1. Generation of reactive species in water films treated by dielectric
barrier discharges with
different gases
Soheila Mohades1, Amanda M. Lietz1, Vesna V. Kovačević2, Bratislav M. Obradović2, Milorad M. Kuraica2 and Mark J. Kushner1
1University of Michigan, Ann Arbor, MI USA
2University of Belgrade, Belgrade, Serbia
International Conference on Plasma Medicine
June 2018
* Work supported by National Science Foundation and Department of Energy Office of Fusion Energy Science
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2. University of Michigan Institute for Plasma Science & Engr.
AGENDA
Falling Water DBD Reactor
Model Description
Plasma with Multiple Pulses with Different Gases
RONS in Gas and Liquid Phase
Circulating Plasma Activated Water (PAW)
Concluding Remarks
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3. University of Michigan Institute for Plasma Science & Engr.
MOTIVATION
Direct or indirect contact of non-thermal plasma sources are used for plasma activation of liquids with applications in degradation of pollutants in solutions and biomedicine.
Understanding chemical processes in plasma − liquid is important to optimize desired properties of plasma activated liquid.
Gas mixture and configuration such as surface/volume ratio are important factors in RONS production.
Different reactors are being developed water treatment:
Kovalova et al. Bioelectrochemistry 112 (2016) 91-99
Selma Mededovic Thagard, MIPSE, University of Michigan, April 4, 2018 
Kovačević et al. J. Phys. D: Appl. Phys. 50 (2017)
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4. FALLING WATER DBD REACTOR
DBD reactor: Two parallel cylindrical glass tubes with bounding electrodes
DBD discharges in the gap in contact with water layer.
Gap 4.3 mm, inner radius 10 mm, outer radius 14.5 mm, water film thickness 0.22 mm.
In experiment PAW was collected in a reservoir and pumped into reactor (recirculated 10 cycles).
In model, we used gas and liquid densities at the end of each cycle as the initial density for the next cycle.
Kovačević et al. J. Phys. D: Appl. Phys. 50 (2017)
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5. DBD OPERATES WITH DIFFERENT GASES
GlobalKin, 0-D
Feed gases: Ar, He, O2, N2, air
Added humid air impurities: 3.5% (N2/O2/H2O=0.8/0.2/2.5)
Liquid: H2O + solvated N2/O2 (~9/5 ppm)
Flow rate: 132 sccm to match gas residence time in the experiment
87 gas and 92 aqueous species
2600 total reactions
21 W, 600 Hz, triangle pulse
1 cycle: 1,500 pulses (2.5 s), 10 s afterglow
Gas/liquid volume ratio ~ 25
Plasma activated water (PAW) recirculates through reactor 10 times (here called 10 cycles) V
Water
Dielectrics
Gas
1 cm
Schematic of the reactor
Gas/liquid volume = 3.42 and cm3
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6. DESCRIPTION OF GlobalKIN
GlobalKIN is a 0-dimensional global model for plasma chemistry, plasma kinetics and surface chemistry.
Complete chemistry can be addressed with rapid computation.
Spatially dependent phenomena are not captured other than by plug flow and diffusion lengths.
Boltzmann Equation Solver for Electrons
Plasma Chemistry Module
Plasma Chemistry Reaction Mechanism
Circuit Module
DVODE ODE Solver for Rate Equations
Ion Mobility Database
Surface Chemistry Module
Electron Cross Section Database
Diffusion Option
Gas Flow Option
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7. GlobalKIN LIQUID MODULE
Liquid is treated as separate "zone" with its own reaction mechanism.
Transport from gas to liquid is through an interfacial surface.
From gas plasma’s perspective, interface is analogous to a reactive surface, with a sticking coefficient and a return flux.
All charged species diffusing to liquid surface solvate.
Neutrals diffusion is restricted by h - Henry’s law constant
Water evaporates into gas phase.
"Sticking" gas phase species enter liquid.
Sticking coefficient, S, based on Henry’s law limited transport into liquid
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8. GlobalKIN LIQUID MODULE
H2O2 O3 H
Charged and Excited Species O
= reaction with H2O
H2O
H3O+ , OH-
HO2 OH O2 M-
O3-
H3O+ H2
O2-
= equilibrium
= slow decay
H, H2
HO2-
O2-,
OH-,
eaq
H2O+ M+ e M*
ONOOH
HO2NO2
NO3-
ONOO-
O2NO2-
HNO2
HNO3
NOx
NO2-
NxOy N N2
HNOx
ROS
RNS
OH,
H, OH,
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9. University of Michigan Institute for Plasma Science & Engr.
PLASMA PROPERTIES
Power (W/cm-3)
1 atm, Argon with saturated water vapor.
40 ns triangle pulse with peak power density at 1340 W/cm-3
Te and ne are from the first pulse
ne peak is at maximum power
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10. Te AND ne IN DIFFERENT GASES
Te and ne are higher in He than other gases.
Highest ne in He then in Ar and lowest in N2.
ne quenches quickly in O2 due to its high electronegativity.
Ne Te inc.
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11. GAS RONS, ARGON PLASMA, 1st CYCLE
Density (cm-3)
“Cycle” refers to one trip of PAW through reactor.
RONS produced due to air impurities.
H2O2 quickly saturates and its highest density is in Ar plasma due to higher OH and HO2 :
OH + OH + M  H2O2 + M H + H2O2  HO2 + H2
Relatively high H and NO3- yields highest HNO3 in Ar.
O/100
H2o2 in.
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12. LIQUID RONS, ARGON, 1st CYCLE
Density (cm-3)
H2O2aq and NO-3aq saturate during pulses and remain stable in afterglow
Highest H2O2aq in Ar plasma due to high H2O2(g) or OHaq.
Highest H3O+aq and lowest pH in Ar plasma. Positive ion from gas quickly charge exchange with water forming H2O+aq and then H3O+aq.
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13. GAS RONS, HELIUM PALSMA, 1st CYCLE
Density (cm-3)
He after Ar has the highest efficiency in OH and H2O2 production which significantly drops during after glow.
H2 and H are higher in He:
H + H + M  H2 + M
Higher NO and HNOx in He and Ar plasma due to higher OH:
NO + OH + M  HNO2 + M
NO2 + OH + M  HNO3 + M
O3 dec.
H2o2 inc.
H2 inc.
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14. LIQUID RONS, HELIUM, 1st CYCLE
Density (cm-3)
RNS aqueous density and trends of decay in He and Ar are similar.
High H2aq in He helps dissociation of H2O2aq into Haq and OHaq and a slight drop in it density during after glow.
Drop in O3aq can be due to drop in O3(g) and diffusing out of liquid which is prominent in thin film configuration (low Vl/Vg) and low Henry’s law species.
N2o increased
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15. GAS RONS, OXYGEN PLASMA,1st CYCLE
Density (cm-3)
Largest O3 production in air and O2 plasmas due to high Oxygen:
O + O2 + M  O3 + M
NO is lowest in O2 plasma due to low N density, as NO is precursor of many RNS reactions lower density of NO2- and NO2 :
O2 + N  NO + O
However, N2O is high which depends on O density on N2 proximity
RNS dec.
ROS =
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16. LIQUID RONS, OXYGEN,1st CYCLE
Density (cm-3)
O3aq saturates quickly and is ~2-log higher in O2 and air plasma
O3aq drops quickly during afterglow by diffusion out, consumed by NO-2, or decomposition to HO2 radical (Equ. slide 18).
Primary source of ONOOH is NO2 and OH and as OH density drops slower in air and O2, ONOOH decay is slower.
dec.
H3O+ dec.
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17. GAS RONS, AIR PLASMA,1st CYCLE
Density (cm-3)
O3 is the dominant ROS in air
Largest production of NO2 and N2O due to high O and N density and lowest in H2 production.
NO quickly reacts with O in each pulse yields to high NO2:
NO + O + M  NO2 + M
NOx then reacts with OH to form HNOx which is relatively high in air.
RNS is lower in T2
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18. LIQUID RONS, AIR,1st CYCLE
Density (cm-3)
O2-aq , OH-aq, and HO2aq density grows during afterglow in air and O2 plasma via chain reactions:
2O3 + H2O 𝐎𝐇− O2 + HO2· + OH·
O2-aq quickly charge exchanges with O3aq. Higher O3-aq in air and O2 increases produces OH during after glow:
O3-aq + H3O+aq  O2aq + H2Oaq + OHaq
Slightly deccreased in RNSL but n2o
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19. GAS RONS, NITROGEN PLASMA,1st CYCLE
Density (cm-3)
N2 is less efficient in RNS production than air (0.2% vs. 20% O2)
NO2 is the most stable RNS in after glow
Least efficient gas in ROS production.
Less RNS
O3 dec.
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20. LIQUID RONS, NITROGEN,1st CYCLE
Density (cm-3)
Decay of HNOxaq is faster in O2 and N2 plasma while it is relatively stable in Ar and He
Thermal decays of HNO4aq in O2, N2 and air results in growth in HO2 :
HNO4aq  HO2aq + NO2aq
NO2aq is consumed to produce HNO2aq and HNO3aq
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21. University of Michigan Institute for Plasma Science & Engr.
RECIRCULATION OF PAW
Number of cycles
Number of cycles
Density of RONSaq at the end of each cycle (2.5 s plasma + 10 s afterglow).
H2O2aq increases with recirculation and max production rate is in Ar plasma in agreement with experimental results.
Recirculation slightly grows OHaq in Ar and drops in N2 and air which correlates to OH gas phase density.
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22. RONS IN LIQUID WITH RECIRCULATION
Number of cycles
Number of cycles
O3aq accumulation is independent of recirculation to some extend with the highest production rate in He which is similar to the experiment results.
Significant accumulation of NO-3aq with all gases. HNO3 is a main source of plasma acidification and is more stable in its conjugate ion form, NO3-. Lowest pH in experiment was measured in Air (pH=2.5).
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23. University of Michigan Institute for Plasma Science & Engr.
SUMMARY
In solution aqueous densities after 10th cycle.
Ar plasma is the most efficient in generating H2O2aq which agrees with experiments.
O2 is the most efficient gas in O3aq and OHaq production which also agrees with experiments.
Our results indicate most efficient NO3−aq and HO2aq production is in Ar.
HNO3(g) which is stable and accumulates with pulsing is higher in Ar.
Most
Least
H2O2aq Ar
air
O3aq O2
OHaq N2
NO3−aq
ONOOHaq
HO2aq
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24. University of Michigan Institute for Plasma Science & Engr.
CONCLUDING REMARKS
With constant applied power for all gases He produces highest ne and Te and supports the highest O2−aq.
Ar plasma is the most efficient in generating H2O2 due to high OH(g) density. He is second most efficient.
Highest solvated O3aq in both gas and liquid phase is O2 plasma followed by air plasma. Lowest O3aq density in Ar.
High HNOxaq in air and Ar is due to high NO and NO2 in gas phase which are source of NO2-aq and NO3- aq in liquid.
Circulation can enhance H2O2aq and NO-3aq density with highest rate in Ar and N2.
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