Mar 23, 2019 | Updated: 12:48 PM EDT

Defects can help soak up pollutant faster in nanomaterials

Mar 14, 2019 02:44 PM EDT

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Cleaning pollutants found in water with a filter that is defective does not sound like it has an impact, but a recent study conducted by the chemical engineers at Rice University found that the right-sized defects helped molecular sieve soak up more PFOS or perfluorooctanesulfonic acid in less time.

In a study in the American Chemical Society journal ACS Sustainable Chemistry and Engineering, Rice University researchers Michael Wong, Chelsea Clark and colleagues showed that a highly porous, Swiss cheese-like nanomaterial called a metal-organic framework (MOF) was faster at soaking up PFOS from polluted water, and that it could hold more PFOS, when additional nanometer-sized holes ("defects") were built into the MOF.

PFOS was used for years in consumer products like fabrics that are stain-resistant and is also the known member of a group of toxic chemicals called PFAS or per-and polyfluoroalkyl substances, which the EPA or the Environmental Protection Agency describes as very persistent in the environment and in the human body, meaning that they do not break down and they can accumulate over time.

Wong, professor and chair of Rice's Department of Chemical and Biomolecular Engineering and a professor of chemistry, said, "We are taking a step in the right direction toward developing materials that can effectively treat industrial wastewaters in the parts-per-billion and parts-per-million level of total PFAS contamination, which is very difficult to do using current technologies like granular activated carbon or activated sludge-based systems."

Wong said that MOFs, three-dimensional structures that self-assemble when the metal ions interact with the organic molecules that are called linkers, are good candidates for PFAS remediation because they are very porous, and they have been used to absorb and to hold significant amounts of specific target molecules in previous applications.

 Some MOFs have a surface area that is larger than a football field per gram, and more than 20,000 kinds of MOFs are documented. Chemists can tune MOF properties, depending on their structure, pore sizes, and their functions, by tinkering with the chemical recipe or the synthesis that produces them.

This was the case with Rice University's PFAS sorbent. Clark, a graduate student in Wong's Catalysis and Nanomaterials Laboratory, began with a well-characterized MOF called UiO-66, and conducted dozens of experiments to see how various concentrations of hydrochloric acid changed the properties of the final product. She found she could introduce structural defects of various sizes with the method-like making Swiss cheese with extra-big holes.

 "The large-pore defects are essentially their own sites for PFOS adsorption via hydrophobic interactions," Clark said. "They improve the adsorption behavior by increasing the space for the PFOS molecules."

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