chem book f(25510)
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14
16
10
6
2
2 6 10 14 18 22
20
-2
0
-6
-10
pH
pe
O x y g e n ( p O 2 = 1 )
H y d r o g e n ( p O 2 = 1 )
0
O x y g e n ( p O 2 = 1 0 - 7 0
)
SO 4-2/H2S line
S O 4 - 2 r e g i o n
H 2 S r e g i o n
M n O 4 - r e g i o n M n + 2
r e g i o n
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CHAPTER-14
POURBAIX DIAGRAM
14.1. POURBAIX DIAGRAM EXPLAINED
During the early part of the 20 th century, Macel Pourbaix, a Belgian born inRussia, invented a powerful graphic technique for studying the equilibrium phases of anaqueous electrochemical system. The resulting pe-pH diagrams, identifying thepredominance region for a chemical species in a specific oxidation state, are named aftertheir inventor.
14.2. POURBAIX DIAGRAM: AN EXAMPLE
In order to understand a Pourbaix Diagram, let us look at Iron, in its variousoxidation states, in an aqueous solution. For this we shall draw various boundary lines ona graph with pH along the horizontal axis and pe along the vertical axis.
14.2.1. THE Fe +2 /Fe BOUNDARY
We start with the pure metal in its lowest oxidation state as Fe o or moresimply Fe. The next higher oxidation state of Iron is +2 and the boundary for theequilibrium is governed by the following equation:
This equation does NOT contain any H + term, and with the pe o value of -
7.45 it converts to:
. This, as shown in the diagram below, is Line (1) parallel to the pH axis
intersecting the pe axis at -7.45.
14.2.2. THE Fe +3 /Fe +2 BOUNDARY
The next item to be considered is the progress of Iron from oxidationstate +2 to oxidation state +3. Again, we first write the reduction equation:
With a pe
o
value of 13.03 the relevant equilibrium equation is:
The curve in this case, again, is a horizontal line plotted in the figure asLine (2).
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14.2.3. THE Fe(OH) 2 PRECIPITATE
We now consider the Fe(OH) 2 and Fe(OH) 3 precipitates. For this, wefirst write the dissolution equations and then use the relevant K sp values. ForFe(OH) 2 the equation is:
Hence:
With a pK sp value of 14.1, under equilibrium conditions, this equation
transforms to:
Using pH + pOH = 14, we, therefore, have the equation of Line (3) as:
14.2.4. THE Fe(OH) 3 PRECIPITATE
We now consider the solubility equilibrium for Fe(OH) 3 as:
As, above, with a pK sp value of 37.2 we have:
. The equation for Line (4), therefore, is:
. The general boundaries, where there is no change in oxidation number
[Fe +2 to Fe(OH) 2 and Fe+3 to Fe(OH) 3], or where there is change ONLY in the
oxidation number (Fe to Fe +2 and Fe +2 to Fe +3), without involvement of ahydrogen ion (H +), have been identified by these four lines. We now proceed withmore complex situations.
14.2.5. THE Fe(OH) 3 /Fe+2 BOUNDARY
For conversion of Fe +2 to Fe(OH) 3 we have the reaction:
The general equation is:
Since the line passes through the point (1.6, 13)
.
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Subtraction of the two equations then gives us the equation for Line (5)as:
. It is important to note here that this line intersects the pH = 7 line at (7, -3.5).
14.2.6. THE Fe(OH) 3 /Fe(OH) 2 LINE
The next line of our interest is the Fe(OH) 3/Fe(OH) 2 line. For this, wewrite the reduction reaction as:
The general equation is:
Since the line passes through the point (7, -3.5):
. Subtraction of the two equations then gives us the equation for Line (6)
as:
.
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14
16
10
6
2
2 6 10 14 18 22
20
-2
0
-6
-10
pH
pe
26
Fe(OH) 2/Fe+2
Fe(OH) 3/Fe+3
Fe +2 /Fe +3
Fe o/Fe +2
1
2
3
5
(7, -7.45)
(1.6, 13)No Fe +2above this pe
No Fe abovethis pe
NoFe(OH) 3
belowthis pH
NoFe(OH) 2
belowthis pH
6
7
H2O/O 2
H2/H2O
(7, -3.5)
4
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14.2.7. THE Fe(OH) 2 /Fe BOUNDARY
This leaves us only with the Fe(OH) 2/Fe boundary for which thereduction equation is:
The general equation is:
Since the line passes through the point (7, -7.45):
. Subtraction of the two equations then gives us the equation for Line (7)as:
. 14.3. PREDOMINANCE REGIONS
With this information we can now clearly identify the predominance regions forthe different Iron species that may be formed under various pe and pH combination inaqueous media. The Pourbaix diagram indicating the predominance regions for Iron isthus shown below:
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14
16
10
6
2
2 6 10 14 18 22
20
-2
0
-6
-10
pH
pe
26
Fe(OH) 2/Fe+2
Fe(OH) 3/Fe+3
Fe +2 /Fe +3
Fe o/Fe +2
1
2
3
5
(7, -7.45)
(1.5, 13)
6
7
H2/H2O
H2O/O 2
Fe(OH) 3
F e ( O H ) 2 Fe
Fe +2
Fe +3
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CHAPTER-15
REACTION RATES AND ORDER
15.1. INTRODUCTION
Chemical reactions have rates that, at a given temperature, are (in exceptionalcases) independent of reactant concentrations, but are more likely to be proportional tothe concentrations of the reactant involved raised to the power of 1 or more. The order ofthe reaction, as shown below, is thus labeled as zero, or 1 and 2 etc., depending onwhether it is concentration independent (0 order) or otherwise:
Reaction Rate = -dC/dt = k (Zero Order) Reaction Rate = -dC/dt = kC a (First Order)
Reaction Rate = -dC/dt = kC a2 or kC aC b (Second Order)
Reaction Rate = -dC/dt = kC a3, kC aCbC c, or kC aC b
2 etc. (Third Order)
15.2. GRAPHICAL PLOT OF A ZERO ORDER REACTION
In a zero order reaction, where the rate is independent of the concentration, aplot of concentration C versus time t yields a straight line, the slope of which yields thevalue of the rate constant k.
15.3. GRAPHICAL PLOT OF A FIRST ORDER REACTION
For a first order reaction the equation, the simple differential equation -dC/dt =kC, has to be solved.
Separation of variables yields:
This implies:
Application of the boundary condition C = C o at t = 0 yields the equation:
Converting to log 10 and rearranging we have:
.
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As indicated by this equation, the plot of log C vs. time would yield a straight linewith the slope giving us the value of the rate constant.
15.4. GRAPHICAL PLOT OF A SECOND ORDER REACTION For the second order, let us consider the simplest case -dC/dt = kC 2.
With the separation of variables, we have:
With the boundary condition C = C o at t = 0 integration leads to:
Thus, if a plot of 1/C versus time yields a straight line, the reaction is of the
second order, with the slope directly yielding the value of the reaction rate constant k.
15.5. CONSECUTIVE REACTIONS
Consecutive reactions are complex reactions of great importance in the field ofEnvironmental Engineering. As the name implies, in such reactions, the products of onereaction become the reactants for the next one and so on. A classic process, fallingunder this category is when the aeration of a water body results in the addition of oxygento it while the degradation of the organic matter in the water consumes this oxygen. Theequation governing this process, called the Streeter-Phelps equation, is well known to allEnvironmental engineers. Starting from the basic kinetic principles, a simple derivation ofthis equation is given below.
15.6. THE STREETER-PHELPS EQUATION Let us consider an element of water in a stream with volume V. Any change in
the oxygen concentration, within this element, would obviously depend on the input andoutput. More precisely:
Now the rate of aeration is linked to the degree of oxygen un-saturation in water.
The higher the un-saturation, the higher the rate of oxygen absorption, by the waterelement, from the atmosphere. Thus, if C s is the saturation concentration and C is theconcentration at any particular time, with k r as the re-aeration rate constant, the aerationrate is given by:
Organic matter in a polluted water body is degraded by microorganisms usingoxygen. In a stable microorganism population, the rate at which the food organic wastein this case is consumed is directly proportional to the concentration of the foodpresent. In Biochemical Oxygen Demand (BOD) studies, this aspect is looked at in terms
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of the Net Oxygen Demand L. Thus, with k d as the oxidation rate constant, the rate ofbiodegradation is given by the rate of change in L as:
With the boundary conditions L = L o at time t = 0, the solution of this simple
differential equation leads to:
Substituting this value of L in the preceding equation yields us the rate of
oxygen consumption for biodegradation as:
Omitting the O 2 subscript from the concentration symbol C, we thus have:
Mathematically, it is more convenient to express everything in terms of the
oxygen deficit D = (C s C). Thus:
Rearrangement yields an easily solved differential equation of the type:
The standard method of solving this equation involves multiplication of each term
by ePt
. In our case the multiplication term istk r
e which leads us to the equation:
The expression on the left, now is simply:
On the right, combining the two exponential terms, we get:
Separating the variables, we have:
The solution of this equation leads to:
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Applying the boundary condition that at time t = 0 the deficit D = D o, we have the
value of the Integration Constant as:
The final relationship giving the oxygen deficit as a function of time known as the
Streeter-Phelps equation is:
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CHAPTER-16
ADSORPTION ISOTHERMS
16.1. APPLICATION OF ADSORPTION
One of the most important uses of adsorption in Environmental engineering hasbeen for the removal of organic material from waters and wastewaters. Examples includethe removal of taste and odour producing organic materials and other trace organiccontaminants such as trihalomethanes, from drinking waters; removal of residual organiccontaminants from treated wastewater effluents; and treatment of leachates, industrial
wastewaters, and hazardous wastes.In such applications activated carbon, granular or powdered, may be mixed with
the water and then removed with the adsorbed materials by settling and filtration. Whenlarge quantities of organic material must be removed, more efficient usage of carbon andhigher quality of water can be obtained by passing the water through a carbon filter bedof large depth. Use of such systems to manage a wide variety of contamination problemsis now quite common.
16.2. THE ADSORPTION PROCESS
The phenomenon of adsorption is easily understood by considering a solution,with high concentration of a contaminant, in contact with a solid adsorbent material. Atthe interface of the two different phases (liquid and solid) the different molecular
interactions make the contaminant molecules to concentrate more within the interface, ascompared to the bulk liquid phase. This phenomenon is called adsorption and theinterface is often referred to as the adsorption space or adsorbed phase . The speciesabsorbed are called adsorbate and the solid localized (non mobile) species, providingthe adsorption surface, is called the adsorbent.
If the mobile molecules can penetrate into the bulk of the other phase, then thisprocess is called absorption . It is sometimes difficult or impossible to distinguishbetween adsorption and absorption and it is convenient to use the wider term sorption .
The solid adsorbents are often characterized by their specific surface area (a s)and pore size distribution. The value of a s refers to unit mass (m) of adsorbent providinga surface are A s :
The pore size distribution provides information on the size and amount of pores
present in the solid adsorbent. Thus Macropores are pores with widths exceeding about50 nm, Mesopores are pores of widths between 2 nm and 50 nm, and Micropores arepores with widths not exceeding 2 nm.
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16.3. ADSORPTION ISOTHERMS
In any scientific field it is always useful if the underlying phenomenon could bedescribed through mathematical equations. This helps, not only in predicting the systembehaviour under different conditions but allows us in modeling and designing efficientprocesses for real life applications.
Like other processes involving molecular interactions, temperature is very criticalin adsorption and even a slight change in temperature can alter the results considerably.Consequently all experimentation, and theoretical development, involving adsorption hasto be done at a known unvarying temperature. Not surprisingly, the equations describingthe adsorption behavior, and the resulting curves, are known as isotherms.
Because of its importance, many attempts have been made to developmathematic models for adsorption. Some of the more important isotherms resulting fromthese efforts are discussed below.
16.4. HENRYS ISOTHERM:
It is very logical to assume that the amount of material adsorbed per unit weightof the adsorbent (generally symbolized by the letter q) on the surface of an adsorbentwould be dependent on the concentration (C) of the material in the solution.
Thus:
With K as the constant of proportionality, the equation becomes:
The equation is referred to as Henrys Isotherm .
16.5. THE FREUNDLICH ISOTHERM :
Henrys isotherm, although found to be applicable to many simple systems,particularly with dilute solutions, is definitely an oversimplification. Through many yearsof experimentation and study of the adsorption behaviour Freundlich developed a purelyempirical relationship.
This relationship, called Freundlich Isotherm states that the mass of adsorbateper unit mass of the adsorbent (q) depends on the concentration C of the contaminantraised to an exponent n. Thus,
The Freundlich isotherm is often expressed in its logarithmic form as:
Experimental data are often plotted in this manner as a convenient way of
determining whether the material removal is taking place or not and if it is following theFreundlich isotherm, what are the values of the constants K (intercept) and n (slope).
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It may be noted that, in the special case when the slope is unity (n = 1), theFreundlich equation reduces to the Henrys isotherm.
16.6. THE LANGMUIR ISOTHERM
Let us suppose that an adsorbent has absorbed q grams of a contaminant pergram of its weight, from a solution in which the molar concentration of the contaminant isC. Now, the rate of adsorption is proportional to the concentration C:
Also, the adsorption rate is proportional to the probability of an adosrbate
molecule striking the adsorbent surface. The latter is, of course the ratio between theamount adsorbed at any one time to the maximum amount that can be adsorbed. Thus:
With the constant of proportionality k a , we can write:
The rate of desorption is, on the other hand, is dependent on the probability of a
molecule leaving the surface which is actually the ratio between the amount adsorbedand the maximum amount absorbable:
Hence, with a constant of proportionality k d we have:
Now, under equilibrium conditions, the rate of adsorption and desorption must beequal. Thus:
Rearrangement leads to:
Taking k d/ka as a new constant a and dividing, throughout by q, followed by
rearrangement, we get:
This equation represents the Langmuir Isotherm in which if C/q is plotted
against C, a straight line would be obtained, from which the constants a and q m can beevaluated using the above equation.
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16.7. THE BET ISOTHERM
The isotherm is so named because it was developed, about a century ago, bythree scientists named Stephen Brunauer, Paul Hugh Emmett and Edward Teller.
Consider a surface of where multilayer adsorption of molecules has taken placein. Limiting ourselves to four layers we represent this phenomenon pictorially as:
16.7.1. FIRST LAYER ADSORPTION:
Adsorbent
MonoLayer
BiLayer
TriLayer
Tetra Layer
A0
A1
A2
A3
A4
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Under equilibrium conditions, when both rates are equal, we have:
16.7.2. SECOND LAYER ADSORPTION:
Under equilibrium conditions, when both rates are equal, we have:
A0
A1Rate of Adsorption
= k 1CA0
Rate of Desorption= k -1 A1
A1
A2
Rate of Desorption= k -2 A2
Rate of Adsorption= k 2CA1
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16.7.3. THIRD LAYER ADSORPTION:
Under equilibrium conditions, when both rates are equal, we have:
16.7.4. DERIVATION OF THE BET EQUATION:
In general, for the ith layer:
With B as a constant of proportionality the Total quantity q of the
material adsorbed is given by the equation:
Replacing with the values found above:
} }
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Now, the amount (q m) adsorbed in a single layer and completely coveringthe surface is related to the total area, i.e.
This implies:
Dividing the expressions for q m and q eliminates the constants A o and
B giving us:
Now consider the solution to be saturated at the saturation concentration
Cs. We now expect a multilayer deposition of a very large amount on thesubstrate. This amount, in comparison with the amount deposited due toadsorption, is infinite. In order for q become infinite, one of the terms in thedenominator on the right of the above expression has to be zero, when C = C s .
Letting (1 bC s) to be zero, gives us the value of b as 1/C s. C s is a constant thevalue of which may, generally be found in the literature, or it can beexperimentally established. Substitution of the value of b now leaves us with anequation which has only two un-known constants q m and a:
In order to factorize the expression on the right hand side, we transferthe higher degree polynomial to the numerator by taking reciprocal on both sides.The resulting equation is:
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The problem now is to transform this equation in such a way that we
have a linear equation through which we could form two independentexpressions that could then be used for determining the values of the variablesa and q m. Rearrangement gives us:
or
This is the BET equation whereby the plot of CC s /q(CC s) against theconcentration C yields a straight line with the slope (aCC s-1)/aC sqm and theintercept 1/aq m. Form the two equations the two constants a and q m can beeasily found.
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CHAPTER-17
WATER DISINFECTION
17.1. NEED FOR DISINFECTING WATER
Contamination of drinking water by microorganisms can cause a variety ofillnesses. It is important, therefore, to control water quality for the prevention of waterborne diseases.
17.1.1. INFECTIOUS AGENTS IN WATERThe infectious agents in water, derived primarily from the faces of the
infected human beings, and animals, may be classified into four groups:
a. Bacteria: Human pathogenic bacteria enter water mainly throughdomestic sewage and are the cause of many diseases likeTyphoid, Cholera, Diarrhea, Tuberculosis and Shigellosis, etc.
b. Viruses: Humans excrete a large number of viruses, capable ofproducing infection. These may cause a number of illnessesincluding Diarrhea, Meningitis, respiratory illness andGastroenteritis, etc.
c. Protozoa: These single celled animals behave as parasites in ahost organism can cause Amoebic Dysentery, AmoebicHepatitis, Cryptosporidiosis and Giardiasis, etc.
d. Intestinal Parasite (Helminthes): Several helminthic parasitesthat can be found in sewage are a potential hazard to publichealth. The most important of these are hookworms, pinworms,intestinal roundworm, threadworm, and the whipworm.
A comprehensive strategy for providing safe non-infective water would,therefore, involve a two-step approach:
a. Reducing the pathogen concentration in treated domesticsewage, before it enters the water bodies that are later to serveas drinking water sources.
b. Inclusion of Disinfection as the final step in the treatment of
water before distribution to the public for domestic use.
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has been receiving considerable attention in the recent years. Many householdunits now incorporate a UV lamp as an essential means for killing bacteria.
Pure water absorbs UV light to a significant level. This absorbanceincreases further in the presence of a number of commonly dissolvedcontaminants. The cross section of flow and the flow speed has, therefore, to beappropriately controlled for contact time optimisation in order to provide effectivedisinfection. Because of these limitations, UV disinfection systems generally findapplications in small, household units only.
17.3.3. PHOTOCHEMICAL DISINFECTION
Following upon the use of n-Type semiconductor particles like TiO 2 forthe oxidation of organic compounds experimental work has shown, in the recentdecades, that in presence of sunlight, or artificial UV radiation, a number ofbacterial species are killed very efficiently. For the practical application of such a
disinfection system, semiconductor particles are immobilized in polymermembranes reactor and the water is allowed to flow over the membrane whilebeing irradiated by a UV lamp. Systems for large-scale commercial applicationsbased on this principle have yet to be developed.
17.4. CHEMICAL DISINFECTION
17.4.1. OZONATION
Ozone (O 3) is an allotrope of oxygen that has a pungent smell and is avery strong oxidizing agent. It is soluble in water and, due to its oxidising effect,burns the pathogens present in water. Ozone is generated by allowing air topass through a 1-3 mm thick electric field whereby the oxygen molecules areconverted to O
o free radicals. These radicals immediately react with the oxygenmolecules converting these to form ozone. Although the method, like UVdisinfection, is energy intensive, easy availability of the raw material like air andoxygen (delivered on site-generated), makes it one of the most attractivealternatives to chlorine (discussed below). The method is thus very commonlyemployed at water treatment plants all over the world.
17.4.2. CHLORINATION
Of the chemical methods of water disinfection, adding chlorine is one ofthe most important ones. Before proceeding further we have, therefore, to studythe chemistry of this compound in detail. Chlorine (Cl 2) in the gaseous orliquefied form is very soluble in water. One reason of this high solubility is the factthat on coming in contact with water it immediately looses its identity and is
converted to the strong acid (HCl) and a comparatively weak but soluble acid(HOCl). The latter, has a pK a value of 7.5. A predominance region diagram forCl2 is shown below:
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17.4.3. CHLORINE DISINFECTION: OPTIMUM pH
From the above diagram, some very useful information can be derived.For one, it is seen that at neutral pH values almost all of the original Cl 2 has beenconverted to other chemical species. The active disinfectant must, therefore, beeither HOCl itself or its anion OCl -. These chlorine species are such strongoxidisers, pe >> pe (oxygen) , that water is effectively oxidised releasing oxygen.
In absence of sunlight (which catalyses the process), this step proceedsslowly but is the major pathway for the degradation of chlorine in water, with time.
The concentration versus pH diagram below also confirms the presence
of HOCl and OCl-
only at neutral pH values. Although both these species aregood disinfectants, the non-ionised HOCl is around one hundred times moreeffective. Efforts are, therefore, made that, during disinfection, and transportationto the consumer, a neutral or slightly acidic pH is maintained in the water.
1 2 3 4 5 6 7 8 9 10 11 12
28
27
26
27
25
24
23
22
20
21
Cl2
Cl -
OCl -
HOCl
p e
pH
Oxygen
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17.4.4. RESIDUAL CHLORINE
Chlorine and hypochlorous acid react with a wide variety of substances
including ammonia. To form monochloramines, diclhoramines, and trichloraminesdepending upon the reactant concentration and pH:
NH3 + HOCl NH 2Cl + H 2O
NH2Cl + HOCl NHCl 2 + H 2O
NHCl 2 + HOCl NCl 3 + H 2O
These amines have a significant disinfecting ability and are, therefore,very important from an Environmental engineering point of view. Whereas freechlorine (Cl 2, HOCl or OCl
-), because of its strong oxidising power, dissipatesvery quickly in water, the combined chlorine, called the residual chlorine, in theform of the chloramines serves a useful purpose as these compounds are fairlystable and may provide a disinfection coverage throughout the water distribution
system. Thus, one of the final steps in water treatment is the addition ofammonia.
17.4.5. CHLORINATION DRAWBACKS
Discussion about chlorine disinfection remains incomplete withoutmentioning the negative aspects of chlorination. In the presence of phenols inwater, chlorophenols are produced which impart taste and odour to the water.Similarly, chlorine reacts with trace organic molecules, present in water, to formtrihalomethanes (THMs) of which chloroform is one example. These compoundsare considered as human carcinogens, and now a days the use of chlorine as adisinfectant is being discouraged in many parts of the world. The benefits fromchlorine, particularly for the developing countries, where the chances of watercontamination are very high, however, outweigh the risks and it is still therecommended method.
2 643 5 7 81
HOCl
Cl2
OCl -
C
Co
pH9 131110 12 14
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CHAPTER-18
SOIL POLLUTION AND DECONTAMINATION
18.1. THE ORIGIN AND CHARACTERISTICS OF SOIL
Soil is the product of weathering process of rocks and contains air spaces, andhas a loose structure. The solid fraction is typically around 5% organic and 95%inorganic. Soils like peat may, however, have 9% organic component.
18.1.1. SOIL HORIZONS
Typical soils exhibit distinctive layers with increasing depth. These layers
are called A, B, C Horizons. Horizon A is the top soil which contains most of theorganic matter and where all biological activity takes place.
Horizon B is the sub-soil receiving the leached material with rainwater,form the top soil.
Horizon C is the weathered parent rock and is called the transition stateof the soil formation. The rock from which soils are formed is very important asthe properties of the soil are inherited from the bedrock.
18.1.2. SIZE FRACTIONS OF SOIL
The size fractions of soil are classified as:
Gravel (2 60 mm)
Sands (0.06 2 mm) Silts (0.002 0.06 mm)
Clays (less than 0.002 mm)
18.1.3. WATER IN THE SOIL
Water in the soil is required for plant material as it is the basic transportsystem taking nutrients from the soil into plant roots and to the leaf from wherewater transpires to the atmosphere.
Due to small pores and capillaries, water is bound in the soil. Water isalso adsorbed on the surface of the clay particles. Generally, a soil is one-thirdair and two- third water. In a waterlogged soil, with the exception of rice, most
plants cannot grow.The aqueous portion of the soil containing dissolved matter in the form of
salts from the products of chemical and biochemical reactions is the soil solution.The productivity and other properties of a soil can thus be studied by extractingthe solution. As it is bound up in pores and capillaries, the extraction of solution is
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not easy and methods may include Centrifugation, Suction, Replacing water withimmiscible fluid and Freezing.
18.1.4. CHEMICALS IN THE SOIL
The soil or the water in it may contain a number of important of inorganicand organic contaminants. These include plant nutrients such as nitrate andphosphate, heavy metals, organic chemicals, inorganic acids, and radionuclides.The sources of these contaminants are fertilizers, pesticides, acidic deposition,agricultural and industrial waste material, and radioactive fallout.
Our atmosphere is 21% O 2 and 0.03 % CO 2. Because of the bacterialactivity the soil may, however, be lower in O 2 (below 15%) and high in CO 2 (5%).
18.2. HUMUS
Of the organics, Soil HUMUS is the most significant component. Decompositionof organics proceeds through the breakdown of layers of polymeric substances to smallerchains containing acidic groups. This portion of the soil, soluble in alkaline solution iscalled HUMUS. A part of Humus, is also soluble in acidic solutions and is called FULVICacid. Most of the C is converted to CO 2 which seeps out of the soil, to the surface, andevaporates. HUMUS thus becomes richer in nitrogen and the N:C ratio becomes 1:10 ascompared to 1:100 in the original plant material and thus serves as a good nitrogenfertilizer.
Humus is very important for the soil as it:
Stabilizes the soil particles
Increases the water holding capacity of the soil
Because of the acid-base behaviour serves as a buffer
Bind, through chelating, heavy metal ions
Traps organic pollutants which are hydrophobic
18.3. MACRONUTRIENTS IN THE SOIL
Macronutrients required by plants substantial amounts include C, H, O, N, P, S,K, Mg and Ca. Soils are generally deficient in N, P, K that need to be supplied throughthe application of artificial fertilizers. For this reason for a soil or compost NPK value isdetermined.
N is carried from dead plants and animals as NH 4+ oxidised to NO 3
-. Plants mayabsorb excess NO 3
- from the soil which may be transferred, through the food chain, tohumans. In addition, when crops are stored in silos nitrate may be reduced to NO 2, whichis toxic. Nitrate pollution, due to fertilizers etc., is thus becoming an Environmentalproblem.
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18.4. SOIL POLLUTION
Pollution of the soil is dependent, to a large degree, on the quality of waterpercolating through the soil. Agricultural runoffs invariably contain Phosphorus andNitrogen. Phosphorus, in general, is tightly bound to the topsoil, whereas the nitrate-N isleached, particularly through sandy soils, and may contaminate the ground water.
The increased use of pesticides has resulted in traces, and higher concentration,of these chemicals being frequently found in water samples from many sources. Soilswith intensive agricultural activity are, therefore, very likely to act as a repository of thesecompounds which are then available to contaminate the irrigation water that reaches theunderground reservoirs by seeping through the soil.
Trace elements, including As, Cd, Cr, Co, Pb, Hg, etc., are importantcontaminants of the soil. The sources of trace elements are soil parent material (rocks),commercial fertilizers, liming material, biosolids, irrigation water, coal combustion
residues, metal-smelting industries, vehicular emissions, etc.The above were mostly non-point sources of soil pollution; amongst the point
sources is essentially the hazardous wastes including mining waste, acid mine drainage;wastes from metal smelting and refining industries; pulp and paper industry waste;petroleum refining wastes; wastes from paint and allied industries; pesticide storagesites; and municipal solid waste sites.
18.5. REMEDIATION OF THE CONTAMINATED SOIL
A number of techniques to decontaminate polluted soils have been looked at.These include:
In Situ Methods
Volatilization Biodegradation
Phytoremediation
Leaching
Vitrification
Isolation/Containment
Passive Remediation
Ex Situ Methods
Land Treatment
Thermal Treatment
Asphalt Incorporation Solidification/ Stabilization
Chemical Extraction
Excavation
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101
CHAPTER-19
ENVIRONMENTALLY SIGNIFICANT ORGANICCOMPOUNDS
19.1. PESTICIDES
Pesticides are defined as materials that are used to prevent damage toagricultural items during the stages of sowing, growth, harvesting and storage.Compounds used to control algal and fungal growth and termite attack on wood, are alsoincluded in this class.
Although some inorganic salts have been occasionally used as pesticides,present day compounds are invariably organic in nature. Pesticides can thus be dividedinto a number of classes the more important ones being:
Algaecides: Control the growth of algal blooms in swimming pools andreflecting ponds
Fungicides: Prohibit fungal growthHerbicides: Help in getting rid of the weeds and other undesirable plantsInsecticides: Control household insects like ants and cockroaches etc.Rodenticides: Kill rodents burrowing the ground or feeding on stored grainTermiticides: Prevent termite attack on wood structures
19.2. ENVIRONMENTAL SIGNIFICANCE OF PESTICIDESThe classical methods of pesticide control involved repelling or killing the
attackers. Recent advances in this field, however, focus on pest attractants for trappingand disrupting the mating process aimed at reducing the population growth; alternatelymating with sterile females of a species also achieves the same objective. In spite ofsuch developments, the use of chemical pesticides, which are highly poisonoussubstances, is prevalent throughout the world. Through bioaccumulation of the parentcompound, or its metabolites, pesticides are likely to cause short and long-term damageto animals and human beings. Their presence in the Environmental systems, therefore, isof serious concern to an Environmental Engineer.
19.3. PHOSPHORUS BASED PESTICIDES
The use of organophosphate compounds as pesticides was an offshoot ofresearch on nerve gas research in Germany during the Second World War. The Germanscientist Gerhard Schrader, in 1941, discovered that the compound octamethylpyro-phosphoric tetra amide had insecticidal properties.
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ir
f op
1
nsconC
Likereversibly in
nctions in in the first or
opular pestici
9.4. CHL
Chloriumber of yeability in theain and cau many such
ow been glohlordane, En
he nerve gactivate the
ects, humananophosphodes, in this cl
Schra
Parath
RINE BA
ated pesticirs are one
envIronment,se damage tcompounds -bally bannedrin, Heptac
ases Sevinenzyme ac
s and manyrus pesticidelass, are Dic
Phosphor
an
ion
ED PEST
des, whichof the mostand ability t the carnivo
DDT being. Other chllor, Mirex, a
102
and VX, ttylcholineste
ther mamms carries thlorvos, Para
s Based Pe
ICIDES
have beenstable famili dissolve ines and the hhe most wellrinated pestd Toxaphen
he organoprase, which
ls. Becausecommon n
hion and Ma
ticides
Dichl
Malat
idely used,s of compo
he body fat,erbivores ali known meicides of noe.
osphorus cis essential
of their discome Schrad
lathion.
rvos
hion
the world ounds. Becauhese travel ue. Productiober of this clte are Dield
ompoundsto nerve
verer, onean. Other
ver, for ae of theirp the foodn and saleass - havein, Aldrin,
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1
h
Dichloro-
9.5. CARThe c
drogen atoethyl, whileore complic
ibenzo-Tric
Aldri
Endri
Mire
AMATErbamates ar s, attachedne of the hyted organic
Chlorine
loroethane (
ESTICIDe derivativesto the nitrog
drogen atomroup. The m
103
Based Pesti
DT)
Sof carbamic
en atom, iss attached toode of action
cides
Die
Chlo
Hept
Toxa
acid H 2NCOreplaced bythe oxygen i of carbama
ldrin
rdane
chlor
phene
OH in whichan alkyl gros replaced be insecticide
one of thep, usuallya longer,is similar
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tp
c
1
nic
pa
that of or hosphorus t
rbamate pe
Ca
9.6. TRIA
If threitrogen atom
ntaining thiidely used hollutant in are difficult to
anophosphaat attacks t
ticides are C
rbofuran (Fur
Aldicarb
Alachlor
ZINE HER
e alternates, the resulti group do srbicides andricultural areiodegrade.
tes. They de acetylcho
arbofuran, C
Carba
adan)
BICIDES
carbon atog compoun
how pesticid is commonlas where it
104
iffer in thatline-destroyi
arbaryl, Aldic
ate Pestici
1-meth
s in a ben is called a
al properties detected in
is used. Like
it is a car g enzyme.
arb, IPC, Ala
es
Carbaryl
ylethyl phen
Metac
ene ring ar Triazine. Ma. Thus Atrazgroundwater
organochlo
bon atom r Important ex
chlor and Me
Sevin)
lcarbamate (
hlor
e substitutedny organic cine is one oand surfaceines these c
ther thanamples of
tachlor.
IPC)
by threeompounds
the mostwater as aompounds
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1
bDee
Ec
9.7. PHE
In coenzene ring
is used to kifective in clamples of s
Thesenvironmentalncern than t
P
OXY HE
mercial her f a phenoxy
ill broad-leafleaning smal
ch herbicide
weed-killer ly, the by-pr he herbicide
henoxyacetic
Triaz
BICIDES
icides somcetic acid aeeds in law
l bushes als are Silvex,
were introducts conta themselves.
Phen
acid
105
ine Herbicid
Atrazine
of the fivee replaced bns, golf cour ng the roa
Mecoprop an
uced at thined in phen
xy Herbicid
(2,
e
remaining hy chlorine. Tes and agris and othe
d MCPA.
end of thoxy herbicid
es
-dichlorophe(2,
ydrogen atous, the comultural fields.r open spa
Seconds are often
noxy) aceticD)
s on theound 2,4-
. 2,4,5-T ises. Other
orld War.of greater
cid
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1
eoh
(2,4,5-tic
9.8. BEN
The cnvironmentallriginal mole
drogen ato
19.8.1
fungici
lorophenox(2,4,5 T)
Mecoprop
ENE BAS
ompound bely significantule itself. Ts are replac
. HEXA CH
Hexachloroide for treatin
) acetic acid
ED POLL
nzene is aderivatives oe fist amod by chlorin
ORO BENZ
benzene, wg seeds, is a
Hexa
106
TANTS
ell knownf the compogst these i atoms.
NE (HCB)
ich is usedhigh potenc
hlorobenze
Sil
M
arcinogen.nd that may
the compo
as a woodcarcinogen
e
ex
PA
here are abe more toxiund where
preservativeith NO safe
number ofc than thell the six
and as alimits.
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19.8.2
hydroreplacthe 10othernumbThesespecifi
them.nearbappro(non-ptheir n
well a
heatcapacipaintsfirst qcapaciIncinedange
fats.furtheingestihumatransf
Exposcancedocu
. POLYCHL
PCBs areen atoms,ed with chlor possible poing. In total 2r of attache
different coc number of
The two rinThe shape o chlorine aimately in tlanar). Theon-coplanar
PCBs ares heat. Thes
ransfer fluiditors. PCBs, and flame r arter of the
itors have siration of murous by-prod
Generallyhis solubility accumulateing contamin body PCBrred from m
Studies onure of the hrs, in groupsented.
RINATED
class of maattached toine atoms.itions which
09 differentchlorine ato
mbinations,hlorine ato
gs in a PCBf the molecutoms so the same plaoplanar PCousins.
ighly un-reae have thus
in large elhave also betardants. Tlast centurynce been r icipal waste
ucts.
CBs are not in fats explalong the foated food a can remaither to child
humans anduman fetus
of humans
107
IPHENYLS
n-made orgthe phenylhlorine atoare number CBs can be
ms, but at dihowever, ar s located at
molecule cale is further it the ringse (coplanar s are consi
tive compoufound wides
lectrical equen used ine commerciand the discleasing thesalso may le
very solubleains why Pod chain. Hund water or in fatty tis
through the
animals hacan affect itexposed co
(PCBS)
nic chemicalrings of as may be pr d 2-6 on onformed. PCferent positi called conpecific positi
rotate arounfluenced byof a specif
) or in a moered to be
nds resistantpread applic
ipment suchproducts sul productionrded and le
e compound to PCB po
in water, buBs can builmans may b inhaling cosues and inlacenta or br
e shown has developmentinuously to
s in which oiphenyl molesent at soe ring, and 2s which havns, are calleeners, each
ions.
d the bondthe repulsio
ic PCB willre perpendicore toxic co
to acids andation as insu
as transfor ch as inks,of PCBs staaking transfos to the enllution along
t are readilyup in ani
e exposed tntaminatedthe liver an
east milk.
mful effectsnt. IncreasePCBs, has
e or morecule, aree or all of-6 on thethe same
d isomers.having a
onnectingn between
either lieular planempared to
bases, aslating and
mers anddhesives,
rted in thermers andIronment.with other
olvable inal fat and
PCBs byir. In the
d may be
on fertility.in certainbeen well
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108
19.8.3. POLY AROMATIC HYDROCARBONS (PAHS)
Benzene compounds, because of their typical aroma are calledaromatic compounds. Organic compounds, such as those shown here,containing more than one benzene rings. BaP is a category A carcinogen knownto cause lung and kidney cancer.
19.9. Dioxins and FuransThe term dioxins commonly refers to polychlorinated dibenzo-dioxins (PCDDs)
and polychlorinated dibenzo-furans (PCDFs) since these are a group of compounds withsimilar chemical structures. Chlorinated chemicals with comparable structural andbiochemical properties, such as certain biphenyls (PCBs) are called dioxin-likecompounds and can act similarly in terms of dioxin like toxicity. Dioxins have no use assuch and are formed as by products during the industrial processes or during incompletecombustion in incinerators, or open garbage burning. Cigarette smoke and vehicle andindustrial exhausts may also contain dioxins.
As discussed above, the compounds 2,4 D and 2,4,5 T are very good defoliantsie. act on the trees and shrubs to force them to shed their leaves. Transported in orangedrums, 2,4,5 T was extensively used by the US forces in Vietnam to clear the jungles to
make it difficult for the enemy soldiers to hide. While the compound (called AGENTORANGE bacuase of the packing) itself may also have harmed the people handling it, itis one of the by-products, 2,3,6,7 Tetrachloro Dibenzo -p-Dioxin (TCDD), that could beformed during production of 2,4 D or 2,4,5 T, and appears as a contaminant, may be aserious problem.
Poly Aromatic Hydrocarbons (PAHs)
Benzene Naphthalene
Pyrene Benzo [a] Pyrene
a
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109
The dioxin TCDD is one of the most lethal substances known to man. Furans arealso a group of compounds that are produced unintentionally along with the dioxins and
the PCBs. Around 135 Furans, with varying toxicity, have been detected persisting in theenvIronment for long periods of time. Many of these compounds are highly carcinogenic.The major source of exposure of man to furans is through the meat and milk of theanimals that have themselves absorbed or ingested the compounds.
19.10. ENDOCRINE DISRUPTORS (EDS)
The endocrine system is a complex system consisting of glands in the body thatproduces hormones. Examples are the thyroid gland in the throat, the pituitary gland inthe brain, the adrenals, pancreas and ovaries in the abdomen, and the testicles, which lieoutside the abdomen.
Hormones act as chemical messengers, controlling many basic functions, suchas growth, development, reproduction, how food is utilised in the body, blood pressure,blood glucose levels and fluid balance. Examples of hormones are insulin from thepancreas, which controls blood glucose, and the sex hormones, oestrogen from the ovaryand testosterone from the testicles, which affect reproductive function.
An endocrine disruptor (ED) is, therefore, defined as a foreign substance ormixture that alters the function(s) of the endocrine system, consequently harming anindividual life form, its offspring, or populations. Many of the chemicals discussed above,particularly the chlorinated ones, act as endocrine disruptors (EDs).
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1
dt
(
2,3,6,7 Tet
9.11. PER
In geegradation bus travel thr ith the receood behavioOPs) have
1
1
O
O
Dioxin
(2,4 D)
rachloro Dib(TCDD)
ISTENT
eral, the chlut are readilugh the footors in theur. After ser een globally
P. Aldrin . DDT . Dioxin
. Furans
. Hexach1. PCBs
Dioxi
nzo- p-Dioxi
RGANIC
orinated pes absorbed i chain. In mystem disruious considebanned. The
rsistent Org
lorobenzene
110
s and Fura
2,3,
OLLUTA
icides and t the adiposn, they mimiting the gro
ration twelvese are:
anic Polluta2.4.6.
8.10.12.
s
Dibe
2
,8-tetrachlor
TS (POP
he PCBs ar e tissue of aic the endocr
th, develop (12) Persis
ts (POPs) ChlordanDieldrin Endrin
HeptachlMirex Toxaphe
Ozo Furan
,4,5 T
dibenzofura
)
resistant toll living orgaine moleculement regeneent Organic
e
r
e
(TCDF)
biologicalnisms ands and bindration andPollutants
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111
CHAPTER-20
Biochemistry
20.1. INTRODUCTION
Biochemistry is the study of the chemical processes in living organisms. Majorclasses of chemicals studied under this subject are proteins, carbohydrates, lipids andnucleic acids. Although there are a vast number of molecules under these classes, theytend to be composed of the same repeating subunits (called monomers ), in differentorder with each class of biomolecules, having different set of subunits.
20.2. CARBOHYDRATES
Carbohydrates are chemical compounds that serve the important function ofstorage and transport of energy in most organisms, including plants and animals. Theycontain carbon hydrogen and oxygen, only, in the ratio 1:2:1. Since the hydrogen-oxygenratio in water is 2:1, from a molecular point of view, these may be said to be composed ofcarbon and water molecules. This forms the basis of their nomenclature, and theirgeneral chemical formula as Cn(H2O) n, the smallest value for "n" being 3.
The general term sugars is used to describe ALL carbohydrate molecular units.The word sugar has its roots in Arabic from the word sukkar , which became sukere inFrench to sugre in classical English and sugar as known today. Again, originating fromthe same roots, the Greek word for sugar, saccharide , is also commonly used in
biochemistry to describe these compounds. The carbohydrate units which may consist ofsingle molecules are thus called monosaccharides, whereas, chemically bonded, twomolecule units are called disaccharides. Similarly, polymers of up to ten units are calledpolysaccharides and higher polymers are known as oligosaccharides.
20.2.1. MONOSACCHARIDES
Depending upon the number of carbon atoms in a molecule, the sugarsare called trioses (3), tetroses (4), pentoses (5), hexoses (6), heptoses (7), andso on. The simplest sugars are of course the trioses, under which class fall thecompounds dihydroxyacetone and glyceraldehyde.
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h
Exam
For th
exoses inclu
Dihydro
ples for tetro
Th
e pentoses, r e compound
Ribo
yacetone
ses include
eose
ibose, arabins such as glu
e
112
hreose and
ose and xylocose, galact
Arabinos
Glyceraldeh
erythrose:
Erythrose
se are threese and fruct
X
de
representativse.
lose
es and the
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20.2.2
disacc(H) frodisacctwo gl
Glucos
Glucos
Glucos
Gluco
. DISACCH
When twoharide is formm one molecharides are scoses). The f
se
RIDES
monosacchad. Such bindle and a hydcrose, (canermula of thes
Fructos
Galactos
Glucose
113
Galactos
ride units biing takes placroxyl group (sugar), lacto
e disaccharid
e
Fr
nd togethere through theH) from thee (milk sugas is C 12H22O1
ctose
by a covaleloss of a hydother. Commr) and maltos1:
Sucrose
Lactose
Maltose
t bond aogen atomnly known(made of
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114
20.2.3. OLIGOSACCHARIDES AND POLYSACCHARIDES
We have seen that two monosaccharide units join together to form adisaccharide. Similarly if more than two nits fuse together, through glycosdicbonds, longer chains of molecules may result. Units containing between threeand nine units are called Oligosaccahride while those containing more than tenmonosaccharide units are known as Polysaccharides.
Polysaccharides represent an important class of biological polymers.Examples include starch, cellulose and glycogen.
20.3. PROTEINS
Proteins are large organic compounds that are essential components of all livingorganisms. The name protein comes from the Greek ("prota"), meaning " of primaryimportance ".
Many proteins are enzymes that catalyze biochemical reactions within the cell.These are made of large number of amino acids, selected from a range of 20 "standard"acids, arranged in a linear chain. Nature has endowed most of the plants andmicroorganisms with the ability to synthesis all of the twenty amino acids. Animals, on theother hand, can synthesis only a faction of these and have to obtain the remaining fromtheir diet. Such external amino acids are known as essential amino acids. The list ofessential amino acids differs from animal to animal and may vary with age.
As discussed above, proteins are linear polymers built from 20 different aminoacids. All amino acids share common structural features that an amino group and acarboxyl group are attached to the same carbon atom, called the alpha carbon. Thesimplest amino acid is the one where hydrogen atoms satisfy the other two carbonbonds, (HOOC(H) 2NH2) and is called glycine. Other amino acids have a variable side
chain R attached to the alpha carbon.It is the three dimensional structure of a protein that allows it to perform the
specific function which it is assigned to do. The amino acid within a protein molecule istermed as a residue and one amino acid is linked to the other through the peptide bond.The chain so formed, through the involvement of the carbon, nitrogen and oxygenatoms,serves as the backbone of the protein molecule.
20.3.1. PROTEIN SYNTHESIS
Proteins are assembled from amino acids using information encoded ingenes (see Section 20.5.2.). Each protein has its own unique amino acidsequence that is specified by the nucleotide sequence of the gene that encodesit.
The size of a protein can be estimated either by the number of aminoacids it contains, or its molecular mass. Conventionally, the protein mass isreported in kilodaltons (kDa) with one Dalton being equivalent to a unit atomicmass. Yeast is a typical small protein containing 466 amino acids having a massof 53 kDa. Proteins, such as titins, contain upto 27,000 amino acids and have a
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1
2
3
4
5
6
7
molec
20.3.2
to thaPrima
Pr isth
Smthflep
Tishapr di
Amino Aci
. Alanine (Al
. Arginine (A
. Asparagine(Asn)
. Aspartic A(Asp)
. Cysteine (
. Glutamic A(Glu)
. Glutamine(Gln)
ular mass of
. PROTEINMost prote
t protein. Pr ily structure i
imary Structcalled the priree-letter codcondary Str
olecule, is cae alpha helixibility of articular functirtiary Structulong and nar lical (primar d may formotein molecuulfide bonds
S
a)
rg)
id
ys)
cid
round 3,000
TRUCTURins, when folotein structus simply the
re: The ordemary structues for the acture: The a
lled the seco and pleatopting manon that the pre: The over
row, or comp structure) p compact te
le is govern etc.
tructure
115
kDa.
ded, form ares are gensequence of
r or sequence. The primaino acids.rrangement,ndary structud sheet. T shapes w
rotein has toall shape ofact and globrotein mayrtiary structud, through
Ami
11. Leu
12. Lysi
13. Met
14. Phe
(Ph
15. Proli
16. Seri
17. Thre
three dimenrally classifi
amino acids i
e of amino ary structure i
in space, ofre. The moste large proich is actu
perform.single protelar is calledend over itse. The interlialt bridges,
no Acid
ine (Leu)
e (Lys)
ionine (Met)
ylalanine)
ne (Pro)
e(Ser)
onine (Thr)
ional structued into fourn the protein
ids in the pr s indicated b
he chain ofcommon ex
tein molecullly determin
in moleculeits tertiary str elf a number nking withinhydrogen bo
Struc
re specifictiers. Thechain.
tein chainusing the
he proteinmples are
has thed by the
hether itucture. Aof times
he samends, and
ure
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8
9
1
2
c
c
bi
btaitt
otelcn
2
cl
. Glycine (Gl
. Histidine (
0. Isoleucine(Ile)
Qmfu
few mas emembthroug
0.4. ENZ
The btalyze cheagnitude (1talyze a spetabolism a
iological entit
The meing a proteiese come in
n enzyme thelf and is ae enzyme.
A protn the Envir mperature, t
lectrolytes.ange in the
ot allow theore in their n
0.5. LIPI
Alongasses of bio
y)
is)
aternary Str ore than onenction as an i
Based on
ain classes.zymes, fibr rane proteinh the cell me
MES
st-known rolical reactio)7. Enzyme
ecific reactiod a cataboli
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olecule onmay consisto contact wit identifies ated upon by
in's (and anonmental che pH, the io change inprotein strucnzyme to fuatural form a
S
ith proteins,logically imp
ucture: It is t protein molndependent
heir typical
lobular protous protein serve as a
mbrane.
e of proteinss increasin are biologi
n only. Withm would not
hich an enz of many huth a substratspecific locathe enzyme
enzymes) stnditions sunic strengthone or mor ure which, ection proper
re termed as
nucleic acidortant molec
116
18. Tryp
(Trp
19. Tyro
20. Vali
e shape thaecules intontity.
ertiary struc
eins are solu are impobarrier, allo
in the cell i their ratescal catalystsut enzymes
proceed at
me acts isdreds of ame. It is the thion on the st. The specifi
ructure androunding thof the fluid,
of these pven if slightlly. It is for thdenatured.
s and carboules or biom
tophan
)
sine (Tyr)
e (Val)
t is formed t larger sub
ure, protein
ble proteinsrtant structuing only ce
s their functiby as muc
with eachmany of thpeeds requi
known as aino acids butree dimensiucture wher location is
ence its actie enzyme.nd the relati
arameters b different thais reason tha
ydrates, lipiolecules. Th
rough the cnit which p
can be divi
many of whiral compontain molecul
on as enzyh as sevenenzyme bei reactions i
red for the s
substrate. Aonly a smallnal protein s the substratalled the ac
ity is highlyMost criticave concentrarings aboutn the nativet proteins wh
s are one of lipids are
llection ofrforms its
ded into a
h functionents, ands to pass
es, whichorders ofg able to
nvolved inrvival of a
n enzymefraction oftructure ofe attachesive site of
dependentl are thetion of thea physicalstate, mayich are no
the majorgroup of
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bater insolublenzene. Lipi
olecular stru F P W St
20.5.1
thiscarbolong
commnumbadipo
20.5.2
glycer placethe ph
20.5.3
estersglycer crawls
20.5.4
struct
substancess found in h
cture:tsospholipids
axeseroids
. FATS
Glycol is aolecule byylic acids ar hain carbox
only have ared), whiche tissue, are
. PHOSPHO
Phospholipol. They diff in the glycer ospholipids t
. WAXES
Waxes arebut unlike th
ol. Wax in h) into the ear
. STEROID
Steroids ar re of the typ
that can beman body c
trihydorxy alrelease of
called trigllic acids ar
chain of 4may be satua major form
LIPIDS
ids and fatsr, however,l molecule bo non-polar
another clasese other lipiuman ear a.
e a class of:
Skeleton S
117
extracted froan be divide
cohol. Longwater moleccerides com also know
to 28 carbrated or uns of energy st
are similaras the formeeing occupieends.
s of lipids. Lids, they involts as a stic
ompounds t
tructure of a
m cells by o into four cl
chain carboxules. The r monly known as fatty ac
ns (usuallyaturated. Trirage in ani
in structure,r contain ond by a phos
e fats and plve monohydky trap for
at have a c
Steroid
ganic solvensses accordi
ylic acids coesulting est by the termids. Natural
unbranchedacylglycerolsals.
as both arely two fattyhate group.
hospholipidsroxy alcoholsnything that
aracteristic c
ts such asng to their
bine withrs of thefats. Theatty acids
and even, stored in
esters ofcids, thirdThis gives
waxes areinstead ofblows (or
arbon ring
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2
ct
sex h
startinit is eformaBlock
0.6. NUC A spe
nvey genetie nucleolus
20.6.1
deoxynitrogand atheirdeoxyare dif while
Steroids crmones (test
One of theg material fossential forion of plaquge of the art
EIC ACIDcial class oc informationf a biologica
. CHEMICA
The most
ribonucleic anous heterophosphate g
chain with tribose. Likeferent: adenihymine is po
denine
mprise fourosterone, pr
more well kn the formatiolife, an excee on the aries supplyi
C
Sf high-mole becausel cell are ca
STRUCTU
common n
cid (DNA).cyclic base,roup. Nucleihe RNA coise, the nitroe, cytosine,
ssible only in
118
groups: chogesterone a
own steroid in of vitamin-ss of theterial wallsg blood to th
olesterol
ular-weightf their char lled nucleic
E
leic acids
ach monoither a purinc acid typesntaining ribgenous basand guanineDNA and ur
Cytosine
lesterol, adr d estradiol),
is Cholestero is found in
aterial, howleading to ce heart often
biochemicalcteristic of gcids.
are ribonuc
er of a nucle or a pyrimdiffer in these and thes possible iare possiblecil is possibl
nocorticoidand the bile
l. Cholesteroall organismver, can re
logging andresults in he
macromolecenerally resi
leic acid (
leic acid coidine; a pentspecific sug DNA cont the two nuin both RNA
e only in RN
Guanine
hormones,cids.
l being the. As such,ult in theblockage.rt attack.
ules, thating within
NA) and
sists of ase sugar,r found inining theleic acidsand DNA,.
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119
Thymine Uracil
To form the DNA and RNA polymers, the nucleotides are hookedtogether. Nucleic acids may be single-stranded or double-stranded. A double-stranded nucleic acid consists of two single-stranded nucleic acids hydrogen-bonded together. RNA is usually single-stranded, but DNA is usually double-stranded.
The key to DNAs functioning is its double-helix structure withcomplementary bases (cytosine and guanine and thymine and adenine pairingtogether through hydrogen bonding) on the two strands. During cell division thetwo strands unwind and new complementary strands are constructed on theunzipped strands. As a result, two double-helix DNA structures that are identicalto the original one appear. This replication makes possible the transmission ofgenetic information when cells divide.
20.6.2. PROTEIN SYNTHESIS
Besides replication, the other major function of DNA is protein synthesis. A given segment of the DNA, called a gene, contains the code for a specificprotein. This code for the primary structure of the protein (the sequence of amino
acids) can be transmitted to the construction site in the cell. DNA stores thegenetic information and RNA molecules are responsible for transmitting thisinformation to ribosomes where the protein synthesis takes place. The processinvolves two types of RNA, the messenger RNA (mRNA) and the transfer RNA(tRNA).
The genetic code is a set of three-nucleotide sets called codons in aDNA molecule that specify particular amino acids to be added to a new proteinchain. Because DNA contains four nucleotides, the total number of possiblecodons is 64 (4 x 4 x 4); hence, there is some redundancy in the genetic codeand some amino acids are specified by more than one codon.
Nucleic acids are primarily biology's means of storing and transmittinggenetic information. It is obvious that any damage to the DNA would result in
deformities in the offspring.
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120
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121
CHAPTER-21
RADIOACTIVITY
21.1. THE PHENOMENON OF RADIOACTIVITY
Radioactivity describes a process whereby atoms of a particular element areconverted to atoms of an element with a lower atomic number. Hence, the process is alsoknown as radioactive disintegration.
21.2. TYPE OF RADIOACTIVE EMISSIONS
A radioactive process results in three types of radiation. The characteristics ofthese and their hazards are listed below:
21.2.1. ALPHA RADIATION
An atom gets its definite character from the number of protons containedin its nucleus called Atomic Number. To lower the atomic number in theradioactivity process protons must, therefore, be eliminated. In radioactivityprotons are ejected as a package in the form a helium nuclei which contains twoprotons and two neutrons. Known as an alpha particle, this packet of particleshas a mass of 4 a.m.u and a charge of +2.
Alpha particles travel with a single velocity characteristic of the element
emitting them. This velocity ranges from 1.5 x 107
m-s-1
to 2 x 107
m-s-1
which isless than one-tenth the speed of electromagnetic radiation (3 x 10 8 m-s -1). As theparticle is large and travels sluggishly, it can be stopped by collision with nitrogenand oxygen molecules in the air. At room temperature, an alpha particle maytravel to a maximum of 10 cm in air. A thin sheet of paper is fairly dense, ascompared to air, and effectively stops these alpha particles.
Since the alpha particles have a large mass, and although their speed isone- tenth the speed of light, they do have a very large momentum. This allowsthem to knock of electrons from the atoms that they might hit (air, paper orbiological material).
21.2.2. BETA RADIATION
Now, during radioactive disintegration, within the nucleus of an atom aneutron may be converted to a proton. Since the neutron has no charge, thisimplies that a negative charged particle must be released. The obvious choice ofthe particle is the electron and when released from an atomic nucleus, becauseof radioactive disintegration, the resulting radiation, in contrast to radiation iscalled beta radiation.
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122
This type of radiation is emitted as a spectrum of velocity ranging fromone third to near the speed of electromagnetic radiation. The spectrum, however,
is characteristic of the radioactive element.Because of their small size and high velocity, these rays travel in air up
to many meters as compared to only 10 cm for alpha particles. These cannot bestopped by a sheet, or even a ream of paper but, on the other hand are stoppedby 1 cm thick sheet of even a lighter element like aluminum. For denser metals,this thickness can be fairly reduced. Although their penetration is high, beta rays,because of their small momentum are not very highly ionizing.
21.2.3. GAMMA RADIATION
Because of the forces existing in a nucleus binding the protons andneutrons together, whenever a nucleus disintegrates a large amount of energy isreleased. This energy is in the form of electromagnetic radiation over a wide
range of wavelength. The radiation of very small wavelength, and hence veryhigh energy, is termed as radiation, is highly penetrating and very damaging.To stop the gamma radiation, very thick blocks of a very dense metal like leadare required.
21.3. RADIATION EFFECTS Alpha particles represent a very small hazard to man since these have difficulty
in penetrating the skin. If, however, such a radioactive substance is ingested, the release particles within the body would start ionizing the tissue of the internal organs resultingin considerable damage.
particles on the other hand are very penetrating and are harmful bothexternally and internally.
radiation obviously is the most dangerous and must be avoided at all costs.
Radiation damages to humans are of two types: Somatic effects include illnesseslike fatigue, skin rash, anemia, loss of hair, damage to the eyesight and development ofcancer in different parts of the body. There can also be long-term damage in terms ofgenetic modification due to mutations in DNA in the reproductive cells. The ensuingdefects are thus transferable to the fetus and the deformed children are born.
21.4. UNITS OF RADIOACTIVITY
Ever since its discovery, about a hundred years ago, different units forquantifying radioactivity have been developed. These are discussed below in detail.
21.4.1. COUNTS PER MINUTE, cpm
The most obvious unit of radioactivity would be disintegrations per unittime. Counts per Minute ( cpm ) was thus one of the early units.
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123
21.4.2. THE CURIE, Ci
Just like many other things, however, the units of radioactivity haveevolved, historically, through many phases. Thus, honouring Pierre and Marie Currie who had discovered the first radioactive element, the number ofdisintegrations by one gram of pure radium in one second was taken as astandard. The name given to the unit was a Curie abbreviated as Ci . Thecurrently accepted value for a Curie is 3.7x10 10 disintegrations per second.
Now, a curie is a very large unit and smaller units are frequently used,such as millicurie (1x10 -3 Ci; mCi), microcurie (1x10 -6 Ci; Ci), nanocurie (1x10 -9 Ci; nCi), picocurie (1x10 -12 Ci; pCi), femtocurie (1x10 -15 Ci; fCi) and attocurie(1x10 -18 Ci; aCi) are used, as appropriate.
21.4.3. THE BECQUEREL Bq
Going back to the counts per unit time, it became apparent thatdisintegrations per second could also be taken as unit. Again, in order to honouranother important scientist who had done pioneering work in the field ofradioactivity, this unit is called a Becquerel ( Bq ). Obviously 1 Ci equals 3.7 x 10 10 Bq! In the International System of Units (SI) the curie has been replaced by thebecquerel (Bq), where 1 becquerel equals= 2.703x10 -11 Ci.
21.5. RADIATION DOSAGE
Due to the hazardous nature of the radioactive radiation, it was soon realized thatmore appropriate units, indicating the interaction of such radiation with the irradiatedsubstance would have to be taken into account to truly estimate the damage that may becaused. Thus, the magnitude of radiation exposures is specified in terms of the radiationdose and there are two important categories of dose: The absorbed dose and thebiological dose.
21.5.1. THE ABSORBED DOSE: THE GRAY
The absorbed dose , or the physical dose , is defined by the amount ofenergy deposited in a unit mass in human tissue or other media. The original unitis the Radiation Absorbed Dose ( RAD ), one unit of which represents theabsorption of energy of 100 erg/g. The SI unit, for the same purpose is the 1 J/kgand is called a gray (Gy ). Obviously 1 gray = 100 rad.
21.5.2. THE ROENTGEN
With the development in the X-ray technology, because of the ionizing
behaviour of these and the -rays, a separate unit had been developed for theionising electromagnetic radiation. This unit was defined as the amount ofradiation required to liberate positive and negative charges of one electrostaticunit (esu) of charge in 1 cm of air at standard temperature and pressure (STP).The unit called a roentgen (R) is named after the German physicist WilhelmRoentgen, the discoverer of X-rays.
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124
Since one esu is defined as 3.34 x 10 -10 C, and the density of dry air, at STP, is1.29 x 10 kg/m 3, 1 R = 2.5810 4 C/kg, which is the accepted SI unit for measuring the
intensity of the ionizing radiation.21.5.3. rem and sievert
In order to assess the effect of radiation, specifically on man, a new unit calledthe Roentgen Equivalent for Man ( rem ) is defined as the exposure of an average adulthuman being to one R of rays. The rem is no longer accepted for use with theInternational System , which uses sievert (Sv ) where one sievert is equivalent to100 rem.
21.6. BIOLOGICAL DOSE OF RADIATION
We can now talk about the biological dose or dose equivalent, which reflects thefact that one type of ionizing radiation may not be as harmful as another type althoughboth may have the same intensity. For example, for a given absorbed dose, alpha
particles, because of their larger momentum, are more damaging than the beta particles,gamma rays, x rays. It is for this reason that radiation dose is expressed as equivalentdose which is the multiplied by a Weighting Factor (WR) which for beta particles(electrons), x-rays and gamma rays is taken to be 1 for, whereas for alpha particles thevalue of WR is 2. Before 1990, this weighting factor was referred to as Quality Factor(QF). Since WR or QF is dimensionless, the unit for equivalent dose is still sievert ( Sv ).
21.7. THE CONCEPT OF HALF LIFE
Radioactive disintegration is a classic example of a process following the firstorder kinetics as the disintegration rate at any particular time is always directlyproportional to the number of atoms of the radioactive species present in a mass ofmaterial at that time. Assuming the number of atoms at timet to be N and the
proportionality constant (the rate constant) as , we have the equation:
By applying the boundary condition that at the start of the process monitoring, the
numerical value of N is No, integration of the above equation yields:
The slope of the plot of 1n(N/N o) vs t gives us the value of which is highly
characteristic of the radioactive substance. In fact determination of the constant can leadto the identity of the radiating material.
In the exponential form the radioactivity equation can be written as:
Plot of this equation for the three materials with values of 1, 2 and 3,
respectively is shown below:
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125
It is clear that with larger the curve falls more sharply and the radioactivematerial depletes more quickly. In all the three cases, however, we can never say when
has ALL the material been totally consumed and the process would stop. In other words,we cannot talk about the life of a lump of radioactive element. We do note, however, thatthe same fraction of N/N o (0.1, 0.2, 0.3, 0.7, 0.9. etc.) is reached earlier by the fasterdepleting (larger ) and latter by the slower depleting (smaller ) materials.
Thus, we can, in sense talk about the life-time of the radioactive substances by interms of what fraction of the original material has been consumed (or is remaining).
Although any arbitrary fraction could be chosen, as a compromise the fraction 0.5 (or )is used in radioactivity (and other first order kinetic processes). We say that half-life of aradioactive element is the time when the amount of material is half of what we startedwith. In the above example the half life periods of the three materials are clearly 0.23,
0.345 and 0.69.If we use the condition that at the half life the amount N is N o/2, we have the
relationship:
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This gives us the relationship:
. Having correlated the disintegration constant and the half life the equation N =
Noe-t can now also be written as:
. / In half-life we now have a very convenient scale, in terms of time, of comparing
the rate at which different radioactive elements disintegrate. One of the biggest problemsfaced by human kind in the peaceful use of nuclear energy is that the waste materialconsists of elements which have half lives of many thousands of years. Effective disposalof this waste, so that our future generations are not harmed, is, therefore, a greatchallenge for Environmental engineers.