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. Dielectric barrier discharge was operated in the aqueous solution and gas boundary. The degradation was measured by high performance liquid chromatography (HPLC). The acceleration of the advanced-oxidation has been investigated by the combination of the anion exchange polymer membrane. The result indicated that the degradation pathways involve a rapid detachment of the nitro group and a slow opening of the aromatic-ring. The hydroxyl radical and the excited hydroxyl anion are responsible for the primary attack of the DNP with the production of dihydroxy- nitrobenzenes . The attack of hydroxyl radical occurs at the benzene ring carbon activated by the presence of a phenolic OH group and a nitro group. The result suggested that the reaction is dominated by a pseudo-first order kinetic reaction. The degradation process is interpreted using Molecular Orbital Theory.
A specific feature of the dielectric barrier discharge electrolysis is the excitation of oxygen molecules in the space surrounding the electrode and the chemical reaction at the interface between the plasma and the aqueous electrolyte. In the primary reaction gaseous region surrounding the electrode, water vapors are dissociated by the collision with excited atoms. In the water region, active species are generated by the dissociation of hydrogen peroxide:
O * H , OH − , H * , O 2 −
Nitrophenols (NP) are industrial products and have been detected in urban and agricultural waste. NPs are intermediates in agricultural chemistry, pesticide and dye synthesis such as artificial-indigo. The Yuzen-Nagashi (washing the Yuzen―printing in the river) was a traditional sight seen in the old KAMOGAWA River. It was not until late 1950s that this tradition was regally prohibited from the environmental point of view. Because of the high stability of the multi-ring aromatic compounds and low efficiency in the bio-degradation, the pollution of drinking-water reservoirs is attracting attentions in recent years. The purification of wastewaters is very difficult since some of multi-ring aromatic compounds are resistant to the bio-degradation and considered priority toxic pollutants, in concern due to the carcinogenic effect and endocrine disruptive effects. The advanced oxidation is attracting attentions as the cutting-edge technology for pollution of drinking-water reservoirs by aromatic compounds.
The decomposition of the aromatic compound is studied in several forms: the streamer discharge in liquid; the direct current contact glow discharge [
Chemical reaction between the organic compound and the active component is discussed with Molecular Orbital Theory, considering the energy level of the Lowest Unoccupied Molecular Orbital (LUMO) and Highest Occupied Molecular Orbital (HOMO) and the Frontier-electron density theory.
center. These components are assembled with a Teflon Cajon-type elbow coupler. The quartz tube of the plasma source is immersed in the water-solution, which is connected to the ground electrode through the capacitive coupling. 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). The discharge voltage was measured with P-3000 high voltage prove and the discharge current was measured with TCP312 current prove clamped to the ground side wire with preamplifier TCPA300 (Tektronix, Beaverton, USA) (
In the dielectric barrier discharge, myriads of spikes are observed in the current as shown in
J/cycle. The electrical power input at 16.5 kHz is 8.25 W.
Discharge Mode | Experimental parameters | ||
---|---|---|---|
Control voltage (V) | Frequency (kHz) | Power (W) | |
Dielectric Barrier Discharge | 6.45 | 16.34 - 94 | 3.69 |
8.66 | 6.96 | ||
9.46 | 9.48 | ||
11.0 a | 12.36 | ||
Glow Discharge | >13 | 16.88 |
a: Operational parameter.
Next figure shows a typical example of gel type poly-styrene base alkalized trimethyl-ammonium polymer (Sanei Chemical, Kumamoto, Japan) NO 3 − anion was exchanged with OH− anion (
The efficiency of the plasma degradation is compared for dinitrophenols: 2,4-DNP, 2,5-DNP, and 3,4-DNP.
Next,
DNP Specie | Experimental result | |||
---|---|---|---|---|
TOC (mg/L) | DNP Concentration (mg/L) | NO 3 − Concentration (mg/L) | pH | |
2,5-DNP | 4.310a | 0 | 20.6 | 3.5 |
3,4-DNP | 5.139 | 0 | 19.0 | 3 |
2,4-DNP | 5.117 | 0.002 | 19.6 | 2.99 |
Apparently, the final concentration of NO 3 − exceeds the original concentration of the nitro-group, and the solution shows acidity.
The weight of nitric anion is calculated, if two nitrogen atoms are converted to NO 3 − , one oxygen comes from the working gas. From the moll ratio, 124/184.106 times [20 mg/L] = 13.47 mg/L. The increase in NO 3 − anion should be attributed to the oxidation in the working gas. This acid solution is not suitable for the release. It would be necessary to improve pH to be neutral, by neutralization using proper reagent after the anion exchange.
The concentration can be expressed as exponential recurrence function as in the following form and plotted along symbols. R squared ranges from 0.9661 to 0.9859. Increase in the discharge current causes the change of the discharge mode to the transfer glow discharge (
ln ( C t C 0 ) = − a t − b , C 0 = 20 [ mg / L ]
The plasma process increases neutral materials in the object, 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−.
The anion-exchange accelerated the plasma-degradation, and the final concentration was 2.9 ppm in the total processing time of 20 minutes. The characteristic yellow color of 2,4-DNP was used as an indicator. 2,4-DNP in liquid shows the absorption spectrum in the visible-UV wave length, as shown by the dark line in
Sample | Recurrent Function | ||
---|---|---|---|
a | b | R-squared | |
2,4-DNP | 0.058 | 0.04 | 0.9661 |
3,4-DNP | 0.099 | −0.200 | 0.9776 |
2,5-DNP | 0.377 | 0.148 | 0.9859 |
Lukes et al. reported the enhancement of *OH radical production and O3 decomposition in the presence of N2 molecules in humid air, through the following reactions [
Dissociation and ionization
H 2 O + e → OH − + H + + e
H 2 O + e → O * H + H * + e
Reactions including excited oxygens
O ( D 1 ) + H 2 O → 2 OH *
O ( D 1 ) + O 2 → O 3
Reactions including N2 molecules:
N 2 + e → 2 N * + e
O ( D 1 ) + NO → NO 2
N * + O 3 → NO + O 2
NO + O 3 → NO 2 + O 2
Reactions including the metastable state of N2 molecules:
N 2 ( A 3 Σ ) + H 2 O → N 2 + O * H + H *
Inhibition of O3 production and enhancement of OH* formation:
OH − + O 3 → O * 2 − + HO 2 *
O 3 + 3 HO 2 * → 3 OH * + 3 O 2
Production of superoxide: Oxygen molecule is the primary acceptor of electrons and forms O * 2 −
O 2 + e → O * 2 −
Production of hydroxyl radicals: super oxide is highly reactive super anions and form hydrogen peroxide in the following chain reaction. Hydroxyl radical and hydroxyl anion are produced in the dissociation of hydrogen peroxide by the electron collision in the discharge area as well as by the decomposition of ozone in water reactions.
O * 2 − + H 2 O → OH − + H * O 2
H * O 2 + H * O 2 → H 2 O 2 + O 2
H 2 O 2 + O * 2 − → O * H + OH − + O 2
H 2 O 2 + e → O * H + OH −
DNP + { O * H + OH − } → Producedsystem
Reactions related to NO 2 − and nitrous acid, related to the production of nitrite ions and ONOO, peroxonitrite:
NO 2 − + H 2 O 2 → ONO 2 −
HNO 2 + H 2 O 2 → HOONO + H 2 O
One of merits of the present multi-gas dielectric barrier discharge is the comparison of the discharge characteristics in air and pure oxygen. Various reactive species are generated in the discharge region and the inhibition of the production of ozone. In the experiment, DNP with the different position of nitro group is examined using 2,4-DNP, 2,5-DNP and 3,4-DNP (Appendix). Using the molecular orbital theory, LUMO and HOMO energy of the starting system and the produced system were calculated considering hydroxyl radical and excited hydroxyl anion as active agents.
In
In
electron density is localized in the ortho and para position with respect to OH group. Nitro group in this position is the target of the electrophilic replacement by OH radical (
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 [
In
species. The LUMO energy of 2,4-DNP is −1.268 eV and excited energy of hydroxyl anion OH− is 4.045 eV. Excited hydroxyl anion is directed to nucleophilic reaction, which depends on the energy difference between the excited OH anion and 2,4-DNP, 5.313 eV. OH* radical is characterized with the singly occupied molecular orbital (SOMO) at 3.121 eV. This electrophilic exchange reaction depends on the difference between the HOMO level of 2,4-DNP, 13.337 eV.
The exchange reactions are limited by the supply of excited hydroxyl radical and excited hydroxyl anion, OH− and OH*. The reverse reaction limited by the energy difference.
In this work, the frontier electron density and HOMO and LUMO energy level are calculated by Biomedical CAChe (Fujitsu, Japan) [
Plasma-degradation of the dinitrophenyl was studied using a compact dielectric barrier discharge in a quartz tube, immersed into the liquid surface. As the dielectric barrier discharge can be operated at low electric power, this method is a feasible solution for the water remediation. In the air-plasma treatment, the accumulation of nitric anion was a problem. A simple solution, the anion exchange was found. This process can interrupt the long degradation process to short solution. The exchange of the nitro system of the DNPs was interpreted by the Molecular Orbital Theory. The LUMO level of the organic compound is lower than the HOMO level of the active component OH−or SOMO level of OH*. The reaction proceeds to the detachment of nitric anion. In the future activities, the adaptive combination of the present method with other difficult materials and the improvement of the efficiency will be the necessary development step.
This research received a fund from Future Science Institute, Tokyo, Japan. The authors express their gratitude to Mr. Kento Nakai and Mr. Kenya Yabu.
The authors declare no conflicts of interest regarding the publication of this paper.
Okawa, H., Kuroda, H., Hirayama-Katayama, K., Kojima, S.-I. and Akitsu, T. (2019) Plasma-Electrolysis of Dinitrophenol in Gas-Liquid Boundary and Interpretation Using Molecular Orbital Theory. World Journal of Engineering and Technology, 7, 141-157. https://doi.org/10.4236/wjet.2019.71010
Dinitrophenol has several types of different structure is available [