1. Field of the Invention
The present invention relates to a forward osmosis membrane for seawater desalination and a method for preparing the same and, more particularly, to a forward osmosis membrane for seawater desalination and a method for preparing the same that facilitates an inflow of water from a feed water to a draw solution to realize high water permeability and excellent contamination resistance and particularly satisfies the property of preventing the back diffusion of solutes of the draw solution in the direction of reverse osmosis, thereby being suitable for desalinating high-concentration seawater.
2. Description of the Prior Art
Forward osmosis is a membrane separation technique that uses the osmotic pressure caused by the difference in concentration between two solutions as a driving force for transport of water from lower-concentration solution to higher-concentration solution through a membrane. As the forward osmosis is just the opposite of the reverse osmosis, the preparation of a forward osmosis membrane is also distinctive from that of a reverse osmosis membrane.
The forward osmosis membrane not only facilitates an inflow of water from feed water to a draw solution through the membrane but also plays an important role in maintaining a constant concentration of the draw solute and a high osmotic pressure. For this, it is of the essence that the forward osmosis membrane is designed to have high water permeability in the direction of osmosis and not to allow the solutes of the draw solution diffused in the direction of reverse osmosis. Likewise, the membrane with minimized contamination is given precedence in the preparation of a forward osmosis membrane. The necessary characteristics of a forward osmosis membrane are as follows.
Firstly, the support layer provided in the forward osmosis membrane is to have high porosity and low pore tortuosity in order to minimize the internal concentration polarization and to increase contamination resistance.
Secondly, the thickness of the forward osmosis membrane is be minimized in order to increase the flux of water passing through the membrane.
Thirdly, hydrophilic materials are used to minimize water-aided filtration resistance.
Fourthly, the solutes of the draw solution are not allowed to diffuse from higher-concentration solution to lower-concentration solution in order to maintain the draw solution at high concentration.
As for the conventional methods for preparing a forward osmosis membrane, U.S. Patent No. 2006-0226067 discloses a preparation method for a forward osmosis membrane using cellulose triacetate as a hydrophilic material. More specifically, solutions of a same material and different concentrations were applied on a support layer 25 to 75 μm thick to form a selective layer 8 to 18 μm thick. An evaluation in the forward osmosis (FO) mode using a draw solution, the membrane completed turned out to be a forward osmosis membrane having a high flux of 11 GFD. However, the membrane undesirably allowed the solutes of the draw solution to diffuse from higher-concentration draw solution to lower-concentration feed water. Such a membrane is impractical in the situation that requires the draw solution to be maintained higher in concentration than high-concentration feed water such as seawater that has a great content of salts.
According to the International Patent No. 2008-137082, a polysulfone solution was cast on a nonwoven fabric to form a membrane like an ultrafiltration membrane. On the surface of the membrane thus obtained, a polyamide reverse osmosis membrane was prepared by carrying out an interfacial polymerization reaction of polyfunctional amine and polyfunctional acyl halide. The membrane removed of the nonwoven fabric was applied to the forward osmosis (FO) system. A property assessment in the FO mode showed that the forward osmosis membrane had a flux of 0.5 GFD and a salt rejection rate more than 99%. The forward osmosis membrane secures a salt rejection rate good enough to separate high-concentration feed water such as seawater but has limitation in practical usage because of low flux.
The membrane prepared from a polysulfone-based polymer according to the prior art is excellent in mechanical strength and thermal and chemical stability and thus can be used as a membrane material. However, this kind of membrane tends to adsorb contaminants according to the characteristic of a hydrophobic membrane, which results in a loss of separation function and consequently reduced life span.