The nanosponge polymer could become a natural extension of every drinking faucet in the world, according to Los Alamos polymer chemist DeQuan Li. It also could be used to clean up organic explosives, remediate underground water and clean up oil or organic chemical spills, especially in water - all while decreasing cleanup costs associated with current technologies.
Conventional processes using either activated carbon or zeolites are inefficient in reducing low-concentration contaminants in water and are totally ineffective in removing organic compounds from water down to the parts-per-trillion level, Li said. The molecular binding between organic contaminants and the polymer is 100,000 times greater than with activated charcoal and the process is 100 percent reversible.
Activated carbon is made from material burnt in a super-heated, oxygen-rich atmosphere creating small holes throughout the grain of charcoal that effectively increases the carbon's surface area, making it more reactive.
Zeolites are a group of crystalline aluminosilicates with three-dimensional frameworks that form uniform surface pores enclosing internal cavities and channels and are used commercially as molecular sieves, mostly for gas separations. The shape and size of the surface pores and internal cavities physically or chemically trap large chemical molecules within their lattice, breaking them into smaller ones.
But both activated charcoal and zeolites have drawbacks in water purification: they are easily deactivated by moisture in the air and cannot function effectively once they are completely saturated with water. Zeolites, for example, are good for absorbing water from organics but not vice versa.
Discovered by Li and graduate student Min Ma, the newly developed material is made of polymeric building blocks called cyclodextrins that form cylindrical cages to trap organics. The nanosponge has a love-hate relationship with water. The polymer's cages have hydrophilic sites that have an affinity for water and hydrophobic sites that are repelled by water. This characteristic makes them useful in water yet able to attract organics.
The hydrophobic sites prefer the organic contaminant over the water. The water actually drives the organic compound into the polymer's cage-like structure. Researchers rinse the saturated polymer with ethanol to release the trapped contaminants and the nanosponge can be reused.
So far the research team has developed polymers to bind with trichloroethylene, toluene, phenol derivatives and a number of dye compounds. Li can fabricate the nanosponge material into granular solids, powders, and optical quality thin films. Such flexibility enables users to customize the polymer for multiple applications and formats accommodating different water-treatment configurations and needs, Li said.
For example, a polymer designed as a membrane could be placed on a household water faucet. This membrane could purify water much like conventional filters do to soften it. The only difference is that water softeners introduce sodium ions in exchange for the calcium ions that make the water hard; the polymers add nothing to the treated water.
Another advantage the polymers have is that the polymers are relatively cheap to manufacture; they are one step away from a commercially available product: cyclodextrins in starch. The conversion is 100 percent effective; all the starch cyclodextrins are converted completely to polymer products. Mass production may bring down the cost below the current price of activated carbon or zeolites, Li said.
Unlike carbon and zeolites, the porous polymers remain effective in air and do not absorb moisture from the air. Activated carbon and zeolites must be "activated" before use or they lose their effectiveness to remove contaminants. But the polymers require no activation and can be placed immediately into the contaminated medium for in-situ treatment.
The polymeric material is transparent and changes color when bound with organic contaminants. This optical characteristic allows the user to determine when the material is almost completely saturated with contaminants and in need of regeneration for reuse.
The tiny sponge-like polymer can be readily incorporated into industrial and municipal online water purification systems, Li said.
Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is operated by Los Alamos National Security, LLC, a team composed of Bechtel National, the University of California, The Babcock & Wilcox Company, and Washington Group International for the Department of Energy's National Nuclear Security Administration.
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