# A Review of Treatment Techniques for Short-Chain Perfluoroalkyl Substances
## 1 Introduction  
Since the 1950s, perfluoroalkyl substances (PFASs) have been widely used in industrial production and commercial products, involving chrome plating, foam extinguishing agents, aviation hydraulic oil, and food packaging paper [1, 2]. They are a class of manade chemicals with all the hydrogen atoms on the carbon skeleton replaced by fluorine atoms, together with a terminal functional group [3]. Abbreviations for different PFASs are shown in Table 1. Due to the strong energy of the C-F bond (536 kJ/mol), PFASs possess exclusive physio-chemical characteristics, including environmental persistence, extraordinary resistance to both environmental and biological degradation, high thermal and chemical stability against oxidation, photolysis, and hydrolysis reactions, hydrophobicity and oleophobicity, as well as multiple toxicities [4]. Moreover, many PFAS (precursors) can easily degrade into persistent PFAS (acids). Therefore, long-chain PFASs (C8-C14) and their sodium, as well as ammonium, salts were added into the candidate list of regulatory substances in the EU, and PFOA and PFOS were added in the Stockholm Convention on Persistent Organic Pollutants (POPs) list [5]. With the ban of long-chain PFASs, short-chain PFASs (PFCAs < C8, PFAS < C7) have been produced and used as substitutes in large quantities. With the improvement of modern analytical techniques such as high-resolution mass spectrometry in non-target and suspect screening approaches in recent years, the researchersfound that long-chain PFASs tended to be adsorbed by solid matter (soil, sediment, etc. ), which made them less mobile. However, short-chain PFASs showed a high polarity and solubility, which rendered them difficult to be removed through environmental adsorption and water treatment processes, thus contributing to long-term mobility in the water cycle through migration of surface water, groundwater, and natural and urban water systems [6; 7; 8; 9]. However, short-chain PFASs show similar properties with long-chain congeners, including being persistent, bioaccumulative, and toxic to a certain extent [10]. Therefore, short-chain PFASs could be classified as persistent and mobile organic compounds (PMOCs) [6]. Large quantity usage of short-chain PFASs could lead to accumulation in the water cycle and water pollution, thereafter threatening drinking water quality. For instance, the concentrations of PFBS and PFBA in Tangxun Lake of Wuhan in China reach up to 3.66 ug/L and 4.77 ug/L, respectively [11]. There was a serious PFBS contamination in groundwater around fluorochemical plants in Fuxin in China, with a concentration up to 31 ug/L [12], exceeding 10 times the health risk limits (HRLs) in drinking water (3 ug/L) issued by the Minnesota Department of Health (MDH). Thus, it is an urgent incident to effectively develop suitable water treatment techniques to regulate short-chain PFAS contaminations in waters. At present, activated carbon (AC) adsorption and ozonation are the commonly applied techniques for organic contaminant elimination in drinking water treatment. However, AC presented low efficiency for very polar compounds [6]. Meanwhile, ozonation generally exhibited poor reactive activity to polar compounds containing acidic functional groups [13], which might be a source of smaller and more polar by-products than the parent compounds [14]. Consequently, the removal efficiency of conventional treatment for short-chain PFASs was undesirable. Moreover, various techniques presented different removal efficiencies for short-chain PFASs, even requiring high energy consumption, strict operating conditions, and releasing a large number of by-pollutants [15]. Recently, the C40 Cities Network of 91 large cities committed to low carbon infrastructure to ensure carbon emissions peak by 2020 and almost halve by 2030 [16; 17; 18]. In 2020, China announced that it would take more aggressive policy measures to achieve peak carbon dioxide emissions by 2030 and achieve carbon neutrality by 2060 [19].