Per- and Polyfluoroalkyl Substances in Air: Size-Segregated Distribution, Gas-Particle Partitioning, Inhalation Hazards and Fluorine Mass Balance


Student thesis: Doctoral Thesis

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Award date31 Aug 2021


Per− and polyfluoroalkyl substances (PFAS) are a class of synthetic chemicals with fully or partially fluorinated carbon chains. They may exist as neutral forms with higher volatility (neutral PFAS, n−PFAS) or as ionic forms with higher water solubility (ionic PFAS, i−PFAS) in the environment. n−PFAS (e.g., fluorotelomer alcohols, FTOHs) are recognized as volatile precursors that may be degraded or metabolized into i−PFAS. As many PFAS are highly persistent and bioaccumulative and have been linked to adverse effects in humans and wildlife, they have received worldwide attention in recent decades. Currently, there are around 5000 PFAS registered in the market, which pose significant challenges for environmental monitoring and management. The atmosphere is a major environmental compartment that receives and transports these chemicals once they are released. However, characteristics and behaviors of PFAS, especially i−PFAS, in the atmosphere have not been well studied. In addition, information about the size distribution of pollutants in atmospheric particulate matter (PM) is essential for estimating their residence time, possible sources, and the inhalation risks they may pose to human health. This study aimed to extend our knowledge on the size−fractionated gas−particle partitioning of both n−PFAS and i−PFAS in the atmosphere and the related human inhalation risk. The mechanisms and vital parameters controlling the size−specific behavior and fate of these environmental contaminants were further examined. In addition, the temporal and spatial distribution of atmospheric PFAS and human exposure risk in the surrounding environment of a point source of pollution were investigated.

The profiles, potential sources, and risks of human inhalation via PFAS in the atmospheric total suspended particles (TSPs) of Karachi, Pakistan, were investigated during the winter. The total concentrations of PFAS ranged from 4.29 to 39.0 pg/m3 during the sampling period. Perfluorobutanoic acid (PFBA) showed the highest concentrations, accounting for 32% of the total PFAS concentrations. Correlation analysis conducted to explore the relationship between individual PFAS and meteorological parameters revealed that perfluorooctane sulfonamides (FOSAs) were closely correlated with one another. Wind speeds were positively correlated with PFAS, while relative humidity showed negative correlations with PFAS. Estimated daily intakes (EDIs) of PFAS were in the range of 0.072–3.98 pg/kg bw/d and 0.006–0.325 pg/kg bw/d for children and adults, respectively. These levels were considerably lower than the acceptable level, indicating that risk to human health was low and inhalation via TSPs was not the major exposure pathway. Further studies should include other exposure pathways (e.g., diet and drinking water consumption) to completely evaluate human exposure to PFAS in the local areas. Overall, this study provides basic information about atmospheric particle−bound PFAS on a regional scale in Pakistan.

Collectively, 248 size−specific PM samples were collected from nine cities in China, Japan, Korea, and India, using a portable four−stage cascade impactor for the analysis of thirty−four i−PFAS. Generally, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) were the predominant compounds across the PM samples, with a range of concentrations of <0.083−77.9 pg/m3 and <0.546−30.1 pg/m3, respectively. An emerging PFAS, hexafluoropropylene oxide dimer acid (HFPO−DA), was quantified in the PM samples for the first time, with concentrations ranging from <0.086 to 21.5 pg/m3. Regarding spatial distribution, PFOA and PFOS predominated in the PM samples collected from China, while PFAS precursors, emerging PFAS (as replacements for PFOS and PFOA), and short−chain PFAS (C < 8) were prevalent in the PM samples collected from India, Japan, and South Korea, respectively. The 72−h backward trajectories of air masses were simulated using the hybrid single−particle Lagrangian integrated trajectory (HYSPLIT) model to explore the origins of the air masses; the highest concentration found in Nanjing, China, was associated with the marine−continental mixed air masses that originated from the Yellow Sea and traveled across several eastern Chinese cities. However, the oceanic air masses exhibited inconsistent effects in different coastal cities, diluting PFAS contamination in Xiamen, China, but contributing to contamination in Gujarat, India. In addition to the influences of air masses, other parameters such as regional climate and local or seasonal emission sources may also affect PFAS profiles across different seasons. Size−dependent distribution of PFAS was further investigated, and the results indicated that the majority of PFAS were predominantly associated with fine particles (PM<1), while PFOS and its alternatives (i.e., 6:2 chlorinated polyfluorinated ether sulfonate, perfluorohexane sulfonic acid, and perfluorobutane sulfonic acid) tended to bind with coarser particles (PM1–2.5 and PM2.5–10). PFOS in size−fractionated particles exhibited seasonal and regional dependency. For example, PFOS was primarily distributed on PM2.5−10 in Xiamen, China, and preferentially partitioned into the coarsest particles (PM>10) in Tsukuba, Japan, irrespective of seasons. Moreover, a migration toward coarser particles from the warm season (summer and autumn) to the cold season (winter and spring) was observed in Jinju, South Korea, which may be attributed to the higher dispersion and long aging process of PM in the warm season or the mixing of local emissions with PM driven across long distance by the movement of air masses. In contrast, PFOA showed consistent prevailing distribution in PM<1 across different seasons and sampling locations.

To expand the knowledge on the size−segregated distribution, gas−particle partitioning, and emission quantity of PFAS from source−related (i.e., landfills and wastewater treatment plants, WWTPs) and other types of land uses, air samples in both particulate and gaseous phases were collected from three sewage treatment works (STWs) and the largest landfill in Hong Kong. An 11−stage MOUDI impactor (for collecting PM) equipped with self−pack polyurethane foam (PUF)/styrene−divinylbenzene resin (XAD)−2/PUF sandwich (for collecting gaseous phase samples) was used to collect both i− and n−PFAS. The samplers were placed beside the STW’s aeration tanks and about 100 m away from the landfill and collected air samples continuously for 48 h. The results showed that ∑FOSAs and ∑(fluorotelomer methacrylates + fluorotelomer acrylates), respectively, dominated in the gaseous (representing 43% of the total PFAS concentrations) and particulate phases (representing 38% of the total PFAS). Compared with previous studies of landfills and WWTPs, relatively lower concentrations of PFAS were observed in this study. Gas−particle partitioning of PFAS was studied and i−PFAS were found in higher particle proportions than n−PFAS. Size−segregated distribution of total perfluoroalkyl carboxylic acids (∑PFCAs) and perfluoroalkane sulfonic acids (∑PFSAs) in different locations were examined using mean−normalized distributions and calculated geometric mean diameter (GMD) values and geometric standard deviation. Although bimodal or multi−modal distributions were observed in most cases, the GMD values indicated that PFAS were preferentially distributed in the coarse, fine, and middle sizes of particles in coastal, urban, and other sites, respectively. Yearly air emissions of PFAS from source−related locations, namely landfill, and two STWs were estimated using a simplified Gaussian dispersion model and observed significantly higher levels of emissions from the landfill (7.71 g/y) than from the two investigated STWs (around 0.02 g/y), implying that the landfill was a potential source of PFAS in the atmosphere. 

Considering that only a small fraction (< 50 compounds) of PFAS is routinely monitored, actual PFAS exposure in the environment may be underestimated, a mass balance approach was developed and applied to evaluate the fraction of unidentified organofluorine (OF) compounds in air samples. A low−volume cascade impactor that consists of four−stage filters, one slice of PUF, and two sheets of newly developed activated charcoal (GAIACTM) felts was used to collect various kinds of OF as well as inorganic fluoride (IF). To enable fast and accurate measurement, a comprehensive workflow to measure a wide array of PFAS, IF, and total fluorine (TF) in the PM and gaseous phase of air samples was proposed, validated, and applied. To further test possible loss during the whole procedure, surrogates were spiked on the top of PUFs, followed by continuously pumping for 48 h. The results showed that except for FOSAs, most PFAS exhibited good recoveries during the whole procedure, indicating the excellent sorption capacity of the sorbent train. 

To apply this sorbent train and airborne−fluorine mass balance approach, indoor (n = 3) and outdoor air (n = 8) samples were collected from three cities in Japan (Tokyo, Ibaraki, and Gifu). Results showed that n−PFAS, particularly 8:2 FTOH (with an average concentration of 2.58 ± 1.67 ng/m3), was the most prevalent group of compounds in the indoor air, while ultra−short−chain i−PFAS such as trifluoroacetic acid (TFA, 1.56 ± 2.75 ng/m3 on average) predominated in the PFAS profile for the outdoor air. Concentrations of OF compounds ranged from 1.74 to 14.3 ng/m3 and from 52.0 to 1,100 ng/m3 in the particulate and gaseous phases, respectively. On this basis, the contribution of measured PFAS to OF compounds was evaluated using the mass balance approach and found that the measured ∑PFAS only contributed to 0.69% and 1.83% of the OF compounds in the PM and gaseous phase, respectively, suggesting that most OF compounds in air samples were unidentified. 

In summary, the findings of this study provide an in−depth understanding of the size−dependent distribution and gas−particle partitioning of PFAS from both regional and global perspectives. The airborne−fluorine mass balance approach proposed in this study could serve as an expeditious and useful tool for making a more comprehensive assessment of OF contamination and the extent of knowledge about PFAS in the atmosphere. For future research directions, the accurate characterization and, ultimately, quantification of unidentified OF compounds should be considered, possibly by applying controlled oxidative experiments and both suspect and nontarget screening techniques using high−resolution mass spectrometry.