chemical energy of another ion flowing down its concentration gradient. A carrier molecule shuttles the ions across. Several years ago, Bend Research found the key to making coupled transport a practical system when then-project director Richard W. Baker originated the idea of working with "supported" liquid membranes, in which the hydrophobic carrier is dissolved in an organic phase held by capillary forces within the pores of a microporous membrane. The first paper, in 1977, dealt with copper separations. Now two more papers, to be published soon in the Journal of Membrane Science, describe the concentration of uranyl sulfate, a combination of U02(S0 4 ) 2 " 2 and U C M S O ^ - 4 , using tertiary amines as a complexing agent in the membrane. From both theory and experiment, the researchers find that there are two modes of coupled transport for uranium, the choice governed largely by the pH on the product solution side. If the pH there is sufficiently high, the uranium complex is "cotransported" across the membrane along with protons from the more acidic feed solution. If the pH is sufficiently low, the complexing agent goes into a protonated form; it then diffuses forward with the uranium, exchanges it for an acid anion (bisulfate), and shuttles the bisulfate back to the feed solution. In this case the "countertransport" is driven by the concentration gradient of the anion. A similar process applies to chromium recovery from electroplating wastes. Under a grant from the Of-
Concentration gradient drives coupled transport Co-transport Feed solution H+ + M •—
Liquid membrane
/T\
Product solution —•H'+M-
\ R H M /
Counter-transport Feed solution A
~*
M->
Liquid membrane
-/RHÀV \RHM/-
H ' — Hydrogen ion M Metal-bearing ion A Acid rain R _ Complexing agent
30
C&ENJuly 14, 1980
Product solution 4 AMM
fice of Water Resources & Technology of the Department of Interior, Bend Research is planning field tests at several electroplating shops. Under a grant from the Bureau of Mines, meanwhile, the company is currently incorporating its uranium separation membranes into a 1000 gal-per-day field test unit, to be located at an uranium mine yet to be selected. Uranium is usually recovered by pouring sulfuric acid over the ore and drawing off a small river of leach solution, notes Babcock. The solution typically contains uranyl sulfate at about 1000 ppm, along with significant amounts of molybdenum, vanadium, and iron. To concentrate the uranium, refining plants use solvent extraction or ion exchange. "We think coupled transport offers an attractive alternative," says Bab-
cock. It can handle dirty solutions without needing prefiltration to remove suspended solids. Moreover, it avoids the phase separation and solvent entrainment problems common with solvent extraction, thus making it easier to meet environmental constraints on wastewater disposal. Babcock estimates that the cost of coupled-transport separation on an industrial scale would be less than that of ion exchange, and about the same as that of solvent extraction. A key problem in the research work so far has been development of membranes that are stable. "Each process is different," Babcock says. "A membrane that is stable for chromium removal won't work for copper." He and his colleagues still don't fully understand what influences a membrane's stability. ϋ
Fluoropolymers coat hard-to-protect surfaces Ship hulls and the graceful columns of the Parthenon don't have much in common except that both are exposed to distinctive environmental scour ges—barnacles and other marinefouling organisms in the first case, and marble-dissolving air pollutants in the second. The surfaces of these structures may one day be protected from their harsh environments by a coating of fluoropolymer. And chemists at the Naval Research Laboratory in Washington, D.C., may help make it possible. For a number of years, NRL chemists have been exploring the physical and chemical properties of highly fluorinated polymers, partic ularly epoxies, polyurethanes, and most recently, acrylics, in the hope of developing superior coating materials that are easy to apply. In the case of ships, marine fouling is caused by barnacles, algae, and other organisms that stick to the ex terior hull. The resulting rough sur face impedes the ship's ability to cut through water. Traditionally, this problem has been tackled by applying an "antifouling" coating that kills marine organisms by slowly leaching toxic heavy metals such as tin into the water. But in these environmentconscious times, many consider this strategy no longer acceptable. Navy chemists are taking a differ ent approach by developing "fouling release" coatings which can be cleaned with little effort. Ideally, such coatings should not allow marine or ganisms to adhere to them at all. When poly(tetrafluoroethylene) or Teflon first became available shortly after World War II, many thought the perfect "fouling release" coating had arrived. But unfortunately, Teflon
has not fulfilled its oceangoing promise. Despite its antistick and low-friction surface, fouling organ isms bond to it surprisingly well. And because Teflon cannot be dissolved in a solvent or melted, there were dif ficulties in applying it conveniently as a common coating. For these and other reasons, it became apparent that pure fluorocarbon polymers like Teflon would not make the most practical coatings. However, scientists found that suspending powdered Teflon in a polymer matrix markedly improves such surface properties as inertness. The problem was in dispersing and wetting the Teflon. Conventional epoxy resins and polyurethanes, which contain no fluorine, are easy to apply and are among the strongest of modern coating materials. But in the liquid, precured state, they are unable to wet the surface of Teflon. NRL chemist James R. Griffith and his colleagues overcame this problem by synthesizing new epoxy and polyurethane resins that contain "substantial" amounts of fluorine and can wet Teflon readily because of the molecular interactions involved. As a result, they were able to disperse powdered Teflon in a fluorinated resin, producing coatings that com bine the outstanding properties of Teflon with the convenient use properties of epoxies and polyure thanes. The big test for such a hybrid ma terial came in October 1977. The ex ternal underwater hull of a tugboat in the Norfolk, Va., harbor was coated with a fluorinated polyurethane containing 38% Teflon by volume. The coating was applied using a con ventional airless spray gun.
When the tugboat was examined after 10 months of service, the hull was found to be covered with a moderate growth of organisms, including barnacles. The barnacles had locked onto the polymer surface by piercing the outer surface of the elastic coating and allowing their shells to grow beneath it. Despite their ingenious adaptation to this unfamiliar surface, the barnacles and other fouling organisms were removed easily by rubbing with a wet rag, although there was shallow local damage to the coating at each barnacle site. In the absence of an organic coating, the barnacles would have been literally cemented to the hull, and removed only with great difficulty. Four months later, a moderately heavy regrowth of fouling was washed away from most of the hull using a stream of water from a fire hose. Now, two and a half years after the coating was applied, it continues to prove durable and easy to clean. Griffith plans to continue monitoring the tugboat's hull for many years, since most marine coatings are "notorious" for not lasting so long as boat owners would like. In the meantime, the coatings could be improved by enhancing their antiadhesive properties and their resistance to undercutting by barnacles. One way to improve the antistick properties of fluoropolymers is to add certain silicones. Dimethylsiloxanes, for instance, can be used to produce relatively soft, "rubbery" surfaces that rival the fluoropolymers in their antiadhesive properties. These silicones cannot be used alone, however, because they are not so durable as epoxies and polyurethanes. But by adding fluoropolymers to silicones, Griffith observes, it's simply "a matter of making something good even better." Navy scientists now are evaluating a number of fluorocarbon-silicone coatings for their ability to resist adhesion by barnacles. These include bonded porous Teflon whose pores are filled with either silicone grease or silicone rubber, and even silicone rubber alone. The addition of silicones to polymer coatings also enhances their resistance to wetting by water and adhesion by ice. Such materials are needed as paints for both lifeboat launching equipment and electric power switches exposed to frigid and wet climes. In a study by the United Nations Educational, Scientific & Cultural Organization, fluoropolymers and silicones were judged also to be the most promising materials to protect the Parthenon and other marble structures from being eaten away by corrosive air pollutants, according to
Griffith: the Acropolis problem
Griffith. A protective coating for such priceless architectural structures must fulfill a number of demanding requirements: • Resistance to ultraviolet light, moisture, sulfur dioxide, and other acidic materials, and degradation by microorganisms. • Resistance to discoloration over a long period of time.
• Minimal effect on appearance, including optical properties of the stone, such as luster. • Renewability without damaging structure. Cupping a 1-inch cube of Acropolis marble in his hand, he observes that the stone is porous—"it breathes"— and that a coating ideally should not interfere with this breathing. The search for a solution to the "Acropolis problem," he notes, is not an established funded program at NRL. Rather, it is an informal "cultural exchange"-type arrangement that has developed over the past four years between NRL and an Athens university. "They send us the marble, we coat it with fluoropolymers, and then we send it back to them for evaluation," explains Griffith. Various kinds of fluoroepoxies, fluoropolyurethanes, and fluoroacrylics are being evaluated, and some look very promising, he says. Polymer coatings, though, are just one avenue that has been suggested to protect the Parthenon, Griffith notes, and the Greek authorities certainly are not committed to it. But if the materials problems are solved, he thinks it might be the best way to preserve the natural beauty of the structures. Ron Dagani, Washington
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July 14, 1980 C&EN
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