Researchers realize perfluoroalkyl mineralization using charged microdroplets

Anthropogenic perfluoroalkyl and polyfluoroalkyl substances (PFAS) are widespread and persistent pollutants that are increasingly subject to stringent regulatory thresholds in water resources. Current nonthermal defluorination strategies have limitations including incomplete mineralization, leaving behind short-chain PFAS byproducts and residual fluoride ions, thereby posing challenges to meeting water quality standards

In a study published in Journal of the American Chemical Society, a team led by Prof. WANG Feng and Assoc. Prof. JIA Xiuquan from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences (CAS), together with Prof. JIANG Guibin's team from the Research Center for Eco-Environmental Sciences of CAS, realized complete perfluorooctanoic acid (PFOA) mineralization and fluoride immobilization using a microcloud enriched with wollastonite-bearing microdroplets.

The microcloud was characterized by fast phase switching of water among the bulk phase, microdroplets, and vapor phase, driven by ultrasonic spray. During this process, positively charged larger droplets and negatively charged smaller droplets were generated, which is a phenomenon known as the Lenard effect.

Electrostatic attraction between the oppositely charged droplets drove their fast coalescence, causing them to rapidly migrate or fall back into the bulk phase. These fast spray-fusion cycles facilitated sustained electron transfer between charged droplets, thereby making the redox reactions thermodynamically feasible.

Researchers found that wollastonite-bearing microdroplets prioritized defluorination over C-C scission in perfluoroalkyl chains through liquid-solid-gas triple-phase contact electrification, resulting in complete PFOA mineralization with hardly detectable shorter-chain anionic PFAS byproducts. Microdroplet-mediated weathering of wollastonite triggered the formation of CaF₂-SiO₂ interfacial structures through Si-F-Ca bonding interactions, enabling fluoride immobilization with negligible leaching.

Moreover, researchers demonstrated that defluorination reactions were initiated by electron attachment coupled to proton or H^• transfer during hydrodefluorination, as well as the ^•OH-mediated C-H bond oxidation process. This approach reduced PFOA concentration to well below 4 ppt, the Maximum Contaminant Level set by the U.S. Environmental Protection Agency, while producing shorter-chain anionic PFAS byproducts with concentrations far below 500 ppt, the proposed total PFAS limit required by the recast Drinking Water Directive by the European Environment Agency. 

Furthermore, researchers found that microdroplet-mediated weathering of CaSiO₃ and in situ generated silica led to the formation of CaF₂-SiO₂ interfacial structures, resulting in F^– residue concentration fluctuating around the 1 ppm regulatory limit set by surface water quality standards. The interaction between microdroplets and minerals enabled efficient C-C bond cleavage, producing syngas with a carbon yield larger than 98% and tunable H₂/CO ratios of 0.5 to 1.

"Our study indicates that beyond potential applications of microdroplets in practical water treatment under ambient conditions, microdroplets from clouds and sea spray may possess a significant yet overlooked, self-cleaning capacity for PFAS pollutants on a global scale," said Prof. WANG.