ce 307 weeks2and3

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WEEKS-2AND 3

CE-307

Dr. Sri Harsha KotaDepartment of Civil Engineering

IITG

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OTHER PHYSICAL EFFECTS

Color

Humic acid present in organic debris imparts

yellowish brown color to water

Iron Oxide Causes reddish color to water.

Manganese Oxides cause brown or blackish color

water.

5 Hazan units (desirable limit.)

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OTHER PHYSICAL EFFECTS

Taste and odor: Water tastes bitter when contaminated with

alkaline impurities and salty when impurities are

metallic salts.

Biological decomposition of organic debris impart aodor of rotten eggs. Which is mainly due to

Hydrogen sulphide (H2S).

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INTRODUCTION TO ENVIRONMENTAL

CHEMISTRY

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CONSTITUENTS OF AN ATOM

Symbol Charge Mass (amu)

Electron e -1 0.0005486

Proton p +1 1.0088925

Neutron n 0 1.0088665

amu= Unified atomic mass unit

1/12 of the mass of an atom in unbound neutral

carbon-12

Atomic number= Number of protons

Mass number= Number of neutrons +protons

Isotopes are forms of an element with same

atomic number but different mass numbers.

For example: Carbon-12 and Carbon-14 are

Isotopes.

An atom is an extremely small particle of matter that retains its

identity during chemical reactions.

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CHEMICAL BONDS AND INTERMOLECULAR

FORCES

Ionic bond

Eg.: HF

Covalent bond

Eg.: H2, CH4, NH3Van der Waals forces

Eg.: Between neutral

molecules of Cl2

Hydrogen bonding Between CH3OH, H2O

Strong bonds between atoms

Weak forces between molecul

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CHEMICAL REACTIONS

2C8H18+25O216CO2+18H2O

Reactants Products

Stoichiometric coefficients

Atoms are neither created nor destroyed

Net charge of reactants=products

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TYPES OF CHEMICAL REACTIONS

Precipitation-Dissolution Reactions CaCl2+Na2CO3 CaCO3(s)+2Na

++2Cl-

If reaction proceeds to right its precipitation (formation of

calcium carbonate), else its dissolution (dissolution of calcite).

Acid-Base Reactions Acid is a substance that can donate a proton to the base.

HCl+H2O H3O++Cl-

Here HCl is acid and water acts like a base.

This reaction leads to a conjugate acid (H3O+), and a base (Cl-)

NaOH+H3O+ 2H2O+Na+

Here NaOH is the base and water acts like an acid.

This reaction leads to a conjugate acid (Na+), and a base (H2O)

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Acid-Base Reactions

pH=-log{H+

} pH in natural waters should be in the range of 6-9 to

support most life.

These reactions which are fast (with half life ofmilliseconds) can be a part of biological reactions, passage ofwater through soils, acid rains, and direct release frommunicipal, households and industries.

Complexation Reactions

It occurs in natural waters whenever the coor-dination oftwo (or more) atoms, molecules, or ions results in theformation of a stable product.

Complex ion + Ligand Stable product They form through coordinate covalent bonding

Ligand is a base attached to the complex ion.

Eg: In Fe(H2O)62+ Fe2+ is the complex ion, and water is the ligand.

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Complexation Reactions

Effects the biological update of the chemical species,

the toxicity, removal efficiency of the metal.

Oxidation-Reduction Reactions

Photosynthesis and respiration are a sequence of

redox (oxidation-reduction) reactions.

Corrosion of iron metal

Fe+H2O+O2 Fe2O3

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Reactions involving gases

Diffusion of oxygen from air to water helps the

survival of aquatic life

Dissolution of CO2 gas in water leads to acidity of

pure rain water (pH=5.6).

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UNITS FOR CONCENTRATION

Weight Percent (P)

Mass per volume (mg/l) Converting mg/l to ppm for water

For dilute solutions, its assumed that water has a density

of 1 g/ml and is not effected by mass of solute.

W=Solvent mass (eg: water,

air)

W0=Solute mass

%1000

WW

WP

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UNITS FOR CONCENTRATION

Molarity

Number of moles per liter of a solution

Mole:Avagadros number of molecules of a substance.

How many molecules does a mole of Benzene have? Whats

its molecular weight?

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Normality

Number of equivalent moles per liter

Equivalent moles=mole/n

n is number of

electrons transferred in redox reactions

Hydrogen ions transferred in acid-base reactions

Hydrogen ions required to replace the cation in

precipitation reactions

Whats a cation?

H2SO4, Ca2+, CO32-, CaCO3

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PROBLEM-CONCENTRATION UNITS

84g of sodium bicarbonate (NaHCO3) added to 1

L of water in a volumetric flask. Express the

concentrations in mg/l, ppm, molarity and

normality.

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CONCENTRATION UNITS

Expressing in CaCO3 equivalent

mg/l as CaCO3=(mg/l as species)(EW of CaCO3)/(EW

of species)

Convert 1M of NaHCO3 into CaCO3 equivalent

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CONCENTRATION UNITS

For gases g/m3 and ppm are usually used

As in dilute aqueous solutions which have a fixed

density of 1g/ml, airs density isnt fixed.

At normal conditions, convert the 1-hr

average Ozone concentration of 300 g/m3

to ppm

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EQUILIBRIUM CONSTANT

Equilibrium in case of solubility calculations

AaBb(s)aAx++bBy-

Mg(OH)2(s)Mg2++2OH-

by

ax

s BAK

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Acid-base reactions

Strong acids completely dissociate in water.

Weak acids only partially dissociate

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PROBLEM

Acetic acid solution was prepared by adding 60

mg of CH3COOH to a volumetric flask, and

adding water to 1 L mark. Despite of this, the

water was neutral. What are the concentrations

of individual constituents in the solution?Assumed that the temperature is 25oC. pKa is

4.75

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Gas-Liquid Equilibrium

The relation between partial pressure of a gas and

its corresponding concentration in aqueous solution

is given by Henrys Law.

Pgas=KHCaq Pgas is partial pressure of a gas (kPa)

Caq is aqueous concentration of dissolved gas in water

(mol/L)

What are units of KH?

Whats the relation when concentration of gas is expressedin mol/m3?

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PROBLEM

The concentration of a gas in water is 1M. Whats

the partial pressure, in the units of kPa, of that

gas in air? Use, Henrys constant for that gas as

1M/atm.

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Gas-Liquid Equilibrium

If concentrations are expressed in mole fractions

x=Pgas/KH,m

Here: x is equilibrium mole fractions

ng is moles of gas

nl is moles of liquid

lg

g

nn

nx

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PROBLEM

Calculate the solubility of air in water at 00C and

1 atm pressure. Assume other dissolved material

is negligible.

KH,m=4.32E+4 atm/mol fraction

M.Wt. of air=28.9 g/mol

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ALKALINITY

Alkalinity is the ability of water bodies to

neutralize added acids.

This acid-neutralizing capacity is important to

figure out how buffered the water is against

sudden changes in pH.

Its primarily due to presence of bicarbonate,

carbonate and hydroxide ions.

Salts of weak acids such as borates, silicates and

phosphates may also contribute, but to a lesser

extent

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ALKALINITY

Prominent ways of carbonates entering into water

bodies:

Water flowing through limestone or marble

CaCO3 (s) Ca2++CO3

2-

CO32-+H2O HCO3-+OH-

Storm water runoff through lawns and agricultural

fields, which use lime (calcite etc.). Lime is used to

neutralize clay soil and ammonia based fertilizers

which produce acid when they are decomposed.

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ALKALINITY

Transfer of CO2 from air to water

CO2+H2O H2CO3

H2CO3 HCO3-+H+

HCO3- CO3

2-+H+

Note: Unlike CaCO3, carbonic acid to

carbonate conversion leads a falls inpH.

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ALKALINITY-MEASUREMENT

Alkalinity is expressed as mg/l of CaCO3 Usually 0.02N H2SO4 is used in the titration

1 ml of acid will neutralize 1mg of alkalinity as CaCO3

H+ ions from acid react with the alkaline species as:

OH-+H+ H2O

CO32-+H+ HCO3

-

HCO3-+H+ H2CO3

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ALKALINITY-MEASUREMENT

Two infliction points

pH=8.3; Phenolphthalein Alkalinity

OH-+1/2 CO32-

Phenolphthalein is used as indicator, and color changes

from pink to colorless pH=4.5; Total Alkalinity

1/2 CO32- + HCO3

-

Methyl orange is used as indicator, and color changes from

orange to red

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PROBLEM

A 200-ml sample of water has an initial pH of 10.

30 ml and 11 ml of 0.02N H2SO4 is required to

titrate the sample to pH of 4.5 and 8.3

respectively. Determine the quantity of each

species and total alkalinity.

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HARDNESS

Hardness is defined as sum of all multivalent

cations in solution.

They are two types:

Temporary Hardness or carbonateHardness (CH):

Carbonates and bicarbonates of calcium and

magnesium.

They can be removed by boiling

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HARDNESS

Permanent Hardness or non-carbonate

hardness (NCH)It is usually caused by the presence of calcium and

magnesium sulfates, nitrate and chlorides in the water,

which become more soluble as the temperature rises.Permanent hardness is hardness of water that cannot be

removed by boiling.

This can be removed using a water softener, or ion

exchange column

NCH=TH-CH

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HARDNESS

Impacts

Excessive soap consumption

2 NaCO2C17H33 +cation2+

cation2+(CO2C17H33)2 +2Na+

Soap Precipitate

Lathering doesnt occur untill all of the hardness ions

are precipitated.

The precipitate formed adheres to surfaces of tubs,

sinks, and dish washers, and may stain clothing,dishes and other items.

Residues of the precipitate may remain in the pores,

so that skin may feel rough and uncomfortable.

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Fouling of water heaters and hot water pipes , because

of scaling of carbonate hardness precipitate.

Magnesium hardness, particularly associated with the

sulfate ion, has laxative effect on persons

unaccustomed to it.

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HARDNESS- MEASUREMENT

TH=Sum of all multivalent ionsAlkalinity=

[HCO3-]+[CO3

2-]+[OH-]-[H+]

@ pH of 6.5-8.3, its assumed that

[OH-]=[H+]

[CO32-]=0

CH is the least of TH or Alkalinity

NCH=TH-CH

Note: Units: mg/l as CaCO3 equivalent

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HARDNESS- MEASUREMENT

EDTA Titrimetric method

Ethylenediaminetetraacetic acid (EDTA) reacts with

multivalent ions to form a complex.

EDTA+M [M.EDTA]complex

We use Eriochrome Black T (EBT)as a titrant to seeif total hardness is removed.

EBT+M Wine red color complex

When TH is converted to EDTA complex, then

aqueous solution changes from wine red to blue.

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Chemical Oxygen Demand (COD)

Nearly all organic compounds can be oxidized

completely using a strong oxidizing agent under

acidic conditions

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COD-MEASUREMENT

A sample is refluxed in strongly acid solution

with a known excess of potassium dichromate

(K2Cr2O7).

As we are using excess potassium dichromate

(K2Cr2O7), not all Cr6+ converts to Cr3+

The left over Cr6+ is estimated by titration with

ferrous ammonium sulfate with ferroin as

indicator.

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BIOCHEMICAL OXYGEN DEMAND (BOD)The amount of oxygen consumed during microbial

utilization of organics, as a food source, is called BODL=oxygen equivalent of organic chemicals remaining

(mg/l)

k= reaction rate constant

L0= Maximum oxygen consumption possible

Are L0 and ThOD same?

0 (1 exp( k ))t

dLkL

dt

BOD L t

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BOD

k is temperature dependent

vant Hoff-Arhenius model

=1.047

20

20k k T

T

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PROBLEM

The BOD5 of a waste water is determined to be

150 mg/l at 200C. What would the BOD8 be if the

test was run at 150C? Assume k=0.23 day-1

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BOD

BOD=CBOD+NBOD

Carbonaceous BOD (CBOD):

Its a result of breakdown of organic molecules like glucose

into CO2 and H2O

Nitrogenous BOD (NBOD):

Its a result of break down of organic molecules like

proteins (which have N) releasing N as NH3 in water.

At normal pH, this ammonia is in the form of NH4+

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NBOD

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PROBLEM

Estimate BOD5 of 113 mg/l of C5H7O2N. Given

decay rate is 0.15 day-1

C5H7O2N+5O2 5CO2+2H2O+NH3

NH3

+2O2

NO3

-+H++H2

O

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DO SAG CURVE

All rivers have a self purifying capacity

As the waste inside a river increases, it looses its

capacity to cleanse itself.

If DO drops below 4 mg/l, most of the aquatic life

will be effected.

But isnt the oxygen in air always in equilibrium

with DO in water?

DO SAG CURVE

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DO SAG CURVESTREETER-PHELPS MODEL

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PROBLEM

The wastewater is found to have a maximum flow rate of15000 m3/day, a BOD5 of 40 mg/l, a dissolved oxygenconcentration of 2 mg/l, and a temperature of 250C. A slowmoving stream, with a flow rate of 0.5 m3/s, a BOD5 of 3mg/l, a dissolved oxygen concentration of 8 mg/l , and a

temperature of 220

C. Complete mixing of wastewater andstream is almost instantaneous, and the velocity ofmixture is 0.2m/s.

Where and when does the maximum DO deficit occur?

Assume:

k1= 0.23day-1

k2=0.4 day-1

Equilibrium concentration of DO at 22.80C= 22.8 mg/l

ce 307 weeks2and3

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