Dr. Hisao Hori and his colleagues in the Potential Pollutants Group of the Research Institute for Environmental Management Technology (Emtech), the National Institute of Advanced Industrial Science and Technology (AIST), have succeeded in developing the method of fully decomposing perfluorocarboxylic acids in water, such as perfluorooctanoic acid (PFOA) into fluoride ions. These compounds are known as typical fluorinated organic compounds, which persist in the environment and accumulate in biological systems.

Fluorinated organic compounds have excellent physical and chemical characters, therefore, their uses have been expanded.

However, some of them, particularly, perfluocarboxylic acids and perfluorosulfonic acids and their salts, have been found in environmental waters and biological systems in global scale.

These compounds consist of carbon-fluorine bonds, the strongest bonds that carbon can form, so that there are no known natural decomposition processes. In attempts to decompose them thermally, a very high temperature (> 1000°C) was required, and owing to poor reactivity with OH radicals, they are hardly decomposed by conventional TiO₂ photocatalyst in water.

The research group of Emtech-AIST has succeeded in fully decomposing PFOA and other perfluorocarboxylic acids into fluoride ions and carbon dioxide in water at the room temperature, by using a heteropolyacid photocatalyst. The fluoride ions can be converted into environmentally harmless CaF₂ through the conventional waste management process.

The future efforts will be focused in the practical application of the reaction system and in the contribution to the reduction of environmental risk of fluorinated organic compounds.

The newly developed process is the first example of photochemical decomposition of PFOA in the world, and will be published in the November issue of the Environmental Science and Technology, American Chemical Society. The electronic edition has already been released on October 9, 2004 at the ACS web site.

Owing to their unique properties, such as thermal and chemical high stabilities and high transparency (no light absorption ability), fluorinated organic compounds, especially perfluorinated acids, have been used for many applications as surfactants: emulsifying agent, water-repellent, fire retardant, wax, carpet cleaner, paper coating, etc. As the use of these compounds has increased, some of them, such as perfluorooctanoic acid (PFOA, C₇F₁₅COOH) and perfluorooctane sulfonate (PFOS, C₈F₁₇SO₃⁻), have recently been detected in environmental waters and in wildlife. Analytical studies have revealed that they bioaccumulate, and their toxicological properties are being clarified. Under such circumstances, in Japan, PFOA and PFOS have been specified under the Chemical Substances Control Law, in December 2002, currently listed in the Type II Monitoring Chemical Substance.

To thermally decomposing them, a very high temperature (>1000°C) is required. Therefore, the development of techniques for decomposing them (as waste, especially in wastewater) to harmless species under mild conditions has been desired as a measure against stationary sources.

 

The research group in the Emtech-AIST has been engaged in developing the method of environmental analysis, understanding the dynamics in environment, decomposing wastes and establishing decomposition techniques for these fluorinated organic chemicals.

They have  focused on the heteropolyacid photocatalyst (H₃PW₁₂O₄₀) for decomposing perfluorocarboxylic acids, including PFOA. The heteropolyacid has many merits, such as high oxidizing ability, high stability under highly acidic condition (pH < 1.0), no possibility to cause coking phenomenon (because it is a homogeneous photocatalyst), ease of recovery from the aqueous reaction solution via selective complexation with ether.

The photochemical reaction has been carried out through the following steps: An aqueous solution including PFOA and the heteropolyacid catalyst was transferred into a pressure-resistant reactor, and then pressurized by oxygen gas. Then, UV-visible light was irradiated to the solution while the reaction temperature was held constant at 25°C. After the irradiation finished, products in the gas and liquid phases were analyzed. (See Photo 1.)

Photo 1. Photoreaction reactor.

Figure 1 shows the dependence of PFOA decomposition on irradiation time. With the light irradiation, the amount of PFOA decreased, while producing carbon dioxide in the gas phase, and fluoride ions in the liquid phase. PFOA completely disappeared after 24 h of irradiation, turning most of fluorine component into fluoride ions. No degradation of the heteropolyacid photocatalyst was observed after the reaction.

Not only PFOA, but also other perfluorocarboxylic acids bearing C1 to C8 perfluoroalkyl groups could be decomposed through this process. In addition, This system produced no environmentally undesirable species such as CF₄ and CF₃H, stable species that have high global-warming potentials (at least 3900 and 9400 times as high as CO₂, respectively), and are often observed in the decomposition of perfluorocompounds by extremely high-energy techniques such as electron-beam irradiation.

Fig. 1. Irradiation-time dependence of PFOA decomposition.