Plasma-Based Water Treatment: Efficient Transformation of

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Plasma-based water treatment: Efficient transformation of perfluoroalkyl substances (PFASs) in prepared solutions and contaminated groundwater Gunnar R Stratton, Fei Dai, Christopher L Bellona, Thomas M. Holsen, Eric Reyvell Velazquez Dickenson, and Selma Mededovic Thagard Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04215 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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Environmental Science & Technology

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Plasma-based water treatment: Efficient transformation of perfluoroalkyl substances

2

(PFASs) in prepared solutions and contaminated groundwater

3 4

Gunnar R. Stratton1, Fei Dai2, Christopher L. Bellona3, Thomas M. Holsen2, Eric R. V.

5

Dickenson4, Selma Mededovic Thagard1,*

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1

7

Engineering, Potsdam, NY 13699 USA

8

2

9

USA

Clarkson University, Plasma Research Laboratory, Department of Chemical and Biomolecular

Clarkson University, Department of Civil and Environmental Engineering, Potsdam, NY 13699

10

3

11

80401 USA

12

4

13

Henderson, NV 89015 USA

Colorado School of Mines, Department of Civil and Environmental Engineering, Golden, CO

Southern Nevada Water Authority, Water Quality Research and Development Division,

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* Corresponding author. Tel.: +1 315 2684423; fax: +1 315 2686654.

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E-mail address: [email protected] (S. Mededovic Thagard)

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Abstract: A process based on electrical discharge plasma was tested for the transformation of

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perfluorooctanoic acid (PFOA). The plasma-based process was adapted for two cases, high

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removal rate and high removal efficiency. During a 30 minute treatment, the PFOA

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concentration in 1.4 L aqueous solutions was reduced by 90% with the high rate process (76.5 W

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input power) and 25% with the high efficiency process (4.1 W input power). Both achieved

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remarkably high PFOA removal and defluorination efficiencies compared to leading alternative

25

technologies. The high efficiency process was also used to treat groundwater containing PFOA

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and several co-contaminants including perfluorooctane sulfonate (PFOS), demonstrating that the

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process was not significantly affected by co-contaminants and that the process was capable of

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rapidly degrading PFOS. Preliminary investigation into the byproducts showed that only about

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10% of PFOA and PFOS is converted into shorter-chain perfluoroalkyl acids (PFAAs).

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Investigation into the types of reactive species involved in primary reactions with PFOA showed

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that hydroxyl and superoxide radicals, which are typically the primary plasma-derived reactive 1 ACS Paragon Plus Environment


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species, play no significant role. Instead, scavenger experiments indicated that aqueous electrons

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account for a sizable fraction of the transformation, with free electrons and/or argon ions

34

proposed to account for the remainder.

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Introduction

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There is considerable and growing concern over perfluoroalkyl substances (PFASs) due to

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their ubiquitous presence and recalcitrance in the environment, and toxicity in humans and

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wildlife.1-5 Manufacture, disposal and use of formulations and products containing PFASs or

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PFAS-precursors (e.g., aqueous film-forming foams) has resulted in PFAS contamination of

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groundwater and drinking water supplies.3,

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acid (PFOA) and perfluorooctane sulfonate (PFOS) is problematic due to their particularly high

42

prevalence, toxicity and resistance to transformation, which has prompted the U. S. EPA to issue

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health advisories for both compounds.8-9

6-7

In particular, the presence of perfluorooctanoic

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Conventional water treatment processes are not effective for the removal of perfluoroalkyl

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acids (PFAAs).10-12 Past research has also demonstrated that commonly used advanced oxidation

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processes (AOPs) such as ultraviolet light (UV) or ozone (O3) with hydrogen peroxide (H2O2)

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are not effective for PFAA transformation, due to the stability of the carbon-fluorine bond.12-13

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While some success has been found for PFAA transformation using alternative processes, such

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as sonolysis, activated persulfate and electrolysis, these typically involve significant chemical

50

and/or energy additions for decomposition reactions to proceed.13-17 As a result, researchers and

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practitioners have focused on the use of adsorbents such as activated carbon, and to a lesser

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extent, reverse osmosis for the treatment of PFAS-contaminated water.12,

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relatively short breakthrough times have been reported for activated carbon for shorter-chain

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PFAAs, and both processes produce a residual requiring disposal or further treatment.12, 18

18-20

However,

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For this work, plasma-based water treatment (PWT) was evaluated for degrading PFOA,

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which served as the model PFAA. Similar to other AOPs, PWT makes use of the highly

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oxidative radicals to oxidize chemical contaminants. Unlike other AOPs however, PWT involves

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the generation of radicals in situ and does not require significant chemical inputs. Additionally,

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plasma is capable of producing a broad range of reactive species (OH, O, H, O3, H2O2, eaq),

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including strong oxidants and reductants.21,22 Previous attempts have been made to utilize plasma

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to degrade PFASs; however, these involved the use of inefficient reactor types and DC

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discharges, which are less efficient than the pulsed discharges used in this study.23-27

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The primary objective of this study was to evaluate the efficacy of PWT for PFOA

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transformation and defluorination compared to the leading alternative technologies by operating

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the PWT process under two sets of parameters. The first was designed to target high removal rate 3 ACS Paragon Plus Environment


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by using high input power (76.5 W); the second was designed to target high removal efficiency

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by using far lower input power (4.1 W). Both cases involved the “laminar jet with bubbling”

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(LJB) reactor, which was found to be the most effective of several reactor types investigated in

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previous work.28 Additional objectives were to 1) determine whether co-contaminants affect the

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performance of PWT by treating samples of PFAS-contaminated groundwater, 2) quantify

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shorter-chain PFAAs that are formed as byproducts from the transformation of PFOA and PFOS,

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and 3) to determine the types of reactive species that play significant roles in the transformation

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of PFOA by PWT.

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Experimental

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Equipment and procedures. A custom-built high voltage (HV) pulsed power supply was

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used to generate the plasma. The electrical and operating parameters (discharge voltage,

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discharge frequency and load capacitance) were varied between experiments (Table 1). The

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voltage and current in the plasma reactor were measured using a Tektronix P6015A high voltage

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probe and a Tektronix P6021 current probe connected to a Tektronix TDS 3032C oscilloscope.

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Voltage and current waveforms are provided in the Supporting Information for the cases of

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laminar jet with bubbling with high rate and high efficiency (Figure S1). The general circuit

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diagram for the HV pulsed power supply can be found in a previous publication.28

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Table 1. Electrical and operational parameters for each set of experiments. Reactor type

Figure

Laminar jet with bubbling (high rate) Laminar jet with bubbling (high efficiency) Laminar jet with bubbling (byproduct trials) Liquid discharge Gas discharge with bubbling

1(a) 1(a) 1(a) 1(b) 1(c)

Discharge Capacitance Discharge frequency (Hz) (nF) voltage (kV) 120 2 +25.0 20 1 +16.0 43 0.94 +20.0 60 1 +18.8 and -16.5 60 1 +18.8 and -16.5

Discharge energy (J) 0.63 0.13 0.19 0.18 and 0.14 0.18 and 0.14

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Three different reactor types were used in this study (Figure 1) and consisted of a 17.3 cm

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diameter glass vessel (total volume = 3.8 L) fitted with an airtight polymer cap, which was

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adapted to allow for sample extraction, solution recirculation, and integration of the electrodes.

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The reactors were operated in semi-batch mode, with liquid recirculating at 1.4 L/min. The liquid

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recirculation loop ensured thorough mixing and included a heat exchanger to keep the solution at

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15°C. The headspace was purged with argon at 3.9 L/min either directly or through a submerged

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diffuser. It must be noted that although the argon was sourced directly from a pressurized

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cylinder, the power required to achieve the same flow rates with a gas pump was included in the

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input power calculations used in the following section (discussed further in the PFAS byproducts

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section). The general characteristics (size, shape, location, etc.) of the plasma discharges in the

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LJB and GDB reactors are shown in Figure S2. Detailed descriptions of each reactor are

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provided in a previous publication.28



98 99 100

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Figure 1. Reactor diagrams: (a) laminar jet with bubbling (LJB), (b) liquid discharge (LD) and (c) gas discharge with bubbling (GDB).

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The high concentration (20 µM) PFOA solutions were prepared by dissolving the PFOA

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(96% purity, Sigma-Aldrich, St. Louis, MO) in deionized water and adjusting the solution

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electrical conductivity to the desired value. For the aqueous electron ( eaq ) scavenger

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experiments, the PFOA solution contained 10 mM sodium nitrate (NaNO3), which yielded a

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conductivity of 1360 µS/cm. For all other experiments with high concentrations of PFOA, NaCl

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was used to adjust the conductivity to 1360 µS/cm. The groundwater experiments were

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conducted using unmodified samples from the effluent of an air stripper within the Former

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NAWC Warminster Groundwater Treatment Plant in Warminster, Pennsylvania. The conductivity

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of the groundwater was 1150 µS/cm. For the complimentary experiments with prepared solutions

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containing low concentrations of PFOA (3.1 nM) and PFOS (0.2 nM), the solutions were

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prepared

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heptadecafluorooctanesulfonic acid potassium salt, Sigma-Aldrich, St. Louis, MO) in deionized

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water and adding NaCl to adjust the conductivity to 1150 µS/cm. The solutions for the byproduct

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experiments were prepared in the same manner as the previous low-concentration solutions, but

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with a conductivity of 300 µS/cm.

−

by

dissolving

PFOA

(same

as

above)

and

PFOS

(98%

purity

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Analysis. Analysis of PFASs was carried out using a Waters Acquity UPLC coupled with a

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Xevo G2 QToF mass spectrometer and equipped with an Acquity HSS T3 (2.1 mm x 100 mm,

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1.8 µm) column and a 100 µL injection loop. The method employed for PFAS analysis has been

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described previously in full detail.29


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For the byproduct quantification experiments, the samples were concentrated using solid

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phase extraction and analyzed using isotope dilution liquid chromatography with tandem mass

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spectrometry (LC/MS-MS); further details of the method have been described previously.12

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Fluoride was analyzed by the EPA method 9214,30 using a Fisher Scientific accumet Excel

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XL60 meter kit with an accumet combination electrode and total ionic strength adjustment buffer

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(TISAB) obtained from VWR Chemicals.



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Results and discussion

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PWT efficacy for prepared solutions. Figure 2 compares the performance of the high

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efficiency and high rate embodiments of the LJB reactor in terms of reduction in PFOA

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concentration and percentage of fluorine recovered as F-.

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131 132

Figure 2. Normalized PFOA concentration and defluorination profiles for the LJB reactor

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configured for high treatment efficiency and high treatment rate.

134 135

The performances of these PWT processes were compared to those of some leading (sonolysis,31

alternative

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treatment17) and DC plasma in O2 bubbles23 (Table 2), in terms of the observed first-order

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removal rate constant (kobs) divided by the power density (PD = input power/treated volume),

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which is a measure of the PFOA removal efficiency. Compared to the other processes, PWT

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performed well, particularly the high efficiency PWT, which was about eight times more

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efficient than activated persulfate, about four times more efficient than electrochemical

142

treatment, and over 57 times more efficient than sonolysis.

treatment

technologies,

activated

persulfate,32

136

electrochemical

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Other performance indicators and corresponding experimental parameters for these seven

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processes are shown in Table 2. Because non-mineralized transformation byproducts may be

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more mobile than the parent compound and may still be harmful to the environment, the rate and

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efficiency of mineralization (transformation of PFOA to F- and CO2) is important. Though CO2

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must be measured to directly confirm that the PFOA is actually mineralized, defluorination has

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been used previously to represent mineralization13 and is used here for the same purpose. 8 ACS Paragon Plus Environment

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However, it must be noted that there may be discrepancies between rates and extents of

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defluorination and mineralization, and so the defluorination results and comparisons presented

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here should not be taken as directly representing mineralization.

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To make fair comparisons between defluorination capabilities, the differences in

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transformation rates must be accounted for. To determine this F50 was defined as the percentage

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of fluorine recovered in the form of fluoride normalized by the time corresponding to 50%

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reduction in PFOA concentration (t50 which is equal to −ln(1/2)/kobs). Therefore, F50/t50

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effectively represents the defluorination rate, while (F50/t50)/PD represents the defluorination

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efficiency. It should be noted that although t50 was not reached during the high efficiency

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treatments, PFOA transformation closely follows pseudo-first-order kinetics and defluorination

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rates are fairly constant, which allowed for t50 and F50 to be obtained by extrapolation.

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The high efficiency PWT process performed well compared with the alternative treatment

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methods, achieving a defluorination efficiency about 30 times greater than that of activated

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persulfate, 10 times greater than that of sonolysis and 15% greater than electrochemical

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treatment. Although the high rate PWT process does not compare as well in terms of efficiency,

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it provides insights into the operational versatility of the PWT and how process performance can

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be varied by changing operational parameters.

166 167

Table 2. Performance indicators and corresponding experimental parameters for high rate PWT,

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high efficiency PWT, sonolysis, UV-activated persulfate, electrochemical treatment and DC

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plasma in O2 bubbles. Parts of this table have been adapted and used in [33] (expected publication

170

date: 2017). Treatment High rate PWT

171 172

[PFOA]0 (µM) 20

PD (W/L) 54.6 a a

kobs (min-1)

k obs  −4 min −1   10 ⋅  PD  W L

F50  %    t 50  min 

0.074

14

2.1

3.8

this work

0.012

41

0.31

11

this work

F50 t 50  −2 % min  10 ⋅  PD  W L 

Ref.

High efficiency PWT

20

2.90

Sonolysis

20

250

0.018

0.72

2.5

0.99

[31]

UV/persulfate

50 b

23

0.012

5.2

0.09

0.38

[32]

Electrochemical

0.031 b

5.0

0.0057

11

0.47

9.5

[17] c

DC plasma in O2

100 b

1550

0.030

0.20

4.4

0.28

[23]

a

Input power includes power requirements for gas pump and plasma generation. b Performance may be sensitive to initial PFOA concentration, thus comparisons are approximate. c For a current density of 10 mA/cm2.



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PWT efficacy for contaminated groundwater. To determine whether the performance of

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PWT would transfer well to practical applications, where co-contaminants may interfere,

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samples of contaminated groundwater were treated using the high efficiency LJB reactor (Figure

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3). In addition to PFOA (~2.4 nM), the groundwater samples contained measurable

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concentrations of PFOS (~0.5 nM) and perfluorohexane sulfonate (PFHxS; ~1.0 nM) as well as

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non-fluorinated co-contaminants such as trichloroethene (3.6 µg/L), tetrachloroethene (0.33

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µg/L), and had a TOC concentration of 0.67 mg/L. For comparison, the high efficiency LJB

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reactor was also used to treat a prepared solution, containing similar concentrations of PFOA

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(3.1 nM) and PFOS (0.2 nM), but without any other co-contaminants.

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PFOA was degraded at about the same rate in both cases (within 2.5%), indicating that the

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non-PFAS co-contaminants present in the groundwater had no significant effect on PWT

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efficacy. Potential reasons for this are discussed in the key reactants section. Compared to the

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previous case of high efficiency LJB reactor with 20 µM PFOA, treatment of lower

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concentrations of PFOA yielded a value for kobs/PD about six times as large (250 min-1/(W/L)).

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While this highlights the magnitude of the influence of initial concentration, the mechanism

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underlying this influence requires further investigation.

189 190

These results also demonstrate that PWT is capable of degrading other PFASs, most notably PFOS, whose transformation rate constant was more than twice that of PFOA.

191 192 193

Figure 3. Normalized concentration profiles for PFASs in the contaminated groundwater and prepared solution.

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PFAS byproducts. Previous studies on the transformation of PFOA and PFOS have cited

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shorter-chain PFAAs as a major class of byproducts. A preliminary investigation was conducted

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to quantify shorter-chain PFAAs produced during treatment of a mixture of PFOS (detection

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limit: 1 ng/L) and PFOA (detection limit: 5 ng/L) in the LJB reactor. The anticipated byproducts

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that were tested for were perfluoroheptanoic acid (PFHpA), perfluorohexanoic acid (PFHxA)

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and perfluoropentanoic acid (PFPnA), which had detection limits of 0.5, 1 and 2 ng/L,

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respectively. Though shorter-chain PFAAs are clearly being produced (Figure 4), the difference

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between the sum of the concentrations of PFOA and PFOS and the sum of the concentrations of

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all PFASs is never greater than 0.1 nmol/L. This indicates that shorter-chain PFAAs account for

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only about 10% of the degraded PFOA and PFOS, which is much lower than for oxidation-based

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processes, where shorter-chain PFAAs account for most of the degraded PFOA (85-95% for

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activated persulfate32). PFAAs smaller than PFPnA may also be formed, however, they would be

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formed via the much slower transformation of PFPnA, thus they are not expected to be produced

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in significant quantities. Further investigation is required to fully understand the byproducts

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produced, particularly those in the gas phase.

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While identification of gaseous byproducts is of interest due to the insights they offer into the

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reaction pathways, they are expected to be of significantly less importance in relation to process

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viability because the process gas is argon, which is neutral and therefore largely unaffected by

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the plasma. This will allow the argon to be recycled (power requirements for gas pump were

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included in the input power calculations for the high rate and high efficiency cases), which will

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allow any gaseous byproducts to be re-treated and further degraded. The quantities of gaseous

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byproducts formed and the extent to which argon recycling will reduce their emission is a subject

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of ongoing investigation.

218



219 220

Figure 4. Concentration profiles showing the reduction in concentrations of PFOA and PFOS,

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and a corresponding increase in concentrations of PFHpA, PFHxA and PFPnA.

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− Key Reactants. Past studies on PFOA transformation have provided evidence supporting eaq

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as the primary reactant for many reduction-based technologies.34,35 In previous plasma studies it

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− has been observed that negative polarity discharges in water generate substantial amounts of eaq ,

225

− while positive polarity discharges do not.36,37 To investigate the potential role of eaq in the

226

transformation of PFOA, experiments were conducted using the LD reactor with both positive

227

and negative polarity. The rate constant for positive polarity is very small compared to that for

228

− negative polarity (Figure 5a), which supports the notion that eaq are important in PFOA

229

transformation. To further test this finding, experiments were performed with negative polarity

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− liquid discharges in the presence of 10 mM NaNO3, which is an effective eaq scavenger (Figure

231

5a).34 The NaNO3 suppressed the transformation of PFOA almost entirely, providing additional

232

− evidence that the eaq produced by the plasma are primarily responsible for degrading PFOA, and

233

likely account for a significant fraction of the overall removal rate achieved by the PWT

234

processes.

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The discrepancy between the rate constants for positive and negative polarity discharges in

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liquid also indicates that hydrogen radicals, hydroxyl radicals and other oxidants generated by

237

the plasma play an insignificant role in initiating primary reactions. These radicals are

238

observably present for both polarities (positive polarity produces greater quantities than


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negative),38-39 and thus their presence alone is not enough to initiate PFOA transformation at an

240

appreciable rate.

241

242 243

Figure 5. (a) Observed PFOA removal rate constants for (+) NaCl: positive polarity LD, (-)

244

− NaCl: negative polarity LD and (-) NaNO3: negative polarity LD with eaq scavenger. (b)

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Normalized observed PFOA removal rate constants for (+) NaCl: positive polarity GDB, (-)

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− NaNO3: negative polarity GDB with eaq scavenger, (-) NaCl: negative polarity GDB and (-)

247

− NaNO3: negative polarity GDB with eaq scavenger. For each polarity, kobs was normalized with

248

− respect to the case without eaq scavenger.

249 250

Conclusions drawn from liquid phase discharge experiments do not necessarily apply to gas

251

phase discharge reactors (LJB and GDB). Therefore, a similar set of experiments was conducted

252

using the GDB reactor. For positive and negative polarity, the presence of NaNO3 caused a

253

significant (at the 90% CI) reduction in the rate of PFOA removal (Figure 5b); however, the

254

effect of NaNO3 is not as extreme as in the case of negative polarity discharges in liquid. This

255

− suggests that, in addition to eaq , there is at least one other reactive species responsible for

256

initiating a significant fraction of the primary reactions with PFOA in the gas phase discharge

257

reactors.

258

PFOA adsorbs to the gas-water interface (the region in which most primary reactions occur

259

in plasma reactors)28 such that much of its hydrophobic tail (5-6 carbons) protrudes into the gas 13 ACS Paragon Plus Environment


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phase,14 thus it is likely that the other reactive species is present in the plasma interior.40 Within

261

the plasma interior, there are two species that are both abundant and likely capable of initiating

262

reactions with the fluorocarbon tail of PFOA: high-energy free electrons and argon ions

263

(evidence of excited-state argon ions is provided in the Supporting Information (Figure S3)).

264

High-energy free electrons may initiate a reaction through excitation or ionization and argon ions

265

may initiate a reaction through charge transfer. The ionization potential of argon (15.7 eV) is

266

much greater than the ionization potential of PFOA (~11 eV, based on calculations carried out by

267

our group using the Gaussian 09 program), which, in the event of charge transfer, will provide

268

the PFOA molecule with a large excess of energy and cause its rapid fragmentation.41 It is also

269

possible that PFOA is thermally decomposed (which occurs at 300-350ºC)13, due to the high

270

temperature in the plasma interior, as this has been confirmed as an important transformation

271

mechanism for PFOS in a different plasma system.42 However, due to the steep temperature

272

gradients near the plasma-liquid interface and the difficulty of obtaining reliable spatially-

273

resolved estimates for the temperature of the plasma interior,43-45 it is uncertain whether the

274

fluorocarbon tail is exposed to temperatures high enough for thermal decomposition to occur in

275

this system. It may be possible to confirm the presence or absence of these proposed mechanisms

276

via our continued investigation of the transformation byproducts.

277

Because most reactions are taking place at or above the gas-liquid interface, PWT should be

278

far less sensitive than most other treatment processes to the presence of co-contaminants, such as

279

NOM or other organic compounds, which is consistent with the results for the groundwater

280

treatment. This lack of sensitivity to co-contaminants coupled with the high PFOA removal and

281

defluorination efficiencies makes PWT a promising technology for remediation of PFAS-

282

contaminated water.

283 284 285

Acknowledgements

286

The authors thank the U.S. EPA for its financial, technical, and administrative assistance in

287

funding and managing this project (Agreement Number 83533201). The comments and views

288

detailed herein may not necessarily reflect the views of the U.S. EPA. The authors thank Bernard

289

Crimmins and Adam Point for their assistance with the UPLC-ToF-MS analysis, Xiangru Fan for

290

calculating the ionization potential of PFOA and Timothy Appleman for helping to arrange our 14 ACS Paragon Plus Environment

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procurement of the groundwater samples. The authors also thank the following personnel at the

292

Southern Nevada Water Authority for analytical support: Brett Vanderford, Oscar Quiñones, and

293

Janie Zeigler-Holady.

294 295

Supporting Information Available

296

This information is available free of charge via the Internet at http://pubs.acs.org



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