Textile Waste water treatment – dye removalby Electro Fenton Method & Photo Electro Fenton

A Thesis Submitted in partial fulfilment of the

requirements for the award of the Degree of

BACHELOR OF ENGINEERING BY

ABHIMANYU KUMAR(BE/10395/2012)

&RAKESH RANJAN(BE/10359/2012)

CHEMICAL & POLYMER ENGINEERING

UNDER THE GUIDANCE OF

Dr. BIDHAN CHANDRA RUIDAS

DEPARTMENT OF CHEMICAL ENGINEERING &

TECHNOLOGY

BIRLA INSTITUTE OF TECHNOLOGY

MESRA-835215, RANCHI

2016

1

DECLARATION CERTIFICATE

This is to certify that the work presented in the thesis entitled “Textile Waste water treatment – dye removalby Electro Fenton Method & Photo Electro Fenton” in partial fulfilment of the requirement for the award of Degree of Bachelor of Engineering in Chemical Engineering of Birla Institute of Technology Mesra, Ranchi is an authentic work carried out under my supervision and guidance. To the best of my knowledge, the content of this

thesis does not form a basis for the award of any previous

Degree to anyone else.

Date: Dr B.C. Ruidas & Dr. M.Mukherjee

Dept. of Chemical Engineering &

Technology

Birla Institute of Technology

Mesra, Ranchi

Head

Dept. of Chemical Engineering &

Technology

Birla Institute of Technology Mesra, Ranchi

2

CERTIFICATE OF APPROVAL

The foregoing thesis entitled “Studies on Nano Heat Transfer

Fluids”, is hereby approved as a creditable study of research topic

and has been presented in satisfactory manner to warrant

its acceptance as prerequisite to the degree for which it has been

submitted. It is understood that by this approval, the undersigned

do not necessarily Endorse any conclusion drawn or opinion

expressed therein, but approve the thesis for the purpose for which

it is submitted.

(Internal Examiner) (External Examiner)

(Chairman)

Head of the Department

3

Acknowledgement

Our endeavour stands incomplete without dedicating our gratitude to a few

people who have contributed a lot towards the successful completion of our

project work. First of all, we are thankful to god almighty for his guidance and

protection throughout the course of our project work

We express our respect and gratitude to our guide, Dr.(Mrs) M. Mukherjee

and Dr. B.C. Ruidas, Professors, Department of Chemical Engineering

and Technology for their constant and valuable guidance. We must mention

the notable contribution Laboratory Technician and other laboratory assistants

from the Department of Chemical Engineering and Technology, BIT Mesra

who helped us learn and use the laboratory instruments as well as for constant

support during the course of my project development.

Our sincere thanks to Dr. (Mr.) Guatam Sarkhel, Head, Department of

Chemical Engineering and Technology for all the opportunity he provided

us to carry out the project successfully. We thank him for extreme

encouragement and kindness.

Finally, we would like to acknowledge our parents, friends and well-wishers

for their love and faith in us.

Abhimanyu Kumar Rakesh Ranjan

BE/10395/2012 BE/10359/2012

Chemical Engineering Chemical Engineering

4

Abstract

The aim of this project was to determine the extent of dye decomposition in

the textile wastewater in Electro Fenton & Photo electro Fenton process using

a lead dioxide anode and graphite cathode. The main processing parameters

were pH of the solution, dye concentration and current applied. It was

confirmed the existence of synergic effects between UVA light photo-

oxidation and/or hydroxyl radicals (⁰OH) formed from water oxidation at the

lead dioxide anode and the Fenton reaction between added Fe2+ and H2O2

produced at the graphite cathode. It was found that the use of EF & PEF in the

technology of textile wastewater treatment was an efficient method for the

decomposition of dye and could be successfully applied as a preliminary stage

prior to further biological treatment. In this thesis detailed coverage of cod

removal has been studied.

Key words: electro-Fenton, anode, cathode, oxidation, hydroxyl radicals,

degradation, dye removal.

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CONTENTS

Sl.No.

Topic Page No.

1 Introduction 8

1.1 Principles of Electrochemical Advanced Oxidation Processes

8

1.2 Criterion for selection of organic dye 11

1.3 Objective thesis 13

2 Literature Review 14

3 Advanced Oxydation Processes(AOPs) 16

3.1 Hydrogen Peroxide Based APOs 16

3.1.1 Hydrogen Peroxide Photolysis 16

3.1.2 Fenton Reaction 17

3.1.3 Photo Fenton 21

3.2 Electrochemical Methods 22

3.2.1 Anodic Oxidation 22

3.2.2 Electro Fenton Process 26

3.2.3 Influence of the experimental parameters on AOPs

27

3.3 Chemical Oxygen Demand 30

6

4 Experiment 32

4.1 Materials & Methods 32

4.2 Procedure 32

4.3 Analytic Methods 33

4.4 COD procedure 33

5 Results 35

5. a) Non UV bases Results 37

5. b) UV bases Results 39

6 Conclusion 41

7 Future Work 42

8 References 43

7

CHAPTER 1. INTRODUCTION

1.1. Principles of Electrochemical Advanced Oxidation Processes

Water is of fundamental importance for life on the Earth. The whole

mechanism of metabolism, the synthesis and structure of colloidal cellular

constituents, the solution and transport of nutrients inside the cells and

interactions with the environment are closely related to the specific

characteristics of water. On the other hand, the part of the freshwater

(groundwater, lakes and rivers, polar ice and glaciers in height) that can be

used by the human beings is only 2.66% of the global water resource.

Furthermore, these freshwater resources, in particular surface water is exposed

to the pollution coming from various human activity. Therefore, in order to

protect natural water resources, it is necessary to treat efficiently wastewater

effluents before their injection in the natural water system.

Common physio-chemical wastewater treatment methods such as activated

carbon adsorption and membrane filtration transform the pollutants from one

phase to another, so they separate but not eliminate the water pollutants.

Ozone and hypochlorite oxidations are efficient methods for water disinfection

but remain inefficient in case of effluents of hard COD (effluents from

industrial or agricultural activities). On the other hand, they are not desirable

because of the high cost of equipment, operating costs and the secondary

pollution arising from the residual chlorine.

Recent progress in the treatment of persistent organic pollutants (POPs) in

water and/or wastewater has led to the development of advanced oxidation

processes (AOPs). These processes involve chemical, photochemical or

8

electrochemical techniques to bring about chemical degradation of organic

pollutants. The most commonly used oxidation processes use H2O2, O3 or O2

as the bulk oxidant to form principal active specie in such systems, i.e., the

hydroxyl radical, •OH, a highly oxidizing agent of organic contaminants.

These radicals react with organic pollutants and thus lead to their degradation

by hydrogen abstraction reaction (dehydrogenation), by redox reaction or by

electrophilic addition to π systems (hydroxylation). The most commonly used

AOPs for the removal of persistent organic pollutants from water are based on

the Fenton’s reaction. However, this reaction has some limitations in

application such as the use of large quantities of chemical reagents, large

production rates of ferric hydroxide sludge and slow catalysis of the ferrous

ions generation. Electrochemical AOPs overcome these drawbacks and offer

many advantages such as low operational cost and high mineralization degree

of pollutants compared to other known chemical and photochemical ones. In

this sense, anodic oxidation and electro-Fenton processes are very commonly

used electrochemical AOPs. In anodic oxidation, pollutants are mineralized by

direct electron transfer reactions or action of radical species (i.e. hydroxyl

radicals) formed on the electrode surface. In this manner, a wide variety of

electrode materials have been investigated recently, but the boron doped

diamond (BDD) has attracted great attention because of its high O2 evolution

overvoltage, high stability and efficiency. This electrode allows to produce

large quantities of hydroxyl radicals from water or hydroxide oxidation

decomposition on the electrode surface (Eqs. 1.1 and 1.2). The formation of

H2O2 is also possible depending on the cathode materials used during the

anodic oxidation process. The oxidation of formed H2O2 to HO2 • (Eq. 1.3) or

to O2 (Eq. 1.4) takes place at anode surface. The formed reactive species may

react with the organics but their oxidation ability are poor compared to

adsorbed •OH radicals.

H2O → ·OHads +H+ +e− (1.1)

OH− → ·OHads +e− (pH≥10) (1.2)

9

H2O2 → HO2· + H+ + e- (1.3)

HO2· → O2 + H+ + e- (1.4)

In the electro-Fenton process, pollutants are destroyed by the action of

Fenton’s reagent in the bulk together with anodic oxidation at the anode

surface in the case of the use of a high O2 evolution overvoltage anode such as

BDD. Fenton’s reagent is formed in the electrolysis medium by the

simultaneous electrochemical reduction of O2 to H2O2 (Eq. 1.5) and Fe (III) to

Fe (II) ions (Eq. 1.6) on the cathode surface. The reaction between these two

species in the homogeneous medium allows the formation of ·OH radicals (Eq.

1.7). The Eqs. 1.3 and 1.4 can also take place during the electro-Fenton

process. Moreover, the oxidation of regenerated Fe2+ to Fe3+ may occur at the

same time (Eq. 1.8) on the anode surface. On the other hand, the existence of

these reactions (Eqs. 1.3, 1.4 and 1.8) are negligible compared to reaction (1.7)

which occurs in the bulk because of the limited surface area of anode. Finally,

iron species (Fe3+/Fe2+) can react with the formed reactive species from anodic

and cathodic reactions (Eq. 1.9-1.11). The overall effect of these reactions

influences the mineralization process of organics in the electro-Fenton

treatment.

O2 + 2H+ + 2e-→ H2O2 (1.5)

Fe(OH)2+ + e-→ Fe2+ + OH- (1.6)

Fe2+ + H2O2 + H+→ Fe3+ + H2O + ·OH (1.7)

Fe2+ → Fe3+ + e- (1.8)

Fe3+ + H2O2·→ Fe2+ + H+ + HO2• (1.9)

Fe3+ + HO2· → Fe2+ + H+ + O2 (1.10)

Fe2+ + HO2· → Fe3+ + HO2- (1.11)

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The hydroxyl radicals (·OH) formed by the electrochemical (Eq. 1.1) or bulk

(Eq. 1.7) are very powerful oxidizing agents. They react unselectively with

organics giving dehydrogenated and/or hydroxylated reaction intermediates

before their total conversion into CO2, water and inorganic ions, when ·OH are

produced in continue. Because ·OH production does not involve the use of

harmful chemical reagents which can be hazardous for the environment,

electrochemical processes can be seen as environmentally friendly techniques.

In conclusion, these processes seem to be very promising for the purification

of water polluted by persistent and/or toxic organic pollutants.

1.2. Criteria for selection of organic dye

Synthetic dyes are widely used for dyeing and printing in a broad range of

industries. Over 100,000 dyes with an estimated production of 700,000 metric

tonnes are produced annually and 2–20% of dyes are directly discharged into

the aquatic environments. Coloured effluents containing dyes are aesthetically

displeasing and can affect the photosynthetic activity of aquatic plants by

reducing light penetration. Furthermore, most dye molecules are complex and

some of them are highly carcinogenic. Thus, purification of dye effluents is

one of the major problems in wastewater treatment.

Azo dyes, generally characterized by the presence of one or more azo bonds (–

N N–) in association with aromatic systems and auxochromes (–OH, –SO3,

etc.), are important synthetic colorants that represent the largest class of dyes

in common use especially applied in textile processing. Textile effluents have

been shown to be toxic, mutagenic, carcinogenic and non-biodegradable,

making it a public health concern. Several conventional techniques including

physical, chemical, and biological processes have been developed to remove

the azo dyes in the aquatic environment, such as adsorption, coagulation,

photo-catalysis and, ozonation and bio sorption. Nevertheless, these adopted

methods have proven to be costly, time-consuming and impractical.

11

The application of acid dyes to protein fibers results in an ionic or salt link

between the dye molecule and the fibre polymer. The point of the fibre

polymer at which the dye is attached is termed the dye site. In wool, the dye

sites are of many amino group of the fibre. Under dyeing conditions, the

amino group becomes positively charged and attracts the negatively charged

dye anion.

There are a large number of amino groups are present in the wool fibre. As a

guide, there are approximately twenty times as many amino groups on wool as

on nylon and five times as many amino groups on wool as on silk. Dark

shades can be readily be obtained on wool because of the highly amorphous

nature of the fibre, which results in relatively easy penetration of the fibre

polymer by the dye molecule and because of the presence of minor groups.

Although silk has an affinity for acid dyes the colors tend to be less fast than

on wool. Silk will exert its affinity for acid dyes at lower temperature than is

the case with wool, and dyeing is usually commenced at 40ºC and the

temperature is not allowed to rise above 85ºC. Glauber‟s salt is not suitable

for use with silk as it diminishes its luster. Sulfuric acid damages the silk. Acid

used should be acetic acid. While using boiled off liquor the bath must be

neutral or only faintly acidic.

The Orange G (Acid Orange 10) dye is a synthetic azo dye used in histology

in many staining formulations. The main use orange G is OG6 Papanicolaou,

to stain keratin and as a major component of Alexander test for pollen

staining. It is also widely used in textile dyes such as wool and silk, used to

dye paper, leather, wood stain, coloring inks and copying pencils. In the

biological stain community, it is the contrast/background stain for

Haematoxylin, Safranin O, Crystal violet, methyl green and Basic Fuchsin.

12

1.3 Objective of the studyThis study is aiming to achieve the following objectives:

a) To investigate the COD removal characteristics of the EF & PEF processes.

13

CHAPTER 2: LITERATURE REVIEWReza Davarnejad et al,2016, used modified nanoparticle graphite electrode

which they proved to be more efficient than the than electrodes.

Liang Ma et al,2016, this study presented a novel vertical-flow electro-Fenton

reactor, composing of 10 cell compartments using PbO2 anode and modified

graphite felt mesh cathode, which was found to be more complete and

efficient in organic pollutants degradation when comparing with the traditional

parallel-flow reactor, using tartrazine as the model pollutant.

Enric Brillas et al,2016, this study established an increasing relative oxidation

power of the EAOPs in the order AO-H2O2 < EF < PEF, in agreement with

their Decolorization trend. PEF was the most powerful EAOP, since the

synergistic action of BDD (radical °OH), radical °OH and UVA light yielded

94% mineralization after 360 min.

Eddy Petit et al.2016, The original coupling of electrochemical and

transmembrane filtration performances of a porous carbon electrode was

successfully demonstrated in this work with the objective of ensuring

degradation of refractory organic matter. The tests performed in dynamic

cross-flow filtration led to significantly higher kinetic rates (almost three

times) compared to batch reactor.

Hannah Roth et al2015, used a micro tube made only of multi-walled carbon

nanotubes (MWCNT) functions as a gas diffusion electrode (GDE) and highly

porous adsorber. In the process, the pollutants were first removed electroless

from the wastewater by adsorption on the MWCNT-GDE. Subsequently, the

pollutants are electrochemically degraded in a defined volume of electrolyte

solution using the electro-Fenton process. Oxygen was supplied into the lumen

of the saturated micro tubular GDE which was surrounded by a cylindrical

anode made of Ti-felt coated with Pt/IrO2 catalysts. At optimal conditions,

complete regeneration of the adsorption capacity of the MWCNT-GDE,

14

complete decolorization and TOC and COD removal rates of 50% and 70%

were achieved, respectively.

Luca Di Palma et al,2015, They compared a graphite, a carbon felt (CF) and a

reticulated vitreous carbon (RVC) electrode with a focus on both electro

generation of hydrogen peroxide and reduction of ferric ions. The results

obtained showed that all the cathodes presented good ability to electrogenerate

hydrogen peroxide. Unlike graphite, the three-dimensional electrodes

exhibited superior performances in air flow and the possibility to adopt higher

currents. Their behaviour differed significantly in the electro regeneration of

ferrous ions. In this case, RVC and CF presented efficiencies and reduction

rates higher than graphite and resulted almost unaffected by the operative

conditions.

E. Bustos et al, 2014, In this work, an air diffusion, high surface area granular

activated carbon electrode was studied in view of its potential use as cathode

for an electro-Fenton process. Using the air-diffusion approach, it was possible

to generate H2O2 concentrations ten times larger than those reached using

dissolved O2. Incorporation of this electrode in an Electro-Fenton system

showed that it was possible to treat a MO loaded effluent and it was shown

that the electrode possesses a relatively good stability.

Carlos Carlesi et al,2016, It was found that a BDD/air-diffusion cell operating

at 100–200 mA is able to accumulate H2O2 concentrations high enough in an

acidic solution of pH 3.0 to degrade a solution with 295 mg/L of Orange G by

EF and EF. The large production of °OH from Fenton’s reaction in both

coupled processes caused a fast discoloration. The discoloration rate was

slightly superior at 50 and 100 mA for PEF as a result of the photolysis of the

Fe(OH)2+ species present in the solution. The relative oxidation power of the

EAOPs increased in the sequence AO-H2O2 < EF < PEF. An almost total

mineralization with 98% TOC reduction was only achieved by the latter

method at 200 mA, showing the synergic oxidation effect of UVA light and

generated °OH.

15

CHAPTER 3: ADVANCED OXIDATION PROCESSES (AOPs)

3.1. HYDROGEN PEROXIDE BASED AOPs

3.1.1. Hydrogen peroxide photolysis (H2O2/UV)

Both, H2O2 and UV irradiation can be used separately to achieve the

degradation of some contaminants, but their combination gives a more

effective mean for water contaminants treatment191,192. UV irradiation of 200-

280 nm (with max = 260 nm) possesses the necessary energy to induce the

homolytic decomposition of hydrogen peroxide193,194 producing hydroxyl

radicals.

H2O2 + h_ 2 •OH (1)

In this case the main oxidant acting on pollutants degradation is hydroxyl

radical, implying that the rate of oxidation depends on the •OH production

rate. But this reaction is limited by the low absorption coefficient of H2O2 (ɛ =

18.6 mol-1 L cm-1 at max = 260 nm) However, it was found that the rate of

H2O2 photolysis is pH dependent and it increases at high pH values. This

happens because at high pH the peroxide anion HO2 - may be formed which

shows a higher molar absorption coefficient (ɛ = 240 mol-1L cm-1) 195 at 254

nm than H2O2.

HO2- + h_ •OH + (1/2)O2•- (2)

The presence of other species in the solution which absorbs the radiation and

turbidity reduce the quantum yield of reactions (1) and (2) and consequently

the efficacy of pollutants removal.

16

3.1.2. Fenton’s reaction

This technique is based on hydrogen peroxide action including catalytic

amounts of Iron (II) salts. The use of this mixture of reagents originates from

early works of Fenton concerning the oxidation of tartaric acid. When in the

tartaric acid solution was added iron sulphate and hydrogen peroxide followed

by alkalisation it got violet coloured. So, Fenton proposed this reaction as an

identification test for tartaric acid. But the use of this mixture of reagents,

H2O2/Fe2+ nowadays called the Fenton’s reagent, is considered for the

oxidation of organic compounds began later by 1930s after a radical

mechanism for the decomposition of H2O2 was proposed197. Afterwards, the

Fenton’s reagent for the use in destruction of toxic organic compounds

became very frequent It has been accepted that Fenton’s reaction includes a

series of reactions initiated by the principal reaction between H2O2 and Fe2+ in

acid medium given below;

H2O2 + Fe2+ _ Fe3+ + •OH + OH- k = 63 L mol-1 s-1 (3)

The generation of hydroxyl radicals (•OH) during this reaction has been

defined205 and confirmed by different methods such as chemical probes or

spectroscopic techniques namely spin-trapping. Also by means of pulse

radiolysis, many works concerning rate constants of the reactions involved in

Fenton’s chemistry have been carried out. For the Fenton’s reaction to take

place, only small quantities of iron salts are needed because iron (II) is

regenerated from the so-called Fenton-like reaction between excess of

hydrogen peroxide and iron (III) formed by reaction (44):

Fe3+ + H2O2 _ Fe2+ + HO2• + H+ (4)

This is not a direct reaction as iron (III) firstly forms an adduct with hydrogen

peroxide, reaction (5) and then this species gives the regenerated iron (II) and

hydroperoxyl radical HO2• (reaction (51)):

17

Fe3+ + H2O2 _ [Fe(HO2)]2+ + H+ k = 3.1x10-3 L mol-1 s-1 (5)

[Fe(HO2)]2+ _ Fe2+ + HO2 • k = 2.7x10-3 L 3mol-1 s-1 (6)

Hydroxyperoxyl radicals HO2• produced in this reaction have less oxidation

power compared OH and do not react strongly with organic molecules.

Considering reaction rate constants we can see that reaction (4) is much slower

than Fenton’s reaction (3), and consequentlyFe2+ regeneration due to this

reaction is not very rapid. Anyways, Fe2+ ion can be regenerated due to some

other very rapid reactions: as Fe3+ reduction by HO2• reaction (7), a reaction(8)

with an organic radical formed during initial organic molecule degradation by

•OH and a

reaction (54) with a superoxide anion (O2•-).

Fe3 + HO2• _ Fe2+ + O2 + H+ k = 2x103 L mol-1 s-1 (7)

Fe3+ + R• _ Fe2+ + R+ (8)

Fe3+ + O2•- _ Fe2+ + O2 k = 5x107 L mol-1 s-1 (9)

The species which contribute in Fe2+ regeneration are produced in reactions

denotedbelow140,211,212:

H2O2 + •OH _ H2O + HO2•- k = 2.7x107 L mol-1 s-1 (10)

HO2•- _ H+ + O2•- pKa = 4.8 (11)

RH + •OH _ R• + H2O k = 107-109 L mol-1 s-1 (12)

ArH + •OH _ ArHOH• k = 108-1010 L mol-1 s-1 (13)

ArHOH• + O2 _ ArOH + HO2•- (14)

18

Although these reactions enable the Fenton’s reaction to proceed for a period

of time, some of them play also a negative role towards the Fenton’s reaction

rate. In the reactions (4) and (10) Fe3+ and •OH act as scavengers of H2O2

destroying it in competition with reaction (3).The organic radical R•

participates in Fe2+ regeneration but also in Fe2+ oxidation by reaction(15),

along with dimerization reaction (16):

R• + Fe2+ + H+ _ RH + Fe3+ (15)

R• + R• _ R-R (16)

Some other reactions involved in Fenton’s chemistry are also141:

Fe2+ + •OH _ Fe3+ + OH- k = 3.2x108 dm3mol-1s-1 (17)

Fe2+ + HO2• + H+ _ Fe3+ + H2O2 k = 1.2x106 L mol-1 s-1 (18)

Fe2+ + O2•- + 2H+ _ Fe3+ + H2O2 k = 1.0x107 L mol-1 s-1 (19)

O2•- + HO2• + H+ _ H2O2 + O2 k = 9.7x107 L mol-1 s-1 (20)

HO2• + HO2• _ H2O2 + O2 k = 8.3x105 L mol-1 s-1 (21)

HO2• + •OH _ H2O + O2 k = 7.1x109 L mol-1 s-1 (22)

O2•- + •OH _ OH- + O2 k = 1.01x1010 L mol-1 s-1 (23)

•OH + •OH _ H2O2 k = 6.0x109 L mol-1 s-1 (24)

The inhibiting role of these reactions restrict the values of several

experimental variables, for instance the occurrence of reaction (22) decreases

the concentration of Fe2+ ions in the medium and along with the reaction (20)

they are the major parasitic reactions that decrease the oxidation power of

19

Fenton reagent. Other reactions (20-25) are not significant because of the

relatively low presence of radical species in the solution in comparison with

other non-radical molecules. It has been proven that radical scavengers play an

important role in the rate of Fenton’s reaction. Such species are chloride,

sulphate and nitrate ions215. Anyways, in many studies this behaviour has not

been observed. The presence of some other oxidizing agents has also been

pointed out216. There have been some experimental works which have brought

some evidence over the existence of high-oxidation state iron complexes under

certain conditions217. So, the formation of mononuclear Fe4+ oxo-complex was

proposed218, which can oxidise organics only by electron transfer:

Fe2+ + H2O2 _ [Fe(OH)2]2+ _ Fe3+ + •OH + OH- (25)

Thus, researchers found an agreement between hydroxyl radical and ferryl

ion-complex mechanisms predominating one or other depending on the

particular operating conditions. The co-generation of •OH and high-oxidation

state oxo-iron complex has been demonstrated by time-resolved laser flash

photolysis spectroscopy219:

[Fe3+-OOH]2+ _ (Fe3+-O•_ Fe4+=O) + •OH (26)

The [Fe3+-OOH]2+ is an excited state species and the overall reaction can be

interpreted as an intraligand reaction. On the basis of these results it has been

proposed that ferryl formation in secondary reactions under classical Fenton

condition cannot be ruled out. The Fenton process efficiency is depended of

many experimental variables141, as: pH, [Fe2+], [H2O2] and temperature. The

concentrations of Fe2+ and H2O2 are the most fundamental parameters. The

efficiency of the process is strongly related to the solution pH. The most

favourable pH values for the Fenton reaction to proceed are 2.8 ≤ pH ≥ 3.0

because at these values the majority of the total iron species in the medium are

present in the form of Fe2+. When the pH is lower than 2.8 the predominant

species of iron present in the solution is Fe3+ as [Fe(H2O)6]3+ or barely Fe3+,

20

deteriorating reaction efficiency. At pH = 1 oxygen concentration does not

change, and this probably because of the stabilisation of H2O2 with H+

in H3O2 + (solvation of H+ with H2O2) which reduces the reaction with Fe2+.

The Fenton’s reaction will also slow down when the pH exceeds the 0 value of

pH 3.5. In the case of pH > 5.0, iron ions will precipitate as Fe(OH)3 thus the

catalyst will be removed from the solution and consequently the Fenton

reaction efficiency slows down. At pH = 4.0 hydroperoxy complexes such as

[Fe(HO2)2]+ and [Fe(OH)(HO2)]+ are the dominant forms of iron. Temperature

is another influencing parameter. The rate of Fenton’s reaction increases with

the temperature but simultaneously the degradation of hydrogen peroxide in

O2 and H2O does. The optimum concentrations of catalyst Fe2+ and H2O2 are

depended on each other and experiments are done in the basis of optimisation

of their ratio instead of studying them separately.

3.1.3. Photo-Fenton (H2O2/Fe2+/hv)

Fenton’s process for polluted water treatment can be improved by combining

with UV photolysis in order to enhance the degradation reaction rate227,228,229.

When the solution under treatment with Fenton’s reagent is irradiated with UV

light, supplementary hydroxyl radicals are obtained from reaction (11)

resulting to the formation of more radicals in the medium. Apart this Fe2+

liberated from [Fe(OH)]2+ will catalyse the Fenton’s reaction (reaction (82)) :

[Fe(OH)]2+ + h_ Fe2+ + •OH (27)

thus avoiding large accumulation of Fe3+ and providing Fe2+ necessary. This

reaction allows maintaining Fenton’s reaction operative for longer time. The

quantum yield for the reaction (27) was found to be 0.14-0.19 at 313 nm.

Additionally, UV irradiation can degrade some oxidation by-products or break

down the bonds (reaction 83) 230 in complexes formed between iron and

carboxylic acids supporting the regeneration of Fe2+.

21

Fe(OOCR)2+ + hv _ Fe2+ + CO2 + R• (28)

The use of irradiation lap (to provide artificial light) with restricted life time is

a drawback of this process as well as considerable hydrogen peroxide

concentration needed. Its cost can be reduced if the UV radiation is replaced

with solar light as it has been shown in some works.

3.2 ELECTROCHEMICAL METHODS

Electrochemical destruction of pollutants in aquatic medium involves, in the

destruction process, the action of electrons, coming from a current source. The

electrochemical treatment is brought about in an electrochemical cell without

the use of specific expensive and relatively dangerous reagents. This permits a

very good compliance with environmental requirements.

Two electrochemical methods are distinguished:

I. Direct oxidation of organic molecules on the anode surface which includes

two mechanisms234 (explained below).

II. Indirect oxidation realized by in-situ generation Fenton’s reagent on the

cathode compartment called electro-Fenton process235.

These two methods constitute the subject of this thesis and will be discussed in

the two subsequent sections.

3.2.1. Anodic oxidation

During the anodic oxidation of organic pollutants the molecules can be

oxidized by two principal mechanisms; direct electrochemical reaction via

electron transfer between electrode (anode) and molecule, and indirect

oxidation via oxidants generated on the anode, called also mediated

oxidation236. The direct electrochemical oxidation occurs below the oxygen

22

onset potential and it subsides above it. At the oxygen evolution potential,

organics oxidation proceeds in competition with oxygen evolution reaction

(OER). Thus, the degradation of pollutants will depend on the mechanism of

OER which strongly varies with the electrode (anode) material237,238,239.

Generally, anodes exhibiting a high over potential for OER show better

efficiency on organics degradation. Many electrode materials have been

studied for their electro-catalytic properties towards organics oxidation. One

of the anodes representing low over potential for OER is iridium dioxide IrO2

based dimensionally stable anode (DSA)250. The evolution of oxygen on these

types of anodes is thought to occur in three steps and involves the change

ofoxidation state of the metal oxide during water discharge according to the

simple reactions (29) - (31). The first step is the charge transfer by the

discharge of water, with the formation of active species on active sites of the

anode surface:

M + H2O _ MOx(OH) + H+ + e- (29)

The second step is a second electron transfer step with the deprotonation of the

adsorbed hydroxy species:

MOx(OH) _ MOx+1 + H+ + e- (30)

And the third one is the formation of oxygen molecules and the regeneration

of two active sites on the surface:

MOx+1 _ MOx + ½ O2(g) (31)

In another work252 a similar scheme for the oxidation of isopropanol on IrO2

based anodes was proposed. Firstly the IrO2 is oxidised to IrO3 via hydroxyl

radicals according to the global reaction (32):

(IrO2)s + H2O _ (IrO3)s + 2H+ + 2e- (32)

23

Then the chemical oxidation of adsorbed isopropanol to acetone by the

electrogenerated IrO3, reaction (33):

(IrO3)s + (CH3CHOHCH3)ads _ (IrO2)s + (CH3COCH3)ads + H2O (33)

And also, oxygen evolution in competition with reaction (33) via

decomposition of surface IrO3 according to the reaction (34):

(IrO3)s _ (IrO2)s + ½ O2(g) (34)

The OER is the prevailing process leading to low degradation efficiencies and

loss of electrical energy. At high oxygen evolution potential electrodes the

organics oxidation process follows a different mechanism. The most

remarkable high oxygen evolution over potential electrode is boron doped

diamond (BDD). This electrode is prepared by chemical vapour deposition of

methane mixed with metallic boron or B(OCH3)3 as dopant. Titanium,

niobium and silicon and other materials can be used as substrate for the

diamond deposition. The water discharge on BDD electrode is thought to

occur through a path giving hydroxyl radicals as intermediate species258. A

simplified mechanism for the organics oxidation on boron doped diamond

electrodes has also been proposed: First the discharge of water molecules

producing hydroxyl radicals chemisorbed on BDD surface as very reactive

oxidising agents (reaction (35)):

BDD + H2O _ BDD(HO•) + H+ + e- (35)

Then the oxidation of organic molecules:

BDD(HO•) + R _ BDD + ROH• (or R• + H2O) (36)

And the competitive oxygen evolution reaction:

BDD(HO•) _ BDD + ½ O2 + H+ + e- (37)

24

BDD electrode is considered a high over potential oxygen evolution anode, so

the oxygen evolution reaction is much less intensive in comparison with case

of DSA type anodes. Nevertheless, a considerable electrical energy is wasted

because of OER. Hydroxyl radicals generated cannot oxidize diamond neither

they are chemically adsorbed on diamond surface but they are physically

adsorbed. The fact that they are loosely adsorbed on the electrode surface let

them quasi free so that they can react with other substances which are found in

the vicinity of the electrode. So the oxidation of organic pollutants by

hydroxyl radicals takes place only at the electrode surface because the

diffusion coefficient of hydroxyl radicals is very low140 (because of its high

reactivity). The pollutant’s degradation takes place in the bulk solution also

via other oxidants generated on the anode. Other oxidants originate from the

supporting electrolyte. If sodium sulphate is used as supporting electrolyte the

peroxydisulphate anions will be present in the solution, following the reaction

(94)

2SO42- _ S2O82- + e- (39)

Whereas when the supporting electrolyte is sodium chloride, Cl- is expected to

be oxidized either by direct electron transfer at anode surface or by a reaction

with •OH in the vicinity of electrode, reactions (39) -(46):

2Cl- _ Cl2 + 2e- (40)

•OH + Cl- _ ClOH•- (41)

ClOH•- _ Cl• + OH- (42)

Cl• + Cl- _ Cl2•- (43)

Cl2•- + •OH _ HOCl + Cl- (44)

Cl2 + H2O _ HOCl + H+ + Cl- (45)

25

HOCl _ H+ + OCl- (46)

Therefore, BDD electrode has very interesting properties which make it

versatile. Its use in polluted water treatment is outstanding and many works

have been dedicated on it.

3.2.2. Electro-Fenton process (Indirect electrochemical oxidation)

The Electro-Fenton process is an indirect electrochemical method for the

destruction of toxic and/or persistent micro-pollutants in contaminated

waters141,235. This method is based on the Fenton’s reaction chemistry198,266.

As described in one of the previous sections Fenton’s reagent (H2O2 + Fe2+) is

used to produce very reactive hydroxyl radicals •OH that are used to eliminate

toxic organic compounds from contaminated waters. In the classical Fenton

process, H2O2 and Fe2+ are externally added to the reaction medium and the

concentration of target molecules is monitored until the depletion of oxidising

agents. As already mentioned the complete mineralisation of pollutants is not

achieved because of the Fe3+ inactivation by ligand action of carboxylic

acids267, but also because of the mere Fenton’s reagent consumption. Whereas

in the electro-Fenton method, Fenton’s reagent is produced directly in the

polluted water to be treated. Fe2+ is added in the solution in a catalytic quantity

as an iron salt and it is continuously regenerated on the cathode surface via the

one electron transfer) (reaction (57)) from Fe3+ formed during Fenton’s

reaction (13):

Fe3+ + e- _ Fe2+ (57)

On the other side H2O2 is also electro-generated at the cathode from the two

electron reduction of oxygen in acidic media (pH3) according to the reaction

(58):

O2 + 2H+ + 2e- _ H2O2 (58)

26

Whereas its reduction to water by reaction (59) is avoided by choosing a

potential (or current) more positive than that of this second reduction step of

O2.

O2 + 4H+ + 4e- _ 2H2O (59)

The oxygen needed for this reaction is introduced in the solution by bubbling

compressed air (or oxygen). Thus, the oxygen reduction includes the

dissolution of oxygen gas in the solution, its transportation to the cathode and

finally the reduction to hydrogen peroxide. Some oxygen also comes from the

naturally oxygen dissolution in water according to the Henry’s law and the

oxygen evolution on the anode from water discharge (reaction (62)):

H2O _ ½ O2 + OH- + e- (60)

Once H2O2 and Fe2+ produced as described above, they react following to the

Fenton’s reaction (reaction (13)) to give hydroxyl radicals which in turn

oxidize organics. Afterwards, Fe3+ generated in reaction (13) reduced to Fe2+

according to reaction (57). On the other hand H2O2 keeps being produced

electrochemically at the cathode. So, the Fenton’s reagent is continuously

supplied in the electrochemical cell in a catalytic way. Apart the

electrogeneration reactions of Fenton’s reagent, parasitic reactions exist too

and their intensity depends on electrochemical cell configuration and other

operation conditions. For example in an undivided cell Fe2+ can be

electrochemically oxidized to Fe3+ at the anode:

Fe2+ _ Fe3+ + e- (61)

Fe3+ can precipitate in the very vicinity or in the pores of three dimensional

cathodes as Fe(OH)3 because of the basic conditions created by water

reduction. Hydrogen peroxide accumulation in the system and its stability

depends on working conditions. Some usual parasitic reactions are reactions

(59) and itself decomposition to oxygen and water (reaction (64)):

27

2H2O2 _ O2 + 2H2O (62)

A parasitic reaction related to the cell configuration is its oxidation on the

anode if an undivided cell is used. This reaction involves hydroperoxyl

radicals as intermediates:

H2O2 _ HO2 • + H+ + e- (63)

HO2• _ O2 + H+ + e- (64)

So all possible parasitic reactions make the accumulation of hydrogen

peroxide be lower than levels expected from its electro generation. Its

identification and dosage in the solution can be done by different methods, one

of them is the spectrophotometric determination based in the Ti(IV)-H2O2

complex which gives a yellow colour and absorbs at 410 nm. It is worth

noting that all parasitic and regeneration reactions of H2O2 and Fe3+ involved

in the Fenton’s chemistry can account for the electro-Fenton process also.

However, some parasitic reactions as those between •OH and H2O2, •OH and

Fe2+ which are the most important ones are reduced or eliminated.

3.2.3. Influence of the experimental parameters on the electro-Fenton

process

Many experimental parameters affect the electro-Fenton efficiency process.

Among them the most important ones are: solution pH, catalyst concentration,

electrode material, applied current, temperature and oxygen or air feed

rate.The influence of pH on Electro-Fenton process efficiency is strongly

dependent on solution pH as already discussed for the Fenton’s chemistry.

Several works have shown that the optimal pH value is 2.8-3 where a

maximum generation of hydroxyl radicals was observed276,277. For pH > 3.5

the rate of mineralisation of organics starts to slow down because a part of

Fe3+ precipitates as Fe(OH)3. At pH < 1 it becomes very slow since Fe2+ forms

complexes with H2O2 and SO4 2-.The nature of acid utilised for pH adjustment

28

as well as the nature of supporting electrolyte affects also the rate of pollutants

degradation via the acid and salt anions involvement in the oxidation

processes. At low pH the formation of iron complexes with Cl- and ClO4– is

also possible whereas SO4 2- apart the complexion action scavenges hydroxyl

radicals too. It has been found that the removal rate of orange II decreases

with the acids utilised for pH adjustment in the order: ClO4 - > Cl- >> SO42- 279.

Catalyst concentration

The catalyst is one of two fundamental reagents of the electro-Fenton process

and its importance is crucial. The rate of degradation reaction increases with

the catalyst concentration until a given value owing to the intensification of

Fenton’s reaction (58). Then after a certain concentration a reverse effect is

observed because of the parasitic reaction (72)which consumes hydroxyl

radicals in competition with organics oxidation following the reactions (67)-

(68). Thus, an optimal concentration of catalyst is required in order to attain

the maximum rate of contaminants oxidation. This optimum concentration

depends on the nature

of the cathode utilized in the process. If a carbon felt cathode is utilized the

optimum concentration for Fe2+ is 0.1-0.2 mmol L-1 at pH = 3 271,281, whereas

higher concentration is required in case of carbon-PTFE gas diffusion

electrodes (GDE), namely 0.5-1.0 mmol L-1 Fe2+ range is the optimum276,282.

Greater concentration of catalyst for the GDE electrodes is necessary because

of their lower ability of Fe2+ regeneration in comparison with carbon felt

cathodes. Moreover, H2O2 is produced in greater extent at GDEs so a greater

concentration of Fe2+ is required to intensify reaction (58), otherwise parasitic

reaction (65) with the production of HO2 •- (week oxidant) can become

important.

29

Applied current

Fenton’s reaction driven by electrical current makes electro-Fenton a

remarkable method for polluted water treatment. The current applied produces

and maintains H2O2 and Fe2+ concentrations during electrolysis. The variation

of current affects the production rate and the concentration of H2O2 and Fe2+

and consequently the rate of degradation of organic molecules. When the

current intensity is increased the quantity of H2O2 in the solution increases

owing to the acceleration of reaction (103). An increase of current intensity

results in a more effective Fe2+ regeneration too (102). Since the concentration

of both H2O2 and Fe2+ is increased with the current intensity, the quantity of

hydroxyl radicals will be higher and as a consequence faster organics removal

are achieved284,285,286,287. Nevertheless, the acceleration of organics

degradation reaction rises until a certain current intensity beyond which no

improvement of the efficacy of process is observed288,289,290. This limiting

degradation current is a consequence of parasitic reactions which compete

with O2 reduction to H2O2 (reaction (103)) namely the hydrogen evolution

reaction on cathode. At high current intensities mass transport of O2 and Fe3+

towards cathode becomes the rate determining step of the electrochemical

reactions of production of H2O2 and Fe2+, thus any increase in current intensity

beyond this limit will lead to a loss of energy without any improvement in the

treatment process. Low current intensities give pollutants removal with higher

electricity effectiveness but longer electrolysis, and if the current intensity is

considerably low no significant remediation of water is attained. The use of

other catalysts other than Fe2+ is also possible. Among them Co2+, Cu2+ and

Mn2+ have been tested showing that optimal concentrations vary from one to

other.

Temperature and oxygen or air feed

Oxygen is feed continuously in the solution by introducing compressed air or

oxygen. This provides a saturated solution with oxygen to reach maximum

30

H2O2 production. Temperatures up to 35-40°C enhance hydroxyl radical

formation, but higher temperatures enhance at the same time hydrogen

peroxide decomposition and other parasitic reactions141.

Electrode material

Electrode (cathode and anode) material plays a very important role on electro-

Fenton process since the principal reagents (oxidants) are generated on. Thus,

this thesis is devoted to study the role of electrode material on electro-Fenton

treatment of polluted waters. Many cathodes have been studied so far for their

performance in the electro-Fenton technology for water treatment, such as:

graphite292,293, mercury294, carbon-PTFE O2 gas diffusion carbon felt132,285,298,

reticulated vitreous carbon (RVC)299, carbon sponge283 and carbon

nanotubes300,301. However, to the best of our knowledge, there has been no a

systematic study to compare the performance of these materials to find the

better one for the process. Therefore, such a study constitutes the subject of

this thesis. A cathode material for electrochemical water treatment must have

some characteristics that make them fit to the electro-Fenton process. A

cathode must have high hydrogen evolution over potential in order to provide

high hydrogen peroxide yield with high current efficiencies, low catalytic

activity for hydrogen peroxide decomposition, chemical and physical stability,

good electrical conductivity and low economical cost. Some materials like

mercury support H2O2 production, however they are very toxic so not useful

for water treatment. Carbon is a very appropriate material for environmental

application as it does not show any toxic effect towards living beings and

represents all the characteristics required for electrochemical water

remediation. Considering the fact that oxygen is poorly soluble in water three

dimensional large surface area cathodes are needed to obtain reasonable

current efficiencies in pollutants removal. Such electrodes are GDEs with thin

and porous structure favouring the circulation of injected oxygen through its

pores until the solution electrode interface. These electrodes allow fast O2

reduction to have H2O2 accumulation owing to high number of active sites on

31

their surface. GDEs are constituted of carbon particles bonded with PTFE in a

cohesive layer. Carbon felt is a three dimensional large specific surface

cathode where the Fenton’s reagent generation takes place very rapidly. In

comparison with GDEs there is a lower accumulation of the H2O2 because its

H2O2 generation ability is lower than that of GDEs. Contrarily, the

regeneration of Fe2+ at carbon felt is faster than at GDE leading to lower

accumulation of H2O2 because hydroxyl radicals are immediately produced

through Fenton’s reaction. Anode material is another source of oxidants that

participate in oxidation of organic matter. Different anodes used in direct

anodic oxidation can be used for electro-Fenton. When a high over potential

oxygen evolution anode is used hydroxyl radicals can be generated from the

water discharge along with other oxidants like S2O82-, ClO- etc. depending on

the supporting electrolyte present in the solution. In fact, the supporting

electrolyte plays always an important role in pollutant degradation279 in extents

varying from anode material. An anode providing high concentration of

hydroxyl radicals is boron doped diamond (BDD) which is widely being used

in environmental studies and also for the particular case of electro-Fenton

thanks to its distinguished performance for water remediation. Nobel metals

represent interesting materials to be used for water remediation owing to their

resistivity in the very oxidising medium in the electrochemical reactor for

organic contaminants destruction. Platinum is one of the preferred anodes as it

does not leave toxic ions in the solution200,235,306,307. Organics are oxidized

directly on its surface by electron transfer or by hydroxyl radicals generated in

low quantities, or by other oxidants in the bulk. Parasitic reactions restrict the

efficiency of oxidation on anodes too. Beyond a given potential, O2 evolution

prevails greatly, reducing the organics oxidation at the anode.

3.3 Chemical Oxygen Demand

Chemical oxygen demand (COD) is a measure of the capacity of water to

consume oxygen during the decomposition of organic matter and the oxidation

of inorganic chemicals such as ammonia and nitrite. COD measurements are

32

commonly made on samples of waste waters or of natural waters contaminated

by domestic or industrial wastes. Chemical oxygen demand is measured as a

standardized laboratory assay in which a closed water sample is incubated

with a strong chemical oxidant under specific conditions of temperature and

for a particular period of time. A commonly used oxidant in COD assays is

potassium dichromate (K2Cr2O7) which is used in combination with boiling

sulfuric acid (H2SO4). Because this chemical oxidant is not specific to oxygen-

consuming chemicals that are organic or inorganic, both of these sources of

oxygen demand are measured in a COD assay.

Chemical oxygen demand is related to biochemical oxygen demand (BOD),

another standard test for assaying the oxygen-demanding strength of waste

waters. However, biochemical oxygen demand only measures the amount of

oxygen consumed by microbial oxidation and is most relevant to waters rich in

organic matter. It is important to understand that COD and BOD do not

necessarily measure the same types of oxygen consumption. For example,

COD does not measure the oxygen-consuming potential associated with

certain dissolved organic compounds such as acetate. However, acetate can be

metabolized by microorganisms and would therefore be detected in an assay

of BOD. In contrast, the oxygen-consuming potential of cellulose is not

measured during a short-term BOD assay, but it is measured during a COD

test.

This test is widely used to determine:

a) Degree of pollution in water bodies and their self-purification capacity,

b) Efficiency of treatment plants,

c) Pollution loads, and

d) Provides rough idea of Biochemical oxygen demand (BOD) which can be

used to determine sample volume for BOD estimation.

The limitation of the test lies in its inability to differentiate between the

biologically oxidizable and biologically inert material and to find out the

system rate constant of aerobic biological stabilization.

33

CHAPTER 4: EXPERIMENT

4.1 Materials & Methods

Materials Required: -

1. Anode: PbO2

2. Cathode: graphite3. Orange G dye4. Ferrous sulphate5. Na2SO46. UV lamp7. Glassware8. NaOH9. H2SO4

Electrochemical cell:

The electrochemical tests were conducted in a 100 mL open Pyrex_ glass cell vessel. The electrodes were square plates of 3cm2 of graphite and lead dioxide. A minimal overpressure of air was maintained on the opposite side of the cathode electrolyte, which pumped air at a rate of 600 mL min–1 .The operative parameters, including total reaction time up to 360 min, stirring at 700 rpm with a magnetic bar, temperature of 35 _C, initial pH of 3.0 and type of electrodes, current applied remained constant in each case, whereas the dye concentration and UV light application is varied. A Philips TL/6 W/08 fluorescent black light blue tube lamp (320–400 nm with maximum intensity at k = 360 nm) was used for PEF treatments, which supplied a photoionization energy of 5Wm–2 detected with a radiometer placed 7 cm above the solution.

4.2 Procedure:

1. A 500ml glass vessel is taken and 300 ml distil water is added.2. 2.04g of Na2SO4 (.05M) is added to the solution.3. The electrodes are the placed on the beaker and connected to the DC power

source to supply 300mA current and left to run for 1 hours. (For the first time only for electrode activation)

4. Electrodes are taken out and the dye is added as per experimental requirement.5. Conc. H2SO4 is added to adjust the pH to 3.0

34

6. 1mM FeSO4 is added to the system and electrodes are put back in and power is back on.

7. Samples are collected at 5 minutes’ interval for first 6 samples and last sample is taken at the end of 60 mins.

4.3 Analytical Methods

Chemical oxygen demand (COD) determination method:

COD degraded performance in mineralize process was assessed by the formula:

4.4 COD Measurement procedure:

1. Homogenize 500 mL of sample for 2 minutes in a blender2. Turn on the DRB 200 Reactor. Preheat to 150 °C.3. Remove the cap of a COD Digestion Reagent Vial for the appropriate range:

4. Hold the vial at a 45-degree angle. Pipet 2.00 mL (0.2 mL for the 0 to 15,000 mg/L range) ofsample into the vial.

5. Replace the vial cap tightly. Rinse the outside of the COD vial with deionized water and wipethe vial clean with a paper towel.

6. Hold the vial by the cap and over a sink. Invert gently several times to mix the contents. Place the vial in the preheated DRB 200 Reactor.

7. Prepare a blank by repeating Steps 3 to 6, substituting 0.2 mL deionized water for the sample.

8. Heat the vials for 2 hours.9. Turn the reactor off. Wait about 20 minutes for the vials to cool to 120 °C or

less.

35

10. Invert each vial several times while still warm. Place the vials into a rack. Wait until the vialshave cooled to room temperature.

11. Use Colorimetric method,0-15,000 mg/L COD12. Colorimetric Determination, 0 to 15000 mg/L COD13. Enter the stored program number for chemical oxygen demand (COD), high

plus range.Press: PRGM and Enter 17

14. Insert the COD/TNT Adapter into the cell holder by rotating the adapter until it drops intoplace. Then push down to fully insert it.

15. Clean the outside of the blank with a towel.16. Place the blank in the adapter. Push straight down on the top of the vial until it

seats solidly into theAdapter.

17. Tightly cover the vial with the instrument cap.18. Press: ZERO The cursor will move to the right, then the display will show: 0

mg/L COD19. Clean the outside of the sample vial with a towel.20. Place the sample vial in the adapter. Push straight down on the top of the vial

until it seats solidly into the adapter.21. Tightly cover the vial with the instrument cap.22. Press: READ The cursor will move to the right, then the result in mg/L COD

will be displayed.

36

CHAPTER 5: Results

a) Non UV based experiments(Electro Fenton):

1. 100mg/l dye conc.Sl.no, Time(mins.

)%COD removal

1 0 02 5 49.73 10 57.34 15 63.15 20 64.16 25 69.57 30 71.78 60 76

2. 150 mg/l dye conc.

Sl.no, Time(mins.)

%COD removal

1 0 02 5 42.83 10 49.74 15 54.15 20 58.36 25 61.27 30 63.88 60 67

37

3. 200 mg/l dye conc.

Sl.no, Time(mins.)

%COD removal

1 0 02 5 38.63 10 46.14 15 495 20 52.36 25 53.67 30 54.98 60 58.5

0 10 20 30 40 50 60 700

10

20

30

40

50

60

70

80

200 mg/l 150mg/l 100 mg/l

Time (mins)

%CO

D re

mov

al

Electro Fenton Result

38

b) UV based Experiments(Photo Electro Fenton)

1. 100 mg/l dye conc.Sl.no, Time(mins.

)%COD removal

1 0 02 5 47.13 10 59.24 15 65.15 20 68.16 25 71.97 30 74.68 60 81

2. 150 mg/l dye conc.Sl.no, Time(mins.

)%COD removal

1 0 02 5 45.63 10 51.24 15 56.15 20 60.26 25 64.17 30 65.78 60 71

3. 200 mg/l dye conc.Sl.no, Time(mins.

)%COD removal

1 0 02 5 40.13 10 47.24 15 52.75 20 56.16 25 58.2

39

7 30 61.38 60 65

0 10 20 30 40 50 60 700

10

20

30

40

50

60

70

80

90

200 mg/l 150mg/l 100 mg/l

Time(min.)

%CO

D re

mov

al

Photo electro Fenton result

40

CHAPTER 6: CONCLUSIONIt has been shown that a lead dioxide/graphite cell operating at 300 mA is able to accumulate H2O2 concentrations high enough in an acidic solution of pH 3.0 to degrade a solution with 100-200 mg/L of Orange G by EF and EF. The large production of °OH from Fenton’s reaction in both coupled processes caused a fast degradation. Final pH of the solution was <3.0 because of production of SO4

2-.

41

Chapter 7: Future work

In continuation to the above study, following are the prospects:

a) Investigation of decolourization efficiency

b) Mineralization study

c) BOD study

d) Investigation of effluents discharged from the textile industry.

42

CHAPTER 8: REFERENCES

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electro-Fenton technique modified by Fe2O3 nanoparticles. Journal of

Environmental Chemical Engineering Volume 4, Issue 2, June 2016, Pages

2342–2349

2. Gengbo Ren, Minghua Zhou, Mengmeng Liu, Liang Ma, Huijia Yang. A

novel vertical-flow electro-Fenton reactor for organic wastewater treatment.

Chemical Engineering Journal Volume 298, 15 August 2016, Pages 55–67

3. Peiying Liang, Matthieu Rivallin, Sophie Cerneaux, Stella Lacour, Eddy

Petit, Marc Cretin. Coupling cathodic Electro-Fenton reaction to membrane

filtration for AO7 dye degradation: A successful feasibility study. Journal of

Membrane Science Volume 510, 15 July 2016, Pages 182–190

4. Alejandro Bedolla-Guzmana, Ignasi Sirésb, Abdoulaye Thiamb, Juan

Manuel Peralta-Hernández, Silvia Gutiérrez-Granadosa, Enric Brillas.

Application of anodic oxidation, electro-Fenton and UVA photoelectro-Fenton

to decolorize and mineralize acidic solutions of Reactive Yellow 160 azo dye.

Electrochimica Acta Volume 206, 10 July 2016, Pages 307–316

5. Elisabetta Petrucci, Anna Da Pozzo, Luca Di Palma. On the ability to

electrogenerate hydrogen peroxide and to regenerate ferrous ions of three

selected carbon-based cathodes for electro-Fenton processes. Chemical

Engineering Journal Volume 283, 1 January 2016, Pages 750–758

6. Gabriel F. Pereira, Abdellatif El-Ghenymy, Abdoulaye Thiam, Carlos

Carlesi, Katlin I.B. Eguiluz, Giancarlo R. Salazar-Banda, Enric Brillas.

Effective removal of Orange-G azo dye from water by electro-Fenton and

Photoelectron-Fenton processes using a boron-doped diamond anode.

Separation and Purification Technology 160 (2016) 145–151

7. Hannah Rotha, Youri Gendelb, Pompilia Buzatua, Oana Davidb, Matthias

Wessling. Tubular carbon nanotube-based gas diffusion electrode removes

43

persistent organic pollutants by a cyclic adsorption – Electro-Fenton process.

Journal of Hazardous Materials Volume 307, 15 April 2016, Pages 1–6

44