Sonochemical degradation of perfluorinated chemicals in aqueous film-forming foams
Graphical abstract
Introduction
Aqueous film-forming foams (AFFFs) are complex mixtures whose major components include a solvent (typically a glycol ether), fluorochemical surfactants (perfluorinated anionic and partially fluorinated amphoteric), and hydrocarbon-based surfactants [1]. Fluorochemical (FC) surfactants represent 1–5% w/w of the AFFF composition and impart properties such as good spreadability, negligible fuel diffusion, and thermal stability to the foam [2]. AFFFs were produced from 1960s to 2001 as important chemical agents for extinguishing hydrocarbon-fuel fires; as of 2004, the US military still possesses almost 11 million liters of these across the country [2]. Continuous use of AFFFs for fire-fighting training and equipment maintenance in airports and former US military fire-fighting training sites [3] has caused groundwater contamination with FC concentrations ranging from 125 to 7090 μg l−1 [1].
The same unique attributes that have made AFFF effective for fighting fires, especially those fueled by petroleum hydrocarbons, have also resulted in concern for these compounds in the environment. AFFFs are persistent in the environment. Concentrations of FCs and FC-precursors have been measured in groundwater, soil and sediments in a US Air Force Base where AFFFs were used until 1990 [4]. Forty to 100% of total initial concentrations of FC in AFFFs were FC-precursors at the site 20 years after use of AFFFs was discontinued. FCs, which were components of AFFFs, are known to be bio-accumulative and to cause toxicity [5], [6], [7], [8]. The presence of FCs in mammals, birds, and fishes was first reported in 2001 [9], [10]. Many studies have investigated the presence of FCs in human tissues and concluded that FCs accumulate in the human body during continuous exposure [11]. Due to the complexity of the AFFF mixture, there is limited information about toxicities of AFFFs [1]. Individual FCs can damage liver, affect the immune system, cause developmental and reproductive toxicology, and be carcinogenic to mammals [11].
Not only are FCs bio-accumulative and toxic, they are generally recalcitrant in the environment and resistant to natural degradation or chemical and biological treatment processes [12], [13], [14]. Effective degradation of individual FCs has been a challenge and often requires use of advanced oxidation processes at higher temperatures and/or pressures [14]. Use of ultrasonic and megasonic irradiation has been suggested as effective technologies to treat FCs [15], [16], [17]. When an acoustic field is applied to a liquid, the sound waves are transmitted with lower pressure (rarefaction) and higher pressure phases [18]. During low pressure cycles, in the bulk of the liquid containing dissolved gas or in some preexisting nuclei in the liquid, such as crevices or small solid particles, small cavities start to form [19]. The bubbles oscillate during several cycles (stable cavitation) and grow due to rectified diffusion until they become unstable and collapse during a higher pressure cycle (transient cavitation) [20]. The stable cavitation though milder can produce temperatures up to 2000 K [21], while collapsing events produce temperatures near 5000 K and pressures close to 1000 bar [19]. Such high temperatures and pressures produced during bubble oscillations or collapsing events will likely degrade FCs in solution, either by direct pyrolysis, or by reaction with hydroxyl radicals formed due to the homolytic decomposition of water under extreme conditions [19]. Sonochemical degradation of FCs would take place at the bubble-water interface, therefore adsorption of molecules on the bubbles is required prior to sonolysis.
When sonochemical degradation of 20 μM each of perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) was investigated at an acoustic frequency of 200 kHz, 60% degradation of PFOS and 85% degradation of PFOA was observed after sonication for 60 min [16]. Degradation of various FCs including perfluorobutanoic acid (PFBA), perfluorobutanesulfonic acid (PFBS), perfluorohexanoic acid (PFHxA) and perfluorohexanesulfonic acid (PFHxS) was observed at concentrations of 0.2–0.5 μM when a wide range of frequencies (202–1060 kHz) was applied [15]. The results of those studies suggested that the rate of sonochemical degradation of FCs was inversely proportional to length of the carbon chain. Since, different FCs would exhibit different degradation rates, sonochemical degradation of AFFF formulations depends on their specific compositions. PFOS (0.13–26.2 μM), PFHxS, PFBS and PFOA in the AFFF FC-600 were readily degraded under a 505 kHz frequency [22]. In a recent study, it was shown that sonochemical degradation of PFOS increased with the initial concentration until reaching a maximum (saturation concentration), and use higher sound frequencies increased the degradation rate but resulted in a less saturation concentration [23]. The main objective of the present study is to investigate sonochemical degradation of two AFFFs, 3M and Ansul, and effects of sound field and solution parameters such as the acoustic frequency (500–1000 kHz) and the initial FC concentration (200–930 times dilution) have on the efficiency of the process. Degradation of AFFFs was quantified by measuring the concentration of fluoride (F−) and sulfate (SO42−) released, as well as the total organic carbon (TOC) content and concentrations of selected FCs (PFBS, PFHxS and PFOS) with several durations of sonication.
Section snippets
Materials
Samples of AFFF, representative of stockpiles of AFFFs present at various Air Force bases in the United States, were provided by Davis-Monthan Air Force base in Tucson (Arizona, USA). The manufacturers of these samples were 3M and ANSUL (USA). Total organic fluorine (TOF) of AFFF was measured at University of Saskatchewan, Canada. TOF was measured by use of combustion ion chromatography (CIC), following methods described by Codling et al. [24]. Undiluted TOF concentrations were 790 mM for 3M and
Sonochemical degradation of AFFFs (3M and Ansul) under a 1 MHz sound frequency
Sonochemical degradation of commercial AFFFs (3M and Ansul) was studied under an acoustic frequency of 1 MHz. The AFFFs were diluted ∼930 times to an initial TOF concentration of 0.8 mM for 3M and 0.9 mM for Ansul. Concentrations of F− ion increased as a function of duration of sonication, from zero to 0.27 ± 0.01 mM and 0.37 ± 0.03 mM for 3M and Ansul, respectively (Fig. 1A). The rate of defluorination of Ansul (2.17 ± 0.17 μM F− min−1) was 45% faster than that of 3M (1.50 ± 0.05 μM F− min−1), leading to a
Conclusions
Degradation of two AFFF formulations, 3M (0.8 mM TOF) and Ansul (0.9 mM TOF), under 1 MHz sound frequency resulted in 45.4% and 34.2% defluorination, respectively after 180 min of sonication. Under similar conditions, measurements of TOC showed 39.0% and 29.4% mineralization for 3M and Ansul, respectively. Due to an increase in the available interfacial sites for the molecules to adsorb and possible increase in the number of cavitation events with increasing acoustic frequency, greater
Acknowledgments
The authors are grateful to the Air Force Civil Engineering Center for providing the funding for this project (FA8903-13-C-0011). We also want to thank Dr. Abrell from the Arizona Laboratory for Environmental Contaminants (ALEC, University of Arizona) for the determination of perfluorinated compounds in some samples. The research was supported, in part, by a Discovery Grant from the Natural Science and Engineering Research Council of Canada (Project # 326415-07) and a grant from the Western
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