TRAINING DEFENCEACADEMY OF MILITARY SCIENCE AND TECHNOLOGY TRAN VAN CONG RESEARCHING HIGH VOLTAGE ELECTROCHEMICAL ENGINEERING OF APPLIED ELECTRODIC PLASMA GENERATION FOR DECOMPOSITION OF
Trang 1TRAINING DEFENCE
ACADEMY OF MILITARY SCIENCE AND TECHNOLOGY
TRAN VAN CONG
RESEARCHING HIGH VOLTAGE ELECTROCHEMICAL ENGINEERING OF APPLIED ELECTRODIC PLASMA GENERATION FOR DECOMPOSITION OF
2,4-DICHLOROPHENOXYACETIC ACID AND
Trang 2ACADEMY OF MILITARY SCIENCE AND TECHNOLOGY,
MINISTRY OF NATIONAL DEFENCE
Scientific supervisors :
1 Prof.Dr.Sci Nguyen Duc Hung
2 Dr Nguyen Van Hoang
Reviewer 1:
Reviewer 2:
Reviewer 3:
Thesis is defended at the doctoral evaluating Council at Institute level, held at the Academy of Military Science and Technology
at :…hour…minute, day…month…year 2022
Thesis can be found at:
- Library of Academy of Military Science and Technology.
- Vietnam National Library.
Trang 31 Necessity of the thesis
Toxic compounds of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) are decomposed of agent orange dioxin residues in Vietnam In addition, in agriculture, the use of 2,4-D chemicals to kill weeds for a long time also causes serious environmental pollution that need to treat
Direct current high voltage electrochemical engineering to create plasma is one of the high effective treatment methods for aromatic ring compounds contain chlorine atom that difficult to decompose and cause polluted environment Therefore, PhD student selected the subject
"Researching high-voltage electrochemical engineering of applied
electrodic plasma generation for decomposition of 2,4-dichlorophenoxy acetic acid and 2,4,5-trichlorophenoxyacetic acid in water environment”
to address the pollutted problem that mentioned above
2 Research objectives
Researching on using electrode plasma engineering to treat 2,4-D, 2,4,5-T contaminant compounds in water environment The polluted substances after treatment are completely decomposed
3 Subjects and scope of the thesis
Thesis studies the use of direct current high voltage electrochemical engineering in the voltage range from 0 to 20 kV to perform an electrochemical reaction to create a gaseous environment on the electrode for the plasma formation process with the aim to decompose polluted compounds 2,4-D, 2,4,5-T in water environment
as COD, TOC Assessment of environmental criteria after treatment
Trang 45 Research methods
Thesis uses the theoretical method overview combined with experiment to conduct the research content
6 Scientific and practical significance of the thesis
Thesis results clarify the factors affecting the formation of plasma in water environment and ability to form active agent
Evaluation of the treatment efficiency of 2,4-D, 2,4,5-T by electrochemical plasma engineering and applicability in practice
7 Thesis layout
The content of the thesis is presented in the regulation structure including: Introduction; Chapter 1 Overview; Chapter 2 Research subjects and methods; Chapter 3 Results and discussion; Conclusion; References; Appendix, as follows:
Chapter 2: Research subjects and methods
Research subjects are the factors affecting the plasma formation andthe decomposition efficiency of 2,4-D, 2,4,5-T
Research method is based on experiment, analysis, evaluation and conclusions with the decomposition of 2,4-D, 2,4,5-T compounds
Chapter 3: Results and discussion
Identifying some results as factors affecting plasma formation, active agents, decomposition efficiency, studying kinetics and mechanism of decomposition process of 2,4-D, 2,4,5-T
Chapter 1 OVERVIEW 1.1 Overview of plasma
x Formation of plasma
Matter exists mainly in three forms, solid, liquid and gases In the
Trang 5gaseous state, if energy is continued to be supplied, the collision between particles in the gases is so strong that they break into pieces, creating charged particles which are ions and electrons or the plasma state The expanded plasma concept considers plasma including particles such as electrons and ions and neutral particles such as atoms and molecules as well as free radicals and photons.
x Classification of plasma
Plasma can be classified according to temperature into two types: thermal plasma and cold plasma or classification based on physical-chemical properties into plasma chemistry or plasma physics
1.2 Electrochemical plasma
x Electrochemical plasma generation engineering
Electrochemical plasma is formed in the solution at high voltage, the electrolysis process generates H2, O2, continues providing energy enough strong to form plasma state that generates OHx free radicals, H2O2 and particles nano
x Formation of active agents in electrochemical plasma
OH• free radical is formed through several mechanisms:
Dissociation: H2O + e- Hx + OHx + e
-Ionization: H2O + e- H2O++ 2e
-H2O++ H2O2 H3O+ + OHxThe emission of UV rays when plasma occurs also contributes to the breaking of O-O bonds leading to the formation of OH• free radical:
Trang 6Gaseous catalysis: Oxygen gases bubbling in the plasma combined with electron creating superoxide free radical (xO2-) leading to the formation of H2O2and OHxfree radical according to reaction mechanism:
in solution via the Fenton reaction:
Chapter 2 RESEARCH SUBJECTS AND METHODS
2.1 Research subjects
Research subjects include: factors affecting plasma formation, efficiency, mechanism, kinetics of 2,4-D, 2,4,5-T decomposition process.Investigate of pollution index such as COD, TOC, TDS, Cl-, pH
2.2 Electrochemical plasma generator
x High voltage direct current power supply
High voltage direct current power from 0 to 20 kV, P = 15kVA
x Structure of reaction vessel and electrode
Reaction vessel is made of heat resistant glass, including double layer The inner layer contains the reaction solution, the outer layer is circulating water through the thermostatic bath
Trang 7Metal anode and cathode electrodes are made of cylindrical shape, wrapping around with epoxy to create an electrode area in contact with the solution.
Figure 2.4 Diagram of electrochemical plasma generation
process treating 2,4-D, 2,4,5-T.
2.3.Chemicals for research
2,4-D, 2,4,5-T acid, Sigma Aldrich and other necessary chemicals
2.4.Research methods
Researching the factors affecting the decomposition process of 2,4-D, 2,4,5-T are evaluated through the decomposition efficiency with the formula:Decomposition efficiency: 0 t
0
C C H(%) = 100%
C uWhere as:
H(%) :decomposition efficiency after t minutes ;
C0: initial concentration, mg/L;
Ct:concentration at time t, mg/L
2.5.Analytical equipment and methods
Analyzing 2,4-D, 2,4,5-T by HPLC 1100, Agilent
Analyzing degradation products by GC-MS 6890-5975, Agilent
Analyzing H2O2, OHx free radical by UV-Vis, UH 5300, HitachiAnalyzing of particle size, Zeta potential by SZ-100, Horiba
Analyzing of particle size, Zeta potential by SZ-100, Horiba
Analyzing metal by ICP-MS 7800/7850, Agilent
Analyzing total organic carbon in solution by TOC-5000A, ShimadzuMeasure electrical conductivity by HI 8733 instrument, Hanna
Measure pH value by HI 8314 instrument, Hanna
Trang 8Chapter 3 RESULTS AND DISCUSSION 3.1 Factors affecting plasma formation on electrode
Researching plasma appearance conditions showed that the plasma appearance depends on factors such as voltage (Figure 3.2), distance between two electrodes (Figure 3.3), electrical conductivity (Figure 3.4.) , solution temperature (Figure 3.5), electrode size and pH value (Table 3.2)
0 20 40 60 80 100 120
Figure 3.2 Plasma appearance on copper, iron and tungsten electrodes depends on the voltage at T= 30 o C, h = 200 mm, pH = 7, EC = 1.4 µS/cm.
0 10 20 30 40 50 60 70 80 90 100 110
Figure 3.3 Appearance plasma on copper, iron, and tungsten electrodes depends on the electrode distance at T=30 o C, V=15 kV, pH=7, EC=1.4 µS/cm.
20 40 60 80 100 120 140 160
Figure 3.4 Appearance plasma on copper, iron and tungsten electrodes depends
on the electrical conductivity at T=30 o C, V=15 kV, pH=7, h=200 mm.
Trang 90 20 40 60 80 100 120 0
50 100 150 200 250
Figure 3.5 Appearance plasma on copper, iron and tungsten electrodes depends
on the solution temperature at V=15 kV, pH=7, h=200 mm, EC=1.4 µS/cm.
Table 3.2 Influence of factors on plasma appearance
U(kV)T(oC) h(mm) pH Ø (mm)
EC(µS/cm) Plasma appearance (min)
(-): Plasma not appearance,
<1:Plasma appearance less than 1 minute
Trang 102.87
10.22 3.67
4.10
23.23 23.30
Figure 3.8 The anodic dissolution of copper, iron and tungsten electrodes
at T=20 o C, h=200 mm, EC=1.4 µS/cm, pH=7, t=120 min.
Amount of dissolved metal from the copper anode was the highest, followed the iron anode , smallest tungsten anode The anodic electrode dissolved with different rates when the voltage change (Figure 3.8)
0 100 200 300 400 500 600 700 800 900
Cu-V Fe-V W-V Cu-T Fe-T W-T
3.2 Characterization of solutions when conducting electrochemical plasma engineering
6.2 6.4 6.6 6.8 7.0
W Fe Cu
Time (min)
Figure 3.10 pH change in double distilled water solution on electrode copper, iron, tungsten at T=30 o C,V=15 kV, h=200 mm, EC=1.4 µS/cm.
Trang 11pH decrease was due to the discharge creating H+ ions The tungsten electrode changed the pH value was the highest, then the iron electrode and the pH value was the lowest on the copper electrode (Figure 3.10).
0 2 4 6 8 10
Figure 3.11 Electrical conductivity change in double distilled water solution
on iron, copper, tungsten electrode at T=30 o C, V=15 kV, h=200 mm, pH=7.
Electrical conductivity of solution using the copper electrode changed the least, followed the iron electrode, the electrical conductivity reached the highest on the tungsten electrode (Figure 3.11)
x Formation of metal nanoparticles in solution
Because the hydrogen atom has a reduction property, so when the hydrogen atom contact with metal ion in the solution will occur reaction:
Men+ + (n/2)H2 Me0
+ nH+Table 3.3 Particle sizes of CuNPs, FeNPs and WNPs: deviation (Dev,nm), at peak (Peak, nm), concentration (Con, nm) and average(Eve, nm)
Trang 12Analyzing size of metal nanoparticles showed that the colloidal system of copper and iron nano solutions at 5 kV and 15 kV was distributed quite concentratedly with the size smaller than 200 nm while with the tungsten electrode there were two distribution region of size were about 100 nm and the larger size region from 500 nm to nearly 1000 nm
At 10 kV, all three electrodes are divided into two size regions (Table 3.3).Table 3.4 Zeta potential value of copper, iron, tungsten metal nano colloidal solutions formed at voltage of 5,10,15 kV
x Formation of OHx free radicals
Analyical results showed that the concentration of OH• free radicals increased rapidly in the first time and then increased slowly The reason of this phenomenon was due to 2,3-DHB, 2,5-DHB have been decomposed(Table 3.6)
Table 3.6 OHxfree radical concentration formed on the iron electrode
2,3-DHB (M) 1,5.10-4 1,8.10-4 2,1.10-4 2,3.10-4 2,4.10-4 2,4.10-42,5-DHB (M) 2,2.10-5 5,8.10-5 9,1.10-5 1,0.10-4 1,2.10-4 1,3.10-4
OHx(M) 1,7.10-4 2,4.10-4 3,0.10-4 3,4.10-4 3,6.10-4 3,7.10-4
Trang 133.3 2,4-D, 2,4,5-T decomposition efficiency by electrochemical plasma engineering
Maximum decomposition efficiency of 2,4-D, 2,4,5-T on iron electrode
at 67.13%, 51.62 %, respectively Followed by copper electrode at 49.75%, 38.91%, minimum tungsten electrode at 29.24% and 25.27% (Figure 3.15)
Figure 3.15 Effect of electrode material on the decomposition efficiency of 2,4-D, 2,4,5-T at V=5 kV, h=300 mm, T=30 o C, EC=38.8 µS/cm, t=60 min.
0 20 40 60 80 100
Figure 3.16 Effect of time on the decomposition efficiency of 2,4-D, 2,4,5-T on iron electrode at V=5 kV, h=300 mm, T=30 o C, EC=38.8 µS/cm, t=60 min.
The decomposition efficiency of 2,4-D: 47.94%, 67.15%, 79.69% and 86.62%, with 2,4,5-T: 36.96%, 51.65%, 61.85%, 71.18%, corressponding to time of 30, 60, 90 and 120 minute Decomposition efficiency increases over time (Figure 3.16)
80.54
67.15
59.95 65.50
51.65
39.46
0 20 40 60 80 100
Figure 3.17 Effect of concentration on the decomposition efficiency of 2,4-D,
2,4,5-T on iron electrode at V=5 kV,h=300 mm,2,4,5-T=30 o C,EC=38.8 µS/cm, t=60 min.
Trang 14The decomposition efficiency of 2,4-D: 80.54%, 67.15%, 59.95%,
while the decomposition efficiency of 2,4,5-T was 65.50%, 51.65% and
39.46% at concentrations of 15, 30, 50 mg/L, respectively The
decomposition efficiency increased when the initial concentration was
small (Figure 3.17)
-100 0 100 200 300 400 500 600 700 800 40
60 80
100
2,4,5-T 2,4-D
Figure 3.18 Effect of air flow on the decomposition efficiency of 2,4-D, 2,4,5-T
Blowing air through the solution with flow: 100 mL/min,
300 mL/min, 500 mL/min, the decomposition efficiency of 2,4-D, 2,4,5-T
increased gradually, with flow of 700 mL/min, the decomposition
efficiency had trend reducing (Figure 3.18)
10 20 30 40 50 60 70 80 90 100
2,4-D 2,4,5-T
Figure 3.19.Effect of voltage on the decomposition efficiency of 2,4-D,2,4,5-T
on iron electrode at h=300 mm, T=30 o C, EC=38.8 µS/cm, t=60 min.
The decomposition efficiency increased when voltage went up due to
increasing ionization ability and increasing OH• free radical and H2O2
(Figure 3.19)
30 40 50 60 70 80 90
Figure 3.20 Effect of temperature on the decomposition efficiency of 2,4-D,
2,4,5-T on iron electrode at V=5kV, h=300 mm, EC=38.8 µS/cm, t = 60 min.
Trang 15Temperature affects electrochemical reactions and plasma formation Increasing temperature leading to increase decomposition efficiency(Figure 3.20).
10 20 30 40 50 60 70 80
2,4-D 2,4,5-T
Figure3.21.Effect of electrodic distance on the degradation efficiency of
2,4-D, 2,4,5-T on iron electrode at V=5 kV, T=30 o C, EC=38.8 µS/cm, t=60 min.
The decomposition efficiency of 2,4-D, 2,4,5-T decreased when increasing the distance between the two electrodes due to the formation process of H2O2and OHxfree radical decrease (Figure 3.21)
35 40 45 50 55 60 65 70 75 80
2,4-D 2,4,5-T
Figure 3.22 Effect of pH on the degradation efficiency of 2,4-D, 2,4,5-T
on iron electrode at V=5 kV, h=300 mm, T=30 o C, t=60 min.
The pH value affected the degradation efficiency The suitable pH value for the decomposition of 2,4-D and 2,4,5-T were in the pH range from 5 to 9 (Figure 3.22)
Figure 3.23 Effect of electrical conductivity on the decomposition efficiency of 2,4-D, 2,4,5-T on iron electrode at V=5 kV, h=300 mm, T=30 o C, t=60 min.
Trang 16When increasing the electrical conductivity with sodium chloride salt, the decomposition efficiency decreased The reason for the decrease in decomposition efficiency was due to the OHxfree radical was quenched by the chlorine anion in solution (Figure 3.23).
20 40 60 80 100 120 140
5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1
Time (min)
2,4-D 2,4,5-T
Figure 3.25 pH change of the 2,4-D, 2,4,5-T solution after treated by plasma on the iron electrode at V=5 kV, h=300 mm, T=30 o C.
The decomposition of 2,4-D, 2,4,5-T into organic acids that changed the pH value in the solution (Figure 3.25)
x Indentifying chemical oxidation demand (COD)
The COD values measured before decomposition of 2,4-D and 2,4,5-T
at concentration of 30 mg/L for each substance were 106 mg/L and 57 mg/L, respectively After the decomposition time of 30, 60, 90 minutes, the COD value of the 2,4-D solution decreased with value of 40.2; 15.9; 6.8 mg/L, respectively Similarly for compound of 2,4,5-T, the COD value reached at: 30.2; 12.4; 8.9 mg/L The COD value decreased sharply over time after 90 minutes from 106 mg/L to 6.8 mg/L for 2,4-D, the