Liquid purification or separation – Filter – Material
Reexamination Certificate
1998-06-03
2001-01-23
Walker, W. L. (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500340, C210S490000, C210S652000, C210S653000
Reexamination Certificate
active
06177011
ABSTRACT:
TECHNICAL FIELD
The present invention relates to improvements in the performance of a reverse osmosis composite membrane such as performance stability and fouling tolerance for selectively separating the components of a liquid mixture. More particularly, the present invention relates to a reverse osmosis composite membrane having a high salt rejection, a high chlorine tolerance, and a high fouling tolerance, which comprises a polymer thin film on the reverse osmosis composite membrane, and to a reverse osmosis treatment method for water using the same.
Such a reverse osmosis composite membrane is suitable for manufacturing ultrapure water, desalinating brackish water, and the like, and it also can contribute to the removal and recovery of the contaminating sources or effective substances from a soil or the like, the cause of pollution in a dyeing waste water system, an electrochemical deposition paint waste water system, or a domestic waste water system to implement a waste water recycling system. In particular, it can operate stably for a long period with respect to the quality of water containing various membrane-fouling substances, such as surfactants and transition metal components including iron, which cause a decrease in flux.
BACKGROUND ART
Examples of the industrially utilized reverse osmosis membranes include Loeb type membranes described in U.S. Pat. Nos. 3,133,132 and 3,133,137 as asymmetric membranes made of cellulose acetate. On the other hand, reverse osmosis composite membranes, in which an active thin film substantially having a selective separation property is formed on a microporous support film, are known as reverse osmosis membranes having a different structure from the asymmetric reverse osmosis membranes.
Presently, a number of such reverse osmosis composite membranes, in which a thin film of polyamide obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional aromatic acid halide is formed on a support film are known (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 55-147106, Publication of Japanese Patent Application (Tokkai Sho) No. 62-121603, Publication of Japanese Patent Application (Tokkai Sho) No. 63-218208, and Publication of Japanese Patent Application (Tokkai Hei) No. 2-187135). Also, those having a thin film of polyamide obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional alicyclic acid halide formed on a support film are known (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 61-42308).
In addition, various methods for after treatment of the reverse osmosis membrane are disclosed. For example, methods using various polymers as a protective layer are disclosed (for example, Publication of Japanese Patent Application (Tokkai Sho) No. 51-13388, Publication of Japanese Patent Application (Tokkai Sho) No. 53-16372, Publication of Japanese Patent Application (Tokkai Sho) No. 62-197105, and Publication of Japanese Patent Application (Tokko Hei) No. 7-90152).
Recently, it has been expected to apply a reverse osmosis membrane to a treatment for water containing fouling substances such as various surfactants, for example, sewage. In addition to the high performance of the reverse osmosis membrane (a high salt rejection and a high water permeability), a high fouling resistance is required to maintain the desired flux for a long period. The above reverse osmosis membranes and the conventional after treatment methods are not sufficient to satisfy these two requirements. Therefore, a reverse osmosis composite membrane having a higher performance has been sought.
One fouling mechanism includes the charge condition of the membrane. For example, the surface of a cross-linked polyamide reverse osmosis membrane obtained by interfacial polymerization of polyfunctional aromatic amine and polyfunctional alicyclic acid halide has a negative charge due to the residual carboxylic acid. The membrane surface having negative charge adsorbs, for example, cationic fouling substances, decreasing the flux. Therefore, a membrane has been required that is neutral in charge and has a high water permeability and a high salt rejection.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide a reverse osmosis composite membrane that has a high salt rejection, a high water permeability, and a high fouling tolerance and permits practical desalination at a low pressure, and a reverse osmosis treatment method for water using the same.
In order to achieve the above object, the present invention provides a reverse osmosis composite membrane comprising a sponge layer and a separation layer formed on a surface of the sponge layer, wherein at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer is present in the separation layer or a surface of the separation layer is coated with at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer, and wherein a surface zeta (&zgr;) potential of the layer in which the at least one substance is present or the coating layer is controlled within±10 mV at pH 6. The surface zeta (&zgr;) potential of such a membrane can be measured by using an electrophoretic light scattering device or the like. Also, the separation layer means a skin layer.
In the reverse osmosis composite membrane of the present invention, the surface zeta potential is within±10 mV preferably in the range of pH 6 to 8, in which pH range the reverse osmosis composite membrane is used, and more preferably, in the range of pH 5to 11.
In the reverse osmosis composite membrane, it is preferable that the reverse osmosis composite membrane comprising the sponge layer and the separation layer formed on the surface of the sponge layer is a reverse osmosis composite membrane in which the absolute value of the surface zeta (&zgr;) potential is greater than±10 mV in the range of pH 5 to 11. Electroneutrality is preferred for controlling the electrical adsorption of membrane-fouling substances having a charge group present in water by the membrane.
In the reverse osmosis composite membrane, it is preferable that the at least one substance selected from the group consisting of an electrically neutral organic substance and an electrically neutral polymer is an organic substance or a polymer that has a nonionic hydrophilic group (for example, a ——OH group). Because, in addition to the electroneutrality, adsorption due to a hydrophobic interaction on the membrane is controlled when the membrane-fouling substances have a hydrophobic group.
In the reverse osmosis composite membrane, it is preferable that the organic substance or the polymer that has a nonionic hydrophilic group is polyvinyl alcohol that is water-insoluble at 25° C. and is water-soluble at 80° C. Such polyvinyl alcohol controls the adsorption of the membrane-fouling substances. On the other hand, polyvinyl alcohol that is water-insoluble at a temperature of more than 80° C. has a small number of alcohol groups, so that such polyvinyl alcohol does not tend to control the adsorption of the membrane-fouling substances.
In the reverse osmosis composite membrane, it is preferable that the polyvinyl alcohol has a saponification degree of 95% or more, because a sufficient number of alcohol groups can be obtained.
In the reverse osmosis composite membrane, it is preferable that the thickness of the organic substance or the polymer that has a nonionic hydrophilic group coating the surface of the separation layer is in the range of 0.001 to 1 &mgr;m. If the thickness is more than 1 &mgr;m, the water permeability obtained after the coating decreases significantly. If the thickness is less than 0.001 &mgr;m, a uniform coating will be difficult.
In the reverse osmosis composite membrane, it is preferable that the reverse osmosis composite membrane in which an absolute value of the surface zeta (&zgr;) potential before treatment is g
Hachisuka Hisao
Ikeda Kenichi
Akin Gump Strauss Hauer & Feld L.L.P.
Nitto Denko Corporation
Ocampo Marianne S.
Walker W. L.
LandOfFree
Composite reverse osmosis membrane having a separation layer... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Composite reverse osmosis membrane having a separation layer..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Composite reverse osmosis membrane having a separation layer... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2454080