22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma-based water treatment: process intensification for the treatment of
mixtures of emerging contaminants
G.R. Stratton1, C.L. Bellona2, Fei Dai2, T.M. Holsen2 and S. Mededovic Thagard1
1
Clarkson University, Plasma Research Laboratory, Department of Chemical and Biomolecular Engineering,
Potsdam, NY, U.S.A.
2
Clarkson University, Department of Civil and Environmental Engineering, Potsdam, NY, U.S.A.
Abstract: We found that organic solute degradation efficiency is proportional to the area
of the plasma-liquid interface. Based on this observation, a new plasma reactor was
developed to maximize plasma-liquid contact. The improved reactor was tested on
common
hazardous
pollutants,
including
perfluorooctanoic
acid
and
perfluorooctanesulfonic acid, which were rapidly degraded, indicating that this reactor
might be viable for real-world application.
Keywords: AOP, perfluorinated compounds, plasma, reactor design, water treatment
1. Introduction
The list of federally regulated water pollutants is ever
expanding and many conventional water treatment
processes are not effective for removing some of these
hazardous compounds [1]. In response to this, several
techniques from the field of advanced oxidation processes
(AOPs) have emerged as feasible alternatives to
conventional processes, and have achieved commercial
success [2]. Pulsed electrical discharge plasma formed
directly in or above water constitutes another, lessdeveloped, AOP, but has received some attention from
researchers due to its unique treatment capabilities. Like
other AOPs, plasma-based water treatment (PWT) makes
use of the highly oxidative hydroxyl (OH) radical to
oxidize chemical contaminants. However, compared to
other AOPs, PWT offers a broader range of treatment
mechanisms, including a wider variety of reactive
chemical species (OH, O, H, O 3 , H 2 O 2 , O 2 , HO 2 ) as well
as physical effects such as generation of ultraviolet-range
radiation (UV), shockwaves capable of inducing
cavitation, and high temperatures capable of thermally
decomposing molecules. Additionally, because the active
chemical species are produced in situ, and directly from
the water, no chemical additives are required for PWT,
which constitutes a major advantage over other AOPs,
which often rely upon chemical feeds such as hydrogen
peroxide (H 2 O 2 ) or ozone (O 3 ).
Despite the apparent promise of PWT, it has not
achieved the same success as other, more mainstream
AOPs, such as those involving various combinations of
O 3 , H 2 O 2 and UV [3]. A major reason for PWT’s relative
lack of success is a lack of direction in the design of
effective plasma reactors. Because plasma reactors feature
a vast array of potential design elements (reactor
geometry, discharge phase, headspace gas composition,
electrode configuration, shape and material, etc.), the
number of different design permutations is practically
limitless, and without the guidance of general design
O-3-2
principles, the most effective permutations can be elusive.
This study investigates how the area of the plasmaliquid interface influences the rate of removal of
Rhodamine B (RhB) dye. The observed relationships are
used to derive a general design principle, which provides
direction for the development of a novel reactor with
greatly improved efficiency. This reactor is tested on a
mixture of 23 potentially hazardous chemicals that are
often found in groundwater, including pharmaceuticals,
perflourinated compounds (PFCs), and disinfection
byproducts. To the best of our knowledge, no previous
study has ever attempted treatment of a mixture with such
a wide variety of different compounds. Among the 23
pollutants are the two PFCs, perfluorooctanoic acid
(PFOA) and perfluorooctanesulfonic acid (PFOS), which
are listed as emerging contaminants by the United States
Environmental Protection Agency due to their toxicity
and their widespread presence and persistence in the
environment. PFOA and PFOS are very resistant to
degradation by conventional water treatment processes as
well as AOPs [1]. Reverse osmosis and activated carbon
have proven capable of removing PFOA and PFOS;
however, these processes simply concentrate the
contaminants, which then necessitates additional
processing (incineration) for actual destruction [4]. The
ability to effectively degrade such compounds using
plasma might renew interest in PWT as a viable
technology.
2. Experimental
Plasma was generated using a custom built high voltage
(HV) pulsed power supply, the circuit of which is shown
in Figure 1. The applied voltage was 20 kV, with a
discharge frequency of 43 Hz, using a 0.94 nF load
capacitor. The voltage and current of the discharges were
measured using a Tektronix P6015A high voltage probe
and a Tektronix P6021 current probe connected to a
Tektronix TDS 3032C oscilloscope. Examples of typical
1
voltage and current waveforms are shown in Figure 2.
Charge Resistor
HVDC
Power Supply
HV
HV
Rotary
Spark Gap
Load
Capacitor
Plasma
Reactor
Argon
Fig. 1. Circuit diagram for pulsed power supply.
(a)
(b)
Fig. 3. Reactor diagrams: (a) gas discharge reactor, (b)
foaming gas discharge reactor.
Fig. 2. (a) Voltage waveform, (b) current waveform.
Two different reactors used, both consisting of a glass
vessel (17.3 cm diameter) with a plastic cap, which had
ports to facilitate sample extraction, regulation of the
headspace gas, solution recirculation and support of the
HV electrode. Argon was fed into the reactor at
2.1 L/min. The solution was recirculated at 1.4 L/min
through a heat exchanger to maintain a temperature of
15 ºC. Both reactors had a point-plate electrode
configuration, with the high voltage point electrode in the
gas (1.2 cm above the liquid surface) and the grounded
plate electrode positioned 1.5 cm beneath the liquid
surface. The “gas discharge” reactor (Fig. 3a) featured a
HV electrode made from sharpened nickel chromium wire
(20 AWG) and a grounded electrode consisting of a
stainless steel (S.S.) plate. Four different sized plates were
used: 0.64, 3.15, 4.75 and 7.60 cm. In the “foaming gas
discharge” reactor (Fig. 3b), the recirculating solution was
pumped through a S.S. tube (2.2 mm inner diameter),
which also served as the HV electrode. The grounded
electrode was an aluminum ring with inner and outer
diameters of 6 and 9.8 cm, respectively. Argon was fed
through a ceramic gas diffuser (9.5 cm diameter) that was
centered directly beneath the hole in the grounded ring.
The treated solutions were prepared from deionized
water, using sodium chloride to adjust the conductivity to
300 µS/cm (pH was 5.4). For RhB degradation
experiments, the initial concentration was 7.5 mg/L. For
experiments involving the 23 model pollutants, the initial
concentrations were varied in accordance with the levels
at which each chemical is typically present in
groundwater (Table 1).
RhB concentration was determined by measuring
absorbance at 554 nm using a Shimadzu UV-1800
spectrophotometer. The concentrations of the 23 model
pollutants were determined at the Southern Nevada Water
Authority Laboratory.
2
3. Results and Discussion
3.1. Interfacial Area
To investigate the influence of the area of the plasmaliquid interface, the gas discharge reactor was operated
with ground plates of four different diameters, which
caused corresponding changes in the length of the leaders
(plasma channels) on the liquid surface, as shown in
Fig. 4. Due to the manner in which leaders branch during
propagation, the liquid surface becomes saturated with
leaders, such that the length of the leaders effectively
represents the radius of a circular region on the liquid
surface, within which plasma channels are quite
uniformly distributed [4]. From photographs of the
discharges, the average leader length was estimated and
used to calculate the area of the circular region, which
will be referred to as the plasma area.
Fig. 4. Gas discharge with four different plate diameters:
(a) 0.64 cm, (b) 3.15 cm, (c) 4.75 cm, and (d) 7.6 cm.
Because the leaders are uniformly distributed within the
discharge region, the plasma area should be proportional
to the area of the plasma-liquid interface. This
O-3-2
proportionality allows plasma area to serve as a
quantifiable surrogate for the area of the plasma-liquid
interface, which is far more difficult to quantify.
RhB degradation experiments were performed using
each of the four ground plates, under the same conditions
as for measuring plasma area. The RhB concentration
data showed that the overall rate of removal followed
first-order kinetics, and the observed rate constant (k obs )
was calculated from the following formula:
of liquid exposed to the plasma.
C 
ln  S,0  = k obs ⋅ t
 CS 
where t (min) is the treatment time, C S,0 (mg/L) is the
initial RhB concentration and C S (mg/L) is the RhB
concentration at time, t. Fig. 5 shows that k obs shares a
distinct linear relationship with the plasma area, and thus
must be proportional to the area of the plasma-liquid
interface.
Fig. 5. Relationship between the observed RhB removal
rate constant and the measured plasma area. Ground plate
diameters: ■ 0.64 cm, ● 3.15 cm, ▲ 4.75 cm, and
▼ 7.6 cm.
3.2. Improved Reactor
That the interfacial area has such a strong influence on
the RhB removal rate indicates that maximizing the
contact between the plasma and liquid should be the key
objective for future improvements to reactor design. This
direction was followed for the development of a novel
foaming gas discharge reactor (Fig. 3b), which performed
far better than the previous gas discharge reactor (Fig. 6),
achieving an observed rate constant more than seven
times greater.
The purpose of pumping the solution through the HV
tube electrode was to churn the bulk liquid in order to
generate a layer of foam on the surface; similarly, bubbling
argon though the gas diffuser was meant to simply increase
the amount of foam. While the precise mechanism for how
the RhB degradation is so greatly enhanced by the foam
has yet to be confirmed, it is likely because the presence of
foam within the discharge region significantly increases the
surface density, which effectively increases the surface area
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Fig. 6. Observed rate constants for both reactors.
3.3. Treating Model Pollutants
Details of the model contaminants as well as the
corresponding initial concentrations and results of the
degradation experiments are provided in Table 1. Results
are reported in terms of percent removal after 60 minutes
of treatment within the foaming gas discharge reactor.
The operating conditions were all the same as for the
previous RhB experiments, and the solution volume was
1.4 L.
While the results show that the reactor was reasonably
capable of removing most pollutants, the 100% removal
of PFOA and PFOS is most noteworthy because these two
contaminants have been found to be extremely resistant to
degradation by several other AOPs, including O 3 , O 3 /UV,
O 3 /H 2 O 2 and Fenton [1]. Due to the novelty of these
results, additional experiments were conducted on
solutions containing just PFOA (initial concentration of
800 ng/L) using the same conditions as before, which
showed that all PFOA was degraded in less than five
minutes.
The efficiency with which PFOA and PFOS were
degraded suggests that the primary mechanism must
involve some species other than OH radicals. It has been
observed that some PFCs are degraded by free electrons
or electronically excited argon [6]. However, if these
mechanisms were valid in this system, it would be
expected that PFBA would have degraded to a more
considerable extent. A more likely mechanism involves
reactions with superoxide and hydroperoxide, both of
which are generated by the plasma [7, 8]. While this
mechanism has been proposed only for the degradation of
PFOA and not PFOS, their chemical structures are quite
similar, particularly in the region where the reaction is
most likely to take place (fluorocarbon tail). The high rate
of PFOA and PFOS degradation is thought to partially
result from the foaming within the reactor, which exploits
the surface-active nature of the two pollutants by
increasing their presence within the surface-dense
discharge region.
3
Table 1. Model pollutants’ names, category, initial concentration and percent removal.
Abbreviation
Pollutant category
Initial concentration
(ng/L)
Acetaminophen
N/A
Pharmaceutical
470
83
Atenolol
N/A
Pharmaceutical
520
78
Caffeine
N/A
Pharmaceutical
420
56
Carbamazepine
N/A
Pharmaceutical
480
89
DEET
N/A
Insecticide
220
95
Fluoxetine
N/A
Pharmaceutical
340
98
Gemfibrozil
N/A
Pharmaceutical
450
99
Ibuprofen
N/A
Pharmaceutical
450
96
Meprobamate
N/A
Pharmaceutical
480
81
Naproxen
N/A
Pharmaceutical
310
86
Primidone
N/A
Pharmaceutical
470
76
Sucralose
N/A
Artificial sweetener
480
55
Sulfamethoxazole
Tris(2-carboxyethyl)phosphine
Triclosan
N/A
Pharmaceutical
Flame retardant
Personal care product
390
320
280
77
42
92
79
Pollutant
Trimethoprim
TCEP
N/A
% removal after
60 min treatment
N/A
Pharmaceutical
390
N-Nitrosodimethylamine
NDMA
Disinfection byproduct
490
34
Perfluorobutanoic acid
PFBA
Perflourinated compound
620
8.9
Perfluorooctanoic acid
PFOA
Perflourinated compound
320
100
Perfluorooctanesulfonic acid
PFOS
Perflourinated compound
170
100
3.7
Monochloroacetic acid
CAA
Haloacetic acid
41000
Dichloroacetic acid
DBAA
Haloacetic acid
44000
13
Trichloroacetic acid
TCAA
Haloacetic acid
44000
4.5
4. Conclusions
It was found that the overall rate of solute degradation
is proportional to the area of the plasma-liquid interface.
This relationship suggests that effort should be put
towards maximizing plasma-liquid contact in order to
design more efficient reactors. This approach was
effective in guiding the development of a novel foaming
gas discharge reactor, which performed very well in
comparison to the standard gas discharge reactor. The
foaming gas discharge reactor was effective when tested
against 23 model contaminants. Though some compounds
were resistant to degradation, most were removed to an
appreciable extent, most notably PFOA and PFOS, which
were rapidly degraded despite their reputation of being
highly resistant to degradation by other processes,
including AOPs. The ability to effectively degrade PFOA
and PFOS makes this a promising technology for future
employment.
(1987)
[3] R. Andreozzi, V. Caprio, A. Insola, R. Marotta,
Catal. Today, 53, 51-59 (1999)
[4]
USEPA,
http://www2.epa.gov/fedfac/emergingcontaminants-perfluorooctane-sulfonate-pfos-andperfluorooctanoic-acid-pfoa. (2014). Accessed: 02/24/15
[5] V. Belosheev, Tech. Phys., 44, 35-40 (1999)
[6] M.B. Chang, J.-S. Chang, Ind. Eng. Chem. Res., 45,
4101-4109 (2006)
[7] M. Ahmad, PhD dissertation, Washington State
University, (2012)
[8] R.P. Joshi, S.M. Thagard, Plasma Chem. Plasma
Process., 33, 17-49 (2013)
5.Acknowledgements
The authors would like to acknowledge the support from
the United States Environmental Protection Agency
(R835332).
6. References
[1] H.F. Schröder, R.J. Meesters, J. Chromatogr. A, 1082,
110-119 (2005)
[2] W.H. Glaze, Environ. Sci. Technol., 21, 335-352
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