ahmedabad - national metallurgical laboratorylibrary.nmlindia.org/fulltext/2007/289-1.pdf ·...

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Final Report

Treatment of Textile Effluents byElectrochemical techniques

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GMDC Science and Research CentreAhmedabad

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National Metallurgical Laboratory(Madras Centre)

CSIR Madras Complex, TaramaniChennai-600113 -

Email:[email protected]: 044-22542077, Fax: 044-22541027

October 2007

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Final Report

Treatment of Textile Effluents byElectrochemical techniques

Sponsored byGMDC Science and Research Centre

Ahmedabad

National Metallurgical Laboratory(Madras Centre)

CSIR Madras Complex, TaramaniChennai-600113 .

Email:[email protected]: 044-22542077, Fax: 044-22541027

October 2007

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CONTENTS

SINO. PAGE NO.SUBJECT

1. Introduction 1

2. Electrochemical cell and experimental methods 15

(i) Electrochemical Cell 15

(ii) Effluent sample 18

(iii) Analytical techniques 18

3. Treatment of effluents from millmanufacturing synthetic polyester fabric (Surat). 19

(i) Electrocoagulation 19(ii) Electrooxidation 22

(iii) Effect of current density 26

4. Treatment of common effluent of textile industries (Tirupur). 34

(i) Electrocoagulation 35

'" (ii) Electrooxidation 39--'

(iii) Effect of current density 42(iv) Effect of anode materials 43

5. Electrooxidation of dyebath wastewater. 49

(i) Effect of anode materials 50(ii) Effect of current efficiency 56(iii) Energy consumption 57

6. Removal of dyes by electrocoagulation 59

(i) Effect of current density 61(ii) Spectral Study 64

7. Summary and Conclusion 79..-

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National Metallurgical Laboratory - Madras Centre

INTRODUCTION

Textile industries are the most polluting sector in terms of the volume and complexity of

its effluent discharge. Textile wastewater comprises a large variety of dyes and other"-

auxiliary chemicals. Most of wastewater from textile industry is generated from dyeing

and finishing operations. In a typical textile industry approximately 1m3 of water is

consumed per ton of cloth processed. Considerable variations are expected in the textile

~

wastewater, as the dyeing process in these industries is batch process. Textile wastewater

is characterized by high Chemical Oxygen Demand (COD), Biological Oxygen Demand

(BOD), Total Organic Compound (TOC), Total Suspended Solids (TSS), Total Dissolved

Solids (TDS) and strong colour, which should be treated before discharging it into the

environment.

..-

Dyes are generally small molecules comprising two key components: the

chromophore, responsible for the colour and the functional group, which bonds the

dye to the fiber. Fastness of color refers to its ability to remain unchanged. Different

dyes of different colors have different degrees of fastness to various conditions. The

adsorption and retention of the dye inside the fiber can be chemical, physical or both,

depending on the textile fiber and dye. The adsorptive strength is controlled by several

factors such as time, temperature, pH and auxiliary chemicals. The effluent

composition is further complicated by the fact that dyes belonging to different

....

chemical classes may be used for a single dyeing operation. The chemical load on

dyeing effluents varies with the chemistry of the process and the mode of operation:

batch or continuous. Values of the liquor ratio and dye exhaustion can be very

different according to the method employed and dyestuff used.

Classification of Dyes

The Colour Index (CI) number, developed by the Society of Dyers and Colourists is

used for dye classification. Once the chemical structure of a dye is known, a five digit

CI number is assigned to it. The first word is the dye classification and the second

word is the hue or shade of the dye. Dyes are broadly classified as natural and...

1

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synthetic dyes. The natural dyes are obtained from flowers, nuts, and berries and from

mineral and animal sources. Some of natural dyes are saffron, indigo and Prussian

'-

blue. The synthetic dyes are produced from variety of chemical substances. According

to their use the dyes are classified as:

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Acetate rayon dye

It is used for cellulose acetate and some synthetic fibers.

Direct dyes

They are azo dyes, and sodium slats, fixing agents, and metallic complexes that are

used generally on cotton-wool, cotton-silk combination. When a dye colors fabric

directly with one operation of impregnation, without the aid of an affixing agent, the

dye is said to be a 'direct dye' for that fiber.

Azoic dyes

The azoic dye contain the azo groups; especially applicable to cotton.

Cationic Dyes

....

The cationic dyes are used with a mordant for cotton and polyesters. The chemical

agent that binds the dye to a fiber, which has no affinity for the dye, is known as a

mordant.

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Basic Dyes

The basic dyes are cationic type with chromophores typically having amino groups.

Acid Dyes

The acid dyes are water soluble anionic dyes with different chromophore groups

substituted with acidic functional groups such as nitro-, carboxyl- and sulfonic acid.

By adding sulfonic group, the water insoluble dye becomes soluble.

Acid-Premetalized Dyes

They require a strong acid bath to get the colour into the fabric. The metal improves

the color fastness.

Neutral -Premetalized Dyes

They have one metal atom usually chromium, bound to two molecules of dyes to

improve fastness properties.'"

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National MetallurgicalLaboratory - Madras Centre

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DisperseDyes

These dyes were developed for dyeing acetate fibers. While they are not soluble in

water, they are supplied in a finely ground form that will disperse in water. The

particles will dissolve in the fibers and by these actions, the fabric is dyed.

Sulfur Dyes~

Sulfur dyes are insoluble in water. Sulfur dyes produce dull colors, such as navy,

brown and black. They are susceptible to chlorine and light.

Vat Dye

The first synthetic vat dye was indigo. Vat dyes are not only resistant to light and acids

and alkalis, but are also equally resistant to strong oxidizing bleaches. They are

insoluble pigments; but they are made soluble in water by the use of a strong reducing

agent. The fabric is immersed in this solution. Subsequent exposure to air or

immersion in an oxidizing bath restores the dye to its insoluble form, as a part of the

,fiber.

Chrome Dyes

.. They are metallic salt formed directly on the fiber by the use of aluminum, chromium

or iron that cause precipitation.

Reactive Dyes

They form chemical compounds with fiber molecules. It is applicable for cellulose,

silk, wool, nylon, acrylics.

Structures of some Dyes

Indigo~o ~

~r=j-( ~Acid violet 1

.~t-q ,,0If ~Q-I

:o-"g- 0

~ ~ N+/j LN= - 0

N-i2 3

fA

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Direct Red 7 /

H2N ?o. H2N "

cQr~~N=N I: N=N ~~~

~~ I~ ~~~

O=~=O O=~OOH OH

Vat Orange 9

~IOI~::::::,... ::;:::;

II I'l~ll"~~

0

Disperse Blue 19

Table 1: Some regular chemicals used in textile industry

4

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Description Chemicals Function

Salts NaC!, Na2S04 To adjust iep of the fiberAcids Acetic acid and H2SO4 pH controlBases NaOH, Na2C03 pH controlBuffers Phosphate pH control

Sequestering agents EDTA Complex hardness; Retarder

Oxidizing agents H202, NaN02 Insolubilise dyes

Reducing agents Sod. Hydrosulphite, Na2S Solublise dyes

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General characteristics of textile wastewaters:

Table 2: Physicochemical characteristics of wastewater generated fromdifferent streams of cotton mill

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Table 3: Concentration of various heavy metals in different streams

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Table 4: Effluents characteristics of various textile mills

5

Source of Appearance pH COD Color TS SS TDSwaste water mgL-1 (Hazen mgL-l mgL-1 mgL-1

Units)De-sizing Turbid, blue 7.9 1180 2085 4880 179 4701Scouring Turbid, brown 10.8 475 1063 4090 130 3952Bleaching Turbid, white 8.7 185 812 886 101 785

Mercerizing Cloudy, creamy 10.2 1740 1436 2266 186 2080Dyeing Dark, blueblack 10.2 763 5882 3197 331 2066Combined Dark blue black 9.8 1100 4010 3870 255 3615Utilities Turbid, mixed 7.5 849 716 644 219 425

Source of CdL+ Cr CuL+ Fej+ NiL+ Pb+ ZnL+

wastewater (total)De-sizing 0.17 0.11 0.13 40.5 0.60Scouring 0.11 0.03 0.19 16.80 0.14 0.80Bleaching 0.60 47.20 0.29 0.13 0.22Mercerizing 0.29 0.53 0.25 31.83 0.67 0.34 0.21Dyeing 0.19 0.67 0.59 51.57 0.38 0.11 0.44Combined 0.09 0.71 0.62 40.40 0.39 0.32 0.34Utilities 0.11 78.0 0.19

Parameters I CottonI Wooen I Syntetic I

AcidicI Viscose I Combinedtextile textIle textIle waste rayon waste

waste9-12 9-10.5 6.5-.0 1-2 9-11 3-41400-1700 2100-2700 500-800 400-800 150-1150 50-2006000-7000 3000-5000 2500-3500 27000-32000 1500-3000 1600-2800200-350 100-150 150-200 800-1050 500-700 200-250.

I Tn (nnm'\ I4500-5500 2600-2900 2200-3200 26000-31000 1000-2200 1500-2500-

175-200 10-20I vllt) \ VVlUJ I 12-30 14-10 110-20-

*.......

National MetallurgicalLaboratory - Madras Centre

Table 5: General standards for discharge of effluents

Parameter.. "-""""' ' "-"'-""--

and surfacewater

jSuspended solids!Imax. (ppm) .

!Particle size---;;i[ 850 mIcronjsuspended solids !Sieve. ~_._;: ~._~_..

value i 5.5 to 9.0" ' ",-", ,...............-.....-................

JOil and grease max.! 10!(ppm) ,

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esidual chlorine. 1.0

ax~.~21I1) 0''''~'''~''.''~''''''''''''~'''-~~''--~~'' i

mo. Nitrogen (as i 50) max. (Pl'm~ :

gen (as,

30

250

0.2

0.01

0.12.0

0.1-2.0

3.0

5.0

0.05

3.0

0.2.~2.0

(ppm) 5.()

hide(ppm) 2.0 - - 5.0olics(ppm) i 1.0 - 5.0ganese(ppm) 2.0 - 2.0

s Fe(ppm): 3.0 - 3.0

~~'(pp~ii:::::::::~~="":O:2 - 0.2,.~(ppm) 10 - - 20

* These standards shall be applicable for industries, operations or processes other than those industries,operations or process for which standards have been specified in Schedule of the Environment Protection

Rules, 1989.

onia (as!. (ppm) i

3.max. (ppm) !

- --.-

(a)100

1.0

---""" "-- ---,-",,,"-"-"'''' ''''''''''-' ""' '.-.'. ................

50 50

100 100

5.0 5.0

350 100

250

0.2

0.01

2.0

2.0

1.0

2.0

3.0

15

0.05

5.0

0.2

15

100

0.2

0.2

6

Land for Marine/ coastal areas

:we.:.L._.l!!..._---,-_....__.._...- .. -.,..... ...--...(b) 1 (c)

solids 3 mm!solids, max 856 microns i

. to 9.0 5.5 to 9.0

10 20

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Wastewater treatment:

The conventional method of textile wastewater treatment consists of chemical

coagulation, biological treatment followed by activated carbon adsorption. While the

conventional coagulation process generates huge volume of sludge, the efficiency of

biological treatment is low because some high molecular weight dyestuffs are

biologically inert.

The advanced oxidation processes such as cavitations, photo-catalysis,' Fenton's process

and the usage of hydrogen peroxide and ozone were tried and the success was found to be

limited. Hybrid processes such as UVlHzOz, Ultrasound! HzOz,Ultrasound! 03, UV/D3,

UV1031HzOz and sono photo catalytic oxidation were also tried but failed to attract

commercial implementation. The continuous depletion of ground water and shortage in

rain fall has necessitated the recycling and reuse of processed water. Therefore new

electrochemical technologies such as Electro-coagulation, electro-flotation and electro-

oxidation were suggested to detoxify the textile wastewater.

The electrochemical methods are effective for the treatment of different effluents

compared to conventional methods. Above all, the electrochemical reactors are compact,

simple and the rate of pollutants removal is very rapid. It was observed that the electro-. .

coagulation is very effective for the de-colorization of Orange II dye and purification of

textile wastewater. The combination of electro-coagulation and electro-oxidation in the

presence of granulated activated carbon packed between stainless steel electrodes was

tried for the total removal of COD from dye wastewater. Activated carbon fiber (ACF),

granulated graphite as cathode for the reduction of vat dyes was also attempted. Though

the carbon electrodes are proved to be effective, their utility in industrial practice was

poor due to low mechanical strength. Alternatively, boron doped diamond electrodes

were developed to overcome the above problem. The electrochemical degradation of

Acid orange 7 and Amaranth dye was studied using boron doped diamond electrodes.

Though these electrodes were proved to be effective, their commercial viability is yet to

be ascertained. The Ti based electrodes were tried due to their stability and long life. The

7

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removal of COD to the extent of 80% from textile was~ewaterusing PtiTi electrode was

reported. The effectiveness of Ti/Ru02, Ti/Pt and Ti/PtlIr for the treatment of textile

wastewater was studied and found that the COD and DOC of the effluents could be

reduced to the extent of 85%. Of the three electrodes tested, the efficiency of organics

removal was found to follow the order: Ti/Ru02 > Ti/Pt > and Ti/PtlIr. The

electrochemical oxidation of organic substances was attributed due to ocr, OR-, nascent

oxygen and other reactive species generated during electrolysis. In the present work, the

effectiveness of electrochemical methods in removing pollutants of textile industry was

studied. The effluents from a textile mill that manufacture synthetic polyester cloth

(Surat, Gujarat) and composite wastewaters generated from the cluster of textile units

(Tirupur, Tamil Nadu) and wastewater of dye bath were processed and the effectiveness

of electrochemical treatment was assessed.

Theoretical background of electrochemical techniques

Electro-coagulation:

It is generally accepted that coagulation is brought about primarily by reduction of net

surface charge to a point where colloidal particles previously stabilized by electrostatic

repulsion can approach closely enough for Vanderwaal' s forces to hold them together and

allow aggregation. The reduction of surface charge is a consequence of the decrease of

the repulsive potential of the electrical double layer by the presence of an electrolyte

having an opposite charge. Electrocoagulation is an electrochemical process where

charged solids are neutralized either at the surface of the electrodes or with the coagulant

generated in-situ by electrolytic oxidation of an appropriate anode material. In most of

the cases either aluminum or iron electrodes are used as anodes to improve the

coagulation of nano and colloidal particles. The electrocoagulation process has an

advantage of removing the smallest colloidal particles because the applied electric field

sets them in faster motion there by facilitating collision between oppositely chargedsolids.

8

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The anodic dissolution of Al and Fe electrodes can be represented as:

Anode: 2Alo --+ 2A13++ 6e-

Cathode: 6H2O + 6e-

2AI + 6H2O

--+ 3H2+ 60H-

--+ 2AI (OH)3+ 3H2 (alkaline conditions)

--+ 2AI (OH)3+ 6H+ (acidic conditions)

Net reaction:

2AI + 6H2O

Similarly the anodic dissolution of iron can be represented as

Anode Feo --+ Fe2++ 2e-

2H2O+ 2e- --+ H2+ 20H-

Feo+ 2H2O --+ Fe (OH)2+ H2(alkaline conditions)

4Fe2++ IOH2O+ O2 --+ 4Fe(OH)3 + 8H+

Below pH 4.0 iron iron can be oxidized to

Feo --+ Fe3++ 3e-

In aqueous solution, iron also can under go hydrolysis reactions

Feo+ 6H2O --+ Fe (H2O)4(OH)2(aq)+ 2H++ 2e-

Feo+ 6HzO --+ Fe (H2O)3(OH)3(aq)+ 3H++ 3e-

The Fe (III) is subsequently forms floc

Cathode

Net reaction

Fe (H2O)3(OHh(aq)--+

2Fe (H2O)3(OH)3(aq)--+

Fe (H2O)3(OH)3(s)

Fe203 (H2O)6

The above reaction is predominant between pH 4.00-7.00. In the pH range of 7-8, iron

hydroxide floc is formed due to polymerization. The de-hydrated iron oxides of different

compounds are also expected according to the following

2Fe (OH)3 --+

Fe (OHh --+

2Fe (OH)3+ Fe (OH)2--+

2Fe (OH)3 --+

Fe203+ 3H2O (Hematite, Maghemite)

FeO + H2O

Fe304 + 4H2O (Magnetite)

FeOOH + H2O (Goethite and Lepidocrocite)

.

9

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Depending on the pH of aqueous solution, the hydrolys~sof aluminum ion (AI3+)initially

leads to mono nuclear complexes as described below.

A13++ H2O -. AIOH2++ H+

AIOH2++ H2O -. AI(OH)2++H+

AI(OH)z++H2O -. AI(OH)3+ H+

AI(OH)3+H2O -. AI(OHh-+ H+

The extent of hydrolysis depends on the total metal concentration, pH as well as amount

of other species present. The solubility of AI(OH)3 is minimum (0.03 ppm) at pH 6.3.

The solubility increases as the solution become more acidic and alkaline. The poly

" nuclear complexes such as AI(OH)n(3-n), Alz(OH)24+ Ah(OH) 174+, AI13(OH)345+,

Ah(OH)l+, AI(OH)63- and other complexes with positive and negative also are formed

that are useful for coagulation.

The suspended solids, emulsified organics and other soluble organic compounds that are

capable to form insoluble complexes with metal ions could be removed during

electrocoagulation. The dissolved organic compounds that contribute to COD could be

coagulated after complexation with metal ions.

=Fe-OH + OH-R

=Fe-OH + OK

=Fe-O-R + H2O

=Fe-OR + OH-

-. (Condensation mechanism)

(Ion exchange mechanism)-.The de-stabilization of suspended solids is achieved by

1) Compression of the diffused double layer around the charged species

2) Charge neutralization of ionic species by counter ions generated by anodic

dissolution. The counter ions helps in reducing the inter particle repulsion so that

VanderWaal's attraction predominates, causing coagulation.

Although electrocoagulation mechanism resembles chemical coagulation, the

characteristics of electrocoagulated floc differ dramatically from that of chemical

coagulation. An electrocoagulated contain less bound water and is readily filterable.

Secondary pollution could be avoided. The gas bubbles produced during electrolysis can

separate the suspended colloidal particles from the aqueous part. Above all, the

equipment is simple and easy to operate. Wastewater treated by electrocoagulation results

10

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in clear, odorless and palatable water. Sludge generated is comparatively less and

amenable to faster filtration. The removal of total dissolved solids is much more than

chemical coagulation.

Electroflotation:

"Flotation by electrically generated bubbles as a means of separating solids from liquid

phase". This technique is particularly effective to separate low density solids, oily

emulsions such as fats, proteins and solids coated with hydrophobic surfactants and fatty

acids. The hydrophilic solid pollutants will become partially/fully hydrophobic by the

adsorption of surfactants and other organic molecules. Such solids tend to float/suspend

in aqueous system even after coagulation. In such circumstances, electroflotation is more

apt technique compared to coagulation. Conventional sedimentation requires long

residence time and thus need large sedimentation tanks. Large volumes of very dilute

sludge which must be removed from the bottom of sedimentation tank at frequent

intervals. If de-sludging is not proper, the whole settling system can go "septic" with

noticeable environmental consequences. Hence separation by flotation is appropriate

alternative for the treatment of effluents containing 1°'Y density solids. Finely dispersed

gas bubbles of oxygen and hydrogen ranging from 10-80 microns can be generated by

adopting stable and insoluble electrodes.

Anode: 2H2O --. 02(g)+ 4H++ 4e-

Cathode 2H2O+ 2e- --. H2(g)+ 20H-

The bubbles generated by conventional techniques are not suitable to separate colloidal

particles. In most of the operations, diffusers are used to generate fine bubbles. The size

of the bubbles emanating from these diffusers is so large making the conventional

flotation process ineffective. From the hydrodynamic point of view, the size of the bubble

-

should be approximately in the same order to that of solid particles. The bubbles

generated by conventional diffusers will be larger than 700 microns where as dispersed

solids in the effluents are usually 0.01 to 100 microns. Since the collision and collection

efficiencies depends on the ratio (dp/dbt with exponent usually in the range of 1.5-2.0,

-

11

:i~~National Metallurgical Laboratory - Madras Centre

electroflotation will be more effective to float fine solids. The dp and db denotes diameter

of particle and bubble.

The advantages of electroflotation compared to conventional flotation are:

I) High dispersion of electrolytic bubbles

2) Uniform distribution of bubbles over the whole mass (approximately 200 billion

bubbles per meter square of electrode surface.

3) Weak coalescence (stable bubbles)

4) High capillarypressure inside electrolyticbubbles ( ~ 1.45xl06 Pa) so that they

can form three phase wetting even on hydrophilic solids.

5) Mild hydrodynamic regimes of mass transfer (turbulence due to bubbles in the

range of 0.01-0.30 Reynolds number).

6) High physico-chemical activity. Due to nascent nature of bubbles oxidation and

reduction are very effective.

7) Contact charging of the particle surface.

Electro-oxidation:

Electro-oxidation is one of the most efficient methods for the mineralization of dissolved

organic pollutants. The simultaneous oxidation and reduction of organic pollutants is an

advantage of this process. Since the pollutants are mineralized no sludge is expected. The

oxidation of organics proceeds by two different mechanisms viz direct and indirect.

i) Direct Oxidation: Oxidation of pollutants on anodes can be accomplished by directly

on the surface of the electrodes due to redox transformations.

ii) Indirect Oxidation: The noble oxide coated catalytic anodes such as Ti02, Ir02, Ru02

and Ta02 (MOx) forms higher oxides (MOx+l)' In the first step, H2O is discharged at the

anode to produce physically adsorbed "active oxygen" (adsorbed OH*radicals) according

-

to the reaction (Ref: M.Panizza et aI., Environ. Sci. Technol, 38,2004,5470:

Ch.Comninellis and A. Nerini, J. Applied Electrochem, 25,1995,23)* +

MOx+ H2O ---+ MOx( OH) + H + e-* +

MOx( OH) ---+ MOx+\+ H + e-

::

At the anode surface the "active oxygen" can be present in two states either as

physisorbed (adsorbed OH*radicals) or land as chemisorbed (oxygen in the oxide lattice,

12

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(MOx+J)). In the absence of any oxidisable organic~, the "active oxygen" produces

dioxygen according to the following reactions.* 1 +-

MOx( OH) MOx+ h02 + H + e

MOx+1 MOx + Ih02

When NaCI is used as supporting electrolyte, cr may react with MOx(OH) to form

adsorbed OCI radicals according to the following reaction

MOx(OH) + cr MOx(OCI) + H++ e"

Further in the presence of cr, the adsorbed hypochlorite radical may interact with the

oxygen already present in the oxide anode with possible transition of oxygen from the

adsorbed hypochlorite radical to the oxide forming the higher oxide MOx+1according to

the following reactions.

MOx( OCI) + cr

MOx(OCI) + cr

MOx+! + H+ + e"

MO + 1/202 + 1/2Ch + e"

i

aret xeIII-

CI:., CI: rn! WI

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i!

IIoaodIIy11",

cr r-a' XeIII-

C1:f-t' C1: 00:!ii

<»1:°\'1

II~ CO:IIua8d8Iy..BuDt BuDt

Scheme of electrochemical oxidation of organic pollutants in the presence of chloride.

In addition to above, oxygen and chlorine liberated from the anode combine to form

different oxidizing agents according to the following reactions

2H2O ---+ 02(g)+ 4W + 4e-

3H2O ---+ 03(g)+6H+ + 6e-l/202+H2O ---+ H202

Fe2++ H202 ---+ Fe3++ OH"+ OH*

2Cr ---+ Ch + 2e-

Ch +H2O ---+ HCI + HOCICa2++ HOCI ---+ CaOCI + H+

13

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Different electrode materials such as SnOz, TiiSnOz, TilIrOz, Ti/RuOz, Ti/TaOz,

Ti/RuOz/TaOzlIrOz, and Titanium based IrOx-SbzOs-SnOz, Sb doped SnOz coated

titanium, Boron doped diamond, TilPt, TilPt-Ir, TilPbOz, graphite and carbon were tried

for the electrochemical oxidation of organic pollutants.

In the present study, three different wastewater samples were collected and processed by

electrochemical techniques.

(I) Effluents of a mill that manufacture synthetic polyester cloth,

(2) Effluents generated from a textile cluster

(3) Wastewater of Dye bath were collected and processed by electrocoagulation and

electrooxidation.

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14

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ELECTROCHEMICAL CELLS AND EXPERIMENTAL METHODS

Electrochemical cells:

Acrylic tank with a working volume of about three litres, fitted with electrodes was used

to conduct the experiments. Mild steel and aluminum rods, with a minimum purity of

98% and each rod measuring 0.6 em diameter and 11 cm length were used as electrodes

for electrocoagulation. Six such rods, connected to a common rod, formed the anode

assembly and an equal number of rods with similar arrangement formed the cathode

assembly. The gap between the anode and cathode was maintained at 2 mm to minimize

the ohmic losses. The entire electrode assembly (undivided electrolytic cell) was placed

on nonconducting wedges fixed to the bottom plate of the electrocoagulation tank. For

electrooxidation, different materials, viz. triple-oxide-coated-titanium rods and graphite

in the form of sheets and also rods were used. The gap between anodes and cathodes was

maintained at 6 mm. The arrangement of the electrodes, both for electrocoagulation!

flotation and electrooxidation, are shown in Figures.

Electro-coagulation / flotation cell

~--------

Electro-oxidation cell withTriple oxide coated Ti as electrodes

15

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Electro-oxidation cellCarbon rods as electrodes Electro-oxidation cell

graphite sheets as electrodes

, Fig 4: Electro-oxidation cellgraphite rods as electrodes

16

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*......

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National Metallurgical Laboratory- Madras Centre

Batch scale experimental set-up

Continuous scale experimental set-up

17

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Effluent sample

The representative wastewater sample was collected over a period of 12hours.

.. Analytical techniques

The aqueous solutions resulting from electrochemical treatment were analyzed for COD,

biological oxygen demand (BODs), suspended solids, chlorides, Total Kjeldahl Nitrogen

(TKN), water hardness and sulfate according to the standard methods suggested by

American Public Health Association. COD was estimated by open reflux method. The

sample was refluxed in an acidic medium with a known excess of potassium dichromate

and the remaining dichromate was titrated with ferrous ammonium sulfate. The

conductivity and suspended solids were estimated using conductivity meter and particle

size analyzer (CILAS 1180, France) respectively.

~,

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18

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TREATMENT OF EFFLUENTS FROM A MILL MANUFACTURINGSYNTHETIC POLYESTER FABRIC (Surat)

The effectiveness of electrochemical methods for the treatment of wastewater from a

textile industry manufacturing 2400 tons per annum of synthetic polyester cloth was

attempted in the present study. The characteristics of raw effluent are presented in table I.

Table I: Physico-chemical characteristics of the raw effluent and

electrocoagulated sample

*Electrocoagulation time: 5 min, Current density: 30 mA cm-2and Aluminumelectrode

Electrocoagulation

In the first stage, the sample was subjected to electrocoagulation to remove the suspended

solids and the COD therein. Experimental results on the kinetics of COD removal at

different current densities are presented in Table 2.

19

Parameter Raw effluentAfter

electrocoagulation *Colour Blackish Pale yellowpH 7.3 8.2

Conductivity (Jlmhos cm:l) 4490 4290

BODs (mg rl) 182 70

COD (mg rl) 1316 428

Total Solids (mg rl) 5104 4208

Total Dissolved Solids (mg rl) 4274 4190

Suspended solids (SS) (mg rl) 830 10

Total Hardness as CaC03 (mg rl) 1500 590

Ca2+(mg rl) 240 92

Mg2+(mg rl) 219 87

Total alkalinity as CaC03 (mg rl) 950 530

Chloride (mg rl) 922 850

Sulphate (mg rl) 1080 960

TKN (mg rl) 70 47.3

Mean size of SS (Jlm) 10.14 -

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Table 2: Effect of current density and anode .materials on COD removal byelectrocoagulation

-

. From the results, it could be seen that the aluminum as anode was more effective

compared to mild steel. The suspended solids were removed to the extent of 99 % from

its initial concentration of 830 mg L'l within 5 minutes. The size distribution of the

suspended solids of the effluent is shown in Fig. 1. From the distribution curve, it is

apparent that the size of the suspended solids varies from 0.04 to 40 ~m with a mean size

-

of 10.14 ~m. Nearly 6% (by volume) of the solids are colloidal that is below 1 /-lm.The

Ae+ or Fe2+ions released by anodic dissolution are expected to destabilize the suspended

solids that lead to coagulation. It is known that the Ae+/ Fe2+ions released from the

anode in turn form oxy-hydroxides, which are good coagulants. In addition to suspended

solids, the dissolved organics that can form insoluble complexes with Fe2+/Fe3+ions

could also be removed during electrocoagulation. The COD removal could be interpreted

either due to the adsorption of dye molecules on oxy-hydroxide or due to interaction of

metal ions forming insoluble metal-dye precipitate or by both mechanisms. In this case,

the COD of the effluent was reduced to 500 mg L'l from its initial concentration of 1316

mg L'l during electrocoagulation.

20

COD (mg rl) COD (mg rl)Aluminum as anode Mild steel as anode

Time Current density (mA cm'2) Current density (mA cm,2)(minutes)

30 24 18 12 30 24 18 12

5 494 503 520 551 612 670 694 702

10 494 503 520 551 595 673 710 740

15 494 503 520 551 - 674 710 793

30 480 489 505 516 579 659 667 793

45 467 461 505 516 - - - 769

60 447 449 498 516 - - - 753

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