Researchers from the University of Illinois, Urbana-Champaign have developed an electrochemical process designed to economically remove up to 86% of ultrashort-chain PFAS trifluoroacetic acid (TFA), an f-gas byproduct, as well as long-chain PFAS from water intended for human consumption.
A study describing the process – “Integrating redox-electrodialysis and electrosorption for the removal of ultrashort- to long-chain PFAS” – was published on September 27 in the journal Nature. The researchers, all in the Department of Chemical and Biomolecular Engineering, include Nayeong Kim, Johannes Elbert, Ekaterina Shchukina and Xiao Su.
While several f-gases degrade to TFA in the atmosphere, HFO-1234yf, used globally in millions of car air-conditioning systems and in commercial refrigerant blends, undergoes 100% transformation to TFA within a few weeks; the TFA then comes down to Earth in rainfall. TFA and its f-gas precursors are considered PFAS (per-and polyfluoroalkyl substances), or “forever chemicals” that last indefinitely in nature, because they have at least one fully fluorinated carbon atom.
Other sources of TFA include the breakdown products of pesticides, pharmaceuticals, fluoropolymers and other PFAS, as well as the direct release of industrially produced TFA and TFA emitted from wastewater treatment plants and landfills.
TFA has been proliferating in growing amounts throughout the environment in surface water, soil, human blood serum, plants and plant-based foods. It has been found in numerous drinking water sources, including seawater, rivers, groundwater and even bottled water, the study said.
Exposure to TFA has not been conclusively associated with health effects, but the German government this year proposed to the EU linking TFA to reproductive toxicity based on evidence of embryo-fetal developmental toxicity in rabbits, and TFA has been connected to liver dysfunction in rats.
Despite the widespread presence of ultrashort-chain PFAS like TFA in drinking water sources and wastewater, “there is relatively limited research on the removal strategies for these hydrophilic [highly water soluble] PFAS compounds,” said the University of Illinois study.
Historically, the most effective way to remove TFA from water – reverse osmosis (RO) – is expensive, requires a substantial amount of energy and typically results in 50% water loss, according to a recent study of TFA in the environment that labels the chemical as a “planetary boundary threat.” Moreover, to remove a full range of PFAS contaminants, multiple water treatment processes may be required in sequence, increasing operational complexity, capital costs, and energy consumption, noted the University of Illinois study.
Inexpensive membranes
The process used in the University of Illinois study to remove TFA and other ultrashort-chain PFAS (less than or equal to four carbons) from water involves what is called redox (reduction-oxidation)-polymer electrodialysis (redox-polymer ED).
Electrodialysis is used to transport salt ions through an ion exchange membrane; in this case an “inexpensive” cellulose-based nanofiltration membrane is used to facilitate the treatment of varied chain lengths of PFAS without membrane fouling (clogging), the study said. Nanofiltration is a membrane filtration process that uses nanometer (nm)-sized pores through which particles smaller than about 1–10nm pass through the membrane.
The polymer used in the process is P(TMA-co-TMPMA-co-METAC); spelled out, this is poly(2,2,6,6-tetramethyl-1-piperidinyloxymethacrylate-co−2,2,6,6-tetramethyl-1-piperidyl methacrylate-co-[2-(methacryloyloxy)ethyl]trimethyl-ammonium chloride)
For removing PFAS with six or more carbons, electrosorption onto carbon electrodes is used. The combined electrodialysis and electrosorption eliminates approximately 90% of ultrashort-chain, short-chain, and long-chain PFAS, while also desalinating water to potable water levels, the study said. In regard to TFA, the system “exhibited over 86% removal.” The redox-active TMA group on the P(TMA-co-TMPMA-co-METAC) serves as the driving force for desalination and the removal of PFAS from the source water.
“The redox-polymer ED exhibits remarkable PFAS removal in real source water scenarios, including from matrices with 10,000 times higher salt concentrations, as well as secondary effluents from wastewaters,” the study said. “Additionally, the removed PFAS is mineralized with a defluorination performance between 76–100% by electrochemical oxidation.”
“Our redox-polymer ED system demonstrates an energy-efficient and process-intensified platform for PFAS removal through solely electrochemical pathways,” said the study.
One of the authors of the study, Xiao Su, Assistant Professor, Chemical and Biomolecular Engineering at University of Illinois at Urbana-Champaign, acknowledged that the study is currently at “bench-scale,” meaning that it is a small-scale lab test. However, it “does demonstrate proof-of-concept for a variety of industrially-relevant questions,” he said.
A key step towards translating it for industrial use would be to study “scale-up pathways and how to increase throughput,” added Su. “We are also looking at doing a more extensive technoeconomic analysis to account for the lifetime use of the membranes and other components of the system and see where we need to improve the stability of the materials.”
Efforts to address PFAS
The EU’s European Chemicals Agency (ECHA) is considering a proposal to regulate PFAS as a category, including f-gases and TFA, under REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals), the EU’s chemicals regulation.
Also in the EU, a new organization, the Coalition for PFAS Free Cooling & Heating, has been launched, with ATMOsphere, publisher of NaturalRefrigerants.com, as a founding member. The organization will be dedicated to promoting natural refrigerants as alternatives to harmful PFAS in cooling and heating systems and supporting an ambitious European PFAS regulation.
The chemical industry addressed the environmental deposition of TFA in an October 2021 study funded by the Global Forum for Advanced Climate Technologies (globalFACT), which represents f-gas producers Chemours, Honeywell, Arkema and Koura. The study concluded that “with the current knowledge of the effects of TFA on humans and ecosystems, the projected emissions through 2040 would not be detrimental.” But the study also acknowledged that “the major uncertainty in the knowledge of the TFA concentrations and their spatial distributions is due to uncertainties in the future projected emissions.
