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Short-chain per- and polyfluoroalkyl substances in aquatic systems: Occurrence, impacts and treatment [1]

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Date: 2020-01-15

Per- and polyfluoroalkyl substances (PFAS) are a class of man-made chemicals with part or all the hydrogen atoms replaced by fluorine atoms on the carbon skeleton and with a terminal functional group [1]. The fluorination stiffens the alkyl chains and compacts the molecules, leading to the formation of a “molecular-brush” structure, which has a dense layer of trifluoromethyl groups that repel both water and oil (i.e., hydro- and oleophobic) [2]. Due to the low molecular polarity, strong C-F bond energy (536 kJ/mol), strong biological resistance, and the amphiphilic nature, PFAS have been used in a variety of industries around the globe and widely distributed in our daily consumer products such as food packaging, pesticide formulations, waterproof fabrics, carpets, non-stick cookware, fume suppressants, photographic films, masking tape, firefighting foams, etc. [3], [4]. Consequently, PFAS have been frequently detected in the aquatic ecosystems, particularly near relevant industries or municipal facilities such as landfill, wastewater treatment plants, or firefighter training sites [5].

In addition to drinking water consumption, human can be exposed to PFAS through a number of important routes, including inhalation of outdoor dusts, dietary intake, and exposure to contaminated soil/sediment [6], [7], [8]. Owing to the extremely high biological resistance, human body can hardly break down PFAS [9]. As a result, PFAS can accumulate and remain in the human body over prolonged times [4]. It was reported that the half-life of perfluorooctane sulfonate (PFOS) is 4–5 years in adult human body and 41 years in the environment [10]. Growing health data have indicated that exposure to PFAS can lead to various adverse health effects, such as low infant birth weight, thyroid hormone disruption, impairment on the immune system, and even cancers [5].

To mitigate human exposure, the United States Environmental Protection Agency (U.S. EPA) released a Drinking Water Health Advisory for perfluorooctanoic acid (PFOA) and PFOS, the two most commonly detected PFAS, which confines the combined concentration of PFOA and PFOS in drinking water to 70 parts per trillion (ppt) [11]. More recently, EPA unveiled a PFAS Action Plan that will move forward with the Maximum Contaminant Level (MCL) process for PFOA and PFOS. As part of the Action Plan, U.S. EPA will continue their enforcement actions and clarify clean up strategies, expand monitoring of PFAS in the environment, and enhance research and the scientific foundation for addressing PFAS by developing new analytical methods and tools.

Internationally, Environment and Climate Change Canada promulgated the Risk Management Approach for long-chain PFCAs in 2012 and prepared a draft screening assessment for PFOA, its salts, and its precursors [12]. In Norway, PFOA had been banned from consumer products since 2013 [13], [14]. In Germany, the MCL for the sum of PFOA and PFOS in drinking water was recommended to be 0.3 ppb [15]. These regulations resulted in a worldwide production shift from long-chain to short-chain PFAS. Due to this manufacturing shift and as short-chain PFAS often result from partial breakdown of long-chain PFAS, it is expected that the impacts of short-chain PFAS will continue to mount, and accordingly, proper regulations will emerge in the near future, although there has been no regulation or advisory available specifically targeting short-chain PFAS [16], [17]. For instance, the two most common short-chain PFAS, perfluorobutane sulfonic acid (PFBS) and perfluorobutanoic acid (PFBA), have been detected widely in drinking water, sediment, sewage sludge, and even snow/ice in the polar area [18], [19], [20], [21], [22]. At a site of Xiaolangdi, China, short-chain PFAS accounted for as high as 89% of the total PFAS in the aquatic environment [3], [23]. The potential health risks of short-chain PFAS, such as perfluorohexanoate (PFHxA), perfluoroheptanoate (PFHpA), and PFBA, are feared at least as high as PFOA [24], [25], [26], [27], [28], [29]. Further, short-chain PFAS are less adsorbable, more persistent and more mobile in groundwater and soil, and thus could transport over a long range of distance and pose lasting environmental impacts [30], [31]. In the 2018 European Chemicals Agency (ECHA) ANNEX XV report, PFHxA was proposed to be added into the list of Article 57 of Regulation No 1907/2006, since the C6 PFCA was as persistent as long-chain PFAS while being mobile in soil and groundwater [32]. In addition, the concentration of PFHxA is expected to increase due to the continuous degradation of longer-chain precursors.

Many treatment technologies have been tested to treat PFAS, though the past efforts have been focused on long-chain PFAS, including adsorption by activated carbon (AC) and ion exchange resins, membrane processes, chemical oxidation/reduction, photochemical decomposition, electrochemical oxidation, sonolysis, and incineration [33], [34], [35], [36], [37]. Yet, cost-effective treatment methods remain lacking to lower the PFAS concentration to the ppt level and/or to destroy or mineralize PFAS. It is particularly challenging to break the highly stable C-F bond (536 kJ/mol) and transform PFAS into innocuous products [3], [29], [38], [39], [40].

While the occurrence, fate, and transformation of long-chain PFAS (mainly PFOA and PFOS) are relatively well documented, much less information is available on occurrence and treatment approaches for short-chain PFAS. In particular, treatment approaches specifically designed for short-chain PFAS are quite limited in the open literature [41], [42]. The goal of the present review was to provide a systematic overview of the latest knowledge on the occurrence, impacts, and engineered treatment of short-chain PFAS. The specific objectives were to: 1) summarize the latest information about the type and level of short-chain PFAS in the aquatic systems; 2) update the knowledge on the toxicity and health risks associated with short-chain PFAS; and 3) overview the available engineered technologies for removal and transformation of short-chain PFAS, including adsorption, oxidation/reduction, photocatalytic degradation, thermolysis, sonolysis, and membrane filtration. The information is important for improved understanding of the environmental impacts of short-chain PFAS and for facilitating development of more cost-effective technologies. As short-chain PFAS are often associated with long-chain PFAS, the knowledge derived would be also useful for long-chain PFAS.

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[1] Url: https://www.sciencedirect.com/science/article/abs/pii/S1385894719319096

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