Sonochemical degradation of perfluorinated chemicals in aqueous film-forming foams

Highlights

• Acoustic treatment at 1 MHz was effective at degrading PFCs in AFFF formulations.

• Defluorination and mineralization rates were higher at 1 MHz compared to 500 kHz.

• Rate of defluorination increased with greater initial concentrations of TOF.

Abstract

Aqueous film-forming foams (AFFFs) are complex mixtures containing 1–5% w/w fluorocarbons (FCs). Here, we have investigated degradation of two commercial AFFF formulations, 3M and Ansul, using sound field at 500 kHz and 1 MHz, with varying initial concentrations ranging from 200 to 930× dilution. The foams were readily degraded by 1 MHz, with percentage of defluorination ranging from 11.1 ± 1.4% (200× dilution of 3M) to 47.1 ± 5.8% (500× dilution of Ansul). Removal of total organic carbon (TOC) ranged from 16.0 ± 1.4% (200× dilution Ansul) to 39.0 ± 7.2% (500× dilution Ansul). Degradation of AFFF was affected by sound frequency with rates of defluorination 10-fold greater when the frequency was 1 MHz than when it was 500 kHz. Mineralization of TOC was 1.5- to 3.0-fold greater under 1 MHz than 500 kHz. Rate of fluoride release was 60% greater for the greatest initial concentration of FC in Ansul compared to the least initial concentration. While the rate of mineralization of AFFF was directly proportional to the initial concentration of Ansul, that was not the case for 3M, where the rates of mineralization were approximately the same for all three initial concentrations. Results of the study demonstrate that sonolysis is a promising technology to effectively treat AFFFs.

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.