The effect of oxygen in the photocatalytic oxidation pathways of perfluorooctanoic acid

•Oxygen influences PFOA photocatalytic oxidation pathways.•Cn → Cn−1 photo-redox and β-scissions pathways are active in PFOA oxidation.•PFOA oxidation can be forced to follow one pathway or the other.•Isolation of COF2 intermediate proved the β-scissions route.•COF2 can be conveniently obtained by waste PFOA.

The influence of oxygen in the photocatalytic oxidation of perfluorooctanoic acid (PFOA) promoted by a commercial nano-sized titanium dioxide was studied by testing the reaction in different conditions: static air, oxygen flux, nitrogen flux and pre-saturated nitrogen flux. The reaction was monitored by Total Organic Carbon (TOC) analysis and Ionic Chromatography (IC). Shorter chain perfluorocarboxylic acids (PFCAs; Cn, n = 1–7) intermediate degradation products were quantitatively determined by High-Performance Liquid Chromatography combined with Mass Spectrometry (HPLC–MS) analysis. The presence of shorter chain PFCAs in solution was also monitored by 19F NMR. The experimental findings are in agreement with two major oxidative pathways: Cn → Cn−1 photo-redox and β-scissions routes mediated by COF2 elimination. Depending on the experimental conditions, the mutually operating mechanisms could be unbalanced up to the complete predominance of one pathway over the other. In particular, the existence of the β-scissions route with COF2 elimination was corroborated by the isolation and characterization of carbonyl difluoride, a predicted fluorinated decomposition by-product.

Graphical abstractThe β-scissions route mediated by COF2 elimination has been proven as active decomposition pathway for the photo oxidation of PFOA. Carbonyl difluoride is an important intermediate for the industrial preparation of key fluoromonomers and its synthesis with good selectivity and high purity proved the possibility to recycle this banned fluorinated surfactant.Figure optionsDownload full-size imageDownload as PowerPoint slide