Molecular Sieves Separate Pollutants from Drinking Water
When Dr. Detlef Knappe saw the news in headlines last year that traces of prescription and over-the-counter drugs were found in the water supplies of dozens of U.S. metropolitan areas, he knew that issue was the proverbial drop in the bucket. The civil and environmental engineering professor has spent several years looking for the best way to remove such trace contaminants from drinking water, from pesticides to antibiotics to fuel additives to algal compounds that make water taste or smell funny. “When you’re talking in parts per billion or per trillion,” he says, “you really need an efficient way to find the molecules you’re looking for.”
“We want to find solutions that improve the overall water quality as efficiently as possible.”
Knappe has zeroed in on a porous mineral known as zeolite. He says that, unlike activated carbon, which most water treatment plants use, zeolites act as “molecular sieves” that can target unwanted compounds in water. Activated carbon grains offer tremendous surface area and effectively attract organic material, but Knappe says carbon doesn’t discriminate enough in what it traps. Therefore, filters can quickly become saturated with natural organic matter from decaying plants and microorganisms, reducing their ability to remove trace pollutants. Zeolites have a well-defined pore size—pores in carbon vary in size—and the mineral can be engineered to obtain the desired size for screening specific molecules. In one of Knappe’s drinking water projects, for example, he used zeolites to remove MTBE, an octane-boosting additive for gasoline that has contaminated aquifers nationwide. As a bonus, the selective screening process also means that zeolite filters can last five times longer than carbon, he says, saving water systems time and money.
“When you’re talking in parts per billion or per trillion, you really need an efficient way to find the molecules you’re looking for.”
Knappe is also working with water system managers across North Carolina to identify taste and odor compounds so he can find targeted treatment solutions. “There are many substances that can change the way water tastes or smells,” he says, “but not every compound corresponds equally well to a particular treatment.” One option his research group is investigating is a catalytic ozone oxidation process in which both ozone and the taste and odor compounds are adsorbed in zeolite pores.
Zeolites are best reserved for situations where specific pollutants need to be removed from water, Knappe says. “Activated carbon still remains the most effective adsorption technology for most water treatment plants,” he says, noting research has shown it removes most pharmaceuticals from water. Still, he is studying other technologies, such as exposing hydrogen peroxide to ultraviolet radiation to create reactive radicals for water purification. “Some treatment systems require a lot of energy. Others produce unwanted byproducts,” he says. “We want to find solutions that improve the overall water quality as efficiently as possible.”