Highly Efficient Preconcentration of Per- and Polyfluoroalkyl Substances (PFAS) by Anodically Generated Shrinking Gas Bubbles

Abstract

Per- and polyfluoroalkyl substances (PFAS) are ubiquitous environmental contaminants that have been used in many applications, including firefighting foams, stain repellents, nonstick coatings, and electroplating. Due to the extremely strong carbon-fluorine bonds in PFAS, they are highly stable and environmentally persistent. Because of their stability, PFAS can lead to bioaccumulation in humans and cause various health issues, including prostate and kidney cancer, thyroid disease, and cardiovascular disease. PFAS widely contaminate the river and lake water systems and hence cause contaminations in the drinking water. Analytical methods used for PFAS detection are currently dominated by chromatography in combination with mass spectrometry. U.S. EPA has established health advisory levels at 70 parts per trillion (70 ng/L) for PFOA and PFOS individually or combined in drinking water. Since this is a very low PFOA/PFOS concentration, preconcentrating the sample up to a detectable level plays a vital role and is an essential step during the analysis. Herein we present a novel preconcentration method for PFAS using electrogenerated shrinking gas bubbles, which can preconcentrate the PFOA and PFOS by 1300 folds within 15 minutes.Our preconcentration method is based on the natural phenomenon called sea-spray aerosol enrichment. Once air gets trapped inside a liquid column, and as these air bubbles start moving to the top through the liquid column, surface-active compounds can be preferentially adsorbed onto the air/liquid interface of the bubble. When these bubbles reach the air/liquid interface at the top of the liquid column, they burst and produce aerosol droplets enriched with surface-active compounds. Rather than using an external gas supply to generate bubbles, we in situ electrogenerated O2 and CO2 bubbles by water oxidation in the presence of 0.2 M ammonium bicarbonate as an electrolyte. The advantages of using anodically enriched bubbles bursting aerosol are 2-fold. The first one is in an ammonium bicarbonate electrolyte; the anodic side generates oxygen bubbles and carbon dioxide bubbles. Since carbon dioxide bubbles get shrunk, the effective surface area reduces. Hence the surface concentration of PFAS increases. With that high surface concentration of PFAS, the droplet's effective concentration becomes higher when it becomes aerosol droplets. The second one is when the electrolysis happens, the anode is charged positively. Due to the charge interactions, PFAS can pre-regulate near the electrode surface and provide additional preconcentration. We tested eight PFAS compounds, including perfluoroalkyl sulfonates and carboxylates. All the perfluoroalkyl carboxylate compounds show an average of 50% increase in the enrichment for the anodically enriched bubble aerosol than the cathodically enriched bubble aerosol. Furthermore, all the perfluoroalkyl sulfonate compounds show an average of 30% enrichment in the anodically enriched bubble aerosol than the cathodically enriched bubble aerosol. Due to its high contribution to environmental contamination, we used perfluorooctanoic acid (PFOA) as our model analyte. Enrichment ratios for different concentrations of PFOA ranging from 10-11 M to 10-7 M show an average of 1050 fold enrichment. To test our bubble shrinking theory, we measured the radius of the bubbles at different heights from the electrode surface. We observed an 87.5% volume reduction when the bubble moves a distance of 17.5 cm from the electrode surface to the air-water interface. We measured the enrichment ratio vs different heights to test the surface interaction between the PFAS molecules and the electrode surface. We obtained the intercept of 400 from the anodic compartment, while from the cathodic enrichment, we obtained the intercept of 200. The intercept (enrichment at zero height) involves two factors. The first one is a higher PFAS content at the air-water interface, which may cause enrichment at zero height. However, this factor is common for both compartments. However, the anodic side electrode is positively charged, and since PFAS are negatively charged, there can be a surface interaction and causing additional enrichment in the anodic side. Furthermore, our method is compatible with the LC/MS/MS method because ammonium bicarbonate is LC/MS/MS friendly solvent. Moreover, we successfully couple this technique with our previously published bubble nucleation-based electrochemical detection. This novel technique may lead to a path toward building a low-cost and fast onsite detection method for PFAS in water.