Plasma and Fusion Research: Regular Articles

Volume 14, 3406071 (2019)

Plasma-Degradation of Dinitrophenols and Interpretation by theMolecular Orbital Theory∗)
Hiroshi OKAWA, Hiroki KURODA, Keiko HIRAYAMA-KATAYAMA1) , Shin-Ichiro KOJIMA2)
and Tetsuya AKITSU1)
Happy Science University, 4427-1 Hitotsumatsu-Hei, Chosei, Chiba 299-4325, Japan1)
University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400-8511, Japan2)
Kyushu University, Fukuoka 812-8581, Japan(Received 8 January 2019 / Accepted 9 March 2019)

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The advanced oxidation of 2, 4 - dinitrophenol (DNP), 2, 5 - DNP, and 3, 4 - DNP in aqueous solution
has been investigated using a multi-gas, dielectric barrier discharge, and the degradation was measured by highperformance liquid chromatography (HPLC). The acceleration of the advanced-oxidation has been investigatedby the combination of the anion exchange polymer. The degradation pathway was suggested involving a rapid
detachment of the nitro group followed by a slow opening of the aromatic-ring. The hydroxyl radical and theexcited hydroxyl anion are responsible for the primary attack of the DNP with the production of dihydroxynitrobenzenes. The attack of hydroxyl radical occurs at the benzene ring carbon activated by the presence of aphenolic OH group and a nitro group. The reaction is dominated by a pseudo-first order kinetic reaction. The
degradation process is interpreted using Molecular Orbital Theory.

ACc 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: dielectric barrier discharge, aromatic compound, dinitrophenol, advanced oxidation, molecular orbital theory, advanced oxidation
DOI: 10.1585/pfr.14.3406071

1. Introduction

RETR

The advanced oxidation is attracting attentions as thetechnology for pollution of drinking-water reservoirs byaromatic compounds. In recent work, we reported thedecomposition of the aromatic compound in comparison
using working gases: argon, nitrogen, oxygen, and air(20% oxygen and 80% nitrogen mixture) [1]. In this work,
we describe the degradation in dielectric barrier discharge
plasma in air. The degradation of three types of dinitrophenols (DNP): 2, 4 - DNP; 2, 5 - DNP; and 3, 4 - DNP will
be compared. Chemical reaction between the organic compound and the active component is discussed with Molecular Orbital Theory. In this proceedings, we present thesummary of the experimental part of this work.

2. Materials and Methods

Reactions including excited oxygens
 1O( D) + H2 O → 2OH∗.
O(1 D) + O2 → O3

Reactions including metastable state of nitrogenmolecules
N2 (A3 Σ) + H2 O → N2 + ∗ OH + ∗ H.
Inhibition of O3 production and enhancement of OH ∗formation:
OH− + O3 → ∗ O−2 + HO∗2.
O3 + 3HO∗2 → 3OH∗ + 3O2
Production of superoxide: Oxygen molecule is the primary accepter of electrons and forms ∗ O−2O2 + e → ∗ O−2 .

∗Lukes et al. [2] reported the enhancement of OH radical production and O3 decomposition in the presence of N2molecules in humid air, through the following reactions.
Dissociation and ionization generating OH anion andradicals


Production of hydroxyl radicals:⎧∗ −⎪O2 + H2 O → OH− + ∗ HO2
⎪
⎪
⎪
⎪
∗
⎪
⎨ HO2 + ∗ HO2 → H2 O2 + O2⎪⎪⎪
H2 O2 + ∗ O−2 → ∗ OH + OH− + O2⎪⎪⎪⎩ H O + e → ∗ OH + OH−2H2 O + e → OH− + H+ + e.
H2 O + e → ∗ OH + ∗ H + e

2DNP + {∗ OH + OH− } → Produced system.

author’s e-mail: hiroshi-okawa@happy-science.university∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

Super oxide is highly reactive anions and form hydrogenperoxide in the chain reaction. Hydroxyl radical and hydroxyl anion are produced in the dissociation of hydrogenperoxide by the electron collision in the discharge area as3406071-1

c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 14, 3406071 (2019)

N2 + e → 2N∗ + e,O(1 D) + NO → NO2 ,N∗ + O3 → NO + O2 ,NO + O3 → NO2 + O2 .
Reactions related to NO−2 and nitrous acid, related tothe production of nitrite ions and ONOO, peroxonitrite:
NO−2 + H2 O2 → ONO−2 ,HNO2 + H2 O2 → HOONO + H2 O.

RETRAC

Peroxonitrite ONO2 is attracting attentions in the advancedoxidation, but the eﬀect is not fully-understood. NO andNO2 cause the electrophilic replacement reaction with hydrogen atoms in the benzene ring. In this experiment,
the reverse reaction was reduced by controlling the temperature of the solution and the anion-exchange using
gel-type poly-styrene-base alkalized trimethyl-ammonium
polymer. (Sanei Chemical, Kumamoto, Japan)
Figure 1 shows the dielectric barrier discharge plasma
source. This plasma source consists of a quartz tube,

3.0 mm outer diameter and 1.4 mm inner diameter and
W/Re (6%) wire, 0.6 mm in diameter coaxially located.
These components are assembled with a Teflon Cajon-typeelbow coupler. The quartz tube of the plasma source isimmersed in the water-solution, which is connected to theground electrode through the capacitive coupling. Figure 2shows the estimation of the discharge power on the basis of
the Q versus V Lissajous curve. The total capacitance CPR
includes the dielectric barrier discharge in the gas-region
and the capacitance across the water region.
When monitoring capacitor is 4200 pF, for example,
the one cycle along the contour, 10−7 C along the verticalaxis multiplied by 5 × 103 V along the horizontal axis isequivalent to 5 × 10−4 J/cycle.
The electrical power input at 16.5 kHz is 8.25 W. The
power source is a quasi-sinusoidal wave inverter, 16.69 16.94 kHz, 8.48 - 7.84 kV, peak to peak, (Type 10AC-24,Logy Electronics, Tokyo, Japan). Table 1 shows the dependence of the output voltage on the control voltage (DCinput) and the discharge power. When the plasma source
was operated at the control voltage of 13 volts and higher,
the discharge formed transfer mode glow discharge.
The discharge voltage was measured with P-3000 highvoltage prove and the discharge current was measured withTCP312 current prove clamped to the ground side wirewith preamplifier TCPA300 (Tektronix, Beaverton, USA)

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well as by the decomposition of ozone in water reactions.
Reactions in air with humidity, one of the merits is the suppression of ozone generation.
Nitrogen reactions including the ozone quench generating the excited state of nitrogen and NO x

Fig. 1 Experimental setup; (a) Layout; (b) Waveforms, voltage:
1 kV/div, current: 100 mA/div. 6.5 kV and 452 mA peakto-peak.

Fig. 2 Typical waveforms and the Q versus V Lissajous curve.
(a) Measurement circuit; (b) Lissajous curve.

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3. Experimental Result

ACThe concentration of the remaining DNP was measured
with liquid chromatograph, L-6000 (Hitachi Co., Japan)
equipped with UV-detector, L-4200, (Hitachi Co., Japan)
with column Waters X-bridge 4.6, 250 mm. TOC measurement was carried out by TOC-LCSH/CSN and auto
sampler, ASI-L, (Shimadzu Co., Japan) Ion chromatography for anion detection, ICS-1100 (Dionex), with column IonPac AS22 (Dionex,) and guard column, IonPac,
AG22 (Dionex). In the ion chromatography, the detector
was replaced to the electro-conductive detector. The reproducibility and the anion exchange was examined using
liquid chromatograph, HPLC Prominence-I LC-2030Plus,(Shimadzu Co., Japan) installed with UV - detector and areverse mode column: Xbridge C-18, 5 µm.

TE
D

Table 1 Estimation of the discharge power.

Fig. 3 Experimental results; (a) HPLC; (b) TOC.

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Figure 3 shows the concentration of DNP and TOC(Total Organic Carbon). TOC is detected as CO2 from thesolid body of the incinerated dried solution. The detachment of NO2 is completed in 10 minutes. However, longer
process is needed for the reduction of TOC, which remains
almost 70 percent of the initial concentration. This result
indicates the production of the intermediate product, suchas hydroquinone. Figure 4 shows the evolution of the concentration of NO−3 and pH. The eﬃciency of the plasma
degradation is compared for dinitrophenols: 2, 4 - DNP,
2, 5 - DNP, and 3, 4 - DNP. Table 2 is the TOC, pH, theconcentration of DNP and NO−3 after 60 minutes.
After 20 minutes of the plasma injection, the pH value
becomes lower than 4, due to the accumulation of nitricacid. Comparing with the initial phase, the concentrationof hydroxyl anion may show proportional reduction in theionization equilibrium. The decomposition reaction is supply dominated by excited OH anions and OH* which aresupplied from the plasma region. The influence of thechange in the ionization balance may be negligible. On
the other hand, the accumulation of nitric acid may cause
the reverse exchange reaction. Nitric anion was reduced by
the anion exchange by gel type poly-styrene base alkalized
trimethyl-ammonium polymer. Apparently, the final concentration of NO−3 exceeds the original concentration of thenitro-group, and the solution shows acidity. The weight ofnitric anion is calculated. If two nitrogen atoms are converted to NO−3 , one oxygen comes from the working gas.
From the moll ratio;
98/184.106 × [20 mg/L] = 10.646 mg/L.

Fig. 4 Ion chromatography and pH measurement; (a) Concentration of NO3 anion; (b) pH.

The increase in NO−3 anion is attributed to the oxidation inthe working gas. This acid solution is not suitable for therelease. Figure 5 shows the evolution of the concentration.

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D

Table 2 Concentration, TOC and pH∗ .

Fig. 5 Exponential recurrence function.

Fig. 7 Energy diagram of LUMO and HOMO state of the starting system (a) excited hydroxyl anion; (b) hydroxyl radical.

AC

Table 3 Coeﬃcients.

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4. Discussions

Fig. 6 Shortcut of degradation by gel type poly-styrene base alkalized trimethyl-ammonium polymer.

The recurrence function can be expressed with followingform (Table 3): Ct= −at − b, C0 = 20 [mg/L].
lnC0The plasma process increases neutral materials in theobject, even if our aim is the ionized or excited radicals.
The anion exchange polymer captured the excessive material, NO−3 and exchanged with hydroxyl anion OH− . Figure 6 shows the shortcut of the degradation using anion
exchange for 2, 4 - DNP.

The eﬀects of the frontier electrons in the Highest Occupied Molecular Orbital (HOMO) of the active
component, OH anion and OH∗ radical and the Lowest Unoccupied Molecular Orbital (LUMO) of the target compound determines the reaction mechanisms, using
Biomedical CAChe [3, 4]. The starting system of 2, 4 DNP shows LUMO energy of −1.268 eV, and HOMO energy −10.210 eV. The excited OH− anion shows LUMO
energy 4.045 eV and HOMO energy −10.375 eV. This reaction is directed to the nucleophilic reaction. OH* radicalshows singly occupied molecular orbit (SOMO) energy of3.121 eV, and this reaction is directed to electrophilic reaction directed to the HOMO level, −10.210 eV. Figure 7shows the energy diagram of 2, 4 DNP with excited hydroxyl anion and hydroxyl radical. Comparing the energy state of OH anion (in NaOH) with 2, 4 - DNP. ThisOH− anion shows HOMO level (−2.61 eV) lower than theLUMO level (−1.268 eV) of 2, 4 - DNP. The diﬀerence isnecessary for the excitation, 1.342 eV. The produced system consists of 1, 2 - OH 4 - nitrobenzene and NO3 anionof HNO3 . Figure 8 shows the energy diagram of the produced system. Although the quantum-chemical calculation
indicates that the exchange reaction is exothermic reaction
including the production system, this reaction was not observed.
This reaction is supply-dominated by OH radicals andanions excited in the plasma region. Figure 9 shows the3D model of the frontier electron density distribution. The
nucleophilic reaction by OH anion targets the negative

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method is a feasible solution for the water remediation. In
the air-plasma treatment, the accumulation of nitric anion
was, solved and realized by shorter solution using the anion exchange. The degradation of the nitro system was interpreted by the Molecular Orbital Theory. The LUMO
energy level of the organic compound is lower than theHOMO energy level of the active component, OH− . The
reaction proceeds to the detachment of nitric anion, and thereverse reaction shows higher activation energy.

Acknowledgement

Fig. 8 Energy diagram of the produced system.

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This research received a fund from Future Science Institute, Tokyo, Japan. The authors express their gratitude
to Mr. Kento Nakai and Mr. Kenya Yabu.

Appendix A

2-D and frontier electron density model of 2, 4 - DNP

Fig A. 1 2-D structures of 2, 4 - DNP.

RETRAC[5].

Fig. 9 Frontier electron density of 2, 4 - DNP; (a) Nucleophilicreaction; (b) Electrophilic reaction.

space charge of electron injected by OH system. Similarly, the electrophilic reaction by OH radical targets thepositive space charge of electron depletion around NO2system. Appendix shows the 3D model of iso-surface ofthe frontier electron density of the LUMO state and theHOMO state, the energy is −1.268 eV and −10.210 eV, respectively.

5. Conclusion
Plasma-degradation of the dinitrophenyl was studied
using a compact dielectric barrier discharge in a quartz
tube, immersed into the liquid surface. As the dielectricbarrier discharge can be operated at low electric power, this3406071-5

Fig A. 2 LUMO and HOMO energy state of 2, 4 - DNP.

Plasma and Fusion Research: Regular Articles

Volume 14, 3406071 (2019)[3] Biomedical CAChe 6.0 Users Guide, 2003, Fujitsu.
[4] K. Somekawa, Molecular Orbital Calculation of Organic
Molecules and the Application, (Kyushu University Press,Fukuoka, Japan, 2013) ISBN 978-4-7985-0089-8.
[5] PUBCHEM https://pubchem.ncbi.nlm.nih.gov/compound/3_4dinitrophenol#section=2D-Structu

RETRACTE
D[1] S. Kojima, K. Katayama-Hirayama and T. Akitsu, World
Journal of Engineering and Technology 4, 423 (2016).
http://DOI.ORG/10.4236/wjet.2016.43042/[2] P. Lukes and B.R. Locke, Industrial and Engineering
Chemistry Research, 44(9), 2921 (2005). https://DOI.
ORG/10.1021/ie0491342

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