Water Treatment by Pulsed Streamer Corona Discharge - LUKEŠ

8/10/2019 Water Treatment by Pulsed Streamer Corona Discharge - LUKE

1/131


2/131


3/131


4/131


5/131


6/131


7/131


8/131


9/131


10/131


11/131


12/131


13/131


14/131


15/131


16/131


17/131


18/131


19/131


20/131


21/131


22/131


23/131


24/131


25/131


26/131


27/131


28/131


29/131


30/131


31/131


32/131


33/131


34/131


35/131


36/131


37/131


38/131


39/131


40/131


41/131


42/131


43/131


44/131


45/131


46/131


47/131


48/131


49/131


50/131


51/131


52/131


53/131


54/131


55/131


56/131


57/131


58/131


59/131


60/131


61/131


62/131


63/131


64/131


65/131

Generation of hydrogen peroxide by the pulsed corona discharge 51

It was shown in Figure 4.6 that phenol had a significant effect on the production ofH2O2. The production of H 2O2 was enhanced in the presence of phenol. Moreover, it wasobserved that the difference in the production of H 2O2 with and without phenol in the solution

increased with the increasing solution conductivity (Figures 4.11 (a)-(d)).

Taking into account the rate constants of phenol and H 2O2 with OH and H radicals(Appendix 3) and an increasing value of the radiant power of the discharge with theincreasing solution conductivity (Table 4.1), it can be inferred that the higher production ofH2O2 in the presence of phenol results from the inhibition of the breakdown of H 2O2 by OHand H radicals directly produced by the discharge (Equations 2.35, 2.10, 4.17) and by OH,HO 2 radicals originated from the photolysis of H 2O2 (Equations 2.9-2.11).

H + H 2O2 H2O + OH (4.17)

It should be noted that an increase of the H 2O2 production due to absorption of UVlight by aromatic ring of phenol was also considered. However, the same results as for phenolwere obtained in the case when KBr salt was added to the solution in amount correspondingto the conductivity of the solution of 50 mS m -1 (Figure 4.12). Since bromides react rapidlywith OH and H radicals (see Appendix 3) and they do not exhibit a high absorbance below300 nm, for the used concentration of phenol that effect could be omitted.

0,0

0,4

0,8

1,2

1,6

0 50 100 150 200 250 300

Energy input [kJ]

c ( H

2 O 2 )

[ m m o l

l - 1 ]

Figure 4.12 Effect of KBr on the production of H 2O2 by the pulsed corona discharge in the solution of theconductivity of 50 mS m -1. Power input 92 W, applied voltage of positive polarity 24.5 kV, pulse repetitionfrequency 30 Hz. , H 2SO 4 0.6 mmol l

-1; , KBr 3.25 mmol l -1.

In addition, although the mechanism of H 2O2 formation by pulsed corona discharge in

water is still not clear (see Section 2.3.3.2.2), the higher production of hydrogen peroxide inthe presence of either phenol or bromides indicates that H 2O2 is formed more likely through


66/131

R ESULTS AND DISCUSSION 52

the other process than by recombination of OH radicals (Equation 2.33) as it was proposed by Sato et al. [54]. Their presumption resulted from the fact that the production of H 2O2 by pulsed corona discharge decreased with the increasing concentration of aliphatic alcohols

(methanol, ethanol and propanol) in the aqueous solution. However, the same effect has beenreported for the photolysis of H 2O2 in the presence of ethyl and isopropyl alcohol [19]likewise for -radiation of aqueous alcohol solutions [38, 136] and it was shown that such

behavior might be attributed to the decomposition of H 2O2 by alcohol radicals (e.g. Equation4.19) formed by reaction of alcohols with OH radical (e.g. Equation 4.18).

HOHCCHOHOHOHCHCH.

3223 ++ (4.18)

OHOHCHOCHOHHOHCCH 23223.

+++ (4.19)

4.2.7 Effect of H 2 and O 2 on H 2O 2 production

In higher conductivities of the solution without presence of phenol it was observedthat the production of H 2O2 achieved a maximum concentration very fast and then dropped toa steady state concentration. A typical example is presented in Figure 4.13 showing the

production of H 2O2 in solution of conductivity 50 mS m -1.

0

0,2

0,4

0,6

0,8

0 200 400 600 800 1000

Energy input [kJ]

c ( H

2 O 2 )

[ m m o l

l - 1 ]

Figure 4.13 Production of H 2O2 in H 2SO 4 solution of the initial conductivity of 50 mS m-1 (c H2SO4 =0.6 mmol l

-1).U =24.5 kV, f =30 Hz, C =10.2 nF, P =92 W.

A similar result was obtained when H 2O2 was added in the beginning to the solution inamount equivalent to the maximum concentration of H 2O2 formed by the discharge (Figure4.13, cmax = 0.7 mmol l -1). Hydrogen peroxide was again initially produced at a rapid rate until


67/131

Generation of hydrogen peroxide by the pulsed corona discharge 53

the concentration reached a maximum almost equal to the double of cmax and then decreasedto a steady concentration analogous to that obtained in the experiment without addition ofH2O2. The independence of the steady concentration of H 2O2 on the initial concentration of

H2O2 led to the conclusion that there must be another effect beside of UV radiation occurringin the pulsed corona discharge the importance of which increased with time.

According to Hochanadel [39], who reported a similar dependence to that one shownin Figure 4.13 for formation of hydrogen peroxide by -radiation of aqueous solution, theH2O2 generation strongly depends on whether the irradiated solution contains oxygen,hydrogen or both gases [137]. Hence, since both gases are produced by the discharge (Table4.12, Appendix 2), the H 2/O2 ratio could play an important role in H 2O2 formation, whenhydrogen peroxide competes with hydrogen and oxygen for hydroxyl radical (Equations 2.10,4.20) and hydrogen radical (Equations 4.17, 4.21), respectively (see Appendix 3).

OH + H 2 H2O + H (4.20)

H + O 2 HO 2 (4.21)

In order to distinguish the effect of H 2 and O 2 on the hydrogen peroxide productiontwo experiments were performed: one with the solution initially saturated either by oxygen orhydrogen and the other with the solution initially degassed by helium. A 0.6 mmol l -1 H 2SO 4 solution of the conductivity of 50 mS m -1 and voltage of the positive polarity of 24.5 kV with

pulse repetition frequency of 30 Hz was used in each case.

0

0,2

0,4

0,6

0,8

1

1,2

0 50 100 150 200 250 300

Energy input [kJ]

c ( H

2 O 2 )

[ m m o l

l - 1 ]

Figure 4.14 Effect of relative hydrogen and oxygen concentrations on H 2O2 production in H 2SO 4 solution of the

conductivity of 50 mS m-1

(c H2SO4 =0.6 mmol l-1

). Power input 92 W, applied voltage of the positive polarity 24.5kV, repetition frequency 30 Hz. , initially H 2-saturated solution; , initially O 2-saturated solution; , initiallydegassed solution; , reference solution without any primary modifications.


68/131


Figure 4.14 shows results for these solutions. It is apparent that there was nosignificant difference in H 2O2 production obtained in initially oxygen-saturated or degassedsolution and in the solution without any primary modifications. A slightly higher production

of H 2O2 was observed in the solution initially saturated by hydrogen. However, in allexperiments the production of H 2O2 reached a maximum and then again decreased. Takinginto account that the solutions were continuously bubbled by the gases produced by thedischarge and treated by the generated shock waves, it seems, that after relatively short timethe concentration of both gases in the solution is controlled by the processes in the dischargeand does not depend on the initial gas concentration.

On the other hand the results obtained in the experiments with an external bubbling[55] do not explain the role of gases dissolved in the solution. According to the microbubble

breakdown theory (see Section 2.2.2) the gas bubbling increases the radical densities since thedischarge can be initiated directly inside the gas bubble. In addition, during bubbling ofoxygen into discharge region ozone is formed and the density of oxygen radical increases.Therefore, a lower yield of hydrogen peroxide should be expected because both ozone and Oradical decompose hydrogen peroxide [16, 58]:

O + H 2O2 OH + HO 2 (4.22)

+

++ + HOHOOOH 32OH

322 . (4.23)

Summary of Section 4.2

It was observed that the production of hydrogen peroxide by the pulsed coronadischarge in water leads to buildup of a steady-state concentration of H 2O2 in solution. Theinitial rate of H 2O2 formation and the yield of H 2O2 formation by the discharge have beendetermined. It was found that k H2O2 increased linearly with the increasing power inputregardless of the energy per pulse, frequency and pulse duration. The yield of hydrogen

peroxide production was independent of the applied voltage however exponentially decreasedwith the increasing solution conductivity. The highest yield (1.5 g/kWh) was obtained for theconductivity of 10 mS m -1. The formation of H 2O2 was found to be independent of pH over

the whole range studied (from 2.8 to 11). The polarity of the applied voltage had a large effecton the production of H 2O2. In general, the yields of H 2O2 for the negative polarity were lessthan half of that for the same voltage of the positive polarity. On the other hand, production ofH2O2 was less dependent on the solution conductivity than in the case of the positive polarity.At higher solution conductivity it was observed that the yield of hydrogen peroxide wasaffected by photolysis caused by the ultraviolet radiation of the discharge. It was inferred thatthe hydrogen peroxide production by the pulsed corona discharge can be separated into two

parts - hydrogen peroxide production at a constant rate by a zero order process and itsdecomposition by breakdown processes such as photolysis due to UV radiation emitted fromthe discharge, thermal decomposition and decomposition by OH, HO 2and H radicals.


69/131

Degradation of phenol by the pulsed corona discharge 55

4.3 Degradation of phenol by the pulsed corona discharge

In this section the results of the oxidation of phenol by the pulsed corona discharge inwater will be presented regarding the formation of oxidation products, effect of pH, iron stateand concentration, solution conductivity and polarity of the applied voltage on phenolremoval efficiency.

4.3.1 Dependence of phenol removal on conditions in the corona discharge reactor

Figure 4.15 summarizes the time dependence of phenol removal (concentration1 mmol l -1) by the pulsed corona discharge for three different conditions. The initial solutionconductivity of 11 mS m -1 and the same power input of 100 W of the positive polarity wereused in each case.

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0 100 200 300 400 500 600

Energy input [kJ]

c ( p h e n o l )

[ m m o l

l - 1 ]

(1)

(2)

(3)

Figure 4.15 Phenol removal for (1)-electrolysis only in 1 mmol l -1 NaCl, (2)-corona discharge in 1 mmol l -1 NaCl, (3)- corona discharge in 0.5 mmol l -1 FeCl 2. Power input 100 W, applied voltage 20 kV, pulse repetitionrate 50 Hz, initial solution conductivity 11 mS m -1, phenol concentration of 1 mmol l -1.

Line (1) demonstrates the negligible role of electrolysis in phenol degradation. In theseexperiments the applied voltage was slightly below the inception value and so, no dischargewas generated. Line (2) corresponds to the corona discharge in the same type of solution (1mmol l -1 NaCl) at the applied voltage near the sparking value, and shows some slowdegradation of phenol, caused apparently by the oxidative action of OH radicals produced by

the discharge. Line (3) demonstrates a significant role of iron ions on phenol removal by thedischarge in the solution of 0.5 mmol l -1 of FeCl 2. In this case hydrogen peroxide generated


70/131


by the discharge was decomposed by the ferrous ions according to Fentons reaction (seeEquation 2.12) and therefore, the yield of OH radicals and the rate of phenol degradationincreased (detailed analysis of effects of iron ions on phenol removal is given in Section

4.3.3.). The phenol removal efficiencies for each case are given in Table 4.9.

Table 4.9 Phenol removal efficiency for different conditions and solution compositions in the reactor. Powerinput 100 W, applied voltage 20 kV, pulse repetition rate 50 Hz, initial conductivity 11 mS m -1.

line system G37% [mol J-1] G37% [g (kWh)

-1]

1 electrolysis / 1 mmol l -1 NaCl 1.9 10 -11 0.006

2 corona discharge / 1 mmol l -1 NaCl 4.1 10 -10 0.138

3 corona discharge / 0.5 mmol l -1 FeCl 2 7.1 10 -9 2.420

4.3.2 Formation of phenol degradation byproducts

4.3.2.1 Formation of aromatic byproducts

1,2-dihydroxybenzene (catechol, CC), 1,4-dihydroxybenzene (hydroquinone, HQ) and1,4-benzoquinone ( p-benzoquinone, BQ), with trace amounts of 1,3-dihydroxybenzene(resorcinol, RS), as primary products of phenol (PH) degradation by the pulsed streamercorona discharge in the presence of iron ions were identified by HPLC analysis (Figure 4.16).This is consistent with the results obtained for the degradation of phenol by ozonation [12,13], sonolysis [35], -radiation [42] or by high energy electron beam irradiation [48].

OH

OH

OH

OH

OH

O

O

OH

OH

phenol catechol hydroquinone p-benzoquinone resorcinol

Figure 4.16 Identified aromatic byproducts formed by the oxidation of phenol by the pulsed corona discharge inthe presence of Fe 2+ ions in the solution .


71/131


Figure 4.17 demonstrates phenol removal and formation of aromatic byproductsduring the degradation of 1 mmol l -1 phenol by the pulsed corona discharge in the presence of0.5 mmol l -1 FeCl 2.

0

0,2

0,4

0,6

0,8

1

0 100 200 300 400 500 600

Energy input [kJ]

c [ m m o l l

- 1 ]

PH

CCHQ

BQ

Figure 4.17 Phenol removal and formation of the aromatic intermediates during the degradation of 1 mmol l -1 phenol by the pulsed corona discharge in the solution of 0.5 mmol l -1 FeCl 2. Initial solution conductivity 10 mSm-1, power input 100 W, applied voltage 20 kV, pulse repetition rate 50 Hz. , phenol; , catechol; ,hydroquinone; , p-benzoquinone.

The formation of dihydroxybenzenes corresponds to the oxidation of phenol byhydroxyl radicals. When OH attack is predominantly by addition on the aromatic ring,initially a dihydroxycyclohexadienyl radical (DCHD) is formed (Figure 4.18) [138, 139].Consequently, depending on pH and availability of suitable oxidants or reductants, DCHDmay react further by: acid-catalyzed elimination of water to form a radical cation (path A);formation of a dioxygen radical adduct in the presence of oxygen (path B); oxidation to acyclohexadienyl cation in the presence of quinone intermediates of phenol (path C, quinone =OX); or-when paths B and C are unavailable - coupling and disproportionation (path D) [139,140]. The production of hydroquinone and catechol is consistent with the -OH group on the

phenyl ring acting as an ortho/para director for the electrophilic addition of hydroxyl radicalson the aromatic ring. The experimental hydroquinone-catechol ratio in the products was 2:3.


72/131


OHOH

OH

OH

OH OH

OH

OH

OH

+ +

Coupling products

(A)

+

OH OH OH

OH

+

+ +

- H +

(- H 2O)- H +

Fe 2+

Coupling ordisproportionation

+ O 2

+ OX

- H +

- HO 2

Ring-openedproducts

(B)

(C)

+

OH

OH

O 2

OH

DCHD

OH

OH

OH

OH

OH OH

OH

(D)

No O 2

No OX

- H +

OH

OH

Figure 4.18 Scheme of OH radical attack by addition on aromatic ring of phenol (DCHD=dihydroxycyclohexadienyl radical) [139].

On the other hand creation of p-benzoquinone in place of hydroquinone in the first

part of experiment indicates possible OH attack of phenol by abstraction of hydrogen atomfrom the O-H bond in the phenol giving a phenoxy radical (Figure 4.19). Phenoxy radical,


73/131


which is resonance-stabilized by delocalization of the unpaired electron over the aromaticring, as shown by structures a-c in Figure 4.19, may undergo further reaction with hydroxyl orhydroperoxyl radical to form dihydroxybenzenes or quinones, respectively. In addition, in the

presence of oxygen a phenoxy radical can be captured by oxygen and peroxi-, hydroperoxi-,and hydroxylated products are formed together with monomeric quinones [141].

[ ]HOO HOO HOO

OH

O

H

O

(a) (b) (c)

OH OH

HO

OO H

OO

OH

H

O

O OH

- H 2O - H 2O - H 2O

OO

O

O

OHOH OH

H

O OH

OHH OH

O

- H 2O

OH

OH

Figure 4.19 Scheme of OH radical attack by abstraction of hydrogen atom from aromatic ring of phenol [141].


74/131


Taking into account the high reactivity of the hydroxyl radicals it is extremely unlikelythat short-lived phenoxy radicals, which are present at very low steady state concentrations,will react with a second hydroxyl radical as required by the proposed mechanism [116].

However, relatively a high concentration of hydrogen peroxide (as a source of HO 2) andoxygen in the solution, both produced by the discharge, support the assumption that thequinones can be formed by the reaction of phenoxy radical either with hydroperoxyl radicalsand oxygen.

The fast loss of p-benzoquinone from the reaction mixture is possibly due to shiftingthe redox equilibrium of dihydroxybenzene/benzoquinone system during the experiment:

OH

OH

O

O

+ 2 H + + 2 e -

(4.24)

likewise because of the involvement of p-benzoquinone in the oxidation of DCHD (Figure4.18, path C). On the other hand, no evidence of o-benzoquinone besides of p-benzoquinoneis probably caused by a lower stability of ortho- form in the solution.

The trihydroxybenzenes as pyrogallol, hydroxyhydroquinone and phloroglucinol(Figure 4.20) [142], which might also be formed as aromatic byproducts of phenoldecomposition by OH radicals, were not detected in these experiments.

OH

OH

OH

OH

OH

OH

OH

OHOH

pyrogallol hydroxyhydroquinone phloroglucinol

Figure 4.20 Trihydroxybenzenes produced by the oxidation of phenol.

4.3.2.2 Formation of aliphatic byproducts

As can be seen from Figure 4.21 giving a comparison of HPLC chromatograms of phenol solution at various time of the treatment by the discharge, in the region of the column

hold-up retention time ( t R0) an increasing peak of products non-retained by C 18 reversecolumn exists.


75/131


0 2 4 6 8 10 12 14 16

tR [min]

R e s p o n s e

0 kJ

60 kJ

240 kJ

600 kJ

HQ

CC

PH

600 kJ

240 kJ

60 kJ

t R0

Figure 4.21 Comparison of HPLC chromatograms ( = 274 nm) obtained for 1 mmol l -1 phenol solution in the presence of 0.5 mmol l -1 FeCl 2 after 0, 60, 240 and 600 kJ of supplied energy by the pulsed corona discharge.Initial solution conductivity 10 mS m -1, power input 100 W, applied voltage 20 kV, pulse repetition rate 50 Hz.tRo - the column hold-up retention time.

0

0,2

0,4

0,6

0,8

1

0 300 600 900 1200 1500 1800 2100

Energy input [kJ]

c

[ m m o l

l - 1 ]

0

20

40

60

80

100

T O C [ % ]

TOCPH

CC

HQ

BQ

Figure 4.22 Phenol removal, formation of the aromatic intermediates and total organic carbon concentration

profile for the degradation of 1 mmol l -1 of phenol by the corona discharge in the solution of 0.5 mmol l -1 Fe(ClO 4)2. Initial solution conductivity 10 mS m

-1, power input 100 W, applied voltage 20 kV, pulse repetitionrate 50 Hz. , phenol; , catechol; , hydroquinone; , p-benzoquinone; , total organic carbon.


76/131


Figure 4.22 demonstrates the concentrations of primary aromatic byproducts and totalorganic carbon concentration profile during the degradation of 1 mmol l -1 phenol in the

presence of 0.5 mmol l -1 Fe(ClO 4)2. The decrease of TOC suggests that the decomposition

pathway of phenol results to ring opened products (35%, 50% and 75% TOC removal for 0.6,1.2 and 2.1 MJ supplied energy into solution, respectively).

Formation of unsaturated and saturated C 1-C6 hydrocarbons by the oxidation of phenolwas reported by many authors. Formic acid, oxalic acid, glyoxylic acid and glyoxal wereidentified as phenol degradation byproducts during the treatment of phenol solution by thecorona discharge in gas phase over the solution [93, 94]. Formaldehyde, acetaldehyde,glyoxal and formic acid were detected using electron beam irradiation [48]. The major

product of the ozonation of phenol was formic acid with smaller amounts of muconaldehyde,muconic acid, maleinaldehyde, glyoxylic acid, glyoxal and oxalic acid [12]. It is expected thatthe similar compounds (Figure 4.23) were formed also during the degradation of phenol bythe pulsed corona discharge in water.

COOH

COOH

COOH

CHO

CHO

COOH

CHO

COOH

CHO

CHO

muconic acid muconoaldehyde maleinaldehyde glyoxylic acid glyoxal

COOH

COOH

CH3CHO

HCOOH HCHO

oxalic acid acetaldehyde formic acid formaldehyde

Figure 4.23 Proposed ring opened products formed during the oxidation of phenol by the discharge.

4.3.2.2.1 Formation of organic acids

Organic acids formed as intermediates of phenol degradation by the corona dischargein water were not detected by used HPLC equipped with a C 18 reverse phase column.However, the proposed formation of organic acids is consistent with pH changes of thesolution during phenol degradation (Figure 4.24). The initial pH value of aqueous solution of

1 mmol l-1

phenol in the presence of 0.5 mmol l-1

Fe(ClO 4)2 was ~ 4.2 and it decreased overthe reaction period to approximately 2.8.


77/131


2,5

3

3,5

4

4,5

0 200 400 600 800 1000 1200

Energy input [kJ]

p H

Figure 4.24 Change of pH value of the solution during degradation of 1 mmol l -1 phenol by the corona dischargein the presence of 0.5 mmol l -1 Fe(ClO 4)2. Initial solution conductivity 10 mS m

-1, power input 100 W, appliedvoltage 20 kV, pulse repetition rate 50 Hz.

It should be noted, that pH value of the solution treated by the discharge decreasedeven without presence of phenol in the solution due to formation of hydrogen peroxide andhydrogen ions (see Appendix 2). However, the change of pH in that case was lower.Typically, the pH of the solution without presence of phenol and in the presence of phenoldecreased after treatment by the discharge by ~ 0.8 and ~ 1.3, respectively (see Table 4.11).

Table 4.10 Negative logarithm of the acid dissociation constants K a of selected organic acids; dibasic acids showa two-step dissociation [115].

compound p K a,I p K a,II

oxalic acid 1.23 4.19

maleic acid 1.83 6.07

glyoxylic acid 3.18 -

formic acid 3.75 -

No change of pH in the case of hydrochloric acid solution and only a little change inthe case of NaOH solution are caused by a lower acidic strength of produced hydrogen

peroxide [p K a=11.62] [115] and carboxylic acids (see Table 4.10). On the other hand a lower

change of pH in the solutions of NaH 2PO 4 and Na 2CO 3 are caused by a buffer capacity ofH2PO 4-/ HPO 4

2- system [p K a=7.21] and HCO 3-/CO 3

2- system [p K a=10.33] [115], respectively.


78/131


Table 4.11 Change of pH of the solution after treatment by the corona discharge in dependence on the solutioncomposition. Initial solution conductivity 10 mS m -1, power input 100 W, applied voltage 20 kV. pH 0 - pH of theuntreated solution, pH 300 - pH of the solution after treatment by the discharge (total supplied energy of 300 kJ).

only + 1 mmol l -1 phenol

solution c [mmol l -1] pH 0 pH 300 pH 0 pH 300

HCl 0.35 3.2 3.2 3.2 3.2

NaCl 1.0 5.6 4.8 5.5 4.2

NaClO 4 1.0 5.6 4.7 5.5 4.2

Na 2SO 4 0.5 5.5 4.6 5.5 4.2

NaH 2PO 4 1.4 5.2 4.9 5.2 4.3

NaNO 3 1.0 5.6 4.6 5.7 4.3

Na 2CO 3 0.5 9.9 9.8 9.6 9.3

NaOH 0.75 10.7 10.5 10.6 10.3

4.3.2.2.2 Formation of carbonyl compounds

The formation of carbonyl compounds during the degradation of phenol by the coronadischarge was proved by the addition of 2,4-dinitrophenylhydrazine to the solution resultingin the creation of brown-orange precipitate of carbonyl-2,4-dinitrophenylhydrazone(carbonyl-DNPH) derivates (see Section 3.3.3).

0,0

0,4

0,8

1,2

1,6

250 300 350 400 450 500 550 600

Wavelength [nm]

A b s o r b a n c e

[ a . u . ]

(a)

(c)

(b)

(e)

(d)

Figure 4.25 Absorption spectra of carbonyl-DNPH mixture in acetonitrile at (a) 150, (b) 300, (c) 600, (d) 1350and (e) 1800 kJ of supplied energy by the corona discharge into solution of 1 mmol l -1 phenol and 0.5 mmol l -1

FeCl 2. Initial solution conductivity 10 mS m-1, P =100 W, U =20 kV, f =50 Hz.


79/131


The mixture of carbonyl-DNPH compounds with absorption peak at 450 nm wasobtained after dissolving of precipitate in acetonitrile. Figure 4.25 shows the change ofabsorption spectra of the carbonyl-DNPH mixture in acetonitrile during the degradation of 1

mmol l-1

of phenol by the discharge in the presence of 0.5 mmol l-1

FeCl 2. Consequently, it isapparent from Figure 4.26 that the formation of carbonyls during phenol degradation followsthe same trend as total organic carbon profile.

0

0,4

0,8

1,2

1,6

0 300 600 900 1200 1500 1800

Energy input [kJ]

A b s o r

b a n c e (

4 5 0 n m

) [ a . u .

]

20

40

60

80

100

T O C [ % ]

TOC

Carbonyl-DNPH

Figure 4.26 Change of absorbance ( = 450 nm) of the carbonyl-DNPH mixture in acetonitrile ( ) and totalorganic carbon profile ( ) during degradation of 1 mmol l -1 phenol by the corona discharge in the solution of 0.5mmol l -1 FeCl 2. Initial solution conductivity 10 mS m

-1, power input 100 W, applied voltage 20 kV.

0 10 20 30 40 50

tR [min]

R e s p o n s e

1

2 3

4

Figure 4.27 HPLC separation of carbonyl-DNPH mixture from phenol solution by the pulsed corona dischargein the presence of 0.5 mmol l -1 FeCl 2. Initial solution conductivity 10 mS m

-1, supplied energy 600 kJ.


80/131


The HPLC separation of carbonyl-DNPH from phenol reaction mixture was achievedand several significant peaks (1 - 4) were detected (Figure 4.27). However, their identificationwas not successful. Comparison of UV spectra of standard formaldehyde-DNPH (Figure

4.28a) and acetaldehyde-DNPH (Figure 4.28b) with formed hydrazones eluting in the similarretention times as standards (peak 1 and 2 in Figure 4.27, respectively) indicates a formationof other products. The UV spectra and retention times of peaks 3 (Figure 4.28c) and 4 (Figure4.28d) were totally different from that obtained for carbonyl-DNPH derivates contained in thestandard carbonyl-DNPH mixture (see Table 3.4).

200 300 400 500 600 Wavelength [nm]

(a) 1

200 300 400 500 600 Wavelength [nm]

(b) 2

200 300 400 500 600 Wavelength [nm]

3(c)

200 300 400 500 600 Wavelength [nm]

4(d)

Figure 4.28 UV spectra of some carbonyl-DNPH derivates (peaks 1-4) separated from phenol solution by HPLC(Figure 4.27). The UV spectra of standard formaldehyde-DNPH (a) and acetaldehyde-DNPH (b) are representedfor comparison as dashed lines.

Attempts have been made to identify the formed carbonyl-DNPH compounds by theapplication of GC/MS. However, no response of these compounds was detected. Thissuggests that higher molecular weight compounds (m/z > 350) containing at least onecarbonyl group with a high boiling point are formed (probably through the condensation offormed byproducts). Their elution (peaks 1 and 2 in Figure 4.27) in the similar retention timesas simple saturated aliphatic aldehydes (formaldehyde and acetaldehyde) by using HPLC

could be consistent with the polarity considerations and the substituent effects.


81/131


4.3.3 Effect of pH on degradation of phenol

Figure 4.29 shows a significant effect of pH of the solution on the phenol removal. Inalkaline solution (pH=10.6) the efficiency of phenol removal ( G37% = 7.9 10

-10 mol J -1) wastwice higher than in acidic conditions ( G37% = 4.0 10 -10 mol J -1) by use of hydrochloric acid(pH=3.2).

0,80

0,85

0,90

0,95

1,00

1,05

0 50 100 150 200 250 300

Energy input [kJ]

c ( p h e n o l

) [ m m o l

l - 1 ]

Figure 4.29 Effect of pH of the solution on phenol removal by the pulsed corona discharge. Initial solutionconductivity 10 mS m -1, power input 100 W, applied voltage 20 kV, pulse repetition rate 50 Hz. , pH = 3.2(0.35 mmol l -1 HCl); , pH = 10.6 (0.75 mmol l -1 NaOH).

According to acid-base equilibrium between the molecular and the anionic form of phenol [p K a = 9.89][115], the phenoxide ion (C 6H5O-) is predominant form in alkalinesolution (83% in the solution of pH =10.6). Taking into account the reactivity of phenol and

phenoxide ion towards OH radicals, the phenoxide ion is 1.5 times more reactive (9.6 109 l

mol -1 s-1) than phenol (6.9 10 9 l mol -1 s-1) [119].

O

OH -

H+

OH .

.+ (O 2 ) -

O

O2

(4.25)

Consequently, especially in alkaline media autooxidation processes of phenol should be considered [141]. In the presence of oxygen the phenoxide ion is oxidized to a phenoxyradical (Equation 4.25), which may undergo a variety of reactions to form oxidized products(see Section 4.3.1).


82/131


Table 4.12 shows concentrations of dissolved oxygen measured in the solutions ofHCl and NaOH after the treatment by the discharge in dependence on the presence of phenolin the solution. The increase of the concentration of dissolved oxygen in the solution treated

by the discharge is consistent with the formation of oxygen by the discharge (see Appendix2). However, in the presence of phenol a lower increase in the concentration of dissolvedoxygen in the solution was found than in the solution without presence of phenol. This is inagreement with the assumption that oxygen is involved in the oxidation pathway of phenol.Consequently, a lower increase of dissolved oxygen in NaOH solution than in HCl solution inthe presence of phenol indicates a higher consumption of oxygen in alkaline solution thanunder acidic conditions.

Table 4.12 Measurements of the concentration of dissolved oxygen [mg l -1] in the solutions of HCl and NaOH independence on the presence of phenol in the solution; cO2,0 initial concentration of O 2 in the solution, cO2,300 concentration of O 2 in the solution after treatment by the discharge (applied energy of 300 kJ). Initial solutionconductivity 10 mS m -1, power input 100 W, applied voltage 20 kV, pulse repetition rate 50 Hz.

solution pH cO2,0 cO2,300

HCl 3.2 9.5 19.1

HCl + PH 3.2 9.6 16.5

NaOH 10.6 9.8 19.6

NaOH + PH 10.6 9.6 14.5

Phenol removal efficiencies in the solutions of various salts of the same initial solutionconductivity of 10 mS m -1 are given in Table 4.13. It is apparent, that the phenol removal wasalmost independent of the type of used salt in the range of pH from 3.2 to 5.7. A higher

phenol removal was observed in carbonate solution due to alkaline properties of carbonateions that caused an increase of pH of the solution to 9.6. Thus in the case of CO 32- asignificant effect of pH of the solution on the phenol removal is evident. The scavenging ofOH radicals by CO 32- was negligible for used concentration of carbonates (see Appendix 3).

Table 4.13 Dependence of phenol removal efficiency on the solution composition. Initial solution conductivity10 mS m -1, power input 100 W, applied voltage 20 kV, pulse repetition rate 50 Hz. pH 0 - initial pH value of thesolution.

solution c [mmol l -1] pH 0 G37% [10-10 mol J -1] G37% [g (kWh)

-1]

HCl 0.35 3.2 4.03 0.136

NaCl 1.0 5.5 4.07 0.138

NaClO 4 1.0 5.5 3.98 0.135

Na 2SO 4 0.5 5.5 3.93 0.133

NaH 2PO 4 1.4 5.2 4.10 0.139

NaNO 3 1.0 5.7 4.00 0.136

Na 2CO 3 0.5 9.6 5.84 0.198

NaOH 0.75 10.6 7.89 0.267


83/131


4.3.4 Effect of iron on degradation of phenol

It was shown in Figure 4.15 that iron has a significant effect on the phenol removal bythe corona discharge. As hydrogen peroxide is produced by the corona discharge, the effect ofiron addition is assumed to be mainly in an increase of a yield of OH radicals by thedecomposing of H 2O2 in Fenton-type reactions by ferrous and ferric ion, respectively(Equations 2.12-2.16) [21, 22].

Nevertheless, the possible reactions of iron with phenol and its degradation productssuch as (i) formation of phenoxy radical by Fe(III) from phenol (Equation 4.26) [141]; (ii)reduction of Fe(III) by dihydroxybenzenes and quinone-intermediates (Equations 4.27-28)[139]; (iii) photolysis of Fe(III)-organic ligands complexes, e.g. organic acids (Equation 4.29)[143], has to be taken into account as well.

.OOH

Fe2+ + + H +Fe3+ +

(4.26)

.O

OH

OH

OH

+ H +Fe2+ +Fe3+ +

(4.27)

.O

OH

O

O

Fe2+ + + H +Fe3+ +

(4.28)

[Fe 3+(RCOO)] 2+ + h Fe 2+ + CO 2 + R (4.29)

This suggests that there is a large set of possible reaction mechanisms that can beinvolved in the removal of phenol by the corona discharge in the presence of iron. Therefore,in order to discuss the role of iron in the phenol degradation it seems better to clarify theeffects of added iron not only on the removal of phenol in itself but as an effect of added ironon the removal of phenol and its degradation byproducts, i.e. all organics presented in thesolution. The calculation of total amount of organic compounds presented in the solution wassimplified by considering only phenol and its primary aromatic byproducts, i.e. p-

benzoquinone, hydroquinone, catechol and resorcinol. Hence, the degree of removal of phenoland its aromatic byproducts from the solution at each moment (%Ar) i can be then defined as

(%Ar) i = 100 ( c PH ,i + cCC,i + c HQ,i + c BQ,i + c RS,i) / c PH,0 (4.30)

where c PH ,i, cCC,i , c HQ,i , c BQ,i , and c RS,i are concentrations of phenol and its byproducts at ithenergy input ( i = 1, 2, 3, , E tot ) and c PH,0 is an initial concentration of phenol.


84/131


4.3.4.1 Effect of iron state

Figure 4.30 shows a comparison of the decrease in (%Ar) i in dependence on the ironstate (ferrous or ferric). In the same initial concentrations of iron (0.125 or 0.25 mmol l -1) inferric or ferrous form in the solution of the same conductivity (10 or 20 mS m -1, respectively),the higher removal of phenol and its byproducts was observed with the ferrous form.

1

10

100

0 120 240 360 480 600

Energy input [kJ]

( % A r )

i

Figure 4.30 Effect of the iron state on degradation of phenol and its aromatic by-products by the coronadischarge in water. Initial phenol concentration 1 mmol l -1; initial solution conductivity of 10 mS m -1 and 20 mSm-1 for iron concentration of 0.125 mmol l -1 and 0.25 mmol l -1, respectively, pH = 3.4. , Fe 3+ 0.125 mmol l -1;

, Fe 3+ 0.25 mmol l -1; , Fe 2+ 0.125 mmol l -1; , Fe 2+ 0.25 mmol l -1.

The calculated phenol removal efficiencies in dependence on the iron state for twodifferent concentrations of ferrous and ferric form are given in Table 4.14. It seems, that theformation of phenoxy radical through the reaction of phenol with the ferric ions (Equation4.26) has for used ferric ions concentrations a negligible effect on the phenol removal.

Table 4.14 Dependence of phenol removal efficiency on the iron state. Power input 100 W, pulse repetition rate50 Hz. is initial solution conductivity and U is applied voltage of positive polarity.

c Fe [mmol l-1] [mS m -1] U [kV] G37% [10

-9 mol J -1] G37% [g (kWh)-1]

0.125 Fe 2+ 10 20 6.3 2.1

0.125 Fe 3+ 10 20 4.4 1.5

0.25 Fe 2+ 20 21 5.7 1.9

0.25 Fe 3+ 20 21 5.4 1.8


85/131


4.3.4.2 Effect of iron concentration

In Figure 4.30 it was shown an increasing effect of iron either in ferrous or ferric formon the phenol removal. Figure 4.31 shows the dependence of degradation of phenol and itsaromatic byproducts on ferrous ions concentration. The solution conductivity of 10 mS m -1 and power input of 100 W were the same in all experiments. The highest rate for thedegradation of 1 mmol l -1 phenol was found for the ferrous ions concentration of 0.5 mmol l -1.

1

10

100

0 120 240 360 480 600

Energy input [kJ]

( % A r ) i

Figure 4.31 Effect of the ferrous ions concentration on degradation of phenol and its aromatic byproducts by thecorona discharge in water. An initial phenol concentration 1 mmol l -1, initial solution conductivity of 10 mS m -1,

pH = 3.4, power input 100 W, applied voltage 21 kV. , Fe 2+ 0.05 mmol l -1; , Fe 2+ 0.125 mmol l -1; , Fe 2+ 0.25 mmol l -1; , Fe 2+ 0.5 mmol l -1; , Fe 2+ 0.75 mmol l -1.

The calculated phenol removal efficiencies in dependence on the ferrous ionsconcentration in the solution are given in Table 4.15.

Table 4.15 Phenol removal efficiency by the discharge in the solutions with different ferrous ions concentration.Initial solution conductivity 10 mS m -1, power input 100 W, applied voltage 21 kV, pulse repetition rate 50 Hz.

c Fe2+ [mmol l-1] G37% [10

-9 mol J -1] G37% [g (kWh)-1]

0.05 4.7 1.6

0.125 6.3 2.1

0.25 6.8 2.3

0.5 7.1 2.40.75 6.9 2.3


86/131


4.3.4.3 Distribution of iron ionic forms during phenol degradation

When iron in the ferrous state was in the beginning added to the solution of phenol,concentration of ferrous ions ( c Fe2+ ) at first dropped to the concentration of 80% value of theinitial ferrous concentration within one minute (supplied energy of 6 kJ) but then rapidrecovered to the same value as in the beginning and then constant level of c Fe2+ was observedduring the presence of aromatic compounds in the solution. It was in contrast with thesituation when only ferrous ions were added to the solution. In that case the ferrous ions werecompletely oxidized to the ferric form in several minutes, which corresponds to the suppliedenergy of ~ 60 kJ (Figure 4.32).

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0 100 200 300 400 500 600

Energy input [kJ]

c ( F e

2 + ) [ m m o l

l - 1 ]

Figure 4.32 Dependence of ferrous ions concentration on the presence of phenol in the solution of 0.5 mmol l -1 FeCl 2 treated by the corona discharge in water Initial phenol concentration 1 mmol l

-1, initial solutionconductivity of 10 mS m -1, pH = 3.4, power input 100 W, applied voltage 21 kV, pulse repetition rate 50 Hz. ,

phenol (initial concentration of 1 mmol l -1); , no phenol.

These results are consistent with the assumption that if phenol is not present in thesolution, the ferrous ions are oxidized by OH radicals (Equation 4.31) and by hydrogen

peroxide to the ferric form (Equation 2.12).

Fe2+ + OH Fe 3+ + OH - (4.31)

When phenol is present in the solution OH radicals preferentially react with phenolover Fe 2+. Therefore, the decline in c Fe2+ observed within first minute of the experiment in the

presence of phenol was due to the consumption of Fe 2+ in the reaction with formed hydrogen peroxide (Equation 2.12). The subsequent recovery in c Fe2+ was due to the reduction of Fe 3+ by produced reducing organic intermediates such as dihydroxybenzenes and quinones (Equations4.27-28). The slow decrease in c Fe2+ during the phenol degradation was due to decreasing levelof reducing intermediates in the solution.


87/131


When iron in the ferric state was initially added to the solution of phenol, the ferrous-ferric ion equilibrium in the concentration ratio 2:3 was established during first five minutes(supplied energy of 30 kJ). When only ferric ions were added to the solution, the ferric form

was stable (Figure 4.33).

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0 100 200 300 400 500 600

Energy input [kJ]

c ( F e

3 + ) [ m m o l

l - 1 ]

Figure 4.33 Dependence of ferric ions concentration on the presence of phenol in the solution of 0.5 mmol l -1 FeCl 3 treated by the corona discharge in water. An initial phenol concentration 1 mmol l

-1, initial solutionconductivity of 10 mS m -1, pH = 3.4, power input 100 W, applied voltage 21 kV, pulse repetition rate 50 Hz. ,

phenol (initial concentration of 1 mmol l -1); , no phenol.

Taking into account the rate constants k (Fe 3+ + H 2O2) 10 -2, k (Fe 2++H 2O2) 76 andk (Fe 2+ + OH) 3 10 8 l mol -1 s -1 [119, 139], it can be inferred that the reduction of Fe 3+ byhydrogen peroxide formed by the discharge is less probably to occur then the oxidation ofFe2+ by OH radicals and H 2O2. This results in the stable ferric form of iron, which was

observed in the solution of FeCl 3 without presence of phenol.

On the other hand, in the presence of phenol the ferric ions are reduced by aromaticoxidation byproducts of phenol to ferrous form (Equations 4.27-28), likewise photolysis ofFe(III) complexes of organic acids (Equation 4.29) by UV light produced by the coronadischarge can occur. Reduction of Fe 3+ by this route is much faster than reduction of Fe 3+ byH2O2. Therefore, ferrous-ferric ion equilibrium was fixed in the concentration ratio of 2:3 as

phenol entered its reaction phase. The same ratio was reported also in Fentons oxidation of phenol by use of ferric ions as an initial iron form [139]. The subsequent slow recovery inc Fe3+ was due to decreasing level of phenol reducing intermediates in the solution.


88/131


4.3.5 Effect of solution conductivity on degradation of phenol

Table 4.16 summarizes the phenol removal efficiencies determined in the solutions ofthe conductivity in the range of 10 - 50 mS m -1. The same power input (92 W) was used ineach case and the applied voltage was set close to the sparkover. It is apparent that the phenolremoval efficiency value remained almost the same for all used solution conductivities.

Table 4.16 Effect of the solution conductivity on the phenol removal efficiency. Power input 92 W, pH ~ 3.5-2.9, solution conductivity adjusted by H 2SO 4 (1:10).

[mS m -1] U [kV] f [Hz] G37% [10 -10 mol J -1] G37% [g (kWh) -1]

10 19.0 50 4.10 0.13920 20.5 43 3.91 0.132

30 21.5 39 4.23 0.143

40 23.0 34 4.35 0.147

50 24.5 30 4.18 0.142

It was shown in Section 4.3.1, that the phenol oxidation by the corona discharge iscaused by OH radical attack on the aromatic ring of phenol. Thus, the phenol removalindependence of the solution conductivity is in contrast to the results reported for the

production of OH radicals that decreased with the increasing solution conductivity [55]. Itwas shown in Sections 4.1.2 and 4.2.6 that UV radiation emitted by the discharge stronglyincreases with the increasing solution conductivity and photolysis of H 2O2 is a significant

process in higher conductivities of the solution. Therefore, similar phenol removal obtained inall solutions of different conductivity suggests that in higher conductivities of the solution

phenol is oxidized by OH radicals that are formed both directly by the discharge and by the photolysis of H 2O2.

On the contrary, Figure 4.34 demonstrates the degradation of 1 mmol l -1 phenol by thecorona discharge in the solutions of the conductivity of 10 and 50 mS m -1 in the presence of0.5 mmol l -1 FeCl 2. A slower rate of phenol removal in the solution of the conductivity of 50mS m -1 is evident. More than 2.5 times higher phenol removal efficiency was determined inthe solution of the conductivity of 10 mS m -1 than under conductivity of 50 mS m -1 (Table4.17).

Table 4.17 Effect of the solution conductivity on the phenol removal efficiency in the presence of 0.5 mmol l -1 FeCl 2. 0 - initial solution conductivity, 277 - solution conductivity after treatment by the discharge (totalsupplied energy of 277 kJ).

0 [mS m-1] 277 [mS m

-1] U [kV] f [Hz] G37% [10-9 mol J -1] G37% [g (kWh)

-1]

10 24.5 19 50 7.1 2.4

50 60 24 35 2.6 0.9


89/131


0

0,2

0,4

0,6

0,8

1

1,2

0 50 100 150 200 250 300

Energy input [kJ]

c ( p h e n o l

) [ m m o l

l - 1 ]

Figure 4.34 Effect of the solution conductivity on the degradation of 1 mmol l -1 phenol by the corona dischargein the presence of 0.5 mmol l -1 FeCl 2. , 10 mS m

-1; , 50 mS m -1.

It is obvious that the effect of the solution conductivity on the oxidation efficiency ofthe corona discharge is more reflected in the presence of iron in the solution. In that case both

OH radicals and hydrogen peroxide (Equation 2.12) are utilized in the degradation of phenol.Thus, the decreasing phenol removal efficiency with the increasing solution conductivity is inagreement with the lower production of OH radicals and H 2O2 by the discharge in highersolution conductivities. The effect of UV radiation emitted by the discharge on the phenoldegradation is for used concentration of phenol negligible (see Section 4.2.6).

Table 4.18 Effect of solution conductivity on phenol removal efficiency ( G PH ) in the presence of 0.5 mmol l-1

FeCl 2 and on yield of hydrogen peroxide ( G H2O2 ). 0 - initial solution conductivity.

0 [mS m-1] G37%, PH [10 -9 mol J -1] G H2O2 [10 -9 mol J -1]

10 7.1 12.2

50 2.6 7.6

Table 4.18 gives the degradation yields of 1 mmol l -1 phenol in the presence of 0.5mmol l -1 FeCl 2 and the yields of hydrogen peroxide production by the discharge in thesolutions of the conductivity of 10 and 50 mS m -1. It is apparent that the degradation yield of

phenol decreased more rapidly with the increasing solution conductivity than in the case ofhydrogen peroxide. Such discrepancy can be caused partly by the consumption of producedOH radicals by phenol byproducts besides of phenol, but the significant change of thesolution conductivity during the degradation of phenol has to be taken into account as well(see Table 4.17).


90/131


4.3.6 Effect of voltage polarity on degradation of phenol

Table 4.19 shows the phenol removal efficiencies determined in the solutions of theconductivity in the range of 10 - 50 mS m -1 under negative polarity of the applied voltage.Comparison of the degradation yields of 1 mmol l -1 phenol vs conductivity of the solution forthe positive and negative polarity of the applied voltage is given in Figure 4.35. It is obviousthat the phenol removal efficiency in the negative polarity was lower. This results from thedifferent discharge development processes under the positive and negative polarity leading tothe generation of the discharges of the different nature. However, the difference between the

phenol degradation yields for the negative and positive polarity was not so great as in the caseof the production of hydrogen peroxide (see Figures 4.9 and 4.35).

Table 4.19 Effect of the solution conductivity on the phenol removal efficiency for applied voltage of thenegative polarity. Power input 92 W, pH ~ 3.5-2.9, solution conductivity adjusted by H 2SO 4 (1:10).

[mS m -1] U [kV] f [Hz] G37% [10-10 mol J -1] G37% [g (kWh)

-1]

10 19.0 50 3.18 0.108

20 20.5 43 3.11 0.105

30 21.5 39 3.23 0.109

40 23.0 34 3.15 0.107

50 24.5 30 3.11 0.105

0

0,04

0,08

0,12

0,16

0 10 20 30 40 50 60

Conductivity [mS m -1]

G P H [ g

( k W h ) - 1 ]

Figure 4.35 Effect of the solution conductivity and polarity of the applied voltage on the degradation yield of 1mmol l -1 phenol. , positive polarity; , negative polarity.


91/131



Conversion of phenol and formation of oxidized reaction byproducts by the pulsedcorona discharge have been determined. Hydroquinone, 1,4-benzoquinone, catechol and traceamounts of resorcinol were detected as primary phenol decomposition products. Theformation of dihydroxybenzenes corresponds to the oxidation of phenol by hydroxyl radicals.The production of hydroquinone and catechol is consistent with the -OH group on the phenylring acting as an ortho/para director for the electrophilic addition of hydroxyl radicals on thearomatic ring. Creation of 1,4-benzoquinone in place of hydroquinone indicates possible OHattack of phenol by abstraction of hydrogen atom from the O-H bond in the phenol. Thedecrease of TOC indicates ring structure cleavage of phenol and further oxidation andmineralization. The formation of carbonyl compounds was proved by derivatization reactionwith 2,4-dinitrophenylhydrazine resulting in the creation of carbonyl-DNPH derivates

Effect of an initial pH value on phenol degradation was observed. Under alkalineconditions the rate of oxidation of phenol was almost twice that under acidic conditions as inalkaline medium phenol exists in the phenoxide form, which reacts faster with OH radicals,and it is also auto-oxidized by dissolved oxygen. The presence of iron significantly enhancedthe rate of phenol degradation. As hydrogen peroxide is produced by the corona discharge, theaddition of iron resulted in an increased concentration of OH radicals by the decomposing ofH2O2 in Fenton-type reactions by ferrous and ferric iron. However, the reactions of ferric ionwith aromatic oxidation byproducts of phenol and photolysis of Fe(III) complexes of organicacids by UV light emitted by the corona discharge take place in the degradation of phenol aswell. Iron in the ferrous state was more efficient than in the ferric state for the oxidation of

phenol. The highest rate for the degradation of 1 mmol l -1 phenol was found for the ferrousions concentration of 0.5 mmol l -1. The yield of phenol removal decreased with the increasingsolution conductivity in the presence of iron. On the other hand, a similar phenol removal wasobtained in solutions of different conductivity without addition of iron. It was inferred that the

photolysis of H 2O2 caused by ultraviolet radiation emitted from the corona dischargesignificantly contributes to the phenol degradation. Higher phenol removal efficiency wasobtained for the positive polarity of the applied voltage than for the negative one.


92/131


4.4 Degradation of monochlorophenols by the pulsed corona discharge

In this section the results of experiments on degradation of 2-, 3- and 4-chlorophenol by the pulsed corona discharge in the presence of Fe 2+ ions are presented regarding formationof aromatic byproducts, mechanism of chlorophenols degradation and effect of chloride group

position on chlorophenol removal.

4.4.1 Degradation of 2-chlorophenol

Figure 4.36 demonstrates 2-chlorophenol removal and formation of aromatic byproducts during the degradation of 500 mol l -1 2-chlorophenol by the pulsed coronadischarge in the presence of 250 mol l -1 FeSO 4.

0

100

200

300

400

500

600

0 20 40 60 80 100 120 140 160 180

Energy input [kJ]

c [ m o l

l - 1 ]

2-CP

CC

CBQ3-CCC

CHQ

Figure 4.36 2-chlorophenol removal and formation of the aromatic intermediates during the degradation of 500

mol l-1

2-chlorophenol by the pulsed corona discharge in the solution of 250 mol l-1

FeSO 4. Initial solutionconductivity 10 mS m -1, power input 54 W, applied voltage 21 kV, pulse repetition rate 35 Hz. , 2-chlorophenol; , 3-chlorocatechol; , chlorobenzoquinone; , chlorohydroquinone; , catechol.


93/131

Degradation of monochlorophenols by the pulsed corona discharge 79

1,2-dihydroxybenzene (catechol, CC), 2-chloro-1,4-dihydroxybenzene (chlorohydro-quinone, CHQ), 2-chloro-1,4-benzoquinone (chlorobenzoquinone, CBQ) and 3-chloro-1,2-dihydroxybenzene (3-chlorocatechol, 3-CCC) as primary products of 2-chlorophenol (2-CP)

degradation by the pulsed streamer corona discharge in the presence of iron ions wereidentified by HPLC analysis (Figure 4.37).

OH

Cl

OH

OH

OH

Cl

OH

Cl

O

O

OH

OH

Cl

2-chlorophenol catechol chlorohydroquinone chlorobenzoquinone 3-chlorocatechol

Figure 4.37 Identified aromatic byproducts formed by the oxidation of 2-chlorophenol by the pulsed coronadischarge in the presence of Fe 2+ ions in the solution .


2-chloro-1,4-dihydroxybenzene (chlorohydroquinone, CHQ), 2-chloro-1,4-benzo-quinone (chlorobenzoquinone, CBQ), 3-chloro-1,2-dihydroxybenzene (3-chlorocatechol, 3-CCC) and 4-chloro-1,2-dihydroxybenzene (4-chlorocatechol, 4-CCC) as primary products of3-chlorophenol (3-CP) degradation by the pulsed streamer corona discharge in the presence ofiron ions were identified by HPLC analysis (Figure 4.38).

OH

Cl

OH

Cl

OH

Cl

O

O

OH

OH

Cl

OH

OH

Cl

3-chlorophenol chlorohydroquinone chlorobenzoquinone 3-chlorocatechol 4-chlorocatechol




94/131


0

100

200

300

400

500

600

0 20 40 60 80 100 120 140 160 180

Energy input [kJ]

c [ m o l

l - 1 ]

3-CP

3-CCCCHQ

4-CCCCBQ

Figure 4.39 3-chlorophenol removal and formation of the aromatic intermediates during the degradation of 500 mol l -1 3-chlorophenol by the pulsed corona discharge in the solution of 250 mol l -1 FeSO 4. Initial solutionconductivity 10 mS m -1, power input 54 W, applied voltage 21 kV, pulse repetition rate 35 Hz. , 3-chlorophenol; , 3-chlorocatechol; , chlorohydroquinone; , 4-chlorocatechol; , chlorobenzoquinone.


1,4-dihydroxybenzene (hydroquinone, HQ), 1,4-benzoquinone ( p- benzoquinone, BQ),and 4-chloro-1,2-dihydroxybenzene (4-chlorocatechol, 4-CCC) as primary products of 4-chlorophenol (4-CP) degradation by the pulsed streamer corona discharge in the presence ofiron ions were identified by HPLC analysis (Figure 4.40).



95/131


OH

Cl

OH

OH

O

O

OH

OH

Cl

4-chlorophenol hydroquinone p- benzoquinone 4-chlorocatechol


0

100

200

300

400

500

600

0 20 40 60 80 100 120 140 160 180

Energy input [kJ]

c [ m o l

l - 1 ]

4-CP

4-CCC

HQBQ

Figure 4.41 4-chlorophenol removal and formation of the aromatic intermediates during the degradation of 500 mol l -1 4-chlorophenol by the pulsed corona discharge in the solution of 250 mol l -1 FeSO 4. Initial solutionconductivity 10 mS m -1, power input 54 W, applied voltage 21 kV, pulse repetition rate 35 Hz. , 4-chlorophenol; , 4-chlorocatechol; , hydroquinone; , p-benzoquinone.


96/131


4.4.4 Mechanism of degradation of chlorophenols by the pulsed corona discharge

The decomposition of chlorophenols by the pulsed corona discharge can be referred,as well as the degradation of phenol, essentially to the OH radical attack forming in the firststep OH-adducts (chlorodihydroxycyclohexadienyl radicals) on ortho - and para - positions tothe phenolic OH-group as the main transients (see Section 4.3.2.1). In the presence of air areversible addition of oxygen to the chlorodihydroxycyclohexadienyl radicals occurs as themajor reaction pathway forming peroxyl radicals. The formation of such intermediate adductswas observed by Getoff and Solar [144, 145] during the radiolytic degradation ofmonochlorophenols. It may be assumed that the same mechanism of oxidation ofchlorophenols by OH radical occurs in the pulsed corona discharge (Equation 4.32,demonstrated for 4-chlorophenol [44]).

OH

Cl

OH

Cl

H

OHH.

OH. O2OH

Cl

OH

.OOH

H

(4.32)

OH

Cl

OH

.OOH

H HO 2.

OH

OH

Cl

+

(4.33)

Subsequently, hydroxylated products on ortho - and para -positions to the phenolicgroup are formed by elimination of HO 2 from peroxyl radicals (Equation 4.33). In the case of2-chlorophenol these products were chlorohydroquinone, 3-chlorocatechol and catechol, from3-chlorophenol these were chlorohydroquinone, 3- and 4-chlorocatechol and from 4-chlorophenol these were 4-chlorocatechol and hydroquinone. Products having the meta OHadducts as precursors were not detected. Similar results have been obtained for the reaction ofOH with phenol where the meta hydroxylation product resorcinol was found only in traces(see Section 4.3.2.1).

OH

Cl

OH

.OOH

HO2

OH

Cl

OH

H

HOO

OOH

.OH

Cl

OH

H

HH. O

O

(4.34)


97/131


In place of reaction 4.33 organic peroxyl radicals may form reversible , '-endoperoxyalkyl radicals. These are supposed to add subsequently another oxygen moleculein an irreversible step (Equation 4.34). The decay of such transients is expected to lead to ring

fragmentation and dechlorination. The proposed formation of ring opened products is inagreement with the results obtained by HPLC analysis. A representative chromatogram isgiven for 3-chlorophenol in Figure 4.42. It is apparent that in the region of very shortretention times, before the hydroxylated products elute, a certain amount of unidentified

products exists, which could be either chlorine free or chlorinated non aromatic compounds.

0 2 4 6 8 10 12 14 16 18 20

tR [min]

R e s p o n s e

CHQ

3-CCC

4-CCC

3-CP

Figure 4.42 Chromatogram at 274 nm of 500 mol l -1 3-chlorophenol solution after treatment by the pulsedcorona discharge in the presence of 250 mol l -1 FeSO 4 at 65 kJ of supplied energy into solution. Initial solutionconductivity 10 mS m -1, power input 54 W, applied voltage 21 kV, pulse repetition rate 35 Hz.

Consequently, the concentration for chloride release given in Table 4.20 suggests thatapproximately 60% dechlorination corresponds to the complete removal of chlorophenolsubstrate from the solution.

Table 4.20 Concentrations for chloride release achieved after complete chlorophenol degradation by the pulsedcorona discharge in the presence of 250 mol l -1 FeSO 4 (total supplied energy of 162 kJ). Concentration ofchlorophenols 500 mol l -1, initial solution conductivity 10 mS m -1, power input 54 W, applied voltage 21 kV,

pulse repetition rate 35 Hz.

cCl [ mol l-1]

2-chlorophenol 293

3-chlorophenol 3104-chlorophenol 268


98/131


The initial formation of chlorobenzoquinone in place of chlorohydroquinone duringdegradation of 2- and 3-chlorophenol and benzoquinone in place of hydroquinone duringdegradation of 4-chlorophenol is derived from formation of phenoxy radicals by H

abstraction from the O-H bond of chlorophenols and subsequent reaction of phenoxy radicalwith HO 2 radicals (see Figure 4.19).

On the other hand, the small amount of identified halogen free aromatic products,catechol and hydroquinone, could arise from the phenoxy radicals resulting from OH attackon the chloro-position of 2- and 4-chlorophenol (Equations 4.35 and 4.36, demonstrated for 2-chlorophenol) [43].

OH

Cl

OH.

OH

H

OH

. Cl.

OH

O

+ HCl

(4.35)

.OH

O

OH

Cl

H.H OH

+ e.g.

OH

OH OH

Cl

OH

+

(4.36)

The fact that for 3-chlorophenol no resorcinol was detected is explainable by minorfavoured OH attack at the chloroposition meta to the phenolic OH-group.

4.4.5 Effect of chlorine atom position on degradation of monochlorophenols

In order to demonstrate the influence of chlorine atom position on the removal ofindividual monochlorophenols by the discharge the initial degradation rate was determined byfitting the experimental data of monochlorophenols degradation as a function of time to afirst-order rate model (see Section 2.3.2).

First-order plot for the degradation of 500 mol l -1 monochlorophenols by the pulsedcorona discharge in the solution of 250 mol l -1 FeSO 4 is given in Figure 4.43. It is apparentthat the initial degradation rate of 3-chlorophenol was the fastest (1.4 10 -3 s -1), followed by2-chlorophenol and 4-chlorophenol (1.1 and 1.0 10 -3 s-1, respectively).


99/131


-1,8

-1,6

-1,4

-1,2

-1

-0,8

-0,6

-0,4

-0,2

0

0 5 10 15 20 25

Time [min]

l n ( c / c

0 )

Figure 4.43. First-order plot for the degradation of 500 mol l -1 monochlorophenols by the pulsed coronadischarge in the solution of 500 mol l -1 FeSO 4. Initial solution conductivity 10 mS m

-1, power input 54 W,applied voltage 21 kV, pulse repetition rate 35 Hz. , 4-chlorophenol; , 2-chlorophenol; , 3-chlorophenol.

Consequently, Table 4.21 summarizes the yields of chlorophenol and phenol removalobtained under the same experimental conditions. The sequence in the removal efficiencyaccording to a presence and position of chlorine atom is 3-CP > 2-CP > 4-CP ~ PH.

Table 4.21 Yields of phenols removal by the pulsed corona discharge in the presence of 250 mol l -1 FeSO 4.Initial concentration of phenols 500 mol l -1, initial solution conductivity 10 mS m -1, power input 54 W, appliedvoltage 21 kV, pulse repetition rate 35 Hz.

G37% [10-9 mol J -1] G37% [g (kWh)

-1]

2-chlorophenol 7.38 3.423-chlorophenol 9.26 4.28

4-chlorophenol 6.57 3.04

phenol 6.51 2.20

A higher removal yield of chlorophenols than of phenol results from the electrondonating character of chlorine which increases the electron density on the aryl ring andrenders the compound more susceptible towards the electrophilic characteristics of oxidizing

agents (e.g. OH radicals) and correspondingly increases the rate of oxidation.


100/131


Consequently, the order 3-CP > 2-CP > 4-CP can be explained on the basis of thedecreased aromatic electron density of individual chlorophenol isomers depending on the

position of chlorine relatively to OH. As both substituents (OH-, Cl-) are ortho / para directors

of electrophilic substitution, the highest electron density on the ortho- and para- position of 3-chlorophenol is evident since those sites correspond to the same effect of both substituents.On the other hand, the contradictory effect of both substituents occurs in the case of 2-and 4-chlorophenol that caused a reduced electron density on these compounds. However, as theelectron donating capacity of hydroxyl group is higher than the chlorine, the furtherelectrophilic substitution is directed in the ortho / para positions with respect to the OH group.Hence, in the case of 4-chlorophenol, where the para position is blocked by chlorine, thelowest electron density on aryl ring from all three chlorophenol isomers should be expected.

OH

Cl

44 %

52 % 45 % 45 %OH

Cl

OH

Cl

33 %

27 % 40 %

Figure 4.44 Ratio of ortho / para isomers formed by degradation of chlorophenols by the corona discharge.

The proposed mechanism of the electrophilic substitution is supported by theexperimental results. Table 4.22 summarizes the yields of chlorophenols and formed

byproducts obtained at 65 kJ of supplied energy by the discharge corresponding to themaximum yield of formed aromatic byproducts. Consequently, the ratio of formed ortho / para isomers is given in Figure 4.44. It is obvious that higher amount of products corresponds toortho substitution of OH radical. For 2- and 3-chlorophenol the ratio of ortho / para productsis 1.2:1 and 2:1, respectively. In the case of 4-chlorophenol this ratio increases to 9:1.

Table 4.22 Yields obtained for degradation of 500 mol l -1 2-, 3-, and 4-chlorophenol solutions by the pulsedcorona discharge in the presence of 250 mol l -1 FeSO 4 at 65 kJ of supplied energy into solution. Initial solutionconductivity 10 mS m -1, power input 54 W, applied voltage 21 kV, pulse repetition rate 35 Hz.

Yields for degradation of phenols and formed products [10 -9 mol J -1]

products 2-chlorophenol 3-chlorophenol 4-chlorophenol

Substrate degradation 6.44 7.11 6.02

Chlorinated products:

3-chlorocatechol 1.77 1.47 -

4-chlorocatechol - 1.00 2.44

chlorohydroquinone 1.50 1.24 -

Chlorine free products:

catechol 0.16 - -hydroquinone - - 0.28


101/131



All three isomeric monochlorophenols were effectively degraded by the pulse coronadischarge in the presence of ferrous ions in the solution. Primary decomposition byproductshave been determined by using HPLC. Main products were the chlorinateddihydroxybenzenes originating from the electrophilic attack of hydroxyl radicals on strictlyortho - and para - positions to the phenolic OH-group when attack on ortho position occurredmost favorably. For 2-chlorophenol these were chlorohydroquinone, chlorobenzoquinone and3-chlorocatechol, for 3-chlorophenol these were chlorohydroquinone, chlorobenzoquinone, 3-and 4-chlorocatechol and for 4-chlorophenol it was 4-chlorocatechol. In addition, chlorinefree aromatic byproducts, catechol for 2-chlorphenol and hydroquinone and 1,4-

benzoquinone for 4-chlorophenol, have been identified in minor amounts. The position ofchlorine relatively to OH strongly influenced the decomposition process. The initialdegradation rate of 3-chlorophenol was the fastest, followed by 2-chlorophenol and 4-chlorophenol. The dechlorination process was less effective. Approximately 60%dechlorination was achieved after complete removal of chlorophenol substrate from thesolution.


102/131


4.5 Comparison between pulsed corona discharge and Fentons process

In the Section 4.3.1 the important role of the Fentons process on the efficiency of phenol removal by the pulsed corona discharge in water has been demonstrated. In order todescribe the magnitude of the contribution of Fentons reaction to the degradation of phenoland monochlorophenols by the pulsed corona discharge in the presence of iron, a comparison

between the pulsed corona discharge and Fentons process is performed. The results ofdegradation of phenol, 2-, 3- and 4-chlorophenol and formation of their aromatic byproducts

by using the Fentons process as a function of the initial H 2O2 concentration are presented.

4.5.1 Degradation of phenols by Fentons system

Figures 4.45-48 show the degradation of phenol, 2-, 3- and 4-chlorophenol andformation of their aromatic intermediates by using the Fentons process as a function of theinitial hydrogen peroxide concentration in the presence of 250 mol l -1 FeSO 4. As expected,the degradation of phenols increased with the increasing concentration of H 2O2. The formedaromatic byproducts were the same as by using the pulsed corona discharge in the presence ofFeSO 4 (see Figures 4.16-17 and 4.36-41). In the case of phenol these products were catechol,hydroquinone and p-benzoquinone, from 2-chlorophenol these were chlorohydroquinone,chlorobenzoquinone, 3-chlorocatechol and catechol, from 3-chlorophenol these werechlorohydroquinone, chlorobenzoquinone, 3- and 4-chlorocatechol and from 4-chlorophenolthese were 4-chlorocatechol, hydroquinone and p-benzoquinone.

0

100

200

300

400

500

600

0 0.5 1 1.5 2

c(H 2O 2) [mmol l-1]

c [ m o l

l - 1 ]

PH

CC HQBQ

Figure 4.45 Phenol removal and formation of the aromatic intermediates during the degradation of 500 mol l -1 phenol with the Fentons process as a function of the initial H 2O2 concentration ( c Fe2+ = 250 mol l

-1). , phenol; , catechol; , hydroquinone; , p-benzoquinone.


103/131

Comparison between pulsed corona discharge and Fentons process 89

0

100

200

300

400

500

600

0 0.5 1 1.5 2


c [ m o l

l - 1 ]

2-CP

CBQ 3-CCCCHQ

CC

Figure 4.46 2-chlorophenol removal and formation of the aromatic intermediates during the degradation of 500 mol l -1 2-chlorophenol with the Fentons process as a function of the initial H 2O2 concentration ( c Fe2+ = 250 mol l -1). , 2-chlorophenol; , 3-chlorocatechol; , chlorobenzoquinone; , chlorohydroquinone; ,catechol.

0

100

200

300

400

500

600

0 0.5 1 1.5 2


c [ m o l

l - 1 ]

3-CP

CHQ 3-CCC4-CCC

CB Q

Figure 4.47 3-chlorophenol removal and formation of the aromatic intermediates during the degradation of 500 mol l -1 3-chlorophenol with the Fentons process as a function of the initial H 2O2 concentration ( c Fe2+ = 250 mol l -1). , 3-chlorophenol; , 3-chlorocatechol; , chlorohydroquinone; , 4-chlorocatechol; ,chlorobenzoquinone.


104/131


0

100

200

300

400

500

600

0 0.5 1 1.5 2


c [ m o l

l - 1 ]

4-CP

4-CCC

HQBQ

Figure 4.48 4-chlorophenol removal and formation of the aromatic intermediates during the degradation of 500 mol l -1 4-chlorophenol with the Fentons process as a function of the initial H 2O2 concentration ( c Fe2+ = 250 mol l -1). , 4-chlorophenol; , 4-chlorocatechol; , hydroquinone; , p-benzoquinone.

4.5.2 Comparison of the specific dose of H 2O 2

In order to describe the magnitude of the contribution of Fentons reaction to thedegradation of phenol and monochlorophenols by the pulsed corona discharge in the presenceof iron, a comparison of the specific dose of hydrogen peroxide ( D H2O2 )37% defined as theamount of H 2O2 required to decrease the concentration of individual phenols to 37% of theinitial concentration was performed.

In the case of Fenton process the ( D H2O2 )37% value for individual phenols was

calculated by using an assumption of pseudo-first-order kinetics of phenols removal by theFentons process (see Sections 2.3.2 and 3.5.2) from the slope of the plot of natural logarithmof individual phenols concentration at current concentration of H 2O2 vs H 2O2 concentration.

In the case of the pulsed corona discharge the specific dose of H 2O2 was estimatedusing Equation (4.37) as

(D H2O2 )37% = k H2O2 t 37% (4.37)

where k H2O2 is initial rate of formation of H 2O2 by the pulsed corona discharge and t 37% is timerequired to decrease the concentration of individual phenols to 37% of the initial

concentration by the discharge.


105/131

Comparison between pulsed corona discharge and Fentons process 91

Combining Equations 3.15, 4.16 and 4.37, we get:

0 H2O2 k

P D

)0116.0exp(1036.1)(

8

%37

=

(4.38)

where P is the applied power input, is average value of the solution conductivity duringthe degradation of individual phenols by the discharge and k 0 is pseudo-first-order rateconstant of the removal by the discharge for individual phenols (see Sections 3.5.2 and 4.4.5).The values of were estimated from the initial solution conductivity and the solutionconductivity achieved after complete removal of individual phenols by the pulsed coronadischarge in the presence of 500 mol l -1 FeSO 4 (total supplied energy of 162 kJ). The initialsolution conductivity was 10 mS m -1 in each case; the final solution conductivity was 22 and34 mS m -1 in the case of phenol and monochlorophenols, respectively.

Table 4.23 summarizes the values of the specific dose of hydrogen peroxide( D H2O2 )37% required for the degradation of phenol, 2-, 3- and 4-chlorophenol to 37% of theinitial concentration by the pulsed corona discharge in the presence of 250 mol l -1 Fe 2+ andFentons processes with the same amount of ferrous ions. The initial concentration ofindividual phenols was 500 mol l -1 in each case.

Table 4.23 Comparison of the specific dose of H 2O2 ( D H2O2 )37% for phenol, 2-, 3- and 4-chlorophenol. Initialconcentration of individual phenols was 500 mol l -1, is average value of the solution conductivity during thedegradation of individual phenols by the discharge, k 0 is pseudo-first-rate constant of removal by the discharge

for individual phenols in the presence of 250 mol l -1 Fe 2+ ( P =54 W, U =21 kV, f =35 Hz). Disch./Fen. is the ratioof ( D H2O2 )37% between the discharge and Fentons process.

(D H2O2 )37% [ mol]

[mS m -1] k 0 [ 10 -3 s-1] Discharge Fenton Disch./Fen. [-]

phenol 16 0.97 546 640 0.85

2-chlorophenol 22 1.10 450 536 0.84

3-chlorophenol 22 1.38 358 494 0.72

4-chlorophenol 22 0.98 505 606 0.83

A comparison of both processes showed a lower consumption of hydrogen peroxide by the pulsed corona discharge than by Fentons process. This is in agreement with theassumption that there are more effects occurring in the pulsed corona discharge, which canenhance the removal efficiency of phenols than in the case of Fentons system. Besides ofdirect production of radicals (mainly OH) and through the decomposition of H 2O2, theimportant role can play also UV radiation of the discharge due to photo-Fenton process (seeEquations 2.12, 2.15 and 2.16) and/or the direct photolysis of organics (see Section 2.1.1).This is evident especially in the case of 3-chlorophenol where the ratio of ( D H2O2 )37% betweenthe discharge and Fenton' process was the lowest from all four phenols.


106/131


On the other hand, taking into account the decreasing production of H 2O2 by thedischarge with the increasing solution conductivity (see Section 4.2.4), a higher increase ofthe solution conductivity during the degradation of chlorophenols compared to phenol

indicates, that the removal efficiency of individual phenols by the discharge is stronglyaffected by the solution conductivity (see Table 4.23). Hence, besides of the initial solutionconductivity the current value of the solution conductivity should be considered incalculations of the removal efficiency of the discharge.

4.5.3 Comparison of estimated operating costs

Table 4.24 summarizes estimated operating costs for the pulsed corona discharge andFentons process. Determination of operating costs for both processes was based on the

calculation of the electrical cost and/or the chemical costs (H 2O2 cost, FeSO 4 7H 2O cost)required for the decrease of the initial concentration of phenol, 2-, 3-, and 4-chlorophenol byfactor e.

Table 4.24 Estimated operating costs of the pulsed corona discharge and Fentons process required for thedecrease of the initial concentration of phenol, 2-, 3- and 4-chlorophenol by factor e ( c0 = 500 mol l -1).Parameters of applied power by the discharge in the presence of 250 mol l -1 Fe2+: P =54 W, U =21 kV, f =35 Hz.

Estimated operating costs

Discharge Fenton

[US$ m -3] [CZK m -3] [US$ m -3] [CZK m -3]

phenol 0.94 14.4 0.037 0.423

2-chlorophenol 0.83 12.7 0.033 0.363

3-chlorophenol 0.66 10.2 0.031 0.338

4-chlorophenol 0.93 14.3 0.036 0.403

The electrical cost associated with the corona discharge process was calculated usingthe energy input E 37% as

V

E 37%6106.3

costPower 1000costElectrical

= (4.39)

where E 37% has been determined from pseudo-first-order rate constant k 0 and power input P (see Table 4.23 and Equations 3.9, 3.10 and 3.15

Documents

Water Treatment by Pulsed Streamer Corona Discharge - LUKEŠ