Fluorine provides a selective affinity toward organic molecules in water

Nanoporous fluoropolymer selectively captures dye and other organic molecules from water based on their size and charge.

Article  |  Spring 2017

Water-soluble, human-made organic pollutants pose a serious threat to global water resources in terms of the increased demand for industrial water consumption and waste disposal. Existing water filters and adsorbents perform adequately with insoluble or slightly soluble, large, organic species, but the removal of highly water-soluble organic pollutants with fine dimensions (especially nanometer scale such as pesticides, artificial dyes, and medicines) is still challenging. The presence of strong intermolecular forces in aqueous media requires new approaches such as charge-specific size-dependent separations. Despite a few attempts to remove organic molecules using nanoporous adsorbents, size-dependent separation and the related chemistry of the interaction between these molecules and sorbents has, thus far, not been well examined.

A team led by Prof. Cafer T. Yavuz (Graduate School of EEWS (Energy, Environment, Water, and Sustainability), KAIST) has developed a nanoporous fluorine-based polymeric network that features a charge-specific size-dependent separation ability of water-soluble molecules from water. The nanoporous fluoropolymer can be simply constructed via one-step self-condensation of tetrafluorohydroquinone, which exhibits high fluorine content with narrow microporosity.

The fluoropolymer adsorbent shows a size-selectivity toward charged organic molecules whose hydrodynamic size is below the accessible pore size of the polymer, which is 1.36 nm. Furthermore, treatment of similarly-sized organic molecules with and without charges shows that the fluorine functionality of the polymer interacts with the charged molecules favorably, resulting in approximately 8 times faster adsorption kinetics compared to its uncharged counterpart. Lack of a σ-hole for fluorine atoms is suggested to be responsible for a fluorine-charge interaction, leading to a first-charge-dependent separation of organic molecules from water using nanoporous polymer.

The fluoropolymer works well even in parts-per billion (p.p.b.) concentration of organic molecules, which is a condition of actual pollution sources. Recyclability and feasibility in lab-scale column separation of fluoropolymer also demonstrates its potential for water purification technology. “Our results show a clear fluorine-cation interaction in size-selective adsorption, which can lead to sorbent designs for size-dependent organic molecule separation from water. Moreover, the fluorine – cation interaction can provide great use in water treatment applications such as mixed matrix membranes and desalination,” commented Prof. Yavuz.

This result was published on November 10th, 2016 in Nature Communications with the title of “Charge-specific size-dependent separation of water-soluble organic molecules by fluorinated nanoporous networks.” (http://dx.doi.org/10.1038/ncomms13377)

Prof. Yavuz explains the technology on the KAIST Podcast: http://www.kaist.ac.kr/Upl/downfile/S07E04_20170320.mp3