Patent Application
20070272608
The invention can be better understood with reference to the following figures and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
The colloidal fibers of the present invention may replace powdered activated carbon in water purification systems. Presently, activated carbon is used during the purification of drinking water for taste and odor control and for the adsorption of natural and synthetic organic pollutants. Rather than being discarded in the water treatment sludge, as conventionally done with activated carbon, the fibers may be regenerated and reused. Additionally, the fibers may be used in place of an activated carbon pre-filter. When the fibers are made from the same or similar polymer as the purification membrane, superior pre-filtration is possible.
In U.S. Pat. No. 6,669,851 sulfone polymer colloids were disclosed that purified water. When these colloidal aggregates were added to water containing organic matter contaminants, the contaminants were adsorbed onto the colloids. Removal of the colloids from the water resulted in the removal of the organic matter, purifying the water. The colloids could be regenerated, or cleaned of the adsorbed organic matter, by exposing them to base. The regenerated colloids could then be reused.
The colloidal fibers of the present invention similarly may be used for the purification of water. The colloidal fibers include sulfone polymer colloidal particles having a colloidal exterior. The colloidal particles forming the fibers impart the contaminant adsorptive properties of the prior aggregates to fibers.
In one aspect, about 20% of the total surface area of the prior colloidal aggregates was lost when colloidal fibers were formed. In another aspect, colloidal fibers have total surface areas of 20 to 100 m2/g. In another aspect, colloidal fibers have total surface areas of 60 to 90 m2/g or 75 to 85 m2/g. These total surface areas are significantly higher than those of non-colloidal polysulfone fibers generally used in size exclusion-type water purification systems.
Hollow polysulfone fibers conventionally have been used for size-exclusion water purification by passing contaminated water through pores in the outer surface of the tubular fibers. If contaminated water is passed through the interior of these hollow fibers under pressure, purified water is provided on the exterior of the fiber. Conversely, when contaminated water is drawn into the tubular fiber from the exterior by vacuum, purified water is provided within the fiber.
Unlike conventional tubular fibers, the colloidal fibers of the present invention include colloidal particles, similar to the colloidal particles described in U.S. Pat. No. 6,669,851. Thus, colloidal fibers purify water in a fundamentally different way than conventional size exclusion fibers. When contaminated water flows over the exterior of the fibers of the present invention, the contaminants are adsorbed by the colloidal particles. While not wishing to be bound by any particular theory, it is presently believed that organic matter diffuses into the interior of the fibers and is adsorbed by the nanometer-sized colloids. The average interstitial distance between the colloidal particles of the present fibers range from 10 to 20 nm. These small features provide a large total surface area, as is desirable when adsorption, as opposed to mechanical exclusion, is performed by the fibers.
While the microstructure of the present fibers is different than that of the prior aggregates described in U.S. Pat. No. 6,669,851 (fibers versus aggregates), the colloidal particles that form the fibers are similar to those that previously formed the aggregates. In this way, the colloidal fibers of the present invention retain the beneficial adsorptive properties of the colloidal aggregates, while allowing the benefits of a larger, elongated microstructure. Furthermore, the colloidal fibers of the present invention may be similarly regenerated as the prior colloidal aggregates.
Compared to the prior aggregates, the present colloidal fibers are larger, having average cross-sectional widths of 35 to 400, 40 to 250, or 50 to 150 μm. Due to the irregular surfaces, average cross-sectional widths for the fibers, determined by analysis of SEM images, were used. The colloidal fibers also have a defined shape in relation to the prior aggregates, having average lengths at least three times, preferably at least 5, 10, 50, or 100 times, their average cross-sectional width.
By selecting the mass of the polysulfone polymer in relation to the mass of the solvent, colloidal fibers of varying lengths may be formed. Unless stated otherwise, all percents are on a weight/weight (w/w) basis. While larger amounts of the polysulfone polymer in relation to the amount of the solvent provides fibers having increased lengths and average cross-sectional widths, the surface area of the fibers, and their corresponding ability to adsorb organic matter, decreases. Thus, depending on the application, the length and average cross-sectional width of the fibers may be tuned in relation to the desired adsorption. In Table I below, the correlation between percent polymer, surface area, and the average interstitial distance between the particles may be seen.
Unlike the prior aggregates that moved relatively freely in liquids, thus being potentially difficult to recover from the water after purification, the present fibers may be packed into columns, formed into woven or unwoven substrates, or otherwise fixed in relation to flowing water. In one aspect, the present fibers may be mixed with or formed in the presence of other fibers, such as non-adsorption fibers, to further increase mechanical strength. Thus, the contaminated water may flow over the colloidal fibers while the fibers are held in a column or in the form of a substrate. By mechanically isolating the adsorptive colloidal particles from the water as fibers, the ease of removing and regenerating the particles in relation to those of the prior “free” aggregates may be substantially increased. Furthermore, because fibers may be packed into columns or formed into substrates, they may mechanically trap larger species, thus functioning as a filter in addition to an adsorbent.
As described in Van Nostrand's Encyclopedia of Chemistry, pp. 272-276 (Douglas M. Considine ed., Van Nostrand Reinhold Co. 1984), colloids are disperse systems with at least one particle dimension averaging in the range of 10−6 to 10−3 mm. Particles may be defined as liquid or solid. Examples include sols (dispersions of solid in liquid), emulsions (dispersions of liquids in liquids), and gels (systems, such as jelly, in which one component provides a sufficient structural framework for rigidity and other components fill the space between the structural units). Preferably, the colloidal particles of the current fibers are sols or sol-gels.
The colloidal particles of the present fibers may be provided by a synthesis that eliminates or substantially reduces the amount of propionic acid (PA) used in relation to conventional methods of producing sulfone polymer colloids. Fritzsche, et al., J. Memb. Sci., 46, pp. 135-55 (1989) describes a conventional polysulfone fiber synthesis where the addition of PA forms a Lewis acid:base pair from the N-methyl-2-pyrrolidine (NMP) and polysulfone components of the reaction mixture. When the resulting polysulfone/NMP/PA acid:base complexes are contacted with water, the NMP dissolves to break the complex and increase the kinetics of the phase separation that forms the colloids. At present, it is believed that a lack of PA significantly slows this phase separation, thus allowing the direct formation of colloidal fibers instead of aggregates.
The colloidal fibers of the present invention may be precipitated when a solution containing a sulfone polymer is added to a liquid in which the polymer has lower solubility than the solvent of the solution. The solution is formed by dissolving the sulfone polymer in a solvent or mixture of solvents that has a higher solubility toward the polymer. Various solvents, solvent mixtures, surfactants, and wetting agents may be used to tailor the morphology of the fibers. While many large-scale production methods may be used, as known to those of ordinary skill in the art, a syringe pump is appropriate on the laboratory scale. A representative laboratory scale synthesis apparatus is shown in
Polymers useful in the present invention include, sulfone homopolymers and copolymers such as polymers of polysulfone, polyethersulfone, polyphenylsulfone, and sulfonated polysulfone; homopolymers and copolymers of cellulose acetate, polyacrylonitrile (PAN), polyetherimide, and poly(vinylidene fluoride) (PVDF); and mixtures thereof. Such polymers may be purchased from AMOCO PERFORMANCE PRODUCTS, INC. (Alpharetta, Ga.) under the trade names of UDEL (polysulfone), MINDEL (sulfonated polysulfone), RADEL-A (polyethersulfone), and RADEL-R (polyphenylsulfone). They also are available from ALDRICH, Milwaukee, Wis.
Suitable average molecular weights (AMW) for polysulfone polymers useful in the present invention range preferably from 10,000 to 45,000, more preferably from 17,000 to 35,000, and most preferably from 26,000 to 27,000. Suitable average molecular weights for polyethersulfone useful in the current invention range from 8,000 to 28,000, preferably from 13,000 to 23,000, and most preferably from 16,000 to 20,000. Suitable average molecular weights for poly(vinylidene fluoride) useful in the current invention range preferably from 100,000 to 600,000, more preferably from 180,000 to 534,000, and most preferably from 275,000 to 530,000. Suitable average molecular weights for polyacrylonitrile useful in the current invention range preferably from 30,000 to 150,000, more preferably from 60,000 to 110,000, and most preferably from 80,000 to 90,000. All average molecular weights are weight average molecular weights.
The solution containing the polymer includes the polymer and one or more solvents in which the polymer demonstrates solubility. Any solvent that permits colloid formation when the polymer solution is added to a liquid in which the polymers have lower solubility may be used. Suitable solvents include N-methyl pyrrolidine (NMP), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, and dioxane, and are available from ALDRICH, Milwaukee, Wis.
Additionally, surfactants may be added to the solution to stabilize the fibers and otherwise vary their morphology. While any surfactant, including anionic, cationic, or non-ionic, may be used, preferable surfactants include sodium lauryl sulfate, TRITON X-45, and TRITON X-100, or mixtures thereof. Wetting agents, such as alcohols, may also be added.
When the fibers of the present invention are added to water containing organic matter, the contaminating organic matter may be adsorbed. The addition may be carried out in any appropriate vessel, reactor, column, and the like. Although variables, including temperature, contaminant concentration, and fiber concentration affect the rate of adsorption, the organic matter is typically adsorbed onto the fibers within minutes to hours, preferably within minutes.
Organic matter includes hydrocarbons, hydrophobic pollutants, or pollutants with mixed hydrophobic/hydrophilic properties that pollute water by imparting an undesirable color, taste, odor, or toxicity, as well as any other carbon-containing compound. Preferably, the present invention removes natural organic matter, such as the carbon containing material typically found in drinking water supplies. Although many types of organic matter contaminants may be found in water, humic acid is one of the most common. Other organic matter contaminants include benzene, toluene, proteins, polyscaharides, lipids, geosmin (a natural organic compound leached from soils), 2-methylisoborneol (MIB) (a natural organic compound of aquatic biological origin), membrane foulants, and endocrine disrupters, such as Atrazine, DDT, dioxin, estradiol, estrone, and testosterone. In the case of the hormones, removal may involve absorption rather than adsorption, as these pollutants have a high relative partitioning in polysulfone Purified water is formed by passing the contaminated water over the fibers.
Generally, membranes fouled with organic matter may be cleaned with acids, bases, or surfactants. In one aspect, the present fibers may be regenerated by isolating the fibers from the source of contaminated water and treating them with an alkali solution. Exposing the fibers to the alkali solution desorbs the organic matter from the fibers. Although the alkali solution may be of any concentration, it preferably has a free hydroxide concentration of 1×10−4 to 10 N, more preferably of 1×10−3 to 5 N, and most preferably of 1×10−2 to 1 N. Any alkali solution may be used, such as a solution of sodium hydroxide, ammonium hydroxide, potassium hydroxide, calcium hydroxide, or mixtures thereof. Sodium hydroxide is presently preferred. Once the contaminants and alkali solution is flushed from the fibers, they may be reintroduced into the source of contaminated water to generate purified water.
The following examples are provided to illustrate one or more preferred embodiments of the invention. Numerous variations can be made to the following examples that lie within the scope of the invention.
A 3% (by weight) solution of polysulfone in NMP was prepared by dissolving 1.65 g of polysulfone powder (Udel P-3500, [C27H22SO4]N-50-80) in 53.35 g NMP (1-methyl-2-pyrrolidone, Sigma-Aldrich, St. Louis, Mo.). Thus, (1.65 g/(1.65 g+53.53 g)*100=3%. Polysulfone solutions including higher concentrations of polysulfone to NMP, such as 4, 6, or 8%, also were prepared in a similar manner. The polysulfone/NMP solution was pumped into a precipitation bath containing deionized water using a syringe pump at the rate of 1.5 mL/hr. The solvent was removed by dialyzing the fiber solution against deionized water.
The surface area of colloidal fibers prepared in accord with Example 1 was measured using nitrogen adsorption. In Table II, below, the results were compared with those obtained from colloidal aggregates synthesized in accordance with U.S. Pat. No. 6,669,851. The Total Surface Area was determined using the Brunauer, Emmett, and Teller (BET) method of nitrogen adsorption. Adsorption was determined using the Barret, Joyner, and Halenda (BJH) method.
The total surface area of the fibers was determined to be approximately 20% smaller than that of the colloidal aggregates. The fibers also demonstrated a smaller effective interstitial distance between particles in relation to the prior aggregates. Further analysis of the nitrogen desorption data suggest that the new fibrous material (
The ability of colloidal fibers prepared in accord with Example 1 to resist fouling was determined using a fouling reduction technique representing the ability of the colloidal fibers to reduce fouling in relation to a conventional water filtration membrane when contacted with a natural water supply. Fifty mg/L of the fibers was mixed with Lake Michigan water for 24 hours and filtered at constant pressure through a 20 kD PES ultra-filtration membrane, as described in Clark et al., “Formation of Polysulfone Colloids for Adsorption of Natural Organic Foulants,” Languir, 21, 7207-13 (2005).
Initial experiments have shown that the kinetics of contaminant adsorption by the fibers may be slower than for the prior colloidal aggregates. This slowing is presently attributed to the fibers being thicker, thus having a larger microstructure, than the aggregates. Having a larger average cross-sectional width than the average diameter of the prior colloidal aggregates, slower adsorption kinetics would be expected for the fibers.
To determine the total surface area using the BET method, the surface area and pore size of the colloidal fibers prepared in accord with Example 1 were measured by N2 adsorption, with an ASAP 2010 instrument, available from Micromeritics, Norcross, Ga. Before surface analysis, the colloidal fibers were prefiltered through a 0.22 μm filter (TCMF, Millipore, Billerica, Mass.) and dried at 65° C. overnight. Degassing was conducted at a temperature of 50° C. overnight. The adsorption data were fitted with the Brunauer-Emmett-Teller (BET) model to calculate the surface area. The interstitial distance between the particles was calculated from the adsorption data using the Barret-joyner-Halenda (BJH) model.
While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that other embodiments and implementations are possible within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.
