A review of the occurrence, transformation, and removal of poly- and perfluoroalkyl substances (PFAS) in wastewater treatment plants

Highlights

• PFAS occurrence information is not available for most developing countries.

• Monitoring is required for ultra-short chain and new generation replacement PFAS.

• Majority of studies report higher PFAS concentrations in effluents than influents.

• Biodegradation transforms PFAS precursors to PFAA at a secondary stage.

• Adsorption, electrochemical, and filtration are the most studied PFAS treatments.

Abstract

Poly- and perfluoroalkyl substances (PFAS) comprise more than 4,000 anthropogenically manufactured compounds with widescale consumer and industrial applications. This critical review compiles the latest information on the worldwide distribution of PFAS and evaluates their fate in wastewater treatment plants (WWTPs). A large proportion (>30%) of monitoring studies in WWTPs were conducted in China, followed by Europe (30%) and North America (16%), whereas information is generally lacking for other parts of the world, including most of the developing countries. Short and long-chain perfluoroalkyl acids (PFAAs) were widely detected in both the influents (up to 1,000 ng/L) and effluents (15 to >1,500 ng/L) of WWTPs. To date, limited data is available regarding levels of PFAS precursors and ultra-short chain PFAS in WWTPs. Most WWTPs exhibited low removal efficiencies for PFAS, and many studies reported an increase in the levels of PFAAs after wastewater treatment. The analysis of the fate of various classes of PFAS at different wastewater treatment stages (aerobic and/aerobic biodegradation, photodegradation, and chemical degradation) revealed biodegradation as the primary mechanism responsible for the transformation of PFAS precursors to PFAAs in WWTPs. Remediation studies at full scale and laboratory scale suggest advanced processes such as adsorption using ion exchange resins, electrochemical degradation, and nanofiltration are more effective in removing PFAS (~95–100%) than conventional processes. However, the applicability of such treatments for real-world WWTPs faces significant challenges due to the scaling-up requirements, mass-transfer limitations, and management of treatment by-products and wastes. Combining more than one technique for effective removal of PFAS, while addressing limitations of the individual treatments, could be beneficial. Considering environmental concentrations of PFAS, cost-effectiveness, and ease of operation, nanofiltration followed by adsorption using wood-derived biochar and/or activated carbons could be a viable option if introduced to conventional treatment systems. However, the large-scale applicability of the same needs to be further verified.

Introduction

Per- and polyfluoroalkyl substances (PFAS), a class of artificially manufactured compounds, have been extensively used in various industrial and commercial products from the mid-twentieth century (Wang et al., 2017). Comprised of a hydrophobic fluorinated alkyl chain at one end and a hydrophilic functional group at the other (Arvaniti et al., 2014), PFAS (C_nF_2n+1-R; R: Functional group) (Meng et al., 2020, Buck et al., 2011) have high thermal and chemical stability owing to the strong C-F bonds (Mulabagal et al., 2018). This unique combination of physical and chemical properties, along with the hydrophobic and lipophobic moieties, make PFAS extremely suitable as water and oil repellants and friction resistants (Kwiatkowski et al., 2020). Due to the desired performance and production suitability, PFAS are manufactured massively for wide-scale industrial (manufacturing of paper, textiles, pesticides, leather, medical aids, oil, and minerals, metal plating etc.) and consumer applications (food packaging, cosmetics and personal `care products, paints, inks, non-stick cooking utensils, surfactants, firefighting foams, and several other waterproof products) (Gagliano et al., 2020; Ji et al., 2020; Wang et al., 2017; Lu et al., 2017; Kotthoff et al., 2015). The general purpose of treating consumer products with PFAS is to impart an extended service life and durability. This practice, in turn, results in prolonged exposure of humans, wildlife, and the environment to PFAS (Herzke et al., 2012).

Although PFAS have been used widely since the 1950s, it was only after the first report of PFAS in wildlife and human serum in the early 2000s that scientific studies on the properties, occurrence, environmental fate, and health effects of PFAS accelerated (Cousins, 2015). Over the past few decades, PFAS have been widely detected in the environment (Nakayama et al., 2019), and they are ubiquitous in almost all aquatic matrices such as drinking water, surface water, groundwater, and coastal water (Wei et al., 2018). Humans can be exposed to PFAS through drinking water and dietary intake (Domingo & Nadal, 2019, Sunderland et al., 2019, Wei et al., 2018). Studies have reported that several acute as well as chronic human diseases, such as thyroid, asthma, anxiety, obesity, pediatric allergies, hyperuricemia, peroxisome proliferation, immune toxicity, kidney disorders, liver damage, and cardiovascular diseases, may be associated with PFAS exposure (Ruan et al., 2019, Jian et al., 2017, Suja et al., 2009). Other studies have demonstrated toxic effects, such as immunotoxicity, carcinogenicity, and hormonal disorders, of some perfluoroalkyl carboxylic acids (PFCAs) on animals (Poothong et al., 2020, Rand & Mabury, 2017).

Today, more than 4,000 PFAS are manufactured and used worldwide, including both conventional (e.g., PFOA and PFOS) and alternative PFAS (short-chain PFAS and newly identified fluorinated replacements of conventional PFAS) (Pan et al., 2019). Anticipating long-term exposure effects due to their persistent and bio-accumulative nature (Ateia et al., 2019a, (Wang et al., 2016), conventional PFAS were voluntarily phased out (Wang et al., 2014b, Krafft & Riess, 2015) due to regulations (e.g., US EPA drinking water guideline for PFOA and PFOS: 70 ng/L) (Li et al., 2020, Ateia et al., 2019b, Pan et al., 2019). Still, many studies have reported high levels of conventional PFAS in WWTP effluents (>100 ng/L) (Table 2 and 3). Alternative PFAS have also been detected abundantly in WWTPs by several studies with concentrations up to some hundred ng/L (Table 2 and 3). Short-chain PFAS, being highly mobile, are detected in all ecosystems (Brendel et al., 2018) and can be equally toxic as their longer-chain counterparts (Li et al., 2020).

The occurrence of PFAS in WWTPs has been reviewed earlier by Arvaniti & Stasinakis, 2015. However, more than 100 new monitoring studies have been published since 2015. Some involve aspects not covered earlier, such as the effect of different treatment stages of a WWTP on removal efficiencies and the transformation of PFAS precursors to perfluoroalkyl acids (PFAAs) during wastewater treatment, that allows a better understanding of the fate of PFAS in WWTPs. Overall, this article aims at providing a systematic overview of the up to date knowledge on the occurrence, fate, transformation, and removal of PFAS in WWTPs.

First, the sampling and analysis methods adopted in monitoring studies are discussed, followed by the global occurrence data of different types of PFAS in the influents and effluents of WWTPs. The removal efficiencies and mechanisms are then discussed in detail, emphasising the fate of PFAS in a full-scale treatment plant and the transformation of PFAS precursors during treatment. This comprehensive review on the levels and fate of conventional and alternative PFAS in WWTPs, along with their removal at different scales, allows the identification of research gaps. Those are compiled in the last section, along with suggestions for future research concerning alternative PFAS (especially ultra-short chain PFAS and PFAS precursors), transformation mechanism of PFAS precursors, and large scale applications of laboratory tested PFAS treatment techniques.