Plasma and Fusion Research: Regular Articles

Volume 14, 3401132 (2019)

Plasma Degradation of Dibromophenols and Interpretation byMolecular Orbital Theory∗)
Tetsuya AKITSU1,3) , Shin-Ichiro KOJIMA2) , Keiko KATAYAMA-HIRAYAMA1) ,Hiroki KURODA3) and Hiroshi OKAWA3)
1)

University of Yamanashi, 4 Takeda, Kofu, Yamanashi 400-8511, Japan
2)
Kyushu University, Fukuoka, Fukuoka 812-8581, Japan
3)
Happy Science University, 4427-1 Hitotsumatsu-Hei, Chosei, Chiba 299-4325, Japan(Received 2 January 2019 / Accepted 17 June 2019)
Dielectric barrier discharge was operated in the liquid-gas boundary and applied to the degradation of Dibromophenol (DBP). The brominated aromatic compound show acute toxicity to aqueous living and, occasionally,
carcinogenic and hormone-disruptive eﬀect for human. The plasma degradation provides the advanced oxidation
process by charged species, anion as well as neutral radicals. Plasma-degradation with the anion-exchange successfully worked and the controlled the production of BrO−3 by NO−3 . Interpretation of the observed diﬀerence inthe resistance is interpreted using the Molecular Orbital Theory.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: plasma electrolysis, aromatic compound, advanced oxidation, molecular orbital theory
DOI: 10.1585/pfr.14.3401132

1. Introduction
Increasing amount of aromatic compounds, with multibenzene rings is produced as flame-retardant plastic materials. 2, 6 - Dibromophenom (2, 6 - DBP) is detected asa degradation product of Tetrabromopisphenol-A (TBBPA) [1,2]. The environmental toxicity is presented by World
Health Organization (WHO) [3]. Concern of the animal
carcinogenicity is expressed for a product in the oxidative degradation: BrO−3 , by the International Agency for
Research on Cancer (IARC) classification Group 2B [4].
Degradation of 2, 6 - DBP have been studied using a dielectric barrier discharge in the gas-liquid boundary [5]. In
this work, we aim at the confirmation of the reproducibility
at lower discharge power, and experimental comparison ofDBPs: 2, 4 - DBP. Degradation at a higher concentration ofDBP solution and a short-cut of the degradation using ananion-exchange polymer have been studied.

Reactions including excited oxygensO(1 D) + H2 O → 2OH∗.
O(1 D) + O2 → O3Reactions 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 .
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− .

2. Materials and Methods
Lukes et al. [6] reported the measurement of *OH radical production and O3 decomposition in the presnce of N2molecules in humid air, through the following reactions.
Dissociation and ionization generating OH anion andradicals


H2 O + e → OH− + H+ + e.
H2 O + e → ∗ OH + ∗ H + eauthor’s e-mail: akitsu@yamanashi.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

22Dibromophenol + {∗ OH + OH− }→ Produced system.
Hydroxyl radical and anion are produced in the dissociation of hydrogen peroxide by the electron collision in thegas discharge. In the reactions, in air with humidity, one of
the merits is the suppression of ozone generation. Ozone
quench is important for the suppression of BrO−3 . The suppression of BrO−3 is realized by NO−3 in the advanced oxidation.

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

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Nitrogen reactions including the ozone quench generating the excited state of nitrogen and NOx
N2 + e → 2N∗ + e,O(1 D) + NO → NO2 ,N∗ + O3 → NO + O2 ,NO + O3 → NO2 + O2 .
Figure 1 shows the experimental layout. The apparatus
consists of a quartz tube 3.0 mm in outer diameter, 1.4 mm
in inner diameter and W-Re electrode inserted coaxially.
This part is submerged in the DBP water solution in a testvial, 100 mL. Discharge is excited with a quasi-sinusoidal
high voltage generated by a fly-back transformer oscillator,
at 16.94 kHz, 7.68 to 7.84 kV. Figure 1 (b) shows the High
voltage is supplied to a W-Re wire inserted along the center. Discharge current was measured with a current probe
and pre-amplifier TCP312 and TCPA300. High-side terminal voltage was measured with P-3000 high voltage probe.
Discharge power was estimated by integrating the multiplied waveform and digitized Q versus V Lissajous figure.
The electronic charge Q was measured as the voltage-drop
across a ceramic capacitor, Cr = 4200 pH connected to theouter electrode surrounding the vial.

In the experiment, DBP sample was solved in NaOH
solution, and diluted with pure-water. The concentration
was measured at 20 mg/L. After plasma-injection, the concentration of the solution of DBP was measured by thehigh-performance liquid chromatograph (HPLC). The total organic carbon (TOC) was measure as the amount of
CO2 from the incinerated organic materials. The concentration of Br− and BrO−3 was measured by the ionchromatography. The concentration of the DBPs weremeasured by HPLC, L-6000 equipped with UV detector,L-4200 (Hitachi Co., Japan) with column X-bridge 4.6,
250 mm (Waters, Japan).
TOC measurement was carried out by TOCLCSH/CSN and auto sampler, ASI-L, (Shimadzu Co.,Japan) Ion chromatography for the anion ICS-1100(Dionex), with column IonPac AS22 (Dionex), and guard
column, IonPac, AG22 (Dionex) installed with the electroconductivity detector. The reproducibility of the time
elapsed measurement and the experiment with high density
samples was examined by HPLC Prominence-I LC-2030
Plus (Shimadzu Co., Japan) installed with UV-detector anda reverse mode column: Xbridge C-18, 5 µm.

3. Experimental Results

Fig. 1 The dielectric barrier electrode and measurement plasma
reactor; (a) Experimental layout; (b) voltage (lower) andcurrent (upper trace) waveform. Vertical unit: 2 kV/div.
50 mA/div. and horizontal unit: 20 µs/div.

Figure 2 shows the time-evolution of 2, 6 - DBP concentration and TOC as a measure of remaining organic
compounds. In Fig. 2 (a), oxygen and air discharge case,
one can find that the bromine detachment is completed
in 20 minutes. The decomposition in nitrogen (greendiamond) and Ar (blue-triangle symbols) discharge are ineﬃcient compared with the oxidative media.
Decrease in the TOC in the oxygen discharge isalmost 100 %. Figure 3 shows the result of the ionchromatography: Br− , and BrO−3 . NO−3 was measured in
the air discharge. In the oxygen discharge, the increase in
the concentration of Br- anion is observed initially. Then,turned to decrease. This change indicates the production ofBrO−3 by oxidation. In the air-discharge, 40 % of the initial
TOC is remaining, but the suppression of BrO−3 productionwas observed. This diﬀerence is attributed to the increasein NO−3 density. The power estimated with the Q versus V
Lissajous is summarized in Table 1.
Next object is the comparison of DBPs structure andthe operation at lower discharge power < 6 W. Figure 4
shows the degradation of 2, 4 - DBP and 2, 6 - DBP, in theinitial 20 minutes.
The power estimated with the Q versus V Lissajous issummarized in Table 2, at low discharge power < 6 W.
In Figs. 4 (b) and (c), our small finding is the bromine
detachment in shorter time in the case of 2, 6 - DBP.
In the industrial bi-product, hazardous concentrationof DBP may be observed. Let’s say 10 times the observed case. Table 3 shows the result of the combination ofthe plasma degradation and the anion exchange using geltype tri-methyl ammonium polymer, after the air-discharge

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Fig. 2 Comparison of the degradation of 2, 6 - DBP by multigas plasma; (a) Concentration of 2, 6 - DBP. Black-crosssymbol indicate the control in a case air bubbling; (b)TOC.
Table 1 Discharge power.

Fig. 3 Concentration of anion; (a) Br− ; (b) BrO−3 ; (c) NO−3 in theair- discharge case. Dotted line: Linear regression.
Table 2 Discharge power.

Table 3 Combined process of plasma-degradation and anion exchange.

degradation in 10 minutes, at discharge power < 13 W.
Using the combination of plasma and anion exchange,high density sample can be degraded to 25 % of the initialconcentration successfully. Thus, a short cut of the degradation have been achieved. Figure 5 shows poly-styrenebase anion exchange polymer, capable to exchange Br−3401132-3

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Fig. 5 Gel-type poly-styrene base, tri-methyl ammonium anion
exchange polymer. (Sanei Chemistry, Japan).

Fig. 4 Experimental comparison of the concentration of DBPs:(a) 2, 6 - DBP, (b) 2, 6 - DBP initial phase, and (c) 2, 4 DBP.

Fig. 6 LUMO and HOMO energy diagram for the Nucleophilic
reaction; (a) Starting system, 2, 4 - DBP + OH− ; (b)Produced system, bromophenol - OH + Br− (in NaBr);
LUMO energy is −0.332 eV and HOMO −10.197 eV.
Empty line symbol indicates LUMO and line with two
spins indicates HOMO state.

and NO−3 anion with OH− anion.

4. Discussion
4.1

Interpretation by frontier electron density calculation

The replacement reaction of Br− and OH− is determined by the energy diﬀerence in the starting system,
DBP+OH-. Consequently, the reversed reaction in the
produced system, bromophenol - OH+Br− (in NaBr). Hydroxyl radical and anion are excited in the gas region. The
excited state of hydroxyl radicals is singly occupied molec-ular orbital (SOMO) and directed to the electrophilic reaction. Figure 6 shows the energy diagram of the excitedstate of hydroxyl anion and DBP. The lowest unoccupied
molecular orbital, LUMO energy of the starting system,
2, 4 - DBP is −0.540 eV and excited energy of OH− is4.045 eV. Diﬀerence between the highest occupied molecular orbital, HOMO and LUMO energy gives the activationenergy, 4.585 eV. LUMO energy of the produced system 1,2 - OH-4-bromophenol is −0.332 eV and HOMO energy ofBr− is −10.197 eV. This calculation indicates the reaction

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Volume 14, 3401132 (2019)tion region localized around the benzene ring.
The frontier electron density was calculated by
Biomedical CAChe (Fujitsu, Japan) [7, 8].

4.2 Interpretation of the control of BrO−3 anion

Fig. 7 LUMO and HOMO energy diagram for the Nucleophilic
reaction; Starting system, 2, 6 - DBP + OH− ; Empty line
symbol indicates LUMO and line with two spins indicates HOMO state.

From the experimental result, the highest concentration of BrO−3 is found in the case of the oxygen-discharge.
Logically, in the air-discharge contains suggests that BrO−3is controlled by NO−3 . A proposal is made by von Gunten that BrO−3 is produced by the cascade oxidation in
the advanced oxidation by oxygen plasma or ozone injection [9–11]. This process is considered as in the following
reaction.
Br− → O3 → HOBr/OBr− → O3 → BrO2→ O3 → BrO−3 .
Concerning the blocking of BrO−3 in the air plasma, a proposal of the following reaction is made by Parrino et al.
[12]NO−3 + 2Br− + 3H+ ↔ Br2 + NHO2 + H2 O.
Air plasma produces NO−3 anions, and H+ is the counterpart of OH− . The accumulation of this anion lowers thepH. of the system and contributes to the decreases in theconcentration of OH− . Nevertheless, the reactant is supplied from the gas-region independently, and NO−3 can contribute to the blocking of the production of undesirable
byproduct BrO−3 .

5. Conclusion

Fig. 8 Iso-surface of 2, 6 - DBP; (a) the nucleophilic susceptibility, (b) electrophilic susceptibility reaction frontier electron density.

proceeds in the forward direction, while supplies of OH∗
radical and excited OH− anion lasts. Figure 7 shows theenergy diagram of the starting system of 2, 6 - DBP+OH− .
LUMO energy is slightly lower.
Figure 8 shows iso-surface of the nucleophilic andelectrophilic reaction frontier electron density of 2, 6 DBP. Excited hydroxyl anion is directed to the nucleophilic
reaction localized around Br atoms, whereas the reaction
with hydroxyl radical is targeted to the electrophilic reac-Plasma degradation of dibromophenols was studied
using a compact dielectric barrier discharge in a quartz
tube, dielectric barrier discharge at the gas-liquid boundary. As the dielectric barrier discharge can be operated at
low electric power, this method is a feasible solution for
the nature remediation. Experimental comparison of working gas revealed the control of hazardous bi-product BrO−3
was realized by the accumulation of nitric anion in airdischarge. The degradation of Br system was interpretedby the Molecular Orbital Theory. The LUMO energy levelof DBP is lower than the excited energy level of the active component, OH− . this result suggests that the electrochemical degradation by the plasma satisfies the requirement for the environmental pollution by the brominatedaromatic compounds. The direction of the forward reaction was explained by the energy-diﬀerence between thestarting system and the produced system.

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

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Appendix A

Appendix B

Figure A shows 3-D LUMO/HOMO model of 2, 6 - DBP.

Figure B shows 3-D LUMO / HOMO model of 2, 4 DBP.

Fig. A

3-D LUMO / HOMO model of 2, 6 - DBP. (a) LUMO surface,−0.737 eV; (b) HOMO surface, −9.481 eV.

Fig. B 3-D LUMO / HOMO model of 2, 4 - DBP (a) LUMO surface−0.540 eV, (b) HOMO surface −9.287 eV.

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Sci. 26, 1284 (2014). http://DOI.ORG/10.1016/S10010742(13)60600-2[4] IARC section 172.730 Potassium Bromate. https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSe
arch.cfm?fr=172.730[5] S. Kojima, K. Katayama-Hirayama and T. Akitsu,World J. Eng. Technol. 4, 423 (2016). http://DOI.ORG/10.4236/wjet.2016.43042[6] P. Lukes and B.R. Locke, Ind. Eng. Chem. Res. 44(9), 2921
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