Mar 14 2019
Using a defective filter to clean pollutants from water may sound like a vague and unoriginal idea, but a new study carried out by Rice University chemical engineers has demonstrated how the right-sized defects can allow a molecular sieve to extract more amounts of perfluorooctanesulfonic acid (PFOS) in a less period of time.
By introducing defects into the structure of a metal-organic framework, or MOF, Rice University researchers found they could increase the amount of toxic pollutants called perfluorooctanesulfonic acid (PFOS) that the MOF could hold, as well as the speed with which it could adsorb them from heavily polluted industrial wastewater. (Image credit: Chelsea Clark)
The results of the study have been reported in the American Chemical Society journal, ACS Sustainable Chemistry and Engineering. Researchers Michael Wong, Chelsea Clark, and coworkers from Rice University demonstrated that an extremely porous, Swiss cheese-like nanomaterial, known as a metal-organic framework (MOF), can soak up PFOS from polluted water in a much faster way, and when the MOF was integrated with more nanometer-sized holes (“defects”), it could even hold more amounts of PFOS.
For many years, PFOS has been utilized in consumer products, including stain-resistant fabrics, and it is also a popular member of a class of harmful chemicals known as “per- and polyfluoroalkyl substances” (PFAS), which the Environmental Protection Agency (EPA) has elucidated as “very persistent in the environment and in the human body—meaning they don’t break down and they can accumulate over time.”
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.
Michael Wong, Professor and Chair, Department of Chemical and Biomolecular Engineering, Rice University.
Wong is also a professor of chemistry.
MOFs are three-dimensional (3D) structures that are capable of self-assembling when an interaction occurs between metal ions and organic molecules known as linkers. Wong informed that MOFs appeared to be excellent candidates for remediation of PFAS because these materials are not only highly porous but have also been used for absorbing and holding a considerable amount of particular target molecules in earlier applications. For instance, certain MOFs possess a surface area that is bigger than a football field per gram; in fact, over 20,000 types of MOFs have been documented so far. Moreover, chemists can also modify the properties of MOFS—their functions, pore sizes, and structure—by tampering with the chemical recipe or synthesis that create them.
This was the case with the PFAS sorbent developed by Rice University. A well-defined MOF, referred to as UiO-66, was initially used by Clark, a graduate student in Wong’s Catalysis and Nanomaterials Laboratory, who performed many numbers of experiments to find out how the properties of the end product are changed by different concentrations of hydrochloric acid. Clark discovered that structural defects of different sizes could be introduced with this technique—similar to making Swiss cheese with additional large holes.
“The large-pore defects are essentially their own sites for PFOS adsorption via hydrophobic interactions,” stated Clark. “They improve the adsorption behavior by increasing the space for the PFOS molecules.”
In order to find out which variety soaks up the most amounts of PFAS from densely polluted water in the least period of time, Clark tested variants of UiO-66 having varied sizes and different amounts of defects.
We believe that introducing random, large-pore defects while simultaneously maintaining the majority of the porous structure played a large role in improving the adsorption capacity of the MOF. This also maintained the fast adsorption kinetics, which is very important for wastewater remediation applications where contact times are short.
Chelsea Clark, Graduate Student, Department of Chemical and Biomolecular Engineering, Rice University.
According to Wong, the study’s emphasis on industrial PFAS concentrations makes it different from a majority of earlier published studies, which have traditionally concentrated on detoxifying polluted drinking water to fulfill the present federal standards of 70 parts per trillion.
In addition, there are several treatment technologies like ion exchange resins and activated carbon that can be used to effectively clean low-level PFAS concentrations from drinking water; however, such methods cannot be effectively used for treating industrial waste that contain high concentrations of PFAS.
Since 2009, the use of PFAS has been largely limited by international treaty, but despite this fact, the chemicals are still being utilized in the chrome plating and semiconductor manufacturing industries, where wastewater can include as high as a single gram of PFAS for each liter of water, or roughly 14 billion times the present EPA limit set for safe drinking water.
In general for carbon-based materials and ion-exchange resins, there is a trade-off between adsorption capacity and adsorption rate as you increase the pore size of the material. In other words, the more PFAS a material can soak up and trap, the longer it takes to fill up. In addition, carbon-based materials have been shown to be mostly ineffective at removing shorter-chain PFASs from wastewater. We found that our material combines high-capacity and fast-adsorption kinetics and also is effective for both long- and short-chain perfluoroalkyl sulfonates.
Michael Wong, Professor and Chair, Department of Chemical and Biomolecular Engineering, Rice University.
Wong further added that it is quite hard to beat materials based on carbon, particularly in terms of cost, because for years, activated carbon has been traditionally used for environmental filtration.
“But it’s possible if MOFs become produced on a large-enough scale,” he stated. “There are a few companies looking into commercial-scale production of UiO-66, which is one reason we chose to work with it in this study.”
Other co-authors include Kimberly Heck and Camilah Powell, both from Rice University. The study was supported by the National Science Foundation (NSF) Graduate Research Fellowship Program and by the NSF’s Nanosystems Engineering Research Center on Nanotechnology-Enabled Water Treatment (NEWT). Based at Rice University, NEWT is a multi-institutional effort established in 2015 to devise water-treatment systems that are mobile, compact, and off-grid and can offer clean water to countless numbers of people and make U.S. energy production more cost-effective and sustainable.