be final project
DESCRIPTION
thesis on dye removal by electrofenton and photo electro fenton- dye removalTRANSCRIPT
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.
5
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)
10
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
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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.
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44