1

1.0 INTRODUCTION

Water is a chemical substance that is essential to all known forms of

life. It covers 71% of Earth's surface. It appears that the available water is

contained in five oceans (over 97 %) and the Antarctic ice sheet (2 %). It

means that all of the rivers, streams, lakes, reservoirs, water present as

clouds, rain water and freshwater aquifers constitute only one percent of

earth’s ‘water’. Water in these bodies perpetually moves through a cycle

of evaporation, precipitation and run-off to the sea.

From a biological standpoint, water has many distinct properties that

are vital both as a solvent in which many of the body's solutes dissolve and

as an essential part of many metabolic processes within the body. Water is

also central to photosynthesis and respiration. The civilization has

historically flourished around rivers and major waterways. Mesopotamia,

the so-called cradle of civilization, was situated between the major rivers

Tigris and Euphrates. Large metropolises like Rotterdam, London,

Montreal, Paris, New York, Shanghai, Tokyo, and Chicago owe their

success in part to their easy accessibility via water and the resultant

expansion of trade. Islands with safe water ports, like Singapore and Hong

Kong, have flourished for the same reason. In places such as North Africa

and the Middle East, where water is more scarce, access to clean drinking

water has been a major factor in human development.

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The use of water in the world has increased by more than 35 times

over the past three centuries. Globally 3240 cubic km of fresh water is

withdrawn and utilised annually, and of this 69 per cent is used for

agriculture, 23 per cent for industry and 8 per cent for domestic purpose2.

Increase in human population and industrial growth over the past 150 years

have now caused a serious world wide water pollution problem. At present,

most of the rivers and lakes of the world are already polluted or at least

threatened with pollutants. Human activities are degrading the quality of

much more water than that was withdrawn and consumed3,4

.

1.1.0 Pollution

The pollution is defined in article 1 of European Economic

Community (EEC) 5

as “the discharge by man, directly or indirectly, of

substances or energy into the aquatic environment, the results of which are

such as to cause potential health hazards to human, harmful to living

resources and aquatic ecosystems, damage to amenities or interferences

with other legitimate uses of water.

1.1.1 Water pollution

When toxic substances enter lakes, streams, rivers, oceans, and other

water bodies, they get dissolved or lie suspended in water or get deposited

on the bed. This results in the pollution of water whereby the quality of

water deteriorates, affecting aquatic ecosystems. The pollutants can also

seep down and affect the groundwater deposits. The most polluting

3

sources are the city sewage, industrial wastes discharged into the rivers and

from natural contaminants. In fact, the water pollution scenario of India is

quite frightening. With the population explosion and spreading of urban

centers there is a greater generation of waste water. The municipalities,

even if they have the most honourable intentions, are not able to find the

resources to treat waste water6. It is noted that 16,000 mld (million litres

daily) of waste water has been generated from class one cities, with a

population of over 1,00,000 and 1600 mld are generated from class two

cities with a population of 50,000 to 1 lakh. Of the 17,600 million litres of

waste water generated in the country every day, only 4,000 million litres

are treated. Due to this, vast quantities of untreated waste water (pollutants)

enter ground water, rivers, and other water bodies. Such water, which

ultimately ends up in households, is often highly contaminated and carries

disease-causing microbes. Some of the other pollutants that enter water

sources are organic matter, nutrients, microbial contamination, toxic

organic compounds, pharmaceutical drugs, suspended particles, nuclear

waste and salinity.

About 97% of water contamination is generated by the chemicals,

paper, petroleum and primary metal sectors. The efforts made by different

industrial sectors to reduce their effluents are impressive. The contributions

of different industrial sectors to water pollution8 are summarized as

follows. Lumber (0.01%), machinery (0.01%), transportation equipment

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(0.01%), textile (0.02%) plastics (0.04%), leather (0.04%), fabricated

metals (0.05%), photographic equipment (0.06%), electrical (0.09%),

stone, clay, glass (0.10%), food (0.53%), primary metals (2.41%),

petroleum (3.21%), paper (8.13%) and chemicals (83.32%).

1.2.0 Heavy Metal Pollution

It is a problem associated with areas of intense industries, but it also

can originate from natural geological weathering, processing of ores, use

of metals, landfill waste, livestock, animal and human excretions, urban

run-off and re-use of drainage water and sewage effluents. The uses of

metal coating, pesticides, fertilizers, etc. in agriculture are the other sources

by which heavy metals enter water. They enter through leaching, diffusion

and infiltration.

A heavy metal is any of a number of higher atomic weight elements,

which has the properties of a metallic substance at room temperature.

There are several different definitions 9, 10

concerning which elements fall

in this class.

A group of elements between copper and bismuth on the periodic

table of the elements with specific gravities greater than 4.0.

A more strict definition increases specificity to metals heavier than

the rare earth metals, which are at the bottom of the periodic table.

None of these are essential elements in biological systems and

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additionally, most of the better known elements are toxic in fairly

low concentrations. Thorium and uranium are occasionally included

in this classification as well, but they are more often referred to as

radioactive metals.

Any metal with density above 7g/cm3

Metals with atomic number greater than that of sodium (23),

starting with magnesium, metals with atomic number more than 20

and specific gravity at least five times greater than that of water,

excluding alkali metals, alkaline earth metals, lanthanides and

actinides are called heavy metals .

The above definition is not based on scientific basis and that there

are a lot of inconsistencies in its usage.

Heavy metals are among the most harmful of the elemental

pollutants. The presence of heavy metals in the environment is a major

concern due to their toxicity. Many industrial processes produce aqueous

effluents containing heavy metal contaminants. According to the World

Health Organization (WHO), the metals of immediate concern are

aluminium, antimony, arsenic, barium, beryllium, cadmium, chromium,

cobalt, copper, iron, lead, manganese, mercury, nickel, palladium,

selenium, tellurium, tin and zinc. Of these some are essential to human

body at trace levels like copper, iron, zinc and trivalent chromium. But

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most of them are highly toxic and dangerous particularly, arsenic,

cadmium, lead and mercury compounds. They have tremendous affinity for

sulphur and disturb enzyme functions by bonding with sulphur groups in

enzymes. Protein carboxylic acids (-COOH) and amino (-NH2) groups are

also chemically bound by heavy metals. Metals like cadmium, copper, lead

and mercury can bind to cell membranes, hindering transport process

through the cell wall. Heavy metals may also precipitate phosphate and

bio-compounds or catalyze their decomposition. The heavy metals are not

usually eliminated from the aquatic ecosystems by natural processes in

contrast to most organic pollutants. They are dangerous because they tend

to bioaccumulate11

. Bioaccumulation means an increase in the

concentration of a chemical in a biological organism over a period of time,

compared to the chemical concentration in the environment. The

compounds accumulated in living things are taken up and stored faster than

they are broken down (metabolized) or excreted. Consequently they cause

acute and chronic toxicity to the human beings and the aquatic life. The

concentration of these pollutants must be reduced by means of treatment to

meet legislative standards. During the past few decades, considering the

nature and seriousness of their effects on human being, federal and state

governments have instituted environmental regulations to protect the

quality of surface and ground water from heavy metal pollutants. They

have specified maximum permissible limits12-14

of such metals in drinking

water which are listed below in Table I.1

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Table I. 1 Maximum permissible limits of some heavy metals in

drinking water

Metals WHO

std. mg/L

US EPA

mg/L

BIS

mg/L

Arsenic 0.010 0.010 0.050

Cadmium 0.003 0.005 0.005

Chromium (Total) 0.050 0.100 0.100

Copper 2.000 1.300 1.500

Lead 0.010 0.050 0.100

Manganese 0.500 0.050 0.100

Mercury 0.001 0.002 0.002

Nickel 0.020 0.100 0.100

Selenium 0.010 0.050 0.050

The most important disasters with heavy metals which highlighted

their toxicity12

are listed below.

Minamata, Japan (1932): The sewage containing mercury was

released by Chisso's chemicals works into Minamata Bay in Japan in 1932.

The mercury accumulated in sea creatures, leading eventually to mercury

poisoning in the population.

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Minamata, Japan (1952): The first incident of mercury poisoning

appeared in the population of Minamata Bay in Japan, caused by

consumption of fish polluted with mercury, bringing over 1000 fatalities.

Sandoz, Mexico (November 1986): Water used to extinguish a

major fire carried approximately 30 tons of fungicide containing mercury

into the Upper Rhine and fishes were killed over a stretch of 100 km.

Coto De Donana, Spain (April 1998): The toxic chemicals in

water from a burst dam belonging to a mine contaminated the Coto De

Donana nature reserve in southern Spain. Approximately 5 million metric

tons of mud containing sulphur, lead, copper, zinc and cadmium flew down

the Rio Guadimar. Experts estimated that Europe's largest bird sanctuary,

as well as Spain's agriculture and fisheries, suffered permanent damage

from this pollution.

In the present study cadmium and chromium were chosen due to

their solubility over a wide range of pH and their presence in several

industrial wastewaters.

1.3.0 Cadmium Occurrence, Use, Toxicity and Effects

1.3.1 Cadmium occurrence

Cadmium displays chemical similarities to zinc and occurs together

with it. The cadmium: zinc ratio in minerals and soils being 1:100 to

9

1:1000, cadmium is obtained as the by-product from the refining of zinc

and other metals, particularly copper and lead 15

. There is no specific

cadmium ore worth mining solely for its content. Of many inorganic

cadmium compounds, acetate, chloride and sulphate are quite soluble.

1.3.2 Cadmium use

Cadmium, its alloys, and its compounds are used in a variety of

consumer and industrial materials16-19

. The use of cadmium compounds

falls into five categories: Active electrode materials in nickel-cadmium

batteries (70% of total cadmium use), pigments used mainly in plastics,

ceramics, and glasses (12%), stabilizers for polyvinyl chloride (PVC)

against heat and light (8%), engineering coatings on steel and some

nonferrous metals (8%) and components of various specialized alloys

(2%).

Cadmium carbonate and chloride were once used as fungicides for

golf courses and lawns20

, but banned by EPA20

in the late 1980s. The

significance of cadmium chloride as a commercial product is declining.

However, it is used in the preparation of cadmium sulphide, in the

manufacture of special mirrors, and in dyeing and calico printing.

Cadmium based colourants are used mainly in engineering plastics,

ceramics, glasses and enamels. Cadmium sulphide and cadmium telluride

are primarily used in solar cells and a variety of electronic devices which

depend on cadmium’s semi conducting properties. The photoconductive

10

and electroluminescent properties of cadmium sulphide have been applied

in manufacturing a variety of consumer goods21

. KDM tag is attributed to

jewellery in which gold is soldered with cadmium. In the traditional

method, silver and copper were used to solder gold. But many jewellers

have switched over to cadmium since it gives good melting fluidity during

the soldering process. Even customers prefer KDM gold, as loss or wastage

of gold during the soldering process is minimum because cadmium gets

readily dissolved in gold. Usage of cadmium in gold jewellery has been

banned in many western nations due to the toxicity of its vapour. World

Gold Council has also imposed restrictions in this regard.

1.3.3 Cadmium exposure

Small amounts of cadmium enter the environment from the natural

weathering of minerals, forest fires, and volcanic emissions, but most is

released by human activities such as mining and smelting operations, fuel

combustion, disposal of metal-containing products, and application of

phosphate fertilizer or sewage sludges. Primary and secondary metal

production, industrial applications, manufacture of phosphate fertilizers,

waste incineration, coal, wood, and oil combustion can all contribute

cadmium to the atmosphere.

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1.3.4 Cadmium in food

Human exposure to cadmium can result from consumption of food,

drinking water, or incidental ingestion of soil or dust contaminated with

cadmium, inhalation of cadmium containing particles from ambient air,

inhalation of cigarette smoke which contains cadmium taken up by tobacco

or working in an occupation involving exposure to cadmium fumes and

dust15

. For non smokers, ingestion of food is the largest source of cadmium

exposure.

Cadmium has been detected in nearly all samples of food analyzed

with sufficiently sensitive methods15

. In foods obtained from unpolluted

areas, the cadmium concentration is usually lower than 0.1 mg/kg fresh

weight. Milk, dairy products, eggs, beef, and fish usually contain less than

0.01 mg/kg, while higher concentrations, 0.01-0.10 mg/kg, are typically

found in vegetables, fruits, and grains16

. As part of the U.S. Food and Drug

administration (FDA) total diet study, the average concentrations of

cadmium in twelve food groups had been analyzed from samples collected

in 27 American cities. Cadmium has been found in nearly all samples, with

the lowest levels in beverages and fruits, and the highest levels in leafy

vegetables and potatoes Table I.2). Watanabe22

measured the cadmium

content in rice samples from various areas in the world during the period

from 1990 to 1995. Twenty-nine samples collected in the United States had

a geometric mean of 7.43 ng Cd/g, with a standard deviation of 2.11 ng

Cd/g.

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Table I.2 Cadmium content in decreasing level for selected foods

Type of food Average

concentration (ppm)

Range of

concentration (ppm)

Potatoes 0.0421 0.016-0.142

Leafy vegetables 0.0328 0.016-0.061

Grain and cereal products 0.0237 0.002-0.033

Root vegetables 0.0159 trace-0.028

Garden Fruits 0.0171 trace-0.093

Oils and fats 0.0108 trace-0.033

Sugar and adjuncts 0.0109 trace-0.053

Meat, Fish and poultry 0.0057 trace-0.014

Legume vegetables 0.0044 trace-0.016

Dairy products 0.0035 trace-0.016

Fruits 0.0021 trace-0.012

Beverages 0.0013 trace

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1.3.5 Effect of cadmium on human beings

As a non-essential metal for human consumption, about 5 % of

cadmium ingested by human beings is absorbed23

, but calcium and iron

deficiency may increase this amount24, 25

. The portion of inhaled cadmium

absorbed is dependent on particle size and solubility. Absorbed cadmium is

mainly stored in kidneys and liver. Excretion is slow, less than 0.01 %

which corresponds to a biological half time of more than 20 years in

human beings26

. In liver and kidneys, cadmium is mainly bound to a low

molecular weight protein –metalothionein – which might also be the

transport protein for cadmium and may be ultimately responsible for the

prominent accumulation of cadmium in the renal cortex27

.The placental

barrier is effective against cadmium and the new born is practically free

from this metal. There is considerable accumulation with age, the mean

kidney concentration at an age of 50 being from 15-50 mg/kg, wet weight

in European countries and USA, whereas considerably higher normal

values have been found in Japan.

Ingestion of highly contaminated food or drink results in acute

gastrointestinal effects. Excessive exposure to cadmium via inhalation may

cause acute or chronic lung disease and chronic renal disease. The later can

also appear after long time exposure via food. The renal damage is

primarily the re-adsorption defect in the proximal tubules. The first sign of

chronic cadmium intoxication is the appearance in urine of low molecular

14

weight protein, known as tubular proteinurea. Aminoaciduria, gulucoseurea

and phosphateurea28

may occur later. Disturbances in mineral metabolism

may cause mineral depletion in bone. Osteomalacia has been found in both

industrially exposed workers and in women exposed to excessive amount

of cadmium in rice, a disease called Itai-Itai disease29

. It is a combination

of severe renal tubular damage and osteomalacia accompanied by various

grades of osteoporosis among people with high cadmium exposure from

food and drinking water. The symptoms are dominated by pains in the back

and legs. When the disease progresses, even mild trauma may give rise to

fractures of various parts of the skeleton .Very large dose of vitamin D is

needed to alleviate the symptoms.

Anaemia and disturbed liver function may also result from

excessive cadmium exposure. Cadmium has given rise to sarcoma at the

site of the injection. Some data indicate that the occupational exposure may

increase the risk of prostrate cancer in man. In animal experiments,

teratogenic effects have been demonstrated after single injections of high

doses of cadmium. Exposure to cadmium causes changes in the distribution

and metabolism of zinc30

. Some of the toxic effects of cadmium are

thought to be due to the interference of the cadmium with zinc enzymes

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1.4.0 Chromium Occurrence, Use, Toxicity and Effects

1.4.1 Chromium occurrence

The only important chromium ore is chromite FeO Cr2O3 which is

never found in pure form. FeO may be replaced with some MgO, and the

mineral also contains silica in varying amounts, as well as small quantities

of other compounds31

. The highest grade of ore contains about 55 %

chromic oxide. Both trivalent and hexavalent chromium are found in nature

but the trivalent is more common form.

The world production of chromite was about 13.78 million tons of

which USA, Russia, Republic of South Africa, and Turkey being the main

producers. Ferrochrome is produced by the direct reduction of the ore.

Chromium metal is prepared by reducing the ore in blast furnace with

carbon (coke) or silicon to form an alloy of chromium called ferrochrome,

which is used as the starting material for many iron containing alloys that

employ chromium. Chromium to be used in iron free alloys is obtained by

reduction on electrolysis of chromium compounds. Chromium is difficult

to work in the pure metal form. It is brittle at low temperatures and its high

melting point makes it difficult to cast. Sodium chromate and dichromate

are produced by roasting chromite ore with soda ash or with soda ash and

lime, followed by chemical treatment for removing impurities. The other

chromium compounds are produced from sodium chromate or dichromate.

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1.4.2 Chromium use

The principal industrial consumers of chromium are the

metallurgical, refractory and chemical industries. The US 1971 figures for

consumption of these industries were 66%, 18 % and 16 % respectively of

the total consumption32

. An important consumer of chromium for many

years has been the tanning industry. Other uses are pigment production and

application, graphics industry and industries using chromium alloys or

plated materials. Ferrochrome and chromium metals are the most important

classes of chromium used in the alloy industry.

1.4.3 Leather tanning industry in India: an overview

The Leather tanning industry occupies a place of prominence in the

Indian economy, in view of its massive potential33

for employment, growth

and exports.

It employs 2.5 million persons.

A large part (nearly 60-65%) of the production is in the

small/cottage sector.

Annual export value poised to touch about 2 billion US dollars.

Amongst top 8 export earners for India.

Endowed with 10% of the world raw material and export constitutes

about 17% of the world trade.

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Has enormous potential for future growth.

Very high value addition within the country.

1.4.4 Estimated production capacities

Tanning industry in India is spread over the following states,

Andhra Pradesh (Hyderabad)

Delhi (Delhi)

Karnataka (Bangalore)

Maharashtra (Mumbai)

Punjab (Jallandhar)

Tamil Nadu (Chennai, Ambur, Ranipet, Vaniyambadi, Trichy,

Dindigul)

Utter Pradesh (Kanpur, Agra)

West Bengal (Kolkatta)

India has processed34

65 million pieces of hides and 170 million

pieces of skins a year. It has produced 776 million pairs of foot wear, 112

million pairs of uppers for shoes, 960 million pairs of non leather foot

wear, 18 million pieces of leather garments, 60 million pieces of leather

goods and 52 million pieces of industry gloves in the year 2005-06. Tamil

Nadu, with around 600 tanning units, releases large amount of effluents

into water sources.

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1.4.5 Chromium in food

The daily intake of chromium from food has been estimated to be in

the range of 0.03-0.1 mg. Since other sources contribute only minor

amounts in relation to these values, they represent also an estimated total

daily intake of chromium for the general population35

. Food items vary

considerably in concentration of chromium. Among large sources are meat,

vegetables, fish and unrefined sugar. The values are reported from non

detectable to about 0.5mg/kg wet weight for various food items.

1.4.6 Importance and deficiency of chromium(III)

Chromium in the trivalent form is an essential trace element to

human. It is involved in the metabolism of glucose36

. Chromium deficiency

may result in impaired glucose tolerance, peripheral neuropathy and

elevated serum insulin, cholesterol and tri glycerides37

similar to those

symptoms observed in diabetic patients. The US National Research

Council has recommended a daily chromium intake of 50mg38

. Chromium

trioxide as Cr(VI) is still being used in some countries as an agent to stop

nose bleeds. However, health authorities in most countries have

recommended to stop that use due to the toxic effect of Cr(VI).

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1.4.7 Effects of chromium(VI) on human beings

Trivalent (chromic) and hexavalent (chromate) are the two species

of chromium of prime interest for toxicity39

. The most biologically active

species, chromates are particularly water soluble compounds readily taken

up by living cells and reduced with in the cell via reactive intermediate to

stable Cr(III) species. Reduction of chromates also takes place in various

body fluids. Hexavalent compounds are characterized by variability in

water solubility and differences that seem to be of major significance to the

detoxification and the bio availability40

.

Both acute and chronic adverse effects of chromium are mainly

caused by hexavalent chromium compounds. Hexavalent chromium causes

skin ulcerations, irritative dermatitis41

, allergic skin reactions and allergic

asthmatic reactions42

. It may also cause ulceration in the mucous

membranes and perforation of the nasal septum. Inhalation of chromium

compounds may cause bronchial carcinomas. The ability of hexavalent

chromium compounds to induce bronchiogenic cancer in humans is well

established. Cr(VI) also seems to be mutagenic43

.

The presence of heavy metals in environment is a potential

problem to water and soil quality due to their high toxicity to plant, animal,

and human life. Moreover, heavy metals cannot be destroyed chemically as

organic pollutants. Increasing strict discharge limits on heavy metals and

their widespread, threatening presence at hazardous waste sites have

20

accelerated the search for advanced yet economically attractive treatment

technologies for their removal. Therefore, several treatment technologies

have been developed for elimination of heavy metal from solution such as

chemical coagulation, precipitation, evaporation, ion-exchange, ultra

filtration, membrane processing, solvent extraction and adsorption.

Following are some of the common treatment methods employed for the

removal of heavy metals.

1.5.0 Popular Heavy Metal Removal Methods

1.5.1 Precipitation

The most popular process for the decontamination of industrial

effluents is the precipitation process8 which is widely used when metal ion

concentration is high, followed by filtration. This method is used to remove

hexavalent chromium from waste water. Here, hexavalent chromium is

reduced to trivalent chromium using a suitable reducing agent and it is

precipitated as chromic hydroxide44

with lime, soda ash, sodium hydroxide

or barium carbonate as the precipitating agent. The presence of any

complexing agent such as cyanide in the waste water inhibits chromic

hydroxide precipitation45

. Sodium bisulphite and hydrogen sulphide are

normally used as reducing agents. Cadmium can be precipitated as its

hydroxide, sulphide or its carbonate. The use of coagulant normally

increases the efficiency of precipitation. It has been reported that lime

cannot be used to precipitate cadmium because this process needs high pH.

21

The removal of cadmium as its sulphide has been the common process in

industry.

The precipitation method generates important volumes of sludge

which contain hydroxides. In other words, heavy metal ions contained in

the effluents are concentrated in the sludge that should be treated or

inertized. The solubility product of some heavy metal compounds can

exceed that defined by the current environmental regulations. The necessity

to reduce Cr(VI) to Cr(III) before its precipitation and the presence of

organo-metallic compounds can reduce its efficiency of precipitation

process. Also the high cost, low efficiency, labour-intensive operation and

lack of selectivity of the precipitation process make it difficult to be used in

modern day effluent treatment plants.

1.5.2 Evaporation

This is energy intensive and hence an expensive process44

, used

when the volume of the waste water to be treated is less and when

recovered solid or the concentrated solutions are reused. e.g. electroplating

waste. This method is also employed for concentrating radioactive liquid

waste. It permits the recovery of wide variety of process chemicals. It is

applicable to remove or concentrate chemicals which cannot be

accomplished by any other means. Disadvantage of this method is that

there is a possibility of retaining all non volatile constituents of the waste

water in the system itself.

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1.5.3 Reduction

Reduction is probably the most commonly used technique for the

treatment of industrial effluents containing chromium. Reduction is

generally carried out by SO2 and is followed by treatment of the chromium

sulphate with Ca(OH)2 and to precipitate Cr(OH)3 in order to meet more

stringent effluent discharge standards46

. The efficiency of reduction

depends on several factors such as the nature of the reducing agent, pH of

the experimental solution, concentration of the reducing agent,

concentration of metal ion in solution and time of contact between metal

ion and the reducing agent.

1.5.4 Electrochemical reduction

This method has been found suitable for reducing hexavalent

chromium to trivalent chromium. It involves the use of consumable iron

electrodes. When electricity is passed through the solution, ferrous ions are

released47

. These ions are responsible for the process of reduction of the

hexavalent chromium to trivalent chromium. It has the same limitations as

encountered in the other conventional methods and, moreover, produces at

least four fold sludge solids. Hence the problem associated with sludge

disposal still exists.

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1.5.5 Electrolytic process

In this method, electrochemical reduction of metal ions to elemental

metal takes place at the cathode47

. This process is used to recover copper,

tin, silver and other metals from plating, etching or pickling baths. The

efficiency of this process has been tremendously increased by innovative

designs such as eco-cell involving rotating electrodes (particularly suited

for concentrated waste) and extended surface electrolysis (especially

suitable for dilute waste). This process consumes high electrical power and

hence cannot be adopted for the treatment of dilute waste. In addition, a

pre-concentration step, ion exchange or evaporative recovery prior to this

process is needed. The use of fluidized semi conducting carbon bed

increases the efficiency of metal removal.

1.5.6 Electrodialysis

This is the process in which colloidal/dissolved species are

exchanged between two liquids through selective ion exchange

membranes48,49

. An electromotive force brings about the separation of the

species according to their charge. A semi permeable membrane allows the

passage of certain charged species while rejecting the passage of oppositely

charged species. Application of this process includes concentration of

rinsed water to desired bath strength, recovery of chromium and cadmium

from automobile plating rinse baths, recovery of valuable metals and

radioactive elements and desalination of water. This method is efficient for

24

effluents containing high metal ion concentrations. The operating costs are

low compared to other methods but it requires qualified labourer to work

with and low current efficiency for dilute solution and also the process is

limited to a flow rate of less than 0.2 m3/hour.

1.5.7 Ion exchange

This is the second most widely used method for metal ions

removal50-53

. This method is the reversible process that facilitates the

removal of anionic and cationic constituents present in water by exchange

with ions of the resins. A variety of synthetic organic resins, inorganic gel

and liquid ion exchangers have been examined for the removal of metal

ions from dilute aqueous solutions. When the resin bed becomes saturated

it is regenerated using acid or alkali. In this method selective extraction of

metallic ions is possible and the decontamination rate of effluent is high.

This method is less efficient for high flow rate of effluents or high metal

ion containing effluents and preliminary elimination of suspended particles

is necessary. Some of the ion exchangers should be waste disposed at the

end of their life time cycle. The economic limitation of the process comes

from the initial investment cost. The presence of complex forming species

however can interfere with the exchange process. In addition, fouling of

resin bed with wetting agents and organic brighteners used in plating,

clogging due to precipitated hardness of water and oxidation of resin by

oxidizing agents if present, are some frequent problems associated with ion

25

exchange method. It can reduce the metal ion concentration to a very low

level. However, ion exchange does not appear to be practicable to

wastewater treatment from a cost stand point.

1.5.8 Reverse osmosis

This is essentially a process in which a semi permeable membrane

allows water molecules through and retains the ions54, 55

. The concentrated

solution is subjected to high pressure as a result of which the solvent is

forced out through a semi permeable membrane to the dilute solution

region. The concentrated solution becomes more concentrated and can

therefore be removed. The three membranes most commonly used are

cellulose acetate, aromatic polyamide and NS-100. The application of this

process for the treatment of cadmium plating and zinc plating solutions and

iron from acid mine drainage has been reported. The serious drawback of

this method is being its high initial capital and operating cost. This method

is not adequate for the treatment of effluents containing high concentration

of metallic ions and preliminary filtration of suspended particles is

necessary for the protection of membrane.

1.5.9 Adsorption

The stringent discharge requirements of wastewater coupled with

rising cost in the treatment of it with the convergence of diminishing

supply of fresh water have prompted many researchers to examine new

avenues for treating it56

. Among the methods, adsorption is the best and the

26

most attractive because it is well established and a powerful technique for

treating domestic and industrial effluents.

Adsorption is a technical term coined to denote the taking up (latin,

sorbere, to suck up) of gases, vapour or liquid by a surface or interface.

Adsorption processes involve an array of phenomena which can alter the

distribution of contaminants between and among the constituent phases and

interfaces of subsurface systems. The interchanges of mass associated with

such processes impact the fate and transport of many inorganic and organic

substances. The effects can be complex, given the diversity, magnitude and

activity of chemical species, phases and interfaces commonly present in

contaminated subsurface environments. Solutes which undergo adsorption

are commonly termed adsorbate, the adsorbing phase the adsorbent, and

the primary phase from which adsorption occurs the solution or solvent.

Two broad categories of sorption phenomena, adsorption and absorption,

can be differentiated by the degree to which the adsorbate molecule

interacts with and is free to migrate between the adsorbent phases. In

adsorption, solute accumulation is generally restricted to a surface or

interface between the solution and adsorbent. In contrast absorption is a

process in which solute transferred from one phase to another

interpenetrates the adsorbent phase by at least several nanometers.

Adsorption results from a variety of different types of attractive

forces between solute molecules, solvent molecules and the molecules of

27

an adsorbent. Such forces usually act in concert, but one type or another is

just as usually more significant than the others in any particular situation.

Accordingly, two loosely defined categories of adsorption – physical and

chemical are traditionally distinguished, according to the class of attractive

force which predominates.

1.5.10 Physical adsorption (physisorption)

If the physical forces of attraction hold the adsorbate to the surface

of the adsorbent, the adsorption is called physical adsorption or

physisorption57-59

. Forces associated with interactions between the dipole

moments of adsorbate and adsorbent molecules commonly underlie

physical adsorption processes. Interactions between polar molecules or

between polar molecules and non-polar molecules in which dipole

moments are thereby induced represent one class of physical adsorption. A

more general class of physical adsorption is associated with London

dispersion forces attributable to rapidly fluctuating or instantaneous dipole

moments resulting from the motion of electrons in their orbital. The

magnitude of physical adsorption can be characterized from low heat of

adsorption.

1.5.11 Chemical adsorption (chemisorption)

If the chemical forces hold the adsorbate molecules to the surface of

the adsorbent, the adsorption is called chemical adsorption or

chemisorption60

. This type of sorption has all of the characteristics of true

28

chemical bonds and is characterized by relatively large heats of sorption.

The reactions may involve substantial activation energies and be favoured

by high temperature. The chemical bonding between an adsorbate

molecule and an adsorbent site can be represented in terms of the Morse

potential energy relationship, which had been developed for a covalent

bond between two identical molecules, and has the functional form

21

errm

mM eD

(1)

The term Dm in equation (1) is the minimum energy, re is the

equilibrium separation of the molecules and m is a constant. The variation

of the Morse potential energy with separation distance exhibits a form

similar to that of the Lennard-Jones potential 60

, although the former yields

a much greater minimum energy and a smaller separation between

molecule and surface sites.

1.5.12 Sorption, occlusion

In many examples of adsorption process, the initial rapid adsorption

is followed by a slow process of absorption of the substances into the

interior of the solid. In this case, the effects of sorption cannot be

distinguished from those of adsorption 57, 60

. Hence a new term called

sorption had been introduced. Sorption is a process in which both

adsorption and absorption take place simultaneously. Sorption of gases on

metals is called as occlusion.

29

1.6.0 Activated Carbon (AC)

In recent years activated carbon adsorbents have become the

accepted medium for the physico-chemical treatment of waste water.

Activated carbon,61

also called activated charcoal or activated coal, is a

general term which covers carbon material mostly derived from charcoal.

For all three variations of the name, "activated" is sometimes substituted

with "active". Activated carbon is predominantly an amorphous solid with

a large surface area and pore volume. By any name, it is a material with an

exceptionally high surface area as one gram of activated carbon has the

surface area of approximately 500 m2.

Adsorption method is used in effluent treatment, since it is less

troublesome, effective and efficient. Adsorption using activated carbon

offers one of the most efficient processes available for removing certain

organics and in-organics from waste water. Several models have been

proposed to describe its structure. The main features common to all

activated carbons are graphite-like planes, which show varying degrees of

disorientation and the resulting spaces between these and planes which

constitute porosity. The units built of condensed aromatic rings are referred

to as Basic Structural Units (BSU) 62

. Activated carbon adsorbs molecules

from both liquid and gaseous phases depending upon the pore size

distribution of the adsorbent and also upon geometry and size of the

adsorbate molecule. The economical aspects of AC adsorption and the

difficulties in the procurement of commercial AC (CAC) have led to the

30

search for alternative low-cost carbonaceous adsorbents. In the past few

years, extensive research had been undertaken to develop low-cost

carbonaceous adsorbents.

A low cost adsorbent is defined62

as one which is abundant in

nature, or is a by-product or waste from industry and requires little

processing. Agricultural waste biosorbents generally used in biosorption

studies are also inexhaustible, low-cost and non-hazardous materials,

which are specifically selective for heavy metals and easily disposed by

incineration. Most of these by-products are considered to be low value

products. Some of the common low cost adsorbents used for removal of

different adsorbates are listed in Table I.3.

31

Table I.3 Low cost adsorbents used for different adsorbate removal.

Adsorbent Adsorbate Reference

2-Mercaptobenzimidazole clay Hg(II) [63]

Activated carbon 2,4-Dichlorophenoxy-acetic acid [64]

Activated carbon Cd(II) [65]

Activated carbon Cd(II) [66]

Activated carbon Cd(II), Ni(II) [67]

Activated carbon Co(II) [68]

Activated carbon Congo red [69]

Activated carbon Hg(II) [70]

Activated carbon Hg(II) [71]

Activated carbon Methylene blue [72]

Activated carbon Paraquat dichloride [73]

Activated carbon Pb(II) [74]

Activated carbon Pb(II), Hg(II), Cd(II), Co(II) [75]

Activated carbon Pb(II) [76]

Activated carbon – granulated Cd(II), Cu(II) [77]

Aeromonas caviae Cr(VI) [78]

Agricultural waste Cr(VI) [79]

Alginate Ni(II) [80]

Almond shell, Olive stone Cd(II), Zn(II), Cu(II) [81]

Anaerobic granular sludges Ni(II), Co(II) [82]

Aspergillus niger Basic blue 9 [83]

Aspergillus niger Pb(II), Cd(II), Cu(II), Ni(II) [84]

Azadirachta indica (Neem) leaf Pb(II) [85]

Bagassee Cd(II), Ni(II) [86]

Bagassee Cd(II), Zn(II) [87]

Baker’s yeast Cd(II) [88]

Banana stalk Musa paradisiacal Hg(II) [89]

Banana Stem Pb(II) [90]

32

Bark (Conifères) Cr(III), Cu(II), Ni(II), Pb(II),

Zn(II)

[91]

Beech leaves Cd(II) [92]

Bentonite Cd(II) [93]

Black gram husk Cd(II) [94]

Bone char Cd(II) [95]

Calabrian pine bark Zn(II), Pb(II) [96]

Calcined alunite Phosphorus [97]

Calcined Mg–Al–CO3

hydrotalcite

Cr(VI) [98]

Carbon cloth - Cd(II) [99]

Cassava waste biomass Cu(II), Cd(II) [100]

Ceiba pentandra hulls Cd(II), Cu(II) [101]

Chitin Cd(II) [102]

Chitin, chitosan, Rhizopus

Arrhizus

Cr(VI), Cu(II) [103]

Chitosan Cu(II) [104]

Chitosan Ni(II) [105]

Clay mineral Cd(II), Zn(II), Cr(II) [106]

Coconut coir pith Cr(VI) [107]

Coconut copra meal Cd(II) [108]

Coconut shell Pb(II) [109]

Coir Cu(II), Pb(II) [110]

Date pits Methylene blue [111]

Date pits Phenol [112]

Diatomaceous clay Methylene blue [113]

Discarded tyres Zn (II) [114]

Dolomite Phosphate [115]

Fish scale Pb(II), Co(II), Zn(II) [116]

Fly ash Congo red [117]

Fly ash (Turkish) Cd(II), Zn(II) [118]

Grafted silica Pb(II), Cu(II) [119]

33

Grape stalks Cr(VI) [120]

Grape stalk Cd(II), Pb(II) [121]

Hazel nut shell Cr(VI) [122]

Human hair Hg(I), Ag(I), Pb(II), Cd(II),

Cu(II), Cr(III),(VI), Ni(II)

[123]

Iron oxide-coated sand As(V), As(III) [124]

Jack fruit peel basic dye [125]

Jack wood saw dust Phenols [126]

Jordanian low-grade phosphate Pb(II) [127]

Juniper fiber Cd(II) [128]

Juniper fiber Phosphorus [129]

Mango seed /shell Cu(II) [130]

Mesoporous silicate Phosphate [131]

Mg–Al–CO3 hydrotalcite Cr(VI) [132]

Microcystis Ni(II), Cr(VI) [133]

Microporous titanosilicate

ETS-10

Pb(II) [134]

Mixed clay/carbon Acid blue 9 [135]

Mucor rouxii Pb(II), Cd(II), Ni(II), Zn(II) [136]

Myriophyllum spicatum Pb(II), Zn(II), Cd(II) [137]

Na-bentonite Oil [138]

Nano particle –TiO2 Cd(II) [139]

Oil shale 4-Nitrophenol [140]

Olive pulp Zn(II) [141]

Orange peel Congo red / rhodamine B [142]

Palm kernel fiber Pb(II) [143]

Palm kernel shell basic dye [144]

Parthenium Cd(II) [145]

Peanut hull carbon Cd(II) [146]

Peanut hull carbon Ni(II) [147]

Peanut hull carbon Pb(II) [148]

Pearl millet husk Basic dye [149]

34

Peas peels, broad bean Cd(II) [150]

Peat Cu(II) [151]

Peat Cu(II) [152]

Peat-resin particle Basic magenta, Basic brilliant

green

[153]

Perlite Cd(II) [154]

Pinus pinaster bark Pb(II) [155]

Pinus pinaster bark Cd(II), Hg(II) [156]

Pinus sylvestris saw dust Cd(II), Pb(II) [157]

Red mud Congo red [158]

Reed leaves Cd(II) [92]

Rhizopus oligosporus Cu(II) [159]

Rhodotorula aurantiaca Pb(II) [160]

Rice bran Cu(II) [161]

Rice husk Cr(VI) [162]

Rice husk Cd(II) [163]

Rice polish Cd(II) [164]

Sago Cu(II), Pb(II) [165]

Sawdust Phenol [166]

Schizomeris leibleinii Pb(II) [167]

Seafood waste Cd(II), Cu(II) [168]

Sepiolite Pb(II) [169]

Silica gel Cd(II), Pb(II) [170]

Soya cake Cr(VI) [171]

Spent grain Pb(II), Cd(II) [172]

Sphagnum moss peat Cu(II), Ni(II) [173]

Sphagnum moss peat Cu(II), Ni(II), Pb(II) [174]

Sugar beet pulp Pb(II) [175]

Sugar beet pulp Pb(II), Cu(II), Zn(II), Cd(II),

Ni(II)

[176]

Sugar cane dust basic dye [177]

Surfactant-modified Phosphate [178]

35

Clinoptilolite

Tree fern Cd(II) [179]

Tree fern Cu(II) [180]

Tree fern Pb(II) [181]

Vermiculite Cd(II) [182]

Waste rubber Hg(II) [183]

Waste tyres, sawdust Cr(VI) [184]

Water hyacinth roots Methylene blue dye [185]

Wheat bran Cd(II) [186]

Wheat straw Cr(VI) [187]

Wollastonite Ni(II) [92]

Wood Basic blue 69, Acid blue 25 [188]

Wood pulp Cr(VI) [189]

Zeolites Zn(II) [190]

1.7.0 Objectives of the Present Work

Tamil Nadu, a state in India, has a large number of battery,

electroplating and tanning industries. Effluents from these industries which

are released into the environment particularly on water bodies contain

cadmium and chromium ions in large concentrations. The serious effects

of these metals on the environment and on human being have made the

researcher to search for a more abundant, low cost adsorbent suitable for

their removal.

In this work, woods of Acacia Nilotica Indica (ANI) and Leucaena

Glauca Benth (LGB) were chosen as precursor for deriving adsorbents and

applying them for the removal of metal ions from aqueous solutions of

36

cadmium and chromium ions and also for the treatment of effluents

containing these two metal ions. A comprehensive literature survey has

revealed little knowledge about these two trees for heavy metal treatments.

It is noteworthy to give a brief description about the raw materials giving

justification for choosing them as adsorbents.

1.8.0 Acacia Nilotica Indica (ANI)

Nine subspecies are currently recognized, only one subspecies,

Acacia Nilotica. Indica (ANI)191

(Fig.I.1) until 1940, was regarded as

Acacia Arabica192

. However, Hill cleared the confusion with nomenclature

and declared the name given by Linnaeus in 1753 as the correct one. The

species produces reasonable quality browse high in tannins193

.

1.8.1 Common names of ANI

Hindi-Babul, Tamil–Karu Velamaram, Karuvelei, Oriya–Bambuda,

Baubra, Malayalam–Karivelan, Kanada- Gobbli, Jali, Kari Jaali.

1.8.2 Morphology of ANI

A moderate sized, generally attain a height of 15m and a girth of 1.2

m, though trees up to 30 m height and girth 1.2 m have also been recorded.

ANI is widely distributed in tropical and subtropical Africa from Egypt and

Mauritania to South Africa. Some subspecies are widespread in Asia as far

east as Burma. ANI grows in Ethiopia, Somalia, Yemen, Oman, Pakistan,

India, Burma, Iran, Vietnam, Australia and the Caribbean. This subspecies

is commonly found on soils with high clay content, but may grow on deep

37

sandy loam in areas of higher rainfall. It commonly grows close to

waterways on seasonally flooded river flats and tolerates salinity well. It

grows in areas receiving less than 350 mm of rainfall to areas receiving

more than 1,500 mm per annum. This species194

is reported to be very

sensitive to frost, but will grow in areas where the mean monthly

temperature of the coldest month is 16°C. But in general it is characteristic

of the dry regions. i.e. arid. There is some evidence that ANI is a weed195

in its native habitat of South Africa, but in different areas of India it is

planted for forestry196

or reclamation of degraded land.

1.8.3 Uses of ANI

The plant and seed pods are eaten by domestic grazers and browsers

such as cattle, sheep, goats and camels. In Africa and the Indian

subcontinent 197-199

, ANI is extensively used as a browse, timber and fire-

wood species. The bark and seeds of ANI are important source of tannin

for leather tanning in Northern India (Kanpur) and is used in villages from

Haryana to West Bengal. The tannin content of bark is as high as 20 %.

The species is also used for medicinal purposes. The bark of ANI has been

used for treating haemorrhages, cold, diarrhoea, tuberculosis and leprosy,

while the roots have been used as an aphrodisiac and the flowers for

treating syphilis lesions200

. The gum of ANI is sometimes used as a

substitute for gum Arabic. The species is suitable for the production of

paper201

and has similar pulping properties to a range of other tropical

38

timbers. The wood 202

is strong, hard, tough, straight or twisted–grained

and coarse textured. It takes up a good polish. It is used in bodies and

wheels of bullock cart, agricultural instruments like plough and harrows,

clod crushers, tool handles, well curbs and Persian wheels. It is also used to

make tent pegs, boat handles, bedsteads, railway wagon buffers, walking

sticks and for carving and turnery. As the wood contains large amounts of

cellulose and lignin, after suitable treatment it can be used as precursor for

adsorbents.

39

Fig I.1 Acacia Nilotica Indica

40

Fig.I.2 Leucaena Glauca Benth

41

1.9.0 Leucaena Glauca Benth (LGB) (Fig.I.2)

1.9.1 Common names of LGB

Tamarind (Corozal, Belize); Lead tree (Florida); Ipil Ipil

(Philippines); Jumby Bbean (Bahamas); False koa, Koa haole

(Hawaii); Tagarai, Nattuccavundal, subabul -Tamil (India); Malayalam-

Ttakaranniram (India),Vaivai (Fiji); cassis (Vanuatu)203-204

.

1.9.2 Morphology of LGB

The shrub or tree up to 18 m tall, forked when shrubby and

branching strongly after coppicing, with grayish bark and prominent

lenticels found throughout the plains of India

LGB is mostly grown in edges of garden and near villages. It is

native to Yucatan Peninsula and the Isthmus of Tehuantepec in Southern

Mexico, widely distributed throughout the tropics, probably introduced into

the Philippines in the 16th

Century as a feed for ruminant livestock and

subsequently spread throughout Asia-Pacific region. Its rugged habit,

abundant seeding and quick growth has enabled it to establish itself even in

rather unfavourable situations.

1.9.3 Uses of LGB

The utility of LGB for afforesting grassland has been proved in

Philippines. It is also planted for filling forest gaps, as windbreak and for

checking soil erosion. It is grown in many countries as shade in plantation

of tea and coffee, coca, rubber, cinchona, teak and sal. It is grown in dense

42

rows as a living fence and used to support vine crops such as pepper and

passion fruit. It sheds its leaves regularly and enriches top soil with

organic matter. Top leaves and pods are relished by cattle, sheep and goats.

Unripe pods and seeds of all subspecies have been used by the

native inhabitants of Mexico and Central America as a food or medicine

since ancient times. Very young shoots are used as a food by villagers in

Thailand. The young pods of this small evergreen bush or tree are eaten as

a vegetable by Malays and the mature dried seeds are eaten raw

LGB is highly valued as ruminant forage and as a fuel wood by

subsistence and semi-commercial farmers throughout South- East Asia and

parts of central Asia and Africa. It is the most commonly researched

species for alley farming systems. It is also been used as a reclamation

species following mining. Barks with high tannin content (~16 %) is used

for tanning leather articles.

The wood of LGB is hard, strong, medium textured and close

grained. It has good quantity of lignin and tannin. It has been tried as a raw

material for paper .

1.9.4 Toxicity LGB

When consumed in excess, all parts of LGB are toxic to

monogastric animals like horse, pigs, rabbits and chickens and cause great

loss of hair205-208

. The toxicity is due to an alkaloid, leucenine or leucenol,

-[N-(3-hydroxy-4-pyridone)]- amino-propionic acid (mp 226-227C)

43

reported to be identical with mimosine, a non-protein amino acid that has

antimitotic and depilatory effects on animals. The concentrations in young

leaf can be as high as 12% and the edible fraction commonly contains 4-

6% mimosine. Mimosine is acutely toxic to animals but is normally

converted to 3-Hydroxy-4(IH)-pyridone (DHP) upon ingestion. DHP is

goitrogenic and, if not degraded, can result in low serum thyroxine levels,

ulceration of the oesophagus and reticulo-rumen, excessive salivation, poor

appetite and low liver weight gains, especially when the diet contains more

than 30% Leucaena. It is reported that LGB has a property of extracting

selenium from the soil and concentrating it in the seeds.

Both the trees grow well in most part of Indian continent and can

withstand arid climatic conditions. Their abundant availability and high

cellulose and lignin content (proximate analyses done by local paper

industry indicate cellulose content of ANI and LGB are between the 65-72

%, lignin content 23-28 %) have made the researcher to try and use these

trees as adsorbent for cadmium and chromium ion removal from aqueous

solutions after acid and pyrolysis treatments.

44

1.10.0 Scope of the Present Work

to prepare the adsorbents from the wood of Acacia Nilotica Indica

and Leucaena Glauca Benth by sulphuric acid treatment and by

pyrolysis process.

to evaluate the efficiency of carbonaceous sorbents prepared from

above steps for the removal of cadmium(II) and chromium(VI) ions

from aqueous solutions and to compare their efficiencies with

commercial activated carbon (CAC).

to characterize the derived adsorbents for various physico–chemical

parameters such as % ash content, bulk density (g/mL), decolorizing

power mg/g, ion exchange capacity, matter soluble in H2O (%),

matter soluble in HCl, % moisture content, pH and surface area.

to carry out ash analysis using X-ray fluorescence spectrometer and

to arrive at the mineral composition of the adsorbents.

to study the surface morphology using scanning electron microscope

to study the structure of adsorbent using FT-IR spectrometer

to find out the effect of the following parameters like pH, particle

size, dose of the adsorbents, contact time, initial metal ion

concentration and temperature by systematic batch mode studies.

45

to model the kinetic experimental points with various equations like

Lagergren's pseudo first order, Yu-Shan Ho and McKay’s pseudo

second order and Webber Morris’s intra particle diffusion .

to match the equilibrium data obtained at different initial metal

concentrations with various isotherms models like Langmuir,

Freundlich, and Redlich–Peterson to find out which one of these

best fits the experimental data.

to find out the stages of adsorption, the rate limiting steps and to

determine diffusion coefficient (film and pore)

to calculate the thermodynamic parameters from temperature

variation studies

to apply adsorbate–adsorbent system to cadmium and chromium

waste water

to perform column experiments with synthetic solution and actual

effluents as a function of flow rate and bed height.

to study the recovery and reuse of adsorbents .