Methods of enhancing water flux of a TFC membrane using oxidizing and reducing agents
Provided is a post-formation process for preparation of a highly permeable thin film composite membranes for reverse osmosis, particularly for use with brackish water at low energy conditions. The process includes contacting a polyamide discrimination layer of a TFC membrane with a solution containing an oxidizing agent to form a treated membrane, followed by contacting the treated membrane with a solution containing a reducing agent. The resulting membrane exhibits enhanced water flux while maintaining salt rejection. Also provided are reverse osmosis membranes prepared in accord with the method, and modules containing the highly permeable thin film composite membranes, and methods of purifying water using the membranes or modules.
1. A process to improve water flux of a thin film composite (TFC) membrane containing a polyamide discrimination layer, the method comprising:
forming a discrimination layer by interfacial polymerization;
modifying the surface chemistry after formation of the discrimination layer by:
treating the TFC membrane after formation of the discrimination layer with an oxidizing solution comprising free chlorine in an amount from about 7 ppm to about 35 ppm at a pH in the range from about 10 to about 11 for a time period of least 5 minutes to react with the polyamide discrimination layer to form an oxidized membrane;
rinsing the oxidized membrane with water;
contacting the oxidized membrane with a reducing solution containing a reducing agent at a pH from about 4 to about 8 for a time period of at least 1 minute to form a treated membrane,
rinsing the treated membrane with water; and
applying a hydrophilic polymer layer comprising polyvinyl alcohol to the surface of the treated membrane followed by application of heat to provide a heat cure of the hydrophilic polymer layer,
wherein the amount of time between the end of the time period during which the membrane is treated with the oxidizing agent, and the beginning of contacting the oxidized membrane with a reducing agent is 60 minutes or less, and
wherein the reducing solution comprises an antichlor in an amount from about 0.1 wt % to about 5 wt % and the antichlor is selected from the group consisting of an alkali-metal bisulfite, an alkaline-earth-metal bisulfite, an alkali-metal meta-bisulfite, an alkali-metal thiosulfate, an alkaline-earth-metal thiosulfate, hydrogen peroxide and combinations thereof.
2. The process of claim 1 , wherein the time period of treatment with the oxidizing solution is from about 5 minutes to about 90 minutes.
3. The process of claim 1 , wherein the time period of treatment with the reducing solution is from about 1 minute to about 60 minutes.
4. The process of claim 1 , wherein the oxidizing solution comprises molecular chlorine, hypochlorous acid, or hypochlorite ion or a combination thereof.
5. The process of claim 1 , wherein the reducing solution comprises an alkali-metal meta-bisulfate.
6. The process of claim 1 , wherein:
the oxidizing solution comprises free chlorine in an amount from about 7 ppm to about 35 ppm;
the reducing solution comprises an alkali-metal meta-bisulfate in an amount from about 0.1 wt % to about 5 wt %.
7. A reverse osmosis membrane prepared according to the process of claim 1 , the membrane exhibiting a water flux greater than the water flux of the untreated TFC membrane.
8. A reverse osmosis module, comprising a reverse osmosis membrane prepared according to the process of claim 1 spirally wound around a central perforated tube.
9. A method of purifying brackish water containing 2,000 ppm or less NaCl, comprising contacting the brackish water with a reverse osmosis membrane of claim 7 .
10. A method of purifying seawater, comprising contacting the seawater with a reverse osmosis membrane of claim 7 .