Module-5

NANO MATERIALS

5.11 INTRODUCTION:
Nanotechnology is an interdisciplinary science involving chemistry,
physics, material science, biology and medicine. In broad sense, nanotechnology is
the study of all the phenomena and processes involved in the synthesis, properties
and applications of nanostructures and nanomaterials.

Now days, every one uses the term ´nano´ for anything which is small. But
in chemistry, a nanomaterial refers to a material with at least one of its dimension
is nanoscale. That is size of nanomaterial various from one nanometre (10-9m, one
billionth of a meter) to hundred nanometres. Every nanomaterials has three -
dimensional structures of nano size; they are classified into zero-, one-, and two-
dimensional materials. Thus classification is based on the restriction imposed by
the size of nanomaterials to the flow of charge carries. Nanoparticles, nano
clusters and nano crystals are called as zero dimensional nanomaterials, because
movement of charge carries in all three directions. While , in case of nanorods,
nanowires, nanotubes movement of charge carries in two directions and hence they
are called as one dimensional nanomaterials. In case of nanofilims, charge carries
are free to move in two directions but confined in only one directions and they are
called two dimensional nanomaterials.
For a chemist, nanotechnology is relatively new, but materials at nanoscale
are not at all news. From centuries chemist have dealt with molecules and ions in
solution, colloidal dispersion, many catalysts, which are all in nanometer scale.
The invention and development of scanning tunneling microscopy (STM), atomic
force microscopy (AFM), transition electron microscopy (TEM) have opened up

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 new possibilities for the characterization, measurement and manipulation of
nanostructures and nanomaterials.
Size dependent properties of nanomaterials
Nowadays, we have very much of importance only to nanomaterials, but not
materials
of their size like micrometer or even smaller than nanometer. Because, materials in
the micrometer scale exhibit physical properties almost same as that of bulk form.
However, materials in the nanometer scale exhibit physical properties distinctively
different from the bulk. For example, melting point of gold remains same whether
the size of gold is 1 gram or 1Kg and a piece of gold is golden in colour however
big or small it is. But in nanoscale range, a material with same structure and
composition will show different properties. A colloid of gold nanoparticles is no
longer ´golden´ but ruby red in colour. Hence only the materials in the nanosize
range exhibit these size dependent properties, because a transition from atoms or
molecules to bulk form takes place this size range. Nanomaterials exhibit several
size dependent properties, such has surface area, electrical properties, optical
properties.

OBJECTIVES:

➢ Students to learn the meaning of Nano materials and its synthesis

➢ Students to know the advantages and applications of Nano materials
➢ Students develops or synthesis new nano materials

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 ➢ To learn this Nano materials applications and it helpful for students
future
➢ Over viewing of synthesis, properties and applications of nanomaterials.

5.12 Synthesis of nano-materials

5.12.1 Sol-Gel process: - Sol-gel technique 8s an important bottom-up approach

for the synthesis of nanomaterials and it consist of a following steps
1. Preparation of sol
2. Conversion of sol to gel
3. Aging of a gel
4. Removal of solvent 5. Heat treatment

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 In Sol-gel synthesis, either a metal salt or metal alkoxide is used as precursor
(starting reactions) to synthesize nanoparticles of a metal oxide. First, a sol is
prepared by dispersing precursors in a solvent. Sol is further converted into a gel
by hydrolysis and condensation of precursors. Hydrolysis and condensation
reactions are initiated by addition of an acid or base catalyst and are complex
multiple - step processes. They occur sequentially and in parallel forming a gel.
Simple hydrolysis and condensation reaction is

Hydrolysis : - M (OC2H5)4 + X H2O → M (OC2H5)4 - X (OH)X + X C2H5OH

5.12.3 Condensation: - M (OC2H5)4 - X (OH)X + M (OC2H5)4 - X (OH)X →

(OC2H5)4 - X (OH)X - 1
M - O - M (OH)X - 1 (OC2H5)4 - X + H2O

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 Gel an aging for a known period of time, finally condenses to nanoscale
clusters of metal hydroxides. The encapsulated liquid can be removed from gel by
evaporative drying. The obtained sample is heated at high temperature from
nanoparticles.

Note: - A sol is a colloidal suspension of small solid particles in a solvent. A gel is

a state where both liquid and solid are dispersed in each other and it is a solid
net`work capturing a liquid.
An example for sol-gel synthesis of nanorods is given below
TiO2 nanorods are prepared by sol-gel method using anodic aluminium
oxide(AAO). A solution is obtained by dissolving titanium isopropoxide in ethanol
and solution containing acetyl acetone dissolved in a mixture of ethanol and water
is mixtued with each other to form sol. Then anodic aluminium oxide membrane is
dipped in this sol. A required volume of HCl is added to control hydrolysis and
condensation reaction resulting in gel. Then, the sample is dried by evaporation
and anodic aluminium oxide is removed by washing with NaOH solution. Nano
rods of TiO2 are obtained by heating at temperature 400oC for 24 hours.
The main advantages of sol-gel process are
1. Nanomaterials of high purity with homogeneity can be obtained.
2. Samples can be prepared at lower temperatures.

5.12.4 Precipitation synthesis: - A precipitating agent like NaOH, NH4OH or

Na2CO3 is added to precursor solution, it changes the PH and causes condensation
of precursors. It also introduces a additional ions into the system. Thus
concentration of solution increases and reaches a critical level called as
supersaturation. At this concentration, nucleus formation is initiated and nucleus
further grows into particles, which gets precipitated. Particle size of the product

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 depends upon the rate of attainment of super saturation and rate of nucleation. If a
solution attains supersaturation level slowly and nucleation occurs longer period
then the precipitated with wide particle size is formed. On the other hand, if
solution reaches supersaturation rapidly nucleation occurs suddenly then nucleus
burst into small size of particles. The product obtained filtered, washed counter
anions of metal salt like nitrate or acetate which are readily decomposed and
finally due to nanoparticles. Precipitation synthesis can be used to prepare
nanoparticles of metal oxides, metal sulphides and metals.

5.12.5 Gas condensation: - Gas condensation is one of the simplest technique for
the production of meta nanoparticles. In this method metal is vaporized by heating
at high temperature inside a chamber. The chamber is previously evacuated and
then, back filled with inert gas to a low-pressure. Metal atoms present in the
vapours collide with the inert gas molecules and lose their energy. Due to
collisions, metal atoms are cooled down rapidly becomes supersaturation and then
nucleus homogeneously producing nanoparticles which are collected at the bottom.

5.12.6 Chemical vapour condensation: - In this method, precursor is vaporized

and mixed with an inert carrier gas like N2 and the mixture is fed into the reactor.

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 Reactor is maintained at a temperature required for the reaction, as result product
produced is deposited over the substrate. The by-product and left overs from the
reactions are passed on to the gas phase, which are easily removed from reaction
chamber. Reaction is frequently catalyzed by the catalyst present on the substrate.

Ex: - Trimethyl gallium and AsH3 are used as precursors and hydrogen is used as a
carrier gas as well as a reducing agent. Reaction occurs at a temperature of 700 oC
at atmospheric pressure. GaAs nano particles are collected on a porous carbon film
downstream at a temperature of 350oC and by-product methane gas is easily
removed out of the reactor by carrier gas.
Me3Ga(g) + AsH3(g) + H2(g) → GaAs(s) + CH4(g)

5.13 NANOSCALE MATERIALS

5.13.1 Nanocrystals and nanoclusters: - Nanoparticle, nanocrystal and
nanocluster are frequently used interchangeable.

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 Nanoparticles are zero dimensional particles with their size varying
between one to hundred of manometers. They are zero dimensional because the
movement of charge carriers is confined in all three dimensions. Most of the
properties of nanoparticles vary wit6h size and can be fine-tuned by variation of
size of particle. But in case of bulk materials properties vary with structure and
composition and not on the size of material.

A nanocluster has a well-defined crystal structure in which the atomic positions

can be exactly determined. But crystal structure of nanocluster is not same as the
corresponding bulk material. For example, a nanocluster of gold formed by 13
atoms (Au13), has icosahedral symmetry while crystal structure of bulk gold is face
centered cubic(FCC). The thermodynamic stability of a nanocluster varies with
size and shape. For example, a nanocluster of gold containing 55 atoms(Au 55),
Au55 is extremely more stable than a nanocluster with 56 atoms(Au56).
Nanoclusters also exhibit properties vary with size and shape.
When the number of atoms in a nanocluster is increased, at some stage a
nanocluster assumes the crystal structure same as that of bulk material. Such
nanocluster are called as nanocrystals. Nanocrystals can be obtained as colloidal
solution (sol) or as solid crystals. The thermodynamic stability and other properties
of a nanocrystal vary smoothly with size and shape. For example, gold
nanocrystals of size 3nm have face centered cubic (FCC) structure which is same
as that of bulk gold metal. If the size of nanocrystals is increased, its structure
remains FCC but the properties vary smoothly with the size of nanocrystal.
5.13.2 Synthesis of Gold Nanocrystal

Tetrachloroaurate(III) anion is taken in a non-aqueous solution of

tetraoctyl ammonium bromide in diethylether. In this stage Au(III) is reduced to

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 Au(0) by using sodium borohydride as a reducing agent. This reduced gold rapidly
nucleates and grows to a size determined by the amount of precursor in solution.
The product is Au(SR) alkanethiolate-capped nanocrystals that is filtered, washed
with acetone.

HAuCl4 + OCl4NBr[Et2O] → OCt4NAuCl4[Et2O]

OCt4NAuCl4[Et2O] + RSH + NaBH4 → Au(SR)
5.13.3 Properties of Nanocrystals
1) Nanocrystals of CdSe have size in the range of 1-10nm
2) CdSe semiconductor nanocrystals exhibit electrolumininescence property.
3) CdSe semiconductor nanocrystals when coated with CdS and silica become
water dispersible. These nanocrystals exhibit size dependent fluorescence.
Nanocrystals size 2nm emits green colour where as with size 4nm emits red
colour radiation.

5.13.4 Applications of nano crystals

1) Nanocrystals in water are used for biolabelling and as scanning agents for
MRI scan.
Ex: - CdSe nanocrystals
2) Nanocrystals display intense and narrow photo luminescence. Due to this
property they are used in LEDs
3) A single nanocrystals can emit radiation of all the colours at different applied
potential. Due to this property they are used in full colour displays applications.

5.14.1 Carbon Nano tubes (CNTs) : - Carbon nanotube is a relatively new carbon
allotropy discovered by S Aumio Iijima in 1999. He found CNTs while observing

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 the by-products of combustion of carbonaceous materials in high - resolution
electron microscope. CNTs are cylinderical tubes with a central hallow core due to
rolling up of graphite sheets. CNTs is a one dimensional material like nanowire but
with the aspect ratio (length/ width) greater than 1000.

Types of CNTs
There are two types of CNTs
1) Single - walled carbon Nano Tubes (SWCNTs) :- They are formed by rolling
up of single graphite layer.
2) Multi - walled carbon Nano Tubes (MWCNTs) :- They consist of two or
more concentric graphene cylinders with vanderwalls forces between adjacent
tubes.

Synthesis of Carbon Nanotubes: - Propylene is fed into the reaction maintained

at 800oC using an anodic aluminum oxide film as template with carrier N 2 gas. It
undergoes paralytic decomposition depositing uniform layer of carbon on the inner

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 wall of the template nano channels. Then, the anodic aluminium oxide template is
removed by washing with NaOH and only CNTs is left as an insoluble fraction.

Properties of Carbon Nanotubes

1) CNTs exhibit high electrical conductivity along the walls and also high thermal
conductivity.
2) They have low density and very high mechanical strength.
3) The structure of CNTs is compared entirely of SP2 carbon - carbon bonds which
stronger than the SP3 bonds found in diamond.

Applications of Carbon Nanotubes

1) They are used for electrode material for lithium ion rechargeable batteries.
2) They are used for metallic interconnects between components in integrated
circuits.
3) It is used cancer thermotherapy to selectively kill cancer cells without affecting
nearby healthy tissues.

5.14.2 Nano Wires:- Laser ablation, Chemical vapour deposition and template
assisted growth are employed for growing nano wires of different materials.
Ex: - Silver seeds are first generated by heating AgNO3 at 160oC in ethylene
glycol, Which serves both as reducing agent and the solvent. If a separate solution
of AgNO3 and poly vinylpyrolidene in ethylene glycol is then added drop wise to
this seed solution, highly elongated silver structures result.

Properties of Nano Wires

1) ZnO nanowires exhibit room temperature ultra-violet laser activity.

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 2) SiC and Si nanowires with sharp tips are good field emission materials

Applications of Nano Wires

1) Semiconductor nanowires have recently been used as building blocks for
assembling a range of nanodevices including Field effect transistors and p-n
diodes.
2) SiC and Si nanowires are used in field emmision X-ray tubes

5.14.3 Nano Rods: - A Nanorod is a one dimensional nanostructure with the

aspect ratio less than 10nm. Aspect ratio is defined as ratio of length of a particle to
its width
Aspect ratio = Length of a particle
Width of a particle

Length and width of nanorods usually vary from 10 to 100nm. The aspect
ratio of a nanorod varies from 0.1 to 10nm. Nanorod is a one dimensional, because
the movement of charge carriers is allowed in only one direction and combined in
two other directions. Nanorods are useful to study the effect of shape and size on

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 the magnetic, electronic, optical and chemical properties of the materials at
nanoscale.

Synthesis of Nanorods : - Nanorods can be synthesized with uniform size

distribution and producible manner by using a structure directing template. A
template is an inert material with uniform pore size and can be removed after
synthesis.
Ex: - TiO2 nanorods are prepared by Sol-gel method using anodic aluminium
oxide as template. By dissolving titanium isopropoxide in ethanol mixtued with
acetyl acetone dissolved in a mixture of ethanol and water to form a Sol. Then
anodic aluminium oxide membrane is dipped in this Sol. A required volume of
HCl is added to control hydrolysis and condensation reactions resulting in gel.
Then the sample is dried by evaporation. Anodic aluminium oxide is removed by
washing with NaOH solution. Nanorods of TiO2 are obtained by the sample at
400oC for 2 hours.

Properties of Nanorods : -
1) Gold nanorods exhibit strong surface plasman resistance in response to
incident light
2) The radiation absorbed by gold nanorod in the near Infra-red region (700
- 1100nm)

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 3) Gold nanorods exhibit very fluorescence intensity

Applications of nanorods
1) Nanorods property is used in cancer thermotherapy to selectively kill
cancer cells without affecting near by heating tissues.
2) Nanorods of alloy like Pt - Ni are used as catalyst in direct methanol
oxygen fuel cell
3) Nanorods of metal oxides like TiO2, V2O5, CuO and MnO2 are used in
catalysts

5.14.4 Nano Composites: - Nano Composites are a class of matrial in which ome
or more phases with nanoscaqledimensions (Zero- dimensional, One - dimensional,
Two - dimensional) are embeded in a metal, ceramic or poymer matrix. Nano
Composite matrial is obtained by inserting nano material in to the matrix layer.

Depending on the dimension of nanomaterials distributed on the matrix,

nanocomposite are follows.
1) Zero- dimensiona Composites: - Isolated nanoparticles are ditributed in a
matrix.

2) One - dimensional Composites: - The nanotubes and nanorods are distributed

in a matrix.

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 3) Two - dimensional Composites: - The thin nanofilms are placed between two
matrix layers.

Properties of Nano Composites

1) Nano Composite is ˝gold ruby glass˶ which was produced by Assyrians in the
seventh century.
2) Composite materials of various nanomatrials with polymer as a marix posses
excellant mechanical and theramal properties.
3) Composites with well-aligned metal nanorods over polymer matrix can exhisit
good electrical conductivity and high tensile strenght.

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 Applications of Nano Composites
1) The excellent mechanical and theramal properties of comoposite ,aterials are
used to wide extent in the automative industry.
2) The coated nanocomposite particcles are widely used in the biology and
medicine.

5.14.5 Syntesis of Fullerenes

Graphite is vapourized by setting up an electric arc between two graphite
eletrodes in a controlled atmosphere of helium gas. The temperature at the tip of
electrode is more than 4000oC and pressure of He gas is 150 - 200 torr. A mixture
of various fullerenes is obtained by condensing the evaporated carbon and the main
product is the fullerene C60. It is extracted and crystallized using benzene as
solvent.

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 Properties of Fullerenes
1) Crystal of C60 have face centered cubic arrangement.
2) Fullerenes C60 molecule can absorb more than 100 photons in a nanosecond and
transfer that energy (23oV) to its vibrational energy.
3) A polymer composite of C60 molecule an d plyvinyl carbazole exhibits very
high photoconductivity.

Fullerenes: - Fullerenes are class of molecules made of only carbon atoms having
cvlosed cage vlike structure. Number of fullerenes molecules with different carbon
atoms like C60, C70, C74, C76, C78, ..ext and C60 molecule is a smallest stable and
also most abundant fullerenes, which is commonly called as Buckminster
fullerenes. The C60 molecule has spherical shape resembling a soccer ball (foot
ball) and size including pi-electron cloud is 1.034nm.

Structure of C60 molecule (Buckminster fullerenes)

C60 molecule has 12 pentagons, 20hexagons, 90edgs and 60 vertices. All
60 carbon atoms are equivale`nt and undergo SP2 hbridized. It has trigonally
bonded similar to graphite.

Applications of Fullerenes
1) Fullerenes C60 molecule is used as optical limiter.
2) Polymer composite of C60 molecule is used in making organic photovolatic
cells.
3) Polymer composite of C60 molecule is also used in photocopying aplications.

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 5.14.6 Dendrimers: - The Dendrimer is derived from greek word, dendors means
ˋtreeˊ and merons means ˋpartˊ . Dendrimers have branched structure like a tree.
They have a central core of multi functional molecule to which branched
molecules are added repeatedly step by step directions like tree limbs. A dendrimer
can be grown up to 10 - 15 generation and its size varies aproximately from 1 -
25nm.

Syntesis of Dendrimers: - There are two methods for the synthesis of dendrimers
In the first step, four acrylonitrile manomers are bonded to two -NH2
groups of core molecule. In the activation step all four CN groups are reduced to
ammine groups (-NH2) by reducing with Co(II) and H2 to give a first generation
dendrimer is obtained.

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 In the second step, eight acrylonitirle monomers are bonded to four - NH2 groups
of first generation dendrimers and then all -CN groups are reduced to amine groups
to get second generation dendrimer

Properties of Dendrimers
1) Dendrimers have less dense central core and larege number of functional groups
at outer surface.
2) They have well-defined size, shape, branching lenght and surface functonality
3) They have nanoscale spherical shape.

Applications of Dendrimers

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 1) Dendrimers are used as antiviral and antimicrobial agents
2) Dendrimers are also used as medical imaging agents in magnetic resonance
imaging (MRI - Scan) of human body.
3) They are useful in cancer teratment for selectively treating only cancer affected
part of the body.

Questions

1) What is a nanomaterial? How they are different from bulk matrials?

2) Explain the Sol-gel method for preparaion of nanomaterial with an example.
3) Explain the precipitation method for preparaion of nanomaterial with an
example.
4) Explain the gas condensiation method for preparaion of nanomaterial with an
example.
5) Explain the CVD method for preparaion of nanomaterial with an example.
6) What are nanpocrystals? Give their properties and applications?
7) What are nanorods? Give the synthesis, properties and applications?
8) What are nanowires? Give the synthesis, properties and applications?
9) Give the synthesis, properties and applications of carbon nanotube.
10) What are nanocomposites? Give the properties and applications?
11) Give the structural features, properties and applications of Buckminster
fullerenes.
12) What is dendrimer? Give the properties and applications?

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MODULE-5

WATER TECHNOLOGY

5.1 INTRODUCTION

Water is nature’s most abundant and useful compound of many essential

things, for the existence of human beings, animals & plants. Water is
rated to be of greatest importance. It covers three fourth of earth’s
surface.

Water is not only essential for animals and plants but also occupies a
unique position in industries. Probably its most importance use as an
engineering material is in steam generation. Water is also used as a
coolant in power & chemical plants. In addition to it water is widely
used in other fields such as production of steel, rayon, papers, atomic
energy, textiles, chemicals, ice & for air conditioning, drinking bathing,
sanitary, washing, irrigation, fire fighting etc.

The main sources of water are

(A) Surface water (i) Rain water

(ii) River water

(iii) Sea water

(B) Underground water - spring & well water

OBJECTIVES

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➢ To Explain Water Technology For Daily Life.

➢ To Known The Pollutants And Impurities In Drinking Water.
➢ To Practically Demonstrate The Water Purification Technique.
➢ To Explain The Basic Parameters Of Drinking Water Like Hardness, Ph,
COD,BOD And TDS ect

5.2 IMPURITIES IN WATER-

All natural water supplies irrespective of the source contain impurities

which may be broadly classified into four categories:

1) Dissolved impurities

2) Suspended impurities

3) Dissolved gases

4) Organic matter

5.2.1 Dissolved Impurities - Dissolved impurities mainly consist of

bicarbonates, chlorides & sulphates of calcium, magnesium and sodium.
In addition, small amts of nitrates, nitrites, silicates, ammonium & ferrous
salts are also present. These salts are derived from rock & soils with
which the water is in contact. Thus water which is in contact with
limestone contains calcium carbonate since the CO2 dissolved in water
interacts with limestone

CaCO3 + H2O + CO2 --- Ca(HCO3)2

Similarly, water which is in contact with magnesite contains magnesium

bicarbonate.

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MgCO3 + H2O + CO2 -- Mg(HCO3)2

In general, ground waters contain more dissolved salts than surface

waters.

5.2.2 Suspended matter - the suspended matter may be of inorganic or

organic nature.

The inorganic materials include particles such as sand, clay, silica,

hydroxides of iron & aluminium etc derived from the erosion of soil.
Some of these particles have large size & therefore settle down readily.
Others are fine particles & colloidal in nature. Such particles do not settle
down easily.

The organic suspensions are decaying vegetable matter & due to

microorganisms. These are also present in colloidal form. The presence
of suspended matter, particularly the colloidal particles impart turbidity to
water.

5.2.3 Dissolved gases: most waters contain dissolved gases such as

oxygen, carbondioxide, sulphur dioxide, ammonia & oxides of nitrogen,
all of which are derived from atmosphere.

5.2.4 Organic matter: organic compounds derived from the decay of

vegetable & animal matter including bacteria may be present in water.
Water also gets contaminated with sewage & human excretal matter etc.
Consequently the pathogenic bacteria such as typhoid bacillus &
commensal bacteria of intestinal origin such as coliform group
(streptococcus faecalis & clostridium welchii) are introduced into water.

5.3 BIOLER FEED WATER

A boiler is a closed vessel which operates under different pressures.

Water heated in boiler under pressure is transformed to steam. The water
used in these types of boilers is called as boiler feed water. (Boiler feed
water is water used to supply a boiler to generate steam or hot water.)A
boiler is a device for generating steam, which consists of two principal

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parts: the furnace, which provides heat, usually by burning a fuel, and the
boiler proper, a device in which the heat changes water into steam. The
steam or hot fluid is then recirculated out of the boiler for use in various
processes in heating applications.

Boiler feed water contains impurities.

These impurities results in many problems.

5.3.1 Boiler Problems

Impurities present in water can cause the following types of problems in

boilers.

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(i) Scale and sludge formation

(ii) Priming and foaming

(iii) Boiler corrosion

5.3.2 Scale and sludge formation

In boilers water is heated under high pressures to high temperatures. As

boiling point of water is much lesser than that of many impurities, the
impurities in water get progressively concentrated inside the boiler. These
impurities get precipitated out on saturation. If the resultant precipitated
is thick deposit is called as scale or if the precipitate is loosely held
known as sludge.

Scales are due to the presence of MgCl2, Mg(HCO3)2, Ca(HCO3)2, CaSO4

and silica in water and sludge is due to CaCl2, MgCl2, MgSO4, MgCO3
etc.

5.3.3 Causes of scale formation

Loss of fuel

(i) Reduction in boiler efficiency

(ii) Boiler explosion

(iii) Decrease in the strength of boiler

(iv) Cleaning process is expensive.

5.3.4 How to remove scales?

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(i) Using wooden scraper or wire brush loose scales can be

removed.
(ii) Blow down operation for loose scales (removal of hard water
from the bottom of the boiler and refilling soft water).
(iii) By thermal shocks, it makes scales brittle.
(iv) Treatment with HCl (5-10%) for carbonates and EDTA for Ca
or Mg salts.

5.3.5 Causes of sludge formation

(i) Loss of fuel

(ii) Reduces boiler efficiency
(iii) Leads to blockage of cooler and distribution pipes.
(iv) Cleaning process is expensive.

5.3.6 How to avoid sludge?

(i) Using soft water

(ii) By removing salty water from boiler time to time.

5.4 Priming and

Priming: It is the process of very rapid boiling of water in the boiler
which makes some water droplets to be carried away along with steam in
the form of spray into the steam outlet.

Priming is due to the presence of suspended and dissolved impurities in

water and very high water level in the boiler and also due to defective
boiler design.

5.4.1 Problems caused by priming

(i) Wet stem reduces the heating efficiency of the steam and causes
corrosion.
(ii) Impurities in water droplets may deposit on turbine blades to bring
down its efficiency.

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5.4.2 Priming prevention

(i) Maintaining low water levels.

(ii) Good boiler design for proper evaporation of water.
(iii) Avoiding rapid discharge of steam.

5.4.3 Foaming

It is the formation of small but persistent bubbles on the surface of boiler

water. These bubbles are carried along with steam leading to excessive
priming.

5.4.4 Prevention of foaming

(i) Using anti foaming agents like castor oil, polyamides etc.
(ii) The removal of silica using ferrous sulphate.
(iii) Removal of oils and grease from sodium aluminate
(iv) Removal of clay and suspended matter using coagulating
agents.

5.5 Boiler corrosion

Corrosion in boilers is due to presence of dissolved oxygen, dissolved

CO2 and MgCl2.

(i) Due to dissolved oxygen

The dissolved oxygen reacts with iron at about 350-450 0C in
the boiler and produces ferrous hydroxide.

2 Fe + O2 + 2 H2O 2 Fe(OH)2

Ferrous hydroxide oxidizes to ferric hydroxide by dissolved

oxygen and deposits. This process repeats till all the dissolved
oxygen is exhausted. The corroded parts are referred to as pits.

(ii) Due to dissolved CO2

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H2O + CO2 H2CO3 H+ + HCO3-

Further HCO3- may combine with Fe2+ giving Fe(HCO3)2 which

decomposes giving Fe(OH)2 and CO2 thus continuing the
process.

Fe(HCO3)2 Fe(OH)2 + 2 CO2

(iii) Due to MgCl2

Mineral acid are produced by the hydrolysis of salts like MgCl 2
and FeCl2 in boiler feed water.

FeCl2 + 2 H2O Fe(OH)2 + 2 HCl

MgCl2 + 2 H2O Mg(OH)2 + 2 HCl

5.6 Determination of Dissolved Oxygen by Winklers or Iodometric

method:

The analysis of dissolved oxygen (DO) in water is a key test to access

raw water quality & to keep a check on stream solution. The DO
forms the basis for Biochemical Oxygen Demand (BOD) which
constitutes an important parameter to evaluate pollution potential of
wastes. DO test is used to control the amount of oxygen in boiler feed
water by physical, chemical & mechanical methods. The
measurements of the amount of oxygen actually dissolved in a water
sample are of great importance as the oxygen content is important for
many biological & chemical processes.

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5.6.1 Principle: The principle involved in the determination of

dissolved oxygen is that the divalent manganese solution along with a
strong alkali is added to water sample. The DO present in water
sample oxidizes divalent manganese to tetravalent manganese. The
basic manganic oxide formed acts as oxygen carrier to enable the
dissolved oxygen in molecular form to take part in the reaction. Upon
acidification, tetravalent manganese reverts to divalent state with the
liberation of nascent oxygen, which oxidizes KI to I2. The liberated
iodine is titrated against standard sodium thiosulphate solution using
starch as indicator.

MnSO4 + 2KOH  Mn(OH)2 + K2SO4

2Mn(OH)2 + O2 - 2 MnO(OH)2 (Basic manganic oxide)

MnO(OH)2 + H2SO4  MnSO4 + 2H20 + [O]

2KI + H2SO4 + [O]  K2SO4 + H20 + I2

I2 + 2Na2S2O3  Na2S4O6 + 2NaI

KI is added as alkaline KI which consists of a solution of sodium

azide, KI & NaOH in water. Sodium azide destroys the nitrites in
water & thereby reduces the error due to nitrites.

NaN3 + H+  HN3 + Na+

HN3 + NO2+ + H+  N2 (gas) + N2O + H2O

5.6.2 Procedure: Pipette out 300 cm3 of water sample into a clean
glass stoppered bottle. Add 3 cm3 of manganous sulphate solution
dipping the pipette below the surface of water. Add 3 cm3 of alkaline
potassium iodide solution. Stopper the bottle & shake well and allow
the precipitate to settle down. Now add 1 cm3 of concentrated
sulphuric acid slowly & mix well until the precipitate dissolves
completely. Pipette out 102 cm3 of this solution into a clean conical
flask & slowly titrate against 0.02N sodium thiosulphate solution
using 2 cm3 of starch indicator near the end point. Record the volume
of sodium thiosulphate solution used.

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5.6.3 Calculation:

Normality x volume of oxygen solution = Normality x volume of

Na2S2O3

Therefore, normality of oxygen solution = Normality x volume of

Na2S2O3

Volume of oxygen solution

= N x V = a

102

Weight of dissolved oxygen/dm3 = Normality x Equivalent weight

of oxygen

= a x 8.0 x 1000 mg/l

5.7 BIOLOGICAL OXYGEN DEMAND :

5.7.1 Definition: BOD is usually defined as the amount of oxygen

required by bacteria while stabilizing decomposable organic matter
under aerobic condition. The BOD Test is widely used to determine
the pollutional strength of domestic & industrial waste in terms of
oxygen that they will require if discharged in natural water sources in
which aerobic condition exits.

Natural water contains dissolved oxygen (8.7ppm or 8.7 mg dm-3) and

the dissolved oxygen (DO) is capable of oxidizing many of these
pollutants particularly the organic wastes such as dead plant matter &
animal wastes. In this way the dissolved oxygen is consumed.
However in running water such as streams and rivers there is a
continuous replenishment of oxygen maintaining the DO level &
hence the degradation is aerobic. The degradation products are CO 2
and water which are harmless. On the other hand, in stagnant waters
such as in lake and well waters, there is gradual decrease in the DO
level ultimately causing anaerobic (absence of air) degradation of
organic wastes releasing obnoxious gases such as H2S, CH4 & NH3.

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BOD is defined as “ the quantity of oxygen required by

microorganisms to oxidize the organic wastes present in one litre of
waster water over a five day period at 20 oC.

5.7.2 Characteristics of BOD parameter:

o The unit of BOD is mg dm-3or ppm.

o It is emperical and semiquantitative.
o Its represents only biodegradable organic load in sewage.
o Strictly aerobic conditions are needed.
o Determination is slow & time consuming method
BOD indicates the amount of decomposable organic matter in the
sewage. It is an expression of how much oxygen is needed for microbes
to oxidize the organic matter in the sewage. It gives information’s about
the following:

▪ Polluting power of sewage or its nuisance value.

▪ The load of organic matter on the sewage treatment
plant.
▪ The amount of clean diluting water required for
disposal of sewage.
It is important to know the BOD of sewage before disposing into rivers or
lakes because dissolved oxygen content in the water will be decreased by
the sewage if its BOD is high resulting in the death of fishes and other
aquatic animals.

Determination of BOD : The parameter is commonly measured by the

quantity of oxygen utilized by aerobic bacteria during 5 days period.

(CHONS) + O2 - CO2 + H2O + NO3- + SO42-

The BOD test is based on determination of oxygen. BOD may be

measured directly in a few sample but in general a dilution procedure is
required.

5.7.3 Direct method: with samples whose 5 day BOD does not exceed
7mg/l it is not necessary to dilute them providing that they are aerated to
bring D.O level nearly to saturation at the start of the test.

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Procedure is to adjust a sample to about 20 C & aerate with diffused air to

increase or decrease the DO gas content of the sample to near saturation.
Two or more BOD bottles are then filled with the sample. Atleast one is
analyzed for BOD immediately & the other are incubated for 5 days at
20 C. After the 5th day the amt of DO remaining in the incubated sample
is determined & the 5th day BOD is calculated by substraction of the 5th
day result from that obtained on the 1st day.

5.7.4 Dilution method: here measuring of BOD is based upon the

fundamental concept. The rate of biochemical degradation of organic
matter is directly proportional to the amt of unoxidised material existing
at that time. According to this concept the rate at oxygen is used in
dilution of waste is in direct ratio to the percentage of waste in the
dilution provided that all factors are equal. The environmental &
nutritional factors that has to be controlled to obtain reproducible result
include.

1. Freedom from toxic material.

2. Favourable pH & osmotic pressure.

3. Presence of available essential nutrients.

4. Std temperature.

5. Presence of significant population of mixed organisms of soil origin.

A synthetic dilution water prepared from distilled water is best for BOD
testing because most of the variables like contamination from chlorine
residue for variation of DO & presence of microorganisms, algae or
nutrients etc can be kept under control. The dilution water should be
seeded with waster water or other materials to ensure a uniform
population of organisms in various dilution & to provide an opportunity
for an organic matter present in the dilution water to be exposed to the
same type of organic matter as those involved in the stabilization of
waste. The dilution water should be aerated to saturate it with oxygen
before use.

The BOD can be calculated as

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BOD = D1 - D2 x B mg. dm-3

Where D1is DO in mg. dm-3 in solution at the start.

D2 is the DO in mg. dm-3 in solution after 5 days.

A is the volume of sample in ml before dilution.

B is the volume of sample in ml after dilution.

5.8 CHEMICAL OXYGEN DEMAND:

BOD refers to biologically oxidisable impurities and does not account for
non-oxidizable & slowly oxidizable impurities. COD is a faster method
of determining the amount of oxygen required to oxidize both the
biologically oxidizable and biologically non-oxidizable but chemically
oxidizable organic & inorganic wastes.

5.8.1 Definition: the amount of oxygen consumed in the chemical

oxidation of organic & inorganic wastes present in 1 litre of waste water.

5.8.2 Characteristics of COD parameter:

1. It is a satisfactory, quantitative method for measuring total organic
load n inorganic load.
2. it is preferable to BOD as the results are reliable.
3. rapidly measurable parameter & needs about 3 hours for
completion.
4. in general COD>BOD since both biodegradable and non
biodegradable organic load are completely oxidized.
5. when use along with BOD test, it gives biologically resistant
organic matter.

5.8.3 Determination of COD:

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5.8.4 Principle: The principle of the method is the oxidation of organic

matter using chemical oxidizing agents such as acidified potassium
dichromate in the presence of a catalyst such as silver sulphate (which
catalyses the oxidation of organic matter) & mercuric sulphate (which
forms a complex with chloride ions present in water thus preventing its
interference).

A typical reaction representing the oxidation of organic matter is given

below:

Ag2SO4

3CH2O + 16 H+ + 2 Cr2O72- ---------- 4Cr3+ + 3CO2 +

11 H2O

HgSO4

The method consists in adding excess of a standard solution of potassium

dichromate acidified with sulphuric acid to a known volume of effluent
sample and back titrating the excess of potassium dichromate against a
standard solution of ferrous ammonium sulphate. COD values are also
expressed in mg dm-3.

5.8.5 Procedure: To a measured volume of waste water sample taken in

a flask, add 10 cm3 of 0.25N K2Cr2O7 solution followed by 30 cm3 of 6N
H2SO4. Add 1g of Ag2SO4 followed by 1g of Hg2SO4. Attach a reflux
condenser & reflux the contents for 2 hrs. Cool and titrate the excess
K2Cr2O7 against ferrous ammonium sulphate solution using ferroin as
indicator till the bluish green colour turns sharply to reddish brown. Let
the volume of titrant required be ‘a’ cm3. Perform a blank titration taking
the same amount of water in place of the waste water. Let the volume
required be ‘b’ cm3.

5.8.6 Calculations:

Volume of K2Cr2O7required for the sample = b-a cm3.

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COD of the sample = N x (b-a) x 8 g dm-3

= N x (b-a) x 8000 mg dm-3

Where N = normality of ferrous ammonium sulphate

V = volume of waste water sample

b = volume of std FAS used in blank titration

a = volume of std FAS used in sample titration

5.9 SEWAGE TREATMENT

The domestic sewage contains heavy load of BOD, pathogenic

bacteria, colour & annoying smell. If such raw waste water is discharged
into natural water bodies they may cause detrimental effects such as
destruction to aquatic life, depletion of dissolved oxygen, disagreeable
colour & odour and not to forget the waterborne diseases caused by the
pathogenic bacteria. Therefore sewage has to undergo proper treatment
before being discharged in natural water bodies.

The sewage treatment is carried out in 3 stages, namely:

1. Primary treatment

2. Secondary treatment

3. Tertiary treatment

5.9.1 Primary treatment (Physical & chemical): It involves

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(a) Screening - physical process of removing large suspended or floating

matter in sewage using bar screens & mesh screens which retain the
matter while allowing the sewage water to pass through them.

(b)Silt & grit removal- is done by passing through grit chambers where
velocity of sewage flow is reduced. Silt & grit being heavier they settle
down in the bottom.

(c) Oil & grease removal- by passing through skimming tanks. Here
compressed air is blown through the sewage water converting it into
soapy mixture & lifted to the surface. The floating mixture is skimmed
off.

(d) Sedimentation process removes finer suspended impurities. This

process may be of 2 types: i) by plain sedimentation which is carried out
in a continuous flow type sedimentation tank. ii) Sedimentation with
coagulation (coagulants like alum, ferrous sulphate etc )

5.9.2 Secondary treatment (biological): After Primary treatment waste

water is made to pass into large tanks where biological treatment is
carried out. This process involves aerobic biochemical oxidation or
aeration. The organic matter is converted into CO2, the nitrogen into
ammonia & finally into nitrites and nitrates. Bases present in the sewage
water form salts like ammonium nitrite, ammonium nitrate, calcium
nitrate, etc.

Secondary or biological treatment is carried out by trickling filter

method or activated sludge process

5.9.3 Trickling filter method - it consists of a rectangular or circular

vessel with a filter bed made of broken bricks or large anthracite coal.
Sewage is sprayed over this bed by means of a rotating distributor. As
the sewage trickles or percolates downwards through the filter bed,
microorganisms grow on the surface of aggregates using organic
materials of the sewage as food. Aerobic conditions are maintained &
purified sewage is removed from the bottom. This method removes 90%
of biologically oxidisable impurities.

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5.9.4 Activated sludge process - it involves extensive aeration of the

sewage water & the process of aerobic oxidation by addition of activated
sludge (i.e., part of sludge previously oxidized) into the sewage water.
Activated sludge contains large number of aerobic bacteria & other micro
organisms.

The sedimented sewage water is mixed with proper quantity of

activated sludge and the mixture is sent to the aeration tank, in which the
mixture is aerated and agitated for several hours. During this process,
organic matters are oxidized. After the process is complete, the effluent
is sent to a sedimentation tank, where sludge is deposited & water free
from organic matter is drawn off. A part of the settled sludge is sent back
for seeding fresh batch of sewage. The activated sludge process operates
at 90-95% efficiency of BOD treatment. If the treated water contains a
high concentration of phosphates, heavy metal ions, colloidal impurities
& non-degradable organic compounds, the water is subjected to tertiary
treatment.

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3) Tertiary treatment: The aim of tertiary treatment is further purification

of waste water as well as its recycling. The tertiary treatment consists of

(a) Removal of phosphate - The phosphates are removed by adding

Ca(OH)2. A flocculant precipitate of calcium phosphate is formed at pH
10-11. At this pH, ammonium salts are converted into ammonia.

3 Ca(OH)2 + 2PO43-  Ca3(PO4)2 + 6OH-

Ca(OH)2 + NH4+  Ca2+ + NH3 + H20 + OH-

(b) Coagulation and Sedimentation: The suspended fine particles are

removed by sedimentation in the presence of coagulants like alum,
ferrous sulphate, etc. The flocculant precipitates of Al(OH)3 or Fe(OH)2
formed by the coagulants entrap the fine particles & help them to settle
down. The highly charged ions of the coagulants also neutralize the
charges on colloidal particles and make them to coagulate and settle
down.

(c) Filtration: the water is passed through conventional sand filter beds to
remove the last traces of suspended matter.

(d) Stripping of ammonia and other gases is done in a degasifier. The

degasifier consists of a large tower fitted with a number of perforated
plates. The hot water trickles through these plates. Large surface area
and higher temperature promote stripping of dissolved gases like NH3,
CO2, H2S, etc.

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(e) Disinfection: The pathogenic bacteria are destroyed by disinfection.

Among many disinfectants, chlorine is cheap & effectivel

Cl2 + H20  HOCl + H+ + Cl- (pH6.5)

Unionised HOCl attacks the cells of bacteria and kills them.

The final composition of tertiary treated waste water is.

BOD < 1ppm

NH4+ < 1ppm

PO43- < 1ppm

The treated water has high clarity free from odour, low BOD &
therefore, it is nearly equivalent to drinking water & can be recycled.

SLUDGE DISPOSAL: Sludge which is collected from sewage treatment

processes are disposed off by the following methods.

(i) Burial at sea: sludge is dumped at places near sea.

(ii) Land spreading: the sludge is uniformly spread over soil, followed by
ploughing. It acts as fertilizer.

(iii) Septic tank treatment & sludge digestion: The sludge is kept in a
closed tank in the absence of air for prolonged period (abt 30 days). The
sludge undergoes anaerobic decomposition producing gases like methane,
H2S, phosphine, etc. The gas can be used as fuel for city supply or power
generation.

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5.10 POTABLE WATER

Water that is fit for human consumption and meets the stringent
microbiological and chemical standards of quality to prevent waterborne
diseases and health risks from toxic chemicals is called potable water.

5.10.1 Desalination: It is the process of partial or complete

demineralization of highly saline water such as a sea water is referred to
desalination. In partial demineralization, the amount of dissolved salts is
reduced to such a level, that water is rendered potable. Several methods
such as flash evaporation, reverse osmosis and electro dialysis are
available for desalination.

5.10.2 REVERSE OSMOSIS:

Reverse osmosis is a separation process that uses pressure to

force a solvent through a membrane that retains the solute on
one side and allows the pure solvent to pass to the other side.
More formally, it is the process of forcing a solvent from a
region of high solute concentration through a membrane to a
region of low solute concentration by applying a pressure in
excess of the osmotic pressure. This is the reverse of the normal

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osmosis process, which is the natural movement of solvent from

an area of low solute concentration, through a membrane, to an
area of high solute concentration when no external pressure is
applied. The membrane here is semipermeable, meaning it
allows the passage of solvent but not of solute.
The membranes used for reverse osmosis have a dense barrier layer in the
polymer matrix where most separation occurs. In most cases the
membrane is designed to allow only water to pass through this dense
layer while preventing the passage of solutes (such as salt ions). Sea
water exerts an osmotic pressure of about 240 psi, brackish water has
significantly lower value. Reverse osmosis can be effected by the use of
pressures in the range of 410 - 540 psi. A reverse osmosis unit consists of
a membrane, a vessel and high pressure pump. The membranes are
generally made up of cellulose acetate or nylon and are usually fabricated
in a cylindrical shape.

This process requires that a high pressure be exerted on the high

concentration side of the membrane, usually 2–17 bar (30–250
psi) for fresh and brackish water, and 40–70 bar (600–1000 psi)
for seawater, which has around 24 bar (350 psi) natural osmotic
pressure which must be overcome.

This process is best known for its use in desalination (removing

the salt from sea water to get fresh water), but has also purified
naturally occurring freshwater for medical, industrial process
and rinsing applications since the early 1970s. In RO, feedwater
is pumped at high pressure through permeable membranes,
separating salts from the water (Figure 1). The feedwater is
pretreated to remove particles that would clog the membranes.
The quality of the water produced depends on the pressure, the
concentration of salts in the feedwater, and the salt permeation
constant of the membranes. Product water quality can be
improved by adding a second pass of membranes, whereby
product water from the first pass is fed to the second pass.

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Flow Diagram of a reverse osmosis system

5.10.3 ELECTRODIALYSIS: Electrodialysis is an electromembrane

process in which ions are transported through ion permeable membranes
from one solution to another under the influence of a potential gradient.
The electrical charges on the ions allow them to be driven through the
membranes fabricated from ion exchange polymers. Applying a voltage
between two end electrodes generates the potential field required for this.
Since the membranes used in electrodialysis have the ability to selectively
transport ions having positive or negative charge and reject ions of the
opposite charge, useful concentration, removal, or separation of
electrolytes can be achieved by electrodialysis.

5.10.4 Principle: Passage of an electric current through a solution of salt

results in migration of cations towards the cathode & anions towards the
anode. The use of semi permeable cation or anion exchange membrane
in an electrolytic vessel permits the passage of only cations or anions
respectively in the solution.

5.10.5 Construction: It consist of a chamber carrying a series of

compartments fitted with closely spaced alternate cation (C) and anion
(A) exchange semipermeable membranes between the electrodes. An
electrodialyzer unit will have 200 to 1000 compartments. The feed water

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is taken in the dialyzer and the electrodes are connected to a source of an

electric current.

The anions pass through the anion permeable membrane towards

the anode. However, these ions do not pass through the next membrane
which is permeable only to cations. Similarly the cations moving in the
other direction will pass through the cation exchange membrane but not
the next. These anions & cations collect in the alternate chambers; the
water in these is enriched with salt while that in the other compartments
is desalinated. Micro porous sieves provided near the electrodes prevent
the re-entry of any deposit, which might have been formed on the
electrodes, into the feed water. The enriched and desalinated waters are
withdrawn separately. The former is rejected and the desalinated water is
recycled to further reduce the salt content.

Figure:

Softening of water by ion exchange process

In this method, softening of water is done by exchanging the ions causing
hardness of water with desired ions from an ion exchange resin. Ion
exchange resins are high molecular weight, cross linked polymers with
porous structure. The functional groups which are attached to the chains
are responsible for ion exchange properties. The resins containing acidic
groups which are capable of exchanging H+ ions for cations (Ca2+ or
Mg2+) present in water are known as cation exchange resins (RH+). The

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rsins containing basic groups which are capable of exchanging OH- for
anions (Cl-, SO42-) present in water are known as anion exchange resins
(ROH-).

Ion exchange process

Process: Water is first passed through a cation exchange resin which

removes the cations present in it.

RH+ + M+ RM+ + H+

2 RH+ + M2+ R2M2+ + 2H+

Where M+ is monovalent like Na+ and M2+ is divalent like Ca2+, Mg2+.

The water is then treated by passing it through an anion exchanger to

remove anions.

ROH- + X- RX- + OH-

2 ROH- + X2- R2X2- + 2 OH-

Where X- and X2- represent the anions Cl-, NO3-, F- and SO42-.

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Thus the cation and anion impurities in water are replaced by an equal
number of H+ and OH- ions respectively.

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