Breakdown of PFOA, PFOS and 6:2FTS Using Acidic Potassium Permanganate as Oxidant

Research Article

Austin Environ Sci. 2016; 1(1): 1005.

Breakdown of PFOA, PFOS and 6:2FTS Using Acidic Potassium Permanganate as Oxidant

Fanga C*, Megharaja M and Naidu R

University of Newcastle, Australia

*Corresponding author: Cheng Fang, Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE), University of Newcastle, Callaghan NSW 2308, Australia

Received: July 11, 2016; Accepted: August 04, 2016; Published: August 08, 2016


We found that inert fluoro-carbon skeletons of Perfluorooctanoic Acid (PFOA), Perfluorooctane Sulfonate (PFOS) and 1H, 1H, 2H, 2H-perfluorooctanesulfonic acid (6:2FTS) could be broken down by potassium permanganate as oxidant in acidic liquid phase at room temperature. This opened a new Approach to the Remediation of Aqueous Film Forming Foams (AFFFs). The breakdown was confirmed from the HPLC-MS and Ion Chromatography (IC) data. Due the oxidization’s contribution, those fluoro-carbon skeletons’ half-life was estimated to be approximately 3 months, much shorter than the several decades that occur in nature.

Keywords: PFOA; PFOS; 6:2FTS; Breakdown; Oxidization; Potassium permanganate


Poly- and Perfluoroalkyl Substances (PFASs) exhibit unique physical and chemical properties, such as hydrophobicity and oleophobicity, which are not evident in other components, and also extreme stability with respect to thermal, chemical and biodegradation [1]. Due to their important anti-wetting and anti-staining properties, they have been used widely and domestically in such activities as clothing, upholstery, carpeting, painted surfaces, food containers, cookware, etc [2]. However, since their fluoro-carbon skeletons are inert and resistant to biodegradation under natural environmental conditions (CF3-CF3 of 99 kcal/mol vs CH3-CH3 of 89 kcal/mol) [3,4] this has led to their global accumulation and distribution in the environment. This has in turn raised serious concerns about their impact on the environment and public health [5-7].

Aqueous Film Forming Foam (AFFF) is a good example that has been widely used to extinguish fires [8,9]. Its main ingredients are anionic fluoro surfactants such as Perfluorooctane Sulfonate (PFOS) and perfluorooctanoic acid (PFOA). Due to serious misgivings about their biological and environmental impact and their persistent nature, PFOS was phased out in the early 2000s. Lots of alternatives were found on the market. These include, for example, 1H, 1H, 2H, 2H-perfluorooctanesulfonic acid (6:2FTS)-, and 1H, 1H, 2H, 2H-perfluorodecane sulfonic acid (8:2FTS)-based fluoro surfactants [10]. Those fluorotelomers were synthesised via telomerisation with linear structures that differ from the products of electrochemical fluorination, such as PFOS containing linear and branched isomers [6,11]. Although they are re-ported to be environmentally safe their fluoro-carbon skeletons still raise concerns about their biodegradability in the natural environment [12]. For example, the half-life of 6:2FTS is estimated to be >10 years 12, which is shorter but still similar to >41 years for PFOS, and >92 years for PFOA (USEPA 505-F-14-001), respectively. It should be noted that those values depend on estimating approach, initial concentration, degradation conditions, etc., and consequently they have varied in the literature [12-14]. New ingredients thus include short chains of the fluoro- carbon (C3-C6) [15,16] and fluorine-free surfactants [17].

Previously we used chemical oxidant (potassium permanganate, KMnO4) to break down the derived groups of fluoro surfactants because the non-fluoro-carbon can be broken down much more easily than fluoro-carbon skeleton [18]. However, we found that the fluorocarbon skeleton could potentially be broken down, although the process was slow. This particular phenomenon is interesting because it might lead to a new degradation approach that is different from previous ones, for example Fenton reaction of hydrogen peroxide [19], persulphate [4], advanced electrochemical oxidization [20], Sonolytic conversion [21] etc. Compared to the heat-up approach [22], this mild condition (occurred at room temperature) offers the promise to scale-up its application. The additional advantages of this oxidant include cost-effectiveness, stability, environmentally safe, easy to operate etc [23,24]. Here we selected 3 common fluoro surfactants - PFOA, PFOS and 6:2FTS - to verify the possibility of breaking down these ingredients using KMnO4.

Materials and Methods

All chemicals including PFOA, PFOS and 6:2FTS, potassium permanganate (KMnO4, ACS reagent, =99.0%), hydrogen chloride (HCl, 37%, w/w, AR), methanol and ammonium acetate (NH4Ac) were purchased from Sigma-Aldrich (Australia). Only polypropylene containers/pipette tips were used throughout to avoid any potential interference from Teflon containers/caps. Milli-Q water was used (> 18 MΩ•cm) in the present study.

All samples were diluted in Milli-Q water in centrifuge tubes (polypropylene) without pre-treatment. 0.1% KMnO4 + 0.36% HCl (w/w) was placed in the tubes for the oxidization process [4,23,24]. The tubes were kept at room temperature (~24°C) and not shielded from the laboratory fluorescent lamp for the purposes of domestic lighting. The tubes were occasionally shaken (once per day) during oxidization. Samples were filtered with nylon syringe filters (0.2μm) prior to HPLC-MS analysis HPLC-MS (Agilent 1260 + Quadrupole 6130) before and after the oxidization [25,26].

For HPLC-MS analysis, we followed the standard method (EPA/600/R-08/092) [27]. In general, 10μL sample solution was injected into Agilent 1260 high-performance liquid chromatography fitted with an XDB-C18 column kept at 40°C. Its dimensions were 2.1 mm internal diameter, 100 mm length and 5μm particle size. The flow rate was 0.5mL/min for gradient mobile phase of methanol: 5mM aqueous NH4Ac for separation. Quadrupole 6130 detector was maintained at 70 V under negative mode for scanning. Extraction of the molecular ions was conducted at m/z 413 for PFOA, 499 for PFOS and 427 for 6:2FTS, respectively. Quantification was done by producing a calibration curve using external standard solutions of PFOA, PFOS (only linear isomers were quantified) and 6:2FTS with correlation coefficients higher than 0.99 and limit of detection being ~0.2 ppb (signal: noise > 3). Blank samples of Milli-Q water and methanol were run prior to each set of test to minimize any background contamination that could have originated from the Teflon components of the HPLC instrument itself. The nebulizer gas (nitrogen) pressure was set at 40 psi, drying gas flow rate was 9L/min and temperature 325 °C, capillary voltage was + 3500 V and skimmer voltage was – 15 V. More details are listed in Ref [21].

Free fluoride (F-) and sulphate (SO4 2-) were detected using Ion Chromatography (IC), which was conducted using DIONEX (ICS- 2000, RFIC, and Thermo Scientific). The ion exchange column was IonPacTM AS18, 2 × 250 mm and kept at 35 °C under 2230 psi pump pressure. Following 25μL sample injection, 10 mM KOH was gradually flow at 0.25mL/min. Conductivity detector was employed with a suppressor of 43mA.

Note that each time we ran at least 6 samples in parallel (3 samples without addition of oxidant as controls and the other 3 samples with oxidant) for quality assurance and quality control (QA/QC) [28].

Results and Discussion

Figure 1 indicates that the colour change depends on the oxidization process. At the beginning, the purple solution confirms the existence of KMnO4. The colour became increasingly darker and changed to brown after 3 months, suggesting the decomposition of oxidant KMnO4. With this decomposition some targets have been oxidized, such as PFOA, PFOS or 6:2FTS in the solution, although the nature of the decomposition of KMnO4 should not be ignored [23,24].