Electrochemical treatment of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) in groundwater impacted by aqueous film forming foams (AFFFs)
Introduction
Groundwater impacts by poly- and perfluoralkyl substances (PFASs) are a growing environmental concern, particularly for perfluoroalkyl acids (PFAAs), a subgroup of PFASs that are extremely persistent in the environment [1]. Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS), with U.S. drinking water provisional health advisory levels of 0.4 and 0.2 μg/L, respectively, are the two PFAAs currently of greatest interest [2]. Release of these compounds into the subsurface is typically associated with firefighter training activities where aqueous film forming foams (AFFFs) have been applied. While some PFAAs are present in the various AFFF formulations that have been used in firefighter training areas [3], PFAAs can be produced in situ from the degradation of the PFASs present in AFFF [4], [5].
Once released to the environment, treatment of PFAAs such as PFOS and PFOA is challenging, as many conventional technologies used to treat organic contaminants in groundwater have proven to be ineffective or inefficient. Treatment approaches such as nanofiltration and advanced UV processes have shown promise for addressing PFAAs [6], [7], [8]. Recently, electrochemical treatment using boron-doped diamond (BDD) anodes in undivided cells has been shown to treat both PFOA and PFOS via first order kinetics [9], [10]. Fluoride generation was observed in these electrochemical experiments, providing clear evidence of PFOA and PFOS defluorination; however, several studies have noted that perchlorate formation can occur during anodic oxidation [11], [12]. Our preliminary studies using BDD anodes for treatment of PFAAs in chloride-containing natural ground waters (which contained approximately 100 mg/L of chloride) resulted in large (>1 mg/L) increases in perchlorate.
Using similar electrochemical treatment approaches in undivided cells, various custom-synthesized mixed metal oxide (MMO) anodes have been shown to treat PFOA, also with apparent first order kinetics and fluoride generation [13], [14], [15]. However, MMO anode materials (e.g., Ti/IrO2 and Ti/RuO2) that are commercially available in large quantities, which would likely facilitate the commercial use of MMO electrochemical treatment technologies for groundwater treatment, were simply described as having “poor performance” with respect to PFOA degradation [13].
While previous electrochemical PFAA decomposition research has shown promise, the research typically has not been performed under environmentally relevant conditions. Rather, such research has typically been conducted using a simplified synthetic ground water matrix with very high PFAA concentrations (e.g., 50 mg/L). Electrochemical treatment typically has consisted of employing an undivided cell using custom-made anodes. Further research is needed to understand electrochemical PFAA decomposition in environmentally and commercially relevant contexts, as several important questions remain unanswered regarding the potential for electrochemical treatment of PFAAs in groundwater. First, natural groundwater is a complex matrix containing many constituents. It currently is unknown, how the natural groundwater matrix will affect electrochemical PFAA treatment. Second, recent studies have shown that PFOA and PFOS contamination in groundwater typically occurs in the presence of a wide range of PFAAs and PFASs [5], [16]. The impacts of these mixtures on PFOA and PFOS treatment have not been investigated. In addition, to date, the electrochemical treatment studies for PFOA and PFOA have been performed at concentrations several orders of magnitude above the PFOS and PFOA concentrations that are typically observed in groundwater plumes (i.e., 1–1000 μg/L). Degradation kinetics may be substantially different at the decreased concentrations that are more typical of AFFF-impacted sites. Finally, previous studies have shown that electrochemical treatment of organic compounds can be more effective in divided electrochemical cells rather than undivided cells [17], [18]. Thus, the use of Ti/IrO2 or Ti/RuO2 anode materials in divided flow cells may result in improved treatment performance with respect to PFOA and PFOS. However, we are unaware of any published studies reporting on the performance of commercially-available MMO anodes for treatment of PFOA and PFOS in divided flow cells.
The overall goal of this research was to evaluate and demonstrate the electrochemical treatment of PFOA and PFOS in a natural groundwater from an AFFF-impacted former firefighter training area. A commercially-available MMO material, for use in a divided electrochemical cell, was selected for this study. PFOA and PFOS treatment rates were investigated as a function of current density, and transformation product formation was investigated. Treatment of other PFAAs present in groundwater also was assessed.
Section snippets
Groundwater collection and synthetic groundwater preparation
Groundwater used in this study was collected from within OU-1 at Ellsworth Air Force Base in South Dakota. A detailed discussion of the PFAS contamination at this site can be found in McGuire et al. [5]. The groundwater was collected from operating extraction wells in the vicinity of a former firefighter training area where AFFF reportedly was used. Groundwater was collected in four 5-gallon gas-tight soda kegs under nitrogen headspace. Collected groundwater was homogenized and stored at 4 °C
General observations
The voltage remained constant (±5%) during each experiment, and also during the duration of all experiments (for each current density tested), indicating that fouling or deactivation processes were not causing increased resistance at the electrode surfaces. Applied cell voltages for the span of current densities tested ranged from 4 to 13 V.
Electrochemical treatment expectedly resulted in a decrease in pH at the anode, and an increase in pH at the cathode. The anolyte pH results for the current
Conclusions
Using a divided electrochemical cell and a Ti/RuO2 anode, defluorination of both PFOA and PFOS was observed in a series of bench-scale experiments. This is, to the best of our knowledge, the first demonstrated application of this approach for perfluorinated compounds in AFFF-impacted groundwater. Other PFAAs also were treated electrochemically, but the shorter chain length PFAAs generally were more recalcitrant. Additional work is needed to further evaluate contaminant transformation pathways,
Acknowledgement
Funding for the project was provided, in large part, through the Air Force Civil Engineering Center (BAA Project 689).
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