Discovered at Argonne National Laboratory, materials that expand under pressure contain large, fluid-filled pores that may enable their use as filter or storage materials. Credit: Argonne.
Various types of ceramic materials have been used to filter everything from water to molten metal to automotive exhaust gases. Here’s a look at two new filtration material developments that, to put it mildly, are on opposite ends of the technology spectrum. One is based on common clay and papaya seeds; the other is a new phase of material that expands when subjected to pressure, seeming to defy the known laws of physics.
First, the low-tech: A proposed new filter media made of kaolinite clay and papaya seeds may offer an inexpensive route to access to clean water for millions in the developing world, according to a press release about new work by a Nigeria–Germany research team.
In an article in the American Chemical Society journal, ACS Sustainable Chemistry & Engineering (DOI: 10.1021/sc400051y), Emmanuel Unuabonah and coauthors say nearly 1 billion people in developing countries lack reliable access to clean water. Among the main contaminants are heavy metals such as lead, cadmium, and nickel.
The proposed solution involves combining kaolinite clay and papaya seeds. Both materials are readily available and have historically been used for water filtration, but had not been combined into a hybrid filtration material until now, the scientists report.
“The hybrid clay adsorbent is a highly efficient adsorbent for heavy metals,” Unuabonah, of the Department of Chemical Sciences at Redeemer’s University (Mowe, Nigeria) and the Institute of Chemistry at the University of Potsdam (Potsdam, Germany) and his coauthors write in the abstract. “With an initial metal concentration of 1 mg/L, the hybrid clay adsorbent reduces the Cd2+, Ni2+, and Pb2+ concentration in aqueous solution to ≤4, ≤7, and ≤20 μg/L, respectively, from the first minute to over 300 min using a fixed bed containing 2 g of adsorbent and a flow rate of ~7 mL/min.” According to the authors, these values, with the exception of Pb2+, meet World Health Organization guidelines for heavy metals, according to the scientists.
The researchers say adsorption on the hybrid clay material is essentially due to ion exchange. The hybrid material is easily regenerated and “has a strong potential for replacing commercial activated carbon in treatment of wastewater in the developing world,” they add.
That’s the low-tech. Here’s the WOW!: a material that expands when placed under pressure.
According to a news release, when Argonne National Laboratory (Lemont, Ill.) chemist Karena Chapman and her colleagues applied pressure of 0.9 to 1.8 GPa to zinc cyanide in a diamond-anvil cell at the lab’s Advanced Photon Source, they created five new phases of material. Two of the novel phases retained their porosity at normal pressure, the release says.
“By applying pressure, we were able to transform a normally dense, nonporous material into a range of new porous materials that can hold twice as much stuff,” Chapman explains in the release. “This counterintuitive discovery will likely double the amount of available porous framework materials, which will greatly expand their use in pharmaceutical delivery, sequestration, material separation, and catalysis.”
Because this behavior runs counter to the known laws of physics, the scientists spent several years testing and characterizing the new materials. They discovered that the morphology of the holes produced under pressure depended on the fluid used to squeeze the zinc cyanide, allowing tailoring of hole shape to trap, store, and filter a variety of materials. “This could not only open up new materials to being porous, but it could also give us access to new structures for selectability and new release rates,” said Argonne chemist Peter Chupas.
Results of the work were recently published in the Journal of the American Chemical Society (DOI: 10.1021/ja4012707).