Removal of perfluoroalkanesulfonic acids (PFSAs) from synthetic and natural groundwater by electrocoagulation

Highlights

• PFSAs in groundwater could be removed by the periodically reverse electrocoagulation.

• Orthogonal experiments were used to confirm the best conditions for the treatment.

• Optimal removal efficiencies of PFSAs in synthetic groundwater were up to 87.4%–100%.

• 59.0%–100% of removal efficiencies reached for PFSAs removal from natural groundwater.

Abstract

Severe contaminations of perfluoroalkanesulfonic acids (PFSAs) existed in the natural groundwater beneath a fluorochemical industrial park (FIP) in Fuxin of China. In the present study, systematic researches were performed to determine the best conditions of efficient treatment for 1 mg L⁻¹ of PFSAs in the synthetic groundwater samples with the periodically reverse electrocoagulation (PREC) using the Al–Zn electrodes. Based upon the orthogonal experiments, the removal efficiencies of perfluorobutane sulfonate (PFBS), perfluorohexane sulfonate (PFHxS), and perfluorooctane sulfonate (PFOS) could reach 87.4%, 95.6%, and 100%, respectively, within the initial 10 min, under the optimal conditions of voltage at 12.0 V, pH at 7.0, and stirring speed at 400 rpm. In addition, the optimized PREC technique was further applied to remove the PFSA contaminations from the natural groundwater samples of the Fuxin FIP, subsequently generating the removal efficiencies of three target PFSA analytes in the range between 59.0% and 100% at 60 min. Moreover, the SEM-EDS analyses showed the hydroxide flocs formed during the process of PREC treatment had clear characteristics of floc aggregates, with the major constituents of O, Al, C, N, Zn, and F elements. As a result, long-chain PFHxS and PFOS tended to be eliminated completely from the natural groundwater by their absorptions on the Al–Zn hydroxide flocs, potentially because of their higher hydrophobicity compared with short-chain PFBS.

Introduction

Perfluoroalkyl substances (PFASs), consisting of perfluoroalkanesulfonic acids (PFSAs), perfluoroalkyl carboxylic acids (PFCAs), perfluoroalkyl phosphinic acids (PFPiAs), perfluoroether carboxylic and sulfonic acids (PFECAs and PFESAs), and their fluorotelomer-based precursors, have been widely manufactured and used as effective and efficient surfactants and surface protectors in a wide variety of domestic and industrial productions around the world, since their initial commercialization seventy years ago, including carpets, leather, paper, packaging, fabric, upholstery, aqueous film-forming foams (AFFFs), mining and oil well surfactants, alkaline cleaners, floor polishes, photographic film, denture cleaners, shampoos, and insecticide (OECD, 2018). It has been well-known that the enormously strong carbon-fluorine (C–F) bonds (115 kcal mol⁻¹) in the structures of perfluoroalkyl moiety contribute to the exclusive physio-chemical characteristics of PFASs, involving extraordinary resistance to both environmental and biological degradation, thermal and chemical stability against oxidation, photolysis, and hydrolysis reactions, as well as both hydrophobicity and oleophobicity (Kissa, 2001). Accordingly, the large-scale productions and widespread applications of PFASs have resulted in the ubiquitous distributions of these compounds in different biotic and abiotic matrices so far (Giesy and Kannan, 2001; Houde et al., 2006, 2011). Moreover, additional studies revealed that PFASs have bioaccumulative ability in diverse living organisms (Sedlak et al., 2017; Letcher et al., 2018) and various toxicities such as developmental toxicity, hepatotoxicity, and immunotoxicity (Lau et al., 2007; Gomis et al., 2018). Consequently, perfluorooctane sulfonate (PFOS), its salts and precursor, perfluorooctane sulfonyl fluoride (PFOSF), were added into Annex B of the Stockholm Convention on Persistent Organic Pollutants (POPs) in 2009, calling for restricted uses worldwide (UNEP, 2009). In the recent years, the Persistent Organic Pollutants Review Committee (POPRC) has also evaluated the proposals for potential inclusions of perfluorohexane sulfonate (PFHxS), perfluorooctanoic acid (PFOA), their salts and PFHxS- and PFOA-related compounds into the Stockholm Convention on POPs (UNEP, 2017, 2019).

As an alternative for long-chain PFSAs (C ≥ 6), short-chain perfluorobutane sulfonate (PFBS) has been adopted for commercial manufacture, since the ban of PFOS production. However, the technical performance of short-chain PFBS are lower than that of long-chain PFHxS and PFOS, much larger quantities of PFBS have thus been employed to achieve a similar performance to PFHxS and PFOS (Lindstrom et al., 2011a). It is evident that short-chain PFASs are highly mobile in the water bodies, and their final degradation products are extremely persistent, hence a lack of proper water treatment techniques for short-chain alternates would bring about the never-ending existence of these contaminations in the aqueous environment (Brendel et al., 2018).

Some further epidemiological studies have shown that populations expose to PFAS contaminations in the environment via several important pathways, such as daily intake of drinking water and dietary (Fromme et al., 2007; Hölzer et al., 2008) and inhalation of indoor dust (Strynar and Lindstrom, 2008), and the human exposure might lead to the increased levels of serum cholesterol, uric acid, liver enzymes, estradiol, and thyroid hormone (Nelson et al., 2010; Steenland et al., 2010; Lopez-Espinosa et al., 2012). Many evidences showed that PFAS contaminations could migrate into the shallow groundwater through the release of aqueous film-forming films (AFFFs) (Moody et al., 2003), the outflow of wastewater from fluorochemical production facilities (Hoffman et al., 2011), or the utilization of biosolids (Lindstrom et al., 2011b). As a result, the US Environmental Protection Agency (U.S.EPA) has issued a health advisory with the levels of 0.070 μg L⁻¹ for both PFOS and PFOA in drinking water originated from surface water and groundwater throughout the nation (U.S.EPA, 2016). In light of historical discharge of PFAS contaminations into local groundwater (ATSDR, 2005), the Minnesota Department of Health (MDH) recently established the health risk limits (HRLs) for both PFOS and PFOA in drinking water with even lower levels at 0.015 and 0.035 μg L⁻¹, respectively (MDH, 2018; 2019a). In addition, the HRLs for the other two PFSAs involving PFHxS and PFBS in drinking water were also issued lately by the MDH at 0.047 and 3 μg L⁻¹, respectively (MDH, 2017; 2019b).

Fuxin fluorochemical industrial park (FIP) has been built in northeastern China since 2004, because of the local abundant resources of mineral fluorite (CaF₂). Our previous study (Bao et al., 2019), focused on the groundwater beneath the FIP, determined that the dominant PFAS contaminants in regional groundwater, including PFOA, PFOS, PFHxS, and PFBS, reached the maximum concentrations up to 2.51, 0.403, 1.14, and 21.2 μg L⁻¹, respectively, all of which remarkably exceeded the updated HRLs from the MDH mentioned above. Furthermore, short-chain PFBS could enter the home-produced vegetables via irrigation with local groundwater.

So far, few studies have been implemented on the efficient removal of PFAS contaminations from natural groundwater, especially short-chain PFASs, most of which concentrating on the treatment of groundwater PFOS and PFOA contaminations. For instance, Schaefer et al. (2015) applied the electrochemical treatment with commercially-produced Ti/RuO₂ anode in a divided electrochemical cell for the decomposition of PFOS and PFOA in the AFFF-impacted groundwater from a former firefighter training area. In 2017, Schaefer et al. further used a nanocrystalline boron-doped diamond (BDD) anode for the electrochemical treatment of PFOS and PFOA in natural groundwater. Furthermore, Xiao et al. (2017) implemented on the removal of PFOS and PFOA contaminations from the AFFF-impacted groundwater by the sorption of biochars and activated carbon. Our recent study (Liu et al., 2018) adopted the periodically reverse electrocoagulation (PREC) with different electrode materials for the removal of PFOA contamination from the groundwater beneath the Fuxin FIP. Compared with the conventional EC technique, the PREC could efficiently eliminate the passivation generated from long-term use of single electrode (Pi et al., 2014). However, further studies would still be required to investigate the removal efficiencies of the PREC technique for the treatment of PFSA contaminations in the natural groundwater around the fluorochemical facilities.

The objectives of the present study were to i) compare the treatment effects of the PREC technique on the removal of three PFSAs from the synthetic groundwater under different conditions, involving the voltage, the pH value, and the stirring speed, thereafter further confirm the optimal treatment parameters with the orthogonal experiments; ii) determine the removal efficiencies of all the PFSA contaminations from the natural groundwater using the optimized PREC technique; and iii) investigate the morphology features of the floc aggregates that generated during the process of PREC treatment based upon the additional analysis.