A New Way To Take Radioactive Contaminants From Water

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A byproduct of the paper industry and crab shells may soon be used to take radioactive contaminants out of water.

Joel Pawlak, an associate professor of forest biomaterials at North Carolina State University, has developed a material similar to foam rubber that absorbs water and attaches to molecules dissolved in it, leaving pure and potable water behind.

The material is a combination of hemicellulose and chitosan. The first comes from wood and is extracted by the ton in the paper-making process. Chitosan is extracted from ordinary crustacean shells - primarily crab, shrimp and lobster - by treatment with hydrochloric acid and then sodium hydroxide.

Mixing the two together creates the foam-like substance, a polymer. The polymer has a lot of positively and negatively charged atoms on the surface, and those charges attract ions in the water. Since they are both positive and negative, there is a wide variety of ions it will stick to.

Thus far the polymer has been tested on potassium iodide, sodium iodide, cadmium, lead and arsenic and salt.

Pawlak notes that while the polymer hasn't been tested on radioactive iodine-131, which is the major contaminant in radioactive water at the nuclear power stations in Fukushima, Japan. But assuming radioactive iodine behaves similarly to the non-radioactive type, there is every reason to believe it will work.

The ability to absorb arsenic also makes this material different - most filters have a tough time removing it from water. This one doesn't.

Generally, Pawlak says, each gram of polymer will remove about 1.3 to 1.7 grams of contaminants. It is especially good at removing salts. Eventually, he wants to get the material to a 100-to-1 ratio, in which a single gram of it can filter 100 grams of water.

In addition to taking out radioactive material, the polymer could be used to make brackish water drinkable. We're working on developing filter for water treatment, Pawlak said. Sort of a one-off personal in line filter. You could pass water into it remove contaminants. Such a device would be useful in situations where one can't carry a lot of water around. Currently Pawlak and his team are working on projects related to desalinization of water combined with heavy metal removal.

There are still obstacles. Concentrating hemicelluloses enough to make producing his polymer economical is currently difficult to do, so to make use of the amounts that get produced there has to be a way to purify it. Currently the hemicellulose is simply burned.

Pawlak said the genesis of the polymer was a search for a sustainable alternative to the materials used in diapers and camping equipment, called polyacrylic acids, or PAA. They are very absorbent but lose some of their efficiency when taking up salty water - like sweat or urine. They are also not as environmentally friendly.

As he was looking for a material that would work, he ran across one that was better at absorbing saline solutions than water. Most polymers that absorb water pick up salty water less efficiently. Initially, he thought the data was simply wrong. But it turned out that they really were seeing a polymer suck up salt water. That led to testing it on other contaminants.

Pawlak worked with N.C. State professor Dr. Richard Venditti. The research was funded by the Consortium for Plant Biotechnology Research, the North Carolina Forestry Foundation and the U.S. Department of Energy.

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