects solute molecules or ions. The semipermeable membrane filters the impurities from a water source (feed solution) which is suspected to contain impurities, leaving purified water on the other side (permeate side) of the membrane called permeate water. The impurities left on the membrane may be washed away by a portion of the feed solution that does not pass through the membrane. The feed solution carrying the impurities washed away from the membrane is also called "reject" or "brine".
[0019] Various embodiments of the invention make use of a forward osmosis (FO) membrane developed as an alternative membrane technology for wastewater treatment due to the low energy requirement as a result of low or no hydraulic pressure applied, high rejection of a wide range of contaminants, and low membrane fouling propensity compared to pressure-driven membrane processes, such as reverse osmosis (RO). FO process uses the osmotic pressure differential across the membrane, rather than hydraulic pressure differential (as in RO processes) as the driving force for transport of water through the membrane. The FO process results in concentration of the feed solution and dilution of a highly concentrated stream (referred to as the draw solution). In other words, the FO process utilizes the natural osmosis phenomenon, which makes use of concentration differences between the two solutions across a semipermeable membrane. The semipermeable membrane acts as a selective barrier between the two solutions, and dominates the efficiency of freshwater transportation in the FO process. A concentrated draw solution on the permeate side of the membrane is the source of the driving force in the FO process. Different terms are used in the literature to name this solution including draw solution, osmotic agent, or osmotic media to name only a few. In a FO process the draw solution has a higher osmotic pressure than the feed solution (or reject or brine).
[0020] Various embodiments provide for a method of preparing a nanocomposite membrane. The method may be a phase inversion type for preparing asymmetric nanocomposite membranes. The membranes may be polymer-based. Further, the membranes may have a dense selective layer disposed adjacent a porous support layer whereby the dense selectively layer is used for rejecting solutes in the wastewaste stream, for example, and the porous support layer is used for providing mechanical strength to the membrane.
[0021] The method may include providing a nanocomposite solution comprising a polymer solution and nanomaterials. The polymer solution is first prepared in a suitable solvent or system of solvents. Suitable solvent or system of solvents include solvents that are miscible with the polymer so that a homogeneous solution may be formed thereafter. In one embodiment, the system of solvents may include N-methyl-2- pyrrolidone, Dimethylformamide, acetone and water (see Examples below).
[0022] In various embodiments, the polymer of the polymer solution is a natural polymer. Examples of the natural polymer include cellulose-based polymers such as cellulose acetate, cellulose triacetate, cellulose acetate proprianate, cellulose butyrate, cellulose acetate propionate, cellulose diacetate, cellulose dibutyrate, cellulose tributyrate, hydroxypropyl cellulose, and nitrocellulose. In one embodiment, the polymer in the polymer solution is cellulose acetate.
[0023] In various embodiments, the polymer of the polymer solution is a synthetic polymer. Examples of the synthetic polymer include polyamide, polybenzimidazole, polyethersulfone, polysulfone, polyvinyl alcohol, polyvinyl pyrrole, polyvinyl pyrrolidone, polyethylene glycol, saponified polyethylene-vinyl acetate copolymer, triethylene glycol, and diethylene glycol.
[0024] In various embodiments, the nanomaterials may be nanoparticles, nanofibers, nanowires, nanotubes, or nanospheres. In one embodiment, the nanomaterials are nanotubes. In another embodiment, the nanomaterials are nanoparticles.
[0025] In various embodiments, the nanomaterials are inorganic materials. Examples of suitable inorganic materials include titanium, silicon, aluminium, zirconium, indium, tin, magnesium, calcium, including the respective oxide thereof and - alloy thereof. In one embodiment, the nanomaterials may be titanium oxide nanotubes or nanoparticles. In another embodiment, the nanomaterials may be tetraethyl orthosilicate.
[0026] In various embodiments, the nanomaterials may be mesoporous materials. Examples of suitable mesoporous materials include aluminosilicate, aluminophopsphate, and zeolite.
[0027] In various embodiments, the nanomaterials may be organic materials. Examples of suitable organic materials include dendrimers, graphite, graphene, carbon nanotube, and fullerene.
[0028] In various embodiments, the nanomaterials are present in the polymer matrix in the range of about 0.1 wt% to about 20 wt% (based on total weight of the polymer matrix).
[0029] In various embodiments, the nanomaterials are in the size from about 1 nm to about 1,000 nm.
[0030] The method may further include homogenizing the nanocomposite solution. In various embodiments, the nanocomposite solution may be stirred continuously until a homogenized nanocomposite solution is obtained. The stirring may be carried out via a mechanical stirrer or magnetic stirrer, for example. In various embodiments, the stirring is continued for more than one day. In one embodiment, the stirring is continued for about two days.
[0031] In various embodiments, the obtained homogenized nanocomposite solution may be subjected to an ultrasound bath to remove air bubbles or partially evaporate volatile components of the solvent. The volatile components may be vaporized at room temperature.
[0032] The method may also include -subjecting the -homogenized nanocomposite solution to a cold water bath to produce the nanocomposite membrane in a gel-like form. The method may further include subjecting the gel nanocomposite membrane to a heat treatment to solidify the nanocomposite membrane, whereby the nanomaterials are dispersed within the polymer matrix of the nanocomposite membrane.
[0033] In various embodiments, after obtaining the homogenized nanocomposite solution and prior to subjecting the homogenized nanocomposite solution to a cold water bath, the homogenized nanocomposite solution may be cast on a plate to form a cast film. For example, Fig. 1 illustrates the general process used for preparing nanocomposite membranes by solution casting. Phase inversion is used for the preparation of flat sheet membranes. In the polymer solution casting, a polymer solution, comprising polymers and nanomaterials (0.1 to 20 wt%, based on total weight of the polymer matrix) in solvents, is stirred continuously until the solution become homogenous. The polymer solution is then casted on a glass plate using a casting knife. The cast film was then immersed into the cold water bath (or coagulation bath) to complete the phase inversion. The membranes were then post-treated in a hot water bath.
[0034] In alternative embodiments, after obtaining the homogenized nanocomposite solution and prior to subjecting the homogenized nanocomposite solution to a cold water bath, the homogenized nanocomposite solution may be extruded through a hollow fiber spinneret. For example, Fig. 2 illustrates the general process used for preparing nanocomposite membranes in the form of hollow fiber by dry- wet spinning. In the dry- wet spinning process, the polymer solution (same as that used for preparing the flat sheet membranes) flows through the ring nozzle of the spinneret while a bore fluid flowed through the inner tube of the spinneret. The fiber passes through a controlled environment air gap before entering -the cold water bath (or coagulation -bath). The hollow fiber filament then passes through a series of rollers in the coagulation bath. Following that, it is then passed through a washing bath. The fully formed hollow fiber is then continuously collected on a wind up drum. Prior to evaluation test, the membranes are subjected to a pretreatment process to reduce pore size of the membranes. In the pretreatment process, the membranes are immersed in a water bath at room temperature and the water bath with the membranes in it are gradually heated from ambient temperature to about 60 to 100 [deg.]C range in about 20 to 30 minutes. The final temperature is kept constant for about 10 minutes. The water bath together with the membranes are then cooled drastically to below 60 [deg.]C by pouring cold water directly into the bath to freeze the membrane structure. The membranes are then ready to undergo solvent exchange to prevent fibers from collapsing during evaluation tests.
[0035] In various embodiments, the nanocomposite membrane prepared thereof by the above-described method may be a porous support layer for providing mechanical strength to subsequent layers disposed thereon. In various embodiments, the nanocomposite membrane may include a porous support layer and a dense selective layer. The dense selective layer is disposed adjacent the porous support layer. The dense selective layer is permeable to water but is impermeable to solutes. The porous support layer comprises nanomaterials dispersed within a polymer matrix. The porous support layer may provide mechanical strength to the nanocomposite membrane. Both the porous support layer and the dense selective layer may be formed of the same polymer. Alternatively, the polymer of the porous support layer and the dense selective layer is different.
[0036] In various embodiments, the membrane may include a smooth layer disposed adjacent the dense selective layer and away from the porous support layer. The smooth layer may be used to prevent or minimize membrane fouling. The materials for the smooth layer can be polyvinyl acetate and polyvinylpyrrolidone.
[0037] In various embodiments, the membrane may also include an intermediate layer between the dense selective layer and the porous support layer. The intermediate layer may be a polymer which is the same as the dense selective layer, the same as the porous support layer, the same as both the dense selective layer and the porous support layer, or different from any of the layer. The mateials for the intermediate layer can be cellulose, polysulfone, polyethersulfone and polyacrylonitrile.
[0038] In various embodiments where the membrane include a dense selective layer and a porous support layer, the dense selective layer may be formed and disposed on the porous support layer by a method selected from the group consisting of phase inversion, interfacial polymerization, spray coating, and dip coating. In various embodiments, the dense selective layer may be formed simultaneously with the porous support layer. In alternative embodiments, the dense selective layer is subsequently formed on the porous support layer.
[0039] In one embodiment, the nanocomposite membranes may be used in forward osmosis processes. For example, the nanocomposite membranes may be used in wastewater treatment, seawater desalination, food and pharmaceutical processing, or power generation via forward osmosis processes.
[0040] The water permeate flux of the nanocomposite membranes were found to be dependent on the amount of nanomaterials used. Compared with the forward osmosis membrane fabricated without nanomaterials, a breakthrough in the water permeate flux of the nanocomposite forward osmosis membranes was achieved, with almost unchanged solute rejection performance.
[0041] In order that the invention may be readily understood and put into<^>practical <~> effect, particular embodiments will now be described by way of the following non- limiting examples.
Examples
Forward Osmosis Tests of Nanocomposite Membranes
[0042] In the following Examples, membranes prepared according to the invention were tested in forward osmosis mode without hydraulic pressure. Each membrane was tested in a crossflow mode with 1M A12(S04)3 as draw solution and DI water as feed solution. The feed solution was directed against the dense selective layer of the membranes and the draw solution was directed against the support layer. Both the feed and draw solutions were maintained at room temperature 25 [deg.]C.
[0043] Results of the solute rejection rate by the nanocomposite membranes of the following Examples are shown in Table 1 below.
Table 1: Solute Rejection Rate of Nanocomposite Forward Osmosis Membranes
Determination Of Water Permeate Flux And Solute Rejection
[0044] In the following Examples, water permeate flux is determined by measuring the weight change of the draw solution over a selected time period. As water transports across the membrane from the feed water into the draw solution, the weight of draw solution was increased. The solute concentration is determined by Inductively Coupled Plasma Emission Spectrometer (ICP). Example 1: Flat Sheet Nanocomposite Membranes with TiO[3/4] Nanotubes Dispersed in Cellulose Matrix
[0045] Ti02 nanotubes were fabricated as follows: Commercial Ti02 (P25, degussa) powders were mixed with a 1 OM NaOH aqueous solution by magnetic stirring, then the mixture was added into a Teflon-lined stainless steel autoclave. The autoclave was heated to 160 [deg.]C from room temperature in 2 h, then maintained at 160 [deg.]C for 48 h and finally cooled to room temperature. The obtained precipitates were firstly washed with distilled water, then adjusted the pH to neutral with 0.1M HC1 solution, and finally were washed repeatedly with distilled water and ethanol until no CF was examined in the solution. After ultrasound treatment for 10 min, the Ti02 nanotubes were obtained. The transmission electron microscopy (TEM) of Ti02 nanotubes is shown in Fig. 3.
[0046] Conventional phase inversion was used for the preparation of flat sheet nanocomposite forward osmosis membranes. A solution comprising cellulose acetate (MW 30,000 g/mol, 39.8 wt% acethyl content) 22.0 wt%, acetone 66.7 wt%, water 10.0 wt%, magnesium perchlorate 1.0 wt% and Ti02 nanotubes (Fig. 3) 0.3 wt% was stirred continuously for two days. The solution was homogenized and put into an ultrasound bath for 30 min to remove air bubbles before casting. Then the casting solution was casted on a glass plate using a casting knife. The cast film was then immersed in an ice water bath to complete the phase separation. The membranes were heat-treated at 80 [deg.]C in a hot water bath for 10 min. Then the membranes were immersed in a 50 wt% glycerol water solution for another 24 h and then were dried in air at room temperature for evaluation tests.
[0047] Under the same forward osmosis experimental conditions, the nanocomposite membrane had a notably higher water flux than commercially available HTI(TM) membrane (Fig. 6).
Example 2: Flat-Sheet Nanocomposite Membranes With T1O2 Nanoparticles Dispersed In Cellulose Matrix
[0048] Conventional phase inversion was used for the preparation of flat-sheet nanocomposite forward osmosis membranes. A solution comprising cellulose acetate (MW 30,000 g/mol, 39.8 wt% acethyl content) 22.0 wt%, acetone 66.7 wt%, water 10.0 wt%, magnesium perchlorate 1.1 wt% and Ti02 nanoparticles (Degussa P25) 0.2 wt% was stirred continuously for two days. The solution was homogenized and put into an ultrasound bath for 30 min to remove air bubbles before casting. Then the casting solution was casted on a glass plate using a casting knife. The cast film was then immersed in an ice water bath to complete the phase separation. The membranes were heat-treated at 80 [deg.]C in a hot water bath for 10 min. Then the membranes were immersed in a 50 wt% glycerol solution for another 24 h and then were dried in air at room temperature for evaluation tests.
Example 3: Flat-Sheet Nanocomposite Membranes With Silica Nanoparticles Dispersed In Cellulose Matrix
[0049] Conventional phase inversion was used for the preparation of flat-sheet nanocomposite membranes. An acetate solution comprising cellulose acetate (MW 30,000 g/mol, 39.8 wt% acethyl content) 22.0 wt%, acetone 66.0 wt%, water 10.0 parts, magnesium perchlorate 1.1 wt% and Tetraethyl orthosilicate 0.9 wt% was stirred continuously for two days. The solution was homogenized and put into an ultrasound bath for 30 min to remove air bubbles before casting. Then the casting solution was casted on a glass plate using a casting knife. The cast film was then immersed in an ice water bath to complete the phase separation. The membranes were heat-treated at 80 [deg.]C in a hot water bath for 10 min. Then the membranes were immersed in a 50 wt% glycerol solution for another 24 h and then were dried in air at room temperature for - evaluation tests.
Example 4: Nanocomposite Membranes In The Form Of Hollow Fiber
[0050] Spinning of the hollow-fiber membrane was based on the dry-wet technique.
A homogenous spinning solution was prepared same as that for the flat-sheet membrane. Afterward, the dope was extruded through a hollow fiber spinneret. Both the core liquid and the external coagulant were pure water, and the external coagulant temperature was controlled at 80 [deg.]C. The air gap which is the distance between the tip of spinneret and the surface of external coagulant was kept 20 cm. After coagulation in iced water, the resulting hollow fiber was wound up with a take-up roller and rinsed with water to remove residual solvents. Then the fibers were annealed in a hot water bath at 80 [deg.]C. Finally, the fibers were immersed in a 50 wt% glycerol solution for another 24 h and then were dried in air at room temperature for evaluation tests.
[0051] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.
WO2012102678
A FORWARD OSMOSIS MEMBRANE
Inventor: SUN DARREN DELAI // LEE TZE SIONG JONATHAN
Applicant: NANO MEM PTE LTD // SUN DARREN DELAI [SG] (+2)
An apparatus for cleaning of wastewater comprising a submerged membrane biological reactor system (MBR) and a submerged membrane module (MBD) wherein the MBR is in fluid communication with the MBD for feeding excess sludge from the MBR to the MBD and wherein the MBR comprises an outlet which releases permeate passed through the membrane of the MBR and the MBD is in fluid communication with the MBR for feeding permeate passed throug the membrane of the MBD back into the MBR and comprises an outlet for releasing gas.
WO2012102677
METHOD AND APPARATUS FOR RECOVERING WATER FROM A SOURCE WATER
Inventor: SUN DARREN DELAI // LEE TZE SIONG JONATHAN
Applicant: NANO MEM PTE LTD // SUN DARREN DELAI
US2010233812
MEMBRANE MADE OF A NANOSTRUCTURED MATERIAL
Inventor:
SUN DARREN DELAI [SG]
ZHANG XIWANG [SG] (+2)
Applicant:
UNIV NANYANG
US2010133182
MICROSPHERIC TiO2 PHOTOCATALYST
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WATER RECLAMATION WITHOUT BIOSLUDGE PRODUCTION
WO2012102678
A FORWARD OSMOSIS MEMBRANE
Inventor: SUN DARREN DELAI // LEE TZE SIONG JONATHAN
Applicant: NANO MEM PTE LTD // SUN DARREN DELAI [SG] (+2)
An apparatus for cleaning of wastewater comprising a submerged membrane biological reactor system (MBR) and a submerged membrane module (MBD) wherein the MBR is in fluid communication with the MBD for feeding excess sludge from the MBR to the MBD and wherein the MBR comprises an outlet which releases permeate passed through the membrane of the MBR and the MBD is in fluid communication with the MBR