Review of Literature - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/8284/11/11_chapter 2.pdfReview of Literature 16 Lead Lead is a bluish-white lustrous metal. It is very soft,

Review of Literature__________

Review of Literature

Heavy metals have a high atomic weight and a density much greater

(at least 5 times) than water. Out of 90 naturally occurring elements, 21 are

non-metals, 16 are light-metals and the remaining 53 (with as included) are

heavy metals. They are highly toxic and can cause damaging effects even at

very low concentrations (Celik et al., 2005). They will accumulate in the food

chain in the body and can be stored in soft tissues like kidney, liver etc. and

also hard tissues like bone.

Some metals are naturally found in the body and are essential to

human health (Kirk et al., 1979). Iron, for example, prevents anemia, and zinc

is a cofactor in over 100 enzyme reactions. Magnesium and copper are other

familiar metals that, in minute amounts, are necessary for proper metabolism

to occur. They normally occur at low concentrations and are known as trace

metals; for example, high levels of zinc can result in a deficiency of copper,

another metal required by the body.

A total of 30 elements are now believed to be essential to life. They can

be divided into the 6 structural elements, 5 macro minerals and 19 trace

elements (Florence, 1989). But there are 12 poisonous heavy metals, such as

Lead, Mercury, Aluminum, Arsenic, Cadmium, Nickel, etc., that act as

poisonous interference to the enzyme systems and metabolism of the body.

The toxicity of heavy metals occurs even in low concentrations of about 1.0-

10 mg/L. The toxicity caused by heavy-metals is generally a result of strong

coordinating abilities (Gadd, 1992).


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Biogeochemistry of Heavy-metals

Some heavy metals occur in environment by geological and biological

means. The biogeochemistry of Zn, Cd, Cu, Hg, and Fe in lakes and streams

polluted by mine and smelter wastes emitted at Flin Flon, Canada, was

investigated. The geological cycle begins when water slowly wears away

rocks and dissolves the heavy-metals. The heavy metals are carried into

streams, rivers, lakes and oceans and may be deposited in sediments at the

bottom of the water body or they may evaporate and be carried elsewhere as

rainwater. The biological cycle includes accumulation in plants and animals

and entry into the food web (Young, 2000).

Heavy-metal Contamination and Toxicity

The situation becomes worst by the addition of heavy-metals to the

environment as a result of both the rapidly expanding industrial and domestic

activities. Heavy metals are subtle, silent, stalking killers. Metal toxicity can be

divided into three categories i.e. blocking the essential biological functional

groups of molecules, displacing the essential metal ion in bio-molecules and

modifying the active conformation of biomolecules (Florence, 1989). The

health hazards are mainly depending on the length of exposure and level of

exposure of heavy metals. These exposures are two kinds namely acute and

chronic. Acute exposure means contact with a large amount of the heavy-

metal in a short period of time. But chronic exposure means contact with low

levels of heavy-metal over a long period of time (Young, 2000).


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Lead

Lead is a bluish-white lustrous metal. It is very soft, highly malleable,

ductile, and a relatively poor conductor of electricity. Isotopes of lead isotopes

are the end products of each of the three series of naturally occurring

radioactive elements.

Properties of lead

Lead is a transition metallic element found in Group IVA of the periodic

table. It has the atomic number 82, atomic mass is 207.2. It is having a melting

point of 327 °C, a boiling point of 1755 °C, a density of 11.34 g.cm-3 at 20°C.

Industrial applications

It is a major constituent of the lead-acid battery used extensively in car

batteries. It is used as a coloring element in ceramic glazes, as projectiles, in

some candles to threat the wick. It is the traditional base metal for organ

pipes, and it is used as electrodes in the process of electrolysis. One of its

major uses is in the glass of computer and television screens, where it shields

the viewer from radiation.

Lead in the environment

Currently lead is usually found in ore with zinc, silver and copper and it

is extracted together with these metals. The main lead mineral in Galena

(PbS) and there are also deposits of cerrussite and anglesite which are

mined. Galena is mined in Australia, which produces 19% of the world's new

lead, followed by the USA, China, Peru' and Canada. Some is also mined in

Mexico and West Germany. World production of new lead is 6 million tones a


17

year, and workable reserves total are estimated 85 million tones, which is less

than 15 year's supply. Lead occurs naturally in the environment. However,

most lead concentrations that are found in the environment are a result of

human activities. Due to the application of lead in gasoline an unnatural lead-

cycle has consisted.

The lead salts enter into the environment through the exhausts of cars.

The larger particles will drop to the ground immediately and pollute soils or

surface waters, the smaller particles will travel long distances through air and

remain in the atmosphere.

Lead has known many applications over the years. It can enter the

human body through uptake of food (65%), water (20%) and air (15%). Foods

such as fruits, vegetables, meats, grains, seafood, soft drinks and wine may

contain significant amounts of lead. Cigarette smoke also contains small

amounts of lead. Lead can enter (drinking) water through corrosion of pipes.

Lead can merely do harm after uptake from food, air or water.

Health effects of lead

Exposure to lead causes a wide range of health effects. The ongoing

exposure to even very small amounts of lead can be harmful especially for

infants and young children compare with adult. Lead is a powerful neurotoxin

that interferes with the development of these systems as well as the kidney

and blood-forming organs. The researches had shown that during pregnancy,

especially in the last trimester, lead can cross the placenta and affect the

unborn child. Even low level lead exposure may harm the intellectual

development, behavior, size and hearing of infants. Female workers exposed


18

to high levels of lead have more miscarriages and stillbirths. Anaemia is

common and lead can also damage the brain and nervous system. Other

symptoms are: appetite loss, abdominal pain, constipation, fatigue,

sleeplessness, irritability, and headache. Short-term exposure to high levels of

lead can cause vomiting, diarrhea, convulsions. Moreover, if continually

exposed to lead as in an industrial setting, it can cause serious health effect

and even also death (http://www.lenntech.com/periodic/elements/pb.htm).

Cadmium

Cadmium is a relatively rare element, naturally presence in the

environment through mainly from gradual phenomena, such as rock erosion

and abrasion, and of singular occurrences, such as volcanic eruptions. It is

normally found in close association with zinc-bearing ores and certain soils

derived from zinc-bearing materials.

Properties of Cadmium

Cadmium is a transition metallic element found in Group IIB of the periodic

table. It has the atomic number 48, and the relative atomic mass 112.41.

Cadmium is a silver-white, soft, malleable metal. It will dull on exposure in moist

air owing to the formation of a thin protective coating of cadmium oxide. The most

remarkable characteristics of cadmium are its great resistance to corrosion, its low

melting-point and excellent electrical conduction. The melting point of cadmium is

321.07 °C. Liquid cadmium boils at 767 °C.


19

Applications of Cadmium

All metal coating and plating have accounts of 35 to 40% of cadmium

usage. Cadmium has some special advantages in this application. It deposits at

high rates and with uniform thickness over intricately shaped objects. The

cadmium coating and plating present higher ductility, good solder ability,

superior resistance to alkali environment, and excellent resistance to salt

water and tropical atmospheres.

Nickel-cadmium rechargeable batteries are vital in daily life. These batteries

have the advantages of long life, good performance under a wide temperature

range, the maximum current delivery with a low voltage drop, low operating

costs, and a low rate of self-discharge. They are safe and recyclable and provide

unique benefits for specific applications. The consumer applications include

power tools, computers, cellular phones, household appliances, etc. The industrial

applications can be found in aircraft and railroad. Cadmium compounds are also

widely used as paints and pigments, plastic stabilizers. These compounds exhibit

excellent resistance to chemicals and to high temperatures.

Health Effect of Cadmium

Cadmium has a long biological half-life, and accumulates in the liver,

kidney, and certain organs. It may ultimately contribute to organic dysfunction.

Excretion is slow, less than 0.01% of the total body burden per day, which

corresponds to a biological half-life of more than 20 years in human beings.

Long-term excessive ingestion of cadmium is only known to take place in Japan.

It has given rise to a renal tubular disease of the same type as in industrial long-


20

term exposure of cadmium. It also causes a severe bone disease, known as Itai-

itai disease .The carcinogenic effects of cadmium for human beings have mainly

been focused on cancer of the prostate (Potts, 1965; Kipling and Waterhouse,

1967; Lemen et al, 1976; and Kjellstrom et al., 1979). A significant increase in

lung cancer incidence rate was found with workers from the nickel cadmium

battery production (Sorahan and Waterhouse, 1983).

Cadmium may be introduced into the environment during the production,

use, and disposal of cadmium-bearing commercial and consumer products. The

most significant source of water-borne cadmium pollution comes from

electroplating shops. The best available technology for the removal of

cadmium includes coagulation or filtration, ion exchange, lime softening and

reverse osmosis.

Copper

Copper is an essential metal. It is one of the oldest metals ever used and

has been one of the important materials in the development of civilization.

Copper is usually found in-nature association with sulfur.

Properties of Copper

Copper is in Group IB of the periodic table, above Silver (Ag) and Gold

(Au). It has the atomic number 29, and the relative atomic mass 63.5. Copper is

malleable, ductile, durable and recyclable. Copper is an excellent conductor for

heat and electricity. Copper also has excellent alloy characteristics and high


21

resistance to corrosion. The melting point of copper is 1083.4 °C. Liquid copper

boils at 2567 °C.

Applications of Copper

Copper is a major industrial metal, ranking third after iron and aluminum in

terms of quantities consumed. The primary use of copper is in electrical

equipment and supplies, including power transmission and generation, building

wiring, electrical and electronic products, and telecommunication. The copper

used in electrical and electronic products accounts for about three quarters of

total copper use.

Copper is also a component of many alloys, where it may occur together

with silver, cadmium, tin, and zinc. The corrosion resistance of copper and its

alloys results in many uses in the construction industries for roofing, plumbing and

for decoration utilitarian items.

Other important applications of copper are in heating, transportation,

industrial machinery, and consumer and general products. Copper salts may serve

as pesticides. Copper byproducts from manufacturing and obsolete copper

products are readily recycled and contribute significantly to copper supply.

Health Effect of Copper

Copper is an essential nutrient to plant, animal and human health. It is

mainly stored in liver and muscles. Excretion is mainly via the bile and only


22

small percentage of the absorbed amount is found in urine. The biological half-

life of copper in human beings is about 4 weeks.

Acute copper poisoning effect includes vomiting and diarrhea in low

ingestion. More serious responses in high ingestion of copper include

hemolysis, hepatic necrosis, gastrointestinal bleeding, oliguria, azotemia,

hemoglobinuria, hematuria, proteinuria, hypotension, tachycardia, convulsions,

coma, or death (Chuttani et al., 1965; and Davenport, 1953). The accumulation

of copper in the lung and liver may cause granulomas and malignant tumors.

Almost all patients with Wilson's disease exhibit a lifelong excess of hepatic

copper (Goldfischer and Sternlieb, 1968).

Nickel

Nickel is a metallic element, making up 0.008 percent of the Earth's crust. If

nickel in the deeper core of the Earth is included, nickel becomes more abundant,

ranking as the fifth most common element after iron, oxygen, silicon and

magnesium.

Properties of Nickel

Nickel is the last member of the first triad in Group VIII of the periodic table.

It has the atomic number 28, and the relative atomic mass 58.69. Nickel is a

silver-white malleable metal. It is found in sulfide ores (which are mainly mined

underground) and in oxide ores (which are mined in open pits). It has a melting

point of 1453° C and relatively low thermal and electrical conductivities. It has

high resistance to corrosion and oxidation, and excellent strength and toughness


23

at elevated temperatures. Nickel is capable of being magnetized. It is attractive and

very durable as a pure metal, and alloys readily with many other metals.

Applications of Nickel

About 85% of the nickel is used in combination with other metals to make

what are known as alloys. Nickel can be alloyed with iron, copper, chromium, and

zinc. These alloys are used in the making of metal coins and jewelry and in

industry for making metal items. As a result of new technology, great efficiency

improvements have been achieved in the manufacture of stainless steel. The

demand for nickel keeps a sustained underlying growth rate of some 5 to 6% per

annum as the result of a number of emerging new applications of stainless steel

and its rapidly-improving price competitiveness.

Nickel and its compounds have no characteristic odor or taste. Nickel

compounds can be used for nickel plating, to color ceramics, to make some

batteries, and as substances known as catalysts that increase the rate of

chemical reactions.

Health Effect of Nickel

Inorganic nickel compounds are absorbed to a few percent from the

gastrointestinal tract. Absorption from the lungs depends on solubility of nickel

compounds. Nickel subsulfide and nickel oxide have low solubility and are

retained in the lungs. Absorbed nickel accumulates in kidneys, livers, and lungs.

The excretion is rapid, chiefly via the urine (Norseth and Piscator, 1979) like

many other trace elements; nickel is widespread in the contemporary human

environment. The available evidence indicates that the natural concentrations of


24

nickel in water, soil, and food do not constitute a biological threat. Epidemiologic

studies of workmen in nickel smelters and refineries revealed significant

incidence of cancers of the lungs and nasal cavities (Committee on Medical and

Biological Effects of Environmental Pollutants, 1975). IARC (International

Agency for Research on Cancer, 1976) concluded that the compounds most

likely causing these cancers were nickel subsulfide and nickel oxide.

Zinc

Zinc is the 23rd most abundant element in the earth's crust. Sphalerite, zinc

sulfide, is and has been the principal ore mineral in the world. Zinc is necessary to

modern living, and, in tonnage produced, stands fourth among all metals in world

production—being exceeded only by iron, aluminum, and copper.

Properties of Zinc

Zinc is a transition metallic element found in Group IIB of the periodic

table. It has the atomic number 30, and the relative atomic mass 65.39. Zinc is

a bluish-white and lustrous metal. It is brittle at ordinary temperatures but

malleable at 100 to 150°C. Zinc is a fair conductor of electricity, and burns in air at

high red heat with evolution of white clouds of the oxide. It exhibits super-

plasticity. Zinc is an essential metal, necessary for the function of various

enzymes. It is the second most common trace metal, after iron, naturally found in

the human body. High zinc concentrations are found in prostate, bone, muscle and

liver. Excretion takes place mainly via the gastrointestinal tract. The biological half-


25

life of retained zinc in humans is in the order of one year (Elinder and Piscator,

1979).

Applications of Zinc

The uses of zinc range from metal products to rubber and medicines.

The applications of zinc depend upon a number of properties. About three-fourths

of zinc used is consumed as metal, mainly as a coating to protect iron and steel

from corrosion (galvanized metal), as alloying metal to make bronze and brass,

as zinc-based die casting alloy, and as rolled zinc. The remaining one-fourth is

consumed as zinc compounds mainly by the rubber, chemical, paint, and

agricultural industries.

Due to its high electrochemical activity, zinc provides cathodic corrosion

protection for iron and steel products. Zinc is extensively used to galvanize other

metals such as iron to prevent corrosion. Zinc can be employed to form numerous

alloys with other metals. Brass, nickel silver, typewriter metal, commercial bronze,

spring bronze, German silver, soft solder, and aluminum solder are some of the

more important alloys.

Zinc has low melting point (419°C), this permits problem-free shaping by

casting. Large quantities of zinc are used to produce die-castings, which are

used extensively by the automotive, electrical, and hardware industries. Zinc

oxide is a unique and very useful material for modern civilization. It is widely used

in the manufacture of paints, rubber products, cosmetics, pharmaceuticals, floor

coverings, plastics, printing inks, soap, storage batteries, textiles, electrical

equipment, and other products. The uses of zinc oxides are based on their


26

properties of opacity to ultraviolet light and their high refractive index. These

provide both durability and high hiding power in paints. Lithopone, a mixture of zinc

sulfide and barium sulfate, is an important pigment. Zinc sulfide is used in making

luminous dials, X-ray and TV screens, and fluorescent lights. The chemical

activity of zinc makes it an essential accelerator and activator in vulcanizing

rubber. The electrostatic and photoconductive properties of zinc are utilized in

photocopying.

Health Effect of Zinc

Repeated intra testicular injections of zinc chloride into chickens and rats

have been reported to produce testicular sarcomas (Sunderman, 1977). A

higher prevalence of chromosome anomalies in leukocytes has been reported to

occur among workers exposed to zinc, and to a lesser extent cadmium and lead,

in a rolling mill (Deknudt and Leonard, 1975). Zinc chloride has also been shown

to induce chromosome aberration in human lymphocytes in vitro (Deknudt and

Deminatti, 1978).

Chromium:

Chromium is a lustrous, brittle, hard metal and can be highly polished

discovered by Vaughlin in 1797. When heated it borns and forms the green

chromic oxide. It has a melting point of 1 9070C, a boiling point of 26720C, a

density of 7.19 gcm-3 at -200C and Standard potential is - 0.71 V (Cr3+ / Cr).


Chromium main uses are in alloys such as stainless steel, in chrome

plating and in metal ceramics. Chromium plating was once widely used to give


27

steel a polished silvery mirror coating. Chromium is used in metallurgy to

impart corrosion resistance and a shiny finish; as dyes and paints, its salts

colour glass an emerald green and it is used to produce synthetic rubies; as a

catalyst in dyeing and in the tanning of leather; to make molds for the firing of

bricks. Chromium (IV) oxide (CrO2) is used to manufacture magnetic tape.

Chromium in the environment

Chromium is mined as chromite (FeCr2O4) ore. Chromium ores are

mined today in South Africa, Zimbabwe, Finland, India, Kazakihstan and the

Philippines. A total of 14 million tones of chromite ore are extracted. Reserves

are estimated to be of the order of 1 billion tones with unexploited deposits in

Greenland, Canada and USA. Most people eating food that contains

chromium (III) is the main route of chromium uptake, as chromium (III) occurs

naturally in many vegetables, fruits, meats, yeasts and grains

Health effects

Long exposure to chromium causes skin rashes ulcers, respiratory

problems and liver damage, weakened immune systems lung cancer and

death. The kidney is the critical target organ for the general population as well

as for occupationally exposed populations. Cadmium is known to accumulate

in the human kidney for a relatively long time, from 20 to 30 years, and, at

high doses, is also known to produce health effects on the respiratory system

and has been associated with bone disease. Most of the available

epidemiological information on cadmium has been obtained from


28

occupationally exposed workers or on Japanese populations in highly

contaminated areas.

Most studies have centered on the detection of early signs of kidney

dysfunction and lung impairment in the occupational setting, and, in Japan, on

the detection and screening for bone disease in general populations exposed

to cadmium-contaminated rice. More recently, the possible role of cadmium in

human carcinogenesis has also been studied in some detail.

Environmental Effects

The main human activities that increase the concentrations of

chromium (III) are steal, leather and textile manufacturing. Through coal

combustion chromium will also end up in air and through waste disposal

chromium will end up in soils. Chromium is not known to accumulate in the

bodies of fish, but high concentrations of chromium, due to the disposal of

metal products in surface waters, can damage the gills of fish that swim near

the point of disposal.

Barium

Barium is a silvery-white metal that can be found in the environment

and is discovered by Sir Humphrey Davy in 1808. It has a melting point of 725

°C, a Boiling point 1640 °C, a density of 3.5 g.cm-3 at 20°C and standard

potential of - 2.90 V.


29


Barium is often used in barium-nickel alloys for spark-plug electrodes

an in vacuum tube as drying and oxygen-removing agent. It is also used in

fluorescent lamps: impure barium sulfide phosphoresces after exposure to the

light. Barium compounds are used by the oil and gas industries to make

drilling mud.

Barium in the environment

Barium is surprisingly abundant in the Earth's crust, being the 14th

most abundant element. High amounts of barium may only be found in soils

and in food, such as nuts, seaweed, fish and certain plants. As a result barium

concentrations in air, water and soil may be higher than naturally occurring

concentrations on many locations.

Health effects

Small amounts of water-soluble barium may cause a person to

experience breathing difficulties, increased blood pressures, heart rhythm

changes, stomach irritation, muscle weakness, changes in nerve reflexes,

swelling of brains and liver, kidney and heart damage. Barium has not shown

to cause cancer with humans, examples are mining, metal production, wood

production and phosphate fertilizer production. World production of copper

amounts to 12 million tones a year and exploitable reserves are around 300

million tones, which are expected to last for only 25 years.


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Arsenic

Arsenic appears in three allotropic forms: yellow, black and grey. The

metallic form is brittle, tharnishes and when heated it rapidly oxidizes to

arsenic trioxide, which has a garlic odor. Its melting point is 814 °C (36 atm), a

boiling point is 615 °C (sublimation), a density of 5.7 g.cm-3 at 14°C and

standard potential of - 0.3 V (As3+/ As


Arsenic compounds are used in making special types of glass, as a

wood preservative and, lately, in the semiconductor gallium arsenade, which

has the ability to convert electric current to laser light. During the 18th, 19th,

and 20th centuries, a number of arsenic compounds have been used as

medicines

Arsenic in the environment

It is found naturally on earth in small concentrations. Arsenic in the

atmosphere comes from various sources: volcanoes release about 3000

tones per year and microorganisms release volatile methylarsines to the

extent of 20.000 tones per year, but human activity is responsible for much

more: 80.000 tones of arsenic per year are released by the burning of fossil

fuels. A little uncombined arsenic occurs naturally as microcrystalline masses,

found in Siberia, Germany, France, Italy, Romania and in the USA. World

resources of arsenic in copper and lead ores exceed 10 million tones.


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Health effects

The World resources of arsenic in copper and lead ores exceed 10

million tones. A very high exposure to inorganic arsenic can cause infertility

and miscarriages with women, and it can cause skin disturbances, declined

resistance to infections, heart disruptions and brain damage with both men

and women. Finally, inorganic arsenic can damage DNA.

Environmental effects

Arsenic is mainly emitted by the copper producing industries, but also

during lead and zinc production and in agriculture. Plants absorb arsenic fairly

easily, so that high-ranking concentrations may be present in food.

Conventional methods for the removal of heavy metals from waste water streams

Living beings of aquatic and terrestrial are affected from heavy metal

pollution. So now a day the removal of heavy metals is mandatory. Several

methods have been devised for the treatment and removal of heavy metals.

Different industries discharge a variety of toxic metals into the environment

(e.g., electroplating, metal finishing operations, electronic –circuit production,

steel and non-ferrous processes and fine-chemical and pharmaceutical

production). The commonly used procedures for removing metal ions from

aqueous streams include chemical precipitation, lime coagulation, ion

exchange, reverse osmosis and solvent extraction (Rich and Cherry, 1987).


32

Reverse Osmosis

It is a process in which heavy metals are separated by a semi-

permeable membrane at a pressure greater than osmotic pressure caused by

the dissolved solids in wastewater. The disadvantage of this method is that it

is expensive (Ahalya et al., 2003).

Electro dialysis

In this process, the ionic components (heavy metals) are separated

through the use of semi-permeable ion selective membranes. Application of

an electrical potential between the two electrodes causes a migration of

cations and anions towards respective electrodes. Because of the alternate

spacing of cation and anion permeable membranes, cells of concentrated and

dilute salts are formed. The disadvantage is the formation of metal

hydroxides, which clog the membrane (Ahalya et al., 2003)

Ultra filtration

They are pressure driven membrane operations that use porous

membranes for the removal of heavy metals. The main disadvantage of this

process is the generation of sludge (Ahalya et al., 2003).

Ion exchange

In this process, metal ions from dilute solutions are exchanged with

ions held by electrostatic forces on the exchange resin. The disadvantages

include: high cost and partial removal of certain ions. Ion exchange resins are

available selectively for certain metal ions. The cations are exchanged for H+

or Na+. The cation exchange resins are mostly synthetic polymers containing

an active ion group such as SO3H. The natural materials such as zeolites can

be used as ion exchange media (Van der Heen, 1977). The modified zeolites


33

like zeocarb and chalcarb have greater affinity for metals like Ni and Pb

(Groffman et al., 1992).The limitations on the use of ion exchange for

inorganic effluent treatment are primarily high cost and the requirements for

appropriate pretreatment systems. Ion exchange is capable of providing metal

ion concentrations to parts per million levels. However, in the presence of

large quantities of competing mono-and divalent ions such as Na and Ca, ion

exchange is almost totally ineffective.

Chemical precipitation

Chemical precipitation is a conventional physiochemical process for

toxic heavy metal removal. It involves the addition of chemicals to alter the

physical state of the dissolved or suspended metals and to facilitate their removal

through sedimentation. Typical chemicals often used to precipitate metal ions

from aqueous streams include caustic soda, lime, sodium sulfide, alum, ferrous

sulfide, ferric chloride and sulfate, soda ash, phosphoric acid/sodium phosphate,

and sodium borohydride. Coagulants or flocculants are often used to destabilize

the colloidal suspension by reducing the repulsive forces for adequate

precipitation. This will help the small unsettleable metal hydroxide ions to

agglomerate into larger and more settleable particles for enhanced removal of the

toxic metal ions (Dohnert, 1978).

Hydroxide precipitation

Now a day chemical precipitation of heavy metals as their hydroxides

using lime or sodium hydroxide is extensively used. Lime is generally favored

for precipitation purposes due to the low cost of precipitant, ease of pH control

in the range of 8.0 –10.0 and the excess of lime also serves as an adsorbent


34

for the removal of metal ions. The efficiency of the process depends on a

number of factors, which include the ease of hydrolysis of the metal ion,

nature of the oxidation state, pH, presence of complex forming ions, standing

time, degree of agitation and settling and filtering and characteristics of the

precipitate. The limitations of this method include difference between metals

in the optimum pH for hydroxide formation may lead to the problems in the

treatment of effluents containing combined metal ions. Variability in metal

hydroxide solubility at a fixed pH is another drawback.

Carbonate precipitation

Carbonate precipitation of metals using calcium or sodium carbonate is

very limited. Patterson et al., 1997 reported improved results using carbonate

precipitate for Cd (II) and Pb (II) from electroplating effluents. When the pH

was brought to 7.5, residual concentration of Pb (II) and Cd (II) were 0.60 and

0.25 mg/L respectively

Sulphide precipitation

Since most of the heavy metals form stable sulphides, excellent metal

removal can be obtained by sulphide precipitation. Treatment with sulphides

is most advantageous when used as a polishing step after conventional

hydroxide precipitation or when very high metal removals are required.

Chemical reduction

Reduction of hexavalent chromium can also be accomplished with

electro-chemical units. The electrochemical chromium reduction process uses

consumable iron electrodes and an electric current to generate ferrous ions


35

that react with hexavalent chromium to give trivalent chromium as follows

(Kiff, 1987).

3Fe2+ + CrO42- + 4H2O → 3Fe3++ Cr3+ + 8OH-

Another application of reduction process is the use of sodium borohydride,

which has been considered effective for the removal of mercury, cadmium,

lead, silver and gold.

Xanthate process

Insoluble starch xanthate (ISX) is made from commercial cross linked

starch by reacting it with sodium hydroxide and carbon disulphide. To give the

product stability and to improve the sludge settling rate, magnesium sulphate

is also added. ISX works like an ion exchanger, removing the heavy metals

from the wastewater and replacing them with sodium and magnesium.

Average capacity is 1.1-1.5meq of metal ion per gram of ISX (Anon, 1978).

ISX is most commonly used by adding to it the wastewater as slurry for

continuous flow operations or in the solid form for batch treatments. It should

be added to the effluent at pH ≥ 3. Then the pH should be allowed to rise

above 7 for optimum metal removal (Wing, 1978). Residual metal ion level

below 50 μg/L has been reported (Hanway et al., 1978, Wing et al., 1978)The

effectiveness of soluble starch xanthate (SSX) for removal of Cd (II), Cr (VI)

and Cu (II) and insoluble starch xanthate (ISX) for Cr (VI) and Cu (II) have

been evaluated under different aqueous phase conditions. Insoluble starch

xanthate had better binding capacity for metals. The binding capacity of SSX

and ISX respectively for different metal ions follows the sequence of Cr (VI)>

Cu (II)> Cd(II) and Cr (VI)> Cu (II) (Tare et al., 1988).


36

Solvent extraction

Liquid-liquid extraction (also frequently referred as solvent extraction)

of metals from solutions on a large scale has experienced a phenomenal

growth in recent years due to the introduction of selective complexing agents

(Beszedits, 1988). In addition to hydrometallurgical applications, solvent

extraction has gained widespread usage for waste reprocessing and effluent

treatment.

Solvent extraction involves an organic and an aqueous phase. The

aqueous solution containing the metal or metals of interest is mixed with the

appropriate organic solvent and the metal passes into the organic phase. In

order to recover the extracted metal, the organic solvent is contacted with an

aqueous solution whose composition is such that the metal is stripped from

the organic phase and is reextracted into the stripping solution. The

concentration of the metal in the strip liquor may be increased, often 110 to

100 times over that of the original feed solution. Once the metal of interest

has been removed, the organic solvent is recycled either directly or after a

fraction of it has been treated to remove the impurities.

Membrane process

Important examples of membrane process applicable to inorganic

wastewater treatment include reverse osmosis and eletrodialysis (EPA, 1980).

These processes involve ionic concentration by the use of selective

membrane with a specific driving force. For reverse osmosis, pressure

difference is employed to initiate the transport of solvent across a

semipermeable membrane and electro dialysis relies on ion migration through


37

selective permeable membranes in response to a current applied to

electrodes. The application of the membrane process described is limited due

to pretreatment requirements, primarily, for the removal of suspended solids.

The methods are expensive and sophisticated, requiring a higher level of

technical expertise to operate.

A liquid membrane is a thin film that selectively permits the passage of a

specific constituent from a mixture (Beszedits, 1988). Unlike solid

membranes, however liquid membranes separate by chemistry rather than

size, and thus in many ways liquid membrane technology is similar to solvent

extraction.

Since liquid membrane technology is a fairly recent development, a

number of problems remain to be solved. A major issue with the use of

supported membranes is the long term stability of the membranes, whereas

the efficient breakup of microspheres for product recovery is one of the

difficulties encountered frequently with emulsion membranes.

Evaporators

In the electroplating industry, evaporators are used chiefly to

concentrate and recover valuable plating chemicals. Recovery is

accomplished by boiling sufficient water from the collected rinse stream to

allow the concentrate to be returned to the plating bath. Many of the

evaporators in use also permit the recovery of the condensed steam for

recycle as rinse water. Four types of evaporators are used throughout the

elctroplating industry (USEPA, 1979a) (I) Rising film evaporators; (ii) Flash

evaporators using waste heat; (iii) submerged tube evaporators; (iv)

Atmospheric evaporators.


38

Both capital and operational costs for evaporative recovery systems are high.

Chemical and water reuse values must offset these costs for evaporative

recovery to become economically feasible.

Cementation

Cementation is the displacement of a metal from solution by a metal

higher in the electromotive series. It offers an attractive possibility for treating

any wastewater containing reducible metallic ions. In practice, a considerable

spread in the electromotive force between metals is necessary to ensure

adequate cementation capability. Due to its low cost and ready availability,

scrap iron is the metal used often. Cementation is especially suitable for small

wastewater flow because a long contact time is required. Some common

examples of cementation in wastewater treatment include the precipitation of

copper from printed etching solutions and the reduction of Cr (VI) in chromium

plating and chromate-inhibited cooling water discharges (Case,

1974).Removal and recovery of lead ion by cementation on iron sphere

packed bed has been reported (Angelidis et al., 1988, 1989).Lead was

replaced by a less toxic metal in a harmless and reusable form

Electro-deposition

Some metals found in waste solution can be recovered by

electrodeposition using insoluble anodes. For example, spent solutions

resulting from sulphuric acid cleaning of Cu may be saturated with copper

sulphate in the presence of residual acid. These are ideal for electro-winning


39

where the high quality cathode copper can be electrolytically deposited while

free sulphuric acid is regenerated.

Adsorption

Since activated carbon also possesses an affinity for heavy metals,

considerable attention has been focussed on the use of carbon for the

adsorption of hexavalent chromium, complexed cyanides and metals present

in various other forms from wastewaters. Watonabe and Ogawa first

presented the use of activated carbon for the adsorption of heavy metals in

1929.

The mechanism of removal of hexavalent and trivalent chromium from

synthetic solutions and electroplating effluents has been extensively studied

by a number of researchers. According to some investigators, the removal of

Cr (VI) occurs through several steps of interfacial reactions (Huang and

Bowers, 1979)

(i) The direct adsorption of Cr6+onto carbon surface.

(ii) The reduction of Cr6+ species to Cr3+ by carbon on the surface.

(iii) The adsorption of the Cr3+ species produced, which occurs to a much

lesser extent than the adsorption of the Cr6+ species.

Adsorption of Cr (III) and Cr (VI) on activated carbon from aqueous

solutions has been studied (Toledo, 1994) Granular activated carbon columns

have been used to treat wastewaters containing lead and cadmium (Reed and

Arunachalam, 1994, Reed et al., 1994) Granular activated carbon was used

for the removal of Pb (II) from aqueous solutions (Cheng et al., 1993) The


40

adsorption process was inhibited by the presence of humic acid, iron (III),

aluminum (III) and calcium (II).

Disadvantages of Conventional Methods

Metals are a class of pollutants, often toxic and dangerous, widely

present in industrial and household wastewaters. Although metal precipitation

using a cheap alkali such as lime (calcium hydroxide) has been the most

favored option, other separation technologies are now beginning to find favor.

Precipitation, by adjusting the pH value is not selective and any iron (ferric

ion) present in the liquid effluent will be precipitated initially followed by other

metals. Consequently precipitation produces large quantities of solid sludge

for disposal, for example precipitation as hydroxides of 100 mg/l of copper (II),

cadmium (II) or mercury (II) produces as much as 10-, 9- and 5 fold mg/l of

sledges respectively. The metal hydroxide sludge resulting from treatment of

electroplating wastewater has been classified as a hazardous waste.

The versatility, simplicity and other technology characteristics will

contribute to the overall process costs, both capital and operational. At

present many of these technologies such as ion exchange represent

significant capital investments by industry. the conventional methods are

ineffective in the removal of low concentrations of heavy metals and they are

non-selective. Moreover, it is not possible to recover the heavy metals by the

above mentioned methods.


41

Biosorption

Biosorption is a process that utilizes the inexpensive biosorbent to

quickly and effectively sequester dissolved toxic heavy metals from dilute

solutions. It is an ideal alternative process for treatment of high volume and

low concentration of industrial waste, streams contaminated with heavy metals.

It accumulate heavy metals from wastewater through metabolically mediated

or physico-chemical pathways of uptake (Fourest and Roux, 1992).

Biosorption has advantages compared with conventional techniques:

Cheap: The cost of the biosorbent is cheap since they often are made from

waste material (Kratochvil and Volesky, 1998 a).

Metal selective: The metal sorbing performance of different types of biomass

can be more or less selective on different metals. This depends on various

factors such as type of biomass, mixture in the solution, type of biomass

preparation and physicochemical treatment.

Regeneration of biosorbents: Biosorbents can be reused, after the recycling

of metal.

No sludge generation: No secondary problems with sludge occur with

biosorption, as is the case with many other techniques in use, for example,

precipitation.


42

Metal recovery: In case of metals, it can be recovered after being sorbed

from the solution. No additional nutritional requirements.

Competitive performance: Biosorption is capable of a performance

comparable to the most similar technique, ion exchange treatment. Ion

exchange is, as mentioned above, rather costly, making the low cost of

biosorption a major factor (Ahalya et al., 2003).

Biosorbents

Typical kinds of these Biosorbents are algae, bacteria, yeast, fungi,

and waste byproducts from food and pharmaceutical industries (Volesky,

1986).

Algae as Biosorbent

algal biomass as a biosorbent is emerging as an attractive,

economical and effective proposition because of certain added advantages of algae

over others(Holan et al, 1994; sing et al, 2001). Algae have low nutrient requirements,

being autotrophic they produce a large biomass, and unl ike other biomass and

microbes, such as bacteria and fungi, they generally do not produce toxic

substances. Binding of metal ions on algal surface depends on different conditions

l ike ionic charge of metal ion, algal species and chemical composition of the

metal ion solution(Sheng et al,2004; Freire et al, 2005; Gupta et al, 2001).

The uptake of Pb by dried biomass of a green alga, Chlorella vulgaris was

investigated in a single-staged batch reactor in the concentration range of 25-

200 mg/L(Holan et al, 1994). The brown seaweed, Sargassum sp. (Chromophyta)

was used as a biosorbent for Cu ions(Antunes et al, 2003). The influence of


43

different experimental parameters such as initial pH, shaking rate, sorption

time, temperature, equilibrium conditions and initial concentration of Cu ions on

Cu uptake was evaluated.

Biosorption of Cr ( I I I ) by Sargassum sp. was studied by Cossich et al .

The results showed that pH has an important effect on Cr biosorption capacity.

The biosorbent size did not affect the Cr biosorption rate and capacity. The

removal of Cr (VI) by Eclonia biomass, the brown seaweed, was examined in a

binary aqueous containing Ni (Park et al, 2006). The removal rate was unaffected

by the presence of Ni (II). Kiran et al reported biosorption of Cr (VI) by native

isolate of an unexplored algal strain, Lyngbya pulealis (HH-15) in batch system

under varying range of pH (2.0-10.0). Maximum metal removal (94.8 %) took

place at pH 3.0 with initial Cr concentration of 50 mg/L, which got reduced

(90.1%) in the presence of 0.2% salts.

Fungi as Biosorbent

Aspergilus niger is fungal biosorbent used for biosorption of heavy metals.

It has a musty odor. It is commonly found in textiles, soils, grains, fruits and

vegetables. Kapoor and Viraraghavan (1997) found that the biosorption of lead,

cadmium and copper was inhibited when carboxyl groups were esterified. This

revealed the important role of carboxyl groups in the biosorption of metals. The

release of calcium, magnesium, and potassium happened along with the

biosorption of lead and cadmium. The metal binding mechanisms of biosorbent

are similar to binding of heavy metals by weakly acidic exchange resins.


44

Bacteria as Biosobent

The use of bacteria is a fast growing field in remediation because of

their small size, their ubiquity, their ability to grow under controlled conditions.

This includes both Gram-positive (e.g., Bacillus, streptomyces) and Gram-

negative (e.g., Pseudomonas, Zoogloea ramigera) bacteria (Addour et al,

1999; Mullen et al, 1989; Nakajima et al, 1986; Norberg et al, 1984,Rydin et

al, 1984; Strandberg et al, 1981). Among all other bacteria, Bacillus sp. has

been identified as having a high potential of metal sequestration and has been

used in commercial biosorbent preparation (Brierly et al, 1986). The

interaction of bacterial surfaces with soluble metals in the aqueous

environments where micro organisms live is inevitable. The biosorption

characteristics of Cd and Pb ions were determined with urpple non-suphur

bacteria Rhodobacter sphaeroides, and hydrogen bacteria, Alcaligenes

eutrophus H16 (Seki et al, 1998). Bacterial exchange of nutrients and wastes

with the surrounding medium occurs, through diffusion both internally and

externally. A polysaccharide from Bacillus firmus is reported to remove metal

ions like lead, copper and zinc from aqueous solution. Enterobacter cloaceae,

a marine bacterium, was tested for its Cr(VI) tolerance and chelation. The

growth of E. cloaceae was observed after incubation period (80h) in control

flasks as well as in the flasks containing metals (Rabbani et al, 2005).

Yeast as Biosobent

Saccharomyces cerevisiae are well-known and commercially

significant yeasts. These organisms have long been utilized to ferment the


45

sugars of rice, wheat, barley, and corn to produce alcoholic beverages. They are

also used in the baking industry to expand, or raise, and dough.

Advantages of S. cerevisiae as biosorbents in metal biosorption

S. cerevisiae is easy to cultivate at large scale. The yeast can be easily

grown using unsophisticated fermentation techniques and inexpensive growth

media (Kapoor and Viraraghavan, 1995). Moreover, the yield of the biomass is

also high. the biomass of S. cerevisiae can be obtained from various food and

beverage industries. S. cerevisiae as a by-product, is easier to get from

fermentation industry, in comparison with other types of waste microbial

biomass. Microorganisms used in enzymatic industry and pharmaceutical

industry are usually involved in the secret of their products, which makes

industries reluctant to supply the waste biomass. The supply of S. cerevisiae as

waste residuals is basically stable. Thirdly, S. cerevisiae is generally regarded

as safe. Therefore, biosorbents made from S. cerevisiae can be easily

accepted by the public when applied practically Fourthly, but not the last, S.

cerevisiae, is an ideal model organism to identify the mechanism of biosorption

in metal ion removal, especially to investigate the interactions of metal-

microbe at molecular level. (Peregol and Howell,1997) reported that the use

of yeasts as model systems is particularly attractive because of the ease of

genetic manipulation and the availability of the complete genomic sequence of S.

cerevisiae. In fact, S. cerevisiae, as a model system in biology, has been

explored fully in molecular biology (Zhou, 2002; Eide, 1997, 1998). Knowledge

accumulated on the molecular biology of the yeast is very helpful to identify

the molecular mechanism of biosorption in metal ion removal . At the same

time, S. cerevisiae can be easily manipulated genetically and morphologically,


46

which is helpful to genetically modify the yeast more appropriate for various

purposes of metal removal.

Forms of S. cerevisiae in biosorption research

S. cerevisiae in different forms has been studied for different purposes

of research. For example, living cell/ dead cell (Kapoor and Viraraghavan,

1995). intact cell/ deactivated cell, immobilized cell/free cell (Veglio and

Beolchini, 1997). raw material/pretreated cell by physicochemical process, wild

type/mutant cell, floccu-lent/non-flocculent cell (Marques et al., 1999). engi-

neered/non-engineered cell, lab culture/waste industrial cell, and cells from

different industries (Park et al., 2003).Comparing the results of metal

biosorption using the different forms of the yeast can give useful information for

understanding the mechanism of metal uptake by S. cerevisiae. For example,

(Ramsay and Gadd, 1997), by examining and comparing the responses of

vacuole-deficient mutants and wild type of S. cerevisiae to several toxic

metals, found that vacuole-deficient strains are more sensitive to Zn, Mn, Co,

Ni with a largely decreased capacity to accumulate these metals than wild type,

but no change for Cu or Cd. The results confirmed the essential role of vacuole

in detoxification for Zn, Mn, Co, Ni, but not for Cu and Cd. Immobilization

technique is one of the key elements for the practical application of

biosorption, especially by dead biomass. Various kinds of immobilized S.

cerevisiae have been studied with different immobilizing materials (Veglio and

Beolchini, 1997; Park et al, 2003).compared two strains of S. cerevisiae for the

biosorption of cadmium. One strain is ATCC 834 which is used for the

production of l-phenylacetyl carbinol (l-PAC) and another strain, ATCC 24858

for ethanol production. They found that the thicker layer and the larger specific


47

surface layer seemed to benefit a larger cadmium uptake capacity for the

strain S. cerevisiae ATCC 834. Free cells appear unsuitable in practical

application, largely due to solid/liquid separation problem. However, Veglio and

Beolchini (1997) pointed out that investigation on the performance of free cells

for metal uptake can provide fundamental information on the equilibrium of the

biosorption process, which is useful for practical application. Meanwhile,

flocculating cell has been suggested for biosorption, attempting to overcome

the separation problem of free cells (Soares et al., 2002).

Whether to employ living cells or non-living cells for biosorption is still at

arguing stage (Suh and Kim, 2000). In the early researches on biosorption of

heavy metal ions, living cells were used. However, dead cells have been

found to have the same or even higher uptake capacity of metal ions,

comparing with living cells. Meanwhile, dead cells can overcome some limits

that living cells are used: nutrition demand, sensitivity to extreme pH value or

higher metal ion concentration, etc. Therefore, biosorption studies involving

dead/pretreated biomass have dominated during 1980s–90s (Malik, 2004).

However, the limitations of the industrial application of biosorption with

immobilized dead cells have been realized from some pilot plants. For

example, the cost for producing the required biosorbents with waste biomass

was too expensive using immobilized techniques and using various pre-

treatment processes. Process of regeneration and re-use is complex and very

expensive. For real effluents, the co-existed ions and organic matters in

aqueous solution made matters even more difficult and more complex. Hybrid

biotechnologies, such as biosorption, bio-precipitation, and bioaccumulation,

using living cells, even together with physicochemical process, are suggested


48

in recent years (Malik, 2004; Tsezos, 2001). As for S. cerevisiae, dead or living

cells are the same important in biosorption studies today. As a waste microbial

biomass from fermentation, study on dead cells of the yeast is also dominant

and necessary. In exploring the mechanism of metal uptake, especially metal–

microbe interactions, living cells of S. cerevisiae should be used inevitably for

specific research at molecular level.

Biosorption mechanism by the cell of S. cerevisiae

The mechanism of metal biosorption is complicated and not fully

understood. The status of biomass (living or non-living), types of biomaterials,

properties of metal-solution chemistry, ambient/environmental conditions such

as pH, will all influence the mechanism of metal biosorption. In the last few

years, some reviews have been published focusing on different aspects of

biosorption mechanism, such as physical–chemical mechanism, metal

detoxification, transfer mechanism and molecular biology development (White

et al., 1995; White and Gadd, 1995; Lovley and Coatest, 1997; Rosen, 2002;

Eide, 1997; Peregol and Howell, 1997; Wang and Yang, 1996). Two types of

metal sequestering are passive mode by dead or inactive cells of S. cerevisiae

and active mode by living cells. Passive mode is independent of energy,

mainly through chemical functional groups of the material, comprising the cell

and particularly cell wall. Active mode is metabolism-dependent and related to

the metal transport and deposition. Of course, passive metal uptake may

occur when the cell is metabolically active (Volesky, 1990b). In this the

mechanisms of metal biosorption will be discussed according to the location

where the metal removed from the solution extracellular


49

accumulation/precipitation, cell surface sorption or precipitation, intracellular or

accumulation (Ve g l i o and Beolchini, 1997).

Extracellular accumulation/precipitation

Some prokaryotic (bacteria, Archaea) and eukary-otic (algae, fungi)

microorganisms can produce or excrete extracellular polymeric substances

(EPS), such as polysaccharides, glucoprotein, lipopolysaccharide, soluble

peptide etc. These substances possess a substantial quantity of anion

functional groups which can adsorb metal ions. References published on

metal biosorption with EPS mainly focus on the bacterial organism, such as

Bacillus megaterium, Acinetobacter, Pseudomonas aeruginosa, sulphate-

reducing bacteria (SRB), Cyanobateria or activated sludge (Liu et al., 2001),

whereas EPS study for fungi and algae is limited (Flemming and Wingender,

2001; Wang and Yang, 1996; Pirog, 1997). The roles of EPS on metal removal

in a biosorption system are usually neglected or ignored, especially in the case

of fungi and yeast. Among the limited studies on metal removal by EPS, most

of them are related to the EPS extracted from intact organism cells, but not the

EPS in living cells. Although conspicuous extracellular layers are mainly

associated with bacterial cells, whether the yeast of S. cerevisiae excretes

EPS is unclear. Suh et al. (1998b) implied that the strain of S. cerevisiae used

in their experiment did not excrete EPS. However, floc-culent strain of S.

cerevisiae has been suggested to be used in metal biosorption due to higher

uptake capacity


50

Cell surface sorption/precipitation

The cell wall tends to be the first cellular structure to come in contact

with metal ions, excluding a possible existing extracellular layer mainly related

to bacterial cells. Two basic mechanisms of metal uptake by cell wall are as

follows: stoichiometric interaction between functional groups of cell wall

composition, including phosphate, carboxyl, amine as well as phosphodiester;

and physicochemical inorganic deposition via adsorption or inorganic

precipitation. Nowadays, complexa-tion, ion exchange, adsorption (by

electrostatic interaction or van der Waals force), inorganic microprecipitation,

oxidation and/or reduction have been proposed to explain metal uptake by

organism (Volesky, 1990a,b; Liu et al., 2002b).

Intracellular accumulation/ precipitation

When the extracellular concentration of metal ions was higher than that of

intracellular, metal ions could penetrate into the cell across the cell wall and

membrane of the biomass by free diffusion. Metal ions can also enter into the

cell if the cell wall was disrupted by natural force (e.g. autolysis) or artificial

force (mechanical force or alkali treatment etc.). The above process is

independent of metabolism. However, the process of intracellular

accumulation/precipitation discussed here mainly relates to the living cells of

biomass, and is an energy-driven process and dependent on active

metabolism. Metal ions transported across the cell membrane, are

transformed into other species or precipitated within the cell by active cells,

including transportation (Eide, 1997, 1998; Portnoy et al., 2001).


51

Biosorption mechanisms

The biosorption mechanisms are not fully understood. Biosorption

mechanisms are classified according to various criteria.

Based on the dependence on the cell's metabolism, biosorption

mechanisms can be divided into two categories (Ahalya et al., 2003).

1. Metabolism dependent.

2. Non -metabolism dependent.

Based on the location where the metal removed from solution is found,

biosorption can be classified as

1. Extra cellular accumulation/ precipitation

2. Cell surface sorption/ precipitation

3. Intracellular accumulation.

Transport across cell membrane

Some mechanisms are used to convey metabolically important ions

such as potassium, magnesium and sodium. Like, the same mechanisms are

used to mediate the heavy metal transport across microbial cell membranes.

The metal transport systems may become confused by the presence of heavy

metal ions of the same charge. So the metabolic activity is not associated with

the same mechanism (Kuyucak and Volesky, 1988).

Metabolism independent binding takes place where the metals are

bound to the cell walls followed by metabolism dependent intracellular uptake,

whereby metal ions are transported across the cell membrane.


52

Physical adsorption

Some weak forces like Van der Waals' forces are helpful in physical

adsorption. Uranium, cadmium, zinc, copper and cobalt biosorption by dead

biomasses of algae, fungi and yeasts takes place through electrostatic

interactions between the metal ions in solutions and cell walls of microbial

cells. Electrostatic interactions have been demonstrated to be responsible for

copper biosorption by bacterium Zoogloea ramigera and alga Chloroella

vulgaris, Chromium biosorption by fungi Ganoderma lucidum and Aspergillus

niger ( Aksu et al., 1992).

Ion Exchange

Most of the micro organisms contain polysaccharides in their cell wall.

the alginates of marine algae occur as salts of K+, Na+, Ca2+, and Mg2+ can

exchange with counter ions such as CO2+, Cu2+, Cd2+ and Zn2+. The copper

uptake by fungi Ganoderma lucidium and Aspergillus niger was also up taken

by ion exchange mechanism (Muraleedharan and Venkobachr, 1990).

Complexation

The metal removal from solution also takes place by complex formation

on the cell surface after the interaction between the metal and the active

groups. (Aksu et al. 1992) hypothesized that biosorption of copper by C.

vulgaris and Z. ramigera takes place through both adsorption and formation of

coordination bonds between metals and amino and carboxyl groups of cell

wall polysaccharides. Complexation was found to be the only mechanism

responsible for calcium, magnesium, cadmium, zinc, copper and mercury


53

accumulation by Pseudomonas syringae. Microorganisms may also produce

organic acids (e.g., citric, oxalic, gluonic, fumaric, lactic and malic acids),

which may chelate toxic metals resulting in the formation of metallo-organic

molecules. These organic acids help in the solubilization of metal compounds

and their leaching from their surfaces. Metals may be biosorbed or complexed

by carboxyl groups found in microbial polysaccharides and other polymers.

Precipitation

Precipitation may be either dependent on the cellular metabolism or

independent of it. In the former case, the metal removal from solution is often

associated with active defense system of the microorganisms. They react in

the presence of toxic metal producing compounds, which favor the

precipitation process. In the case of precipitation not dependent on the cellular

metabolism, it may be a consequence of the chemical interaction between the

metal and the cell surface (Eide, 1997, 1998).

Metal ion uptake and interaction of metal ions with microorganisms

Heavy metals can be present in wastewater in two forms, i.e. paniculate

form and solubilized form. Heavy metals of solubilized form exist as free metal

ions or as complexed ions by forming metal-ligand complex with inorganic or

organic ligands. Heavy metals in the form of paniculate include heavy metals

present in colloidal form and heavy metals adsorbed on paniculate matter.

Heavy metals can be taken up by microorganisms in many ways, which

were summarized by Gadd (1988) and Brierley (1990). Once heavy metal ions


54

are entrapped in the cellular structure of a microorganism, they will be bound to

the binding sites. This passive uptake, independent of the biological metabolism,

is termed as biosorption, which is "a non-directed physico-chemical interaction

that may occur between metal/radionuclide species and the cellular compounds

of biological species" (Shumate and Strandberg, 1985). In the case of live

microorganisms, under the effect of cell metabolic cycle, some heavy metal ions

will go through cell membranes and enter cells. This metal uptake is referred

to as an active uptake or intracellular uptake. The passive and active uptakes

consist of what is termed as "bioaccumulation". Thus, metal uptake by dead

cells is through passive uptake (extracellular uptake) and metal uptake by live

cells involves both passive and active uptakes.

Metal uptake by living cells

Living cells of many fungal strains have been shown to accumulate metal

ions. Brown et al. (1974) reported on the adsorption of Hg by live cells of

Saccaromyces cerevisiae. Kojo and Lodenius (1989) found that macrofungi

Agarictis was able to accumulate cadmium and mercury. Mullen et al. (1992) used

A. niger and M. rouxii in the biosorption of heavy metals and found that

biosorption of metals decreased in the order La > Ag > Cu > Cd. P. spinulosum

and A. niger removed copper most effectively, while it removed cadmium,

manganese and zinc moderately (Ross and Townsley, 1986). A. oryzae can

adsorb cadmium up to 9 mg/g (Kiff and Little, 1986). Live Trkhoderma harzianum

was found to be capable of biosorbing uranium, so was the immobilized


55

biomass in a column (Khalid et al.., 1993). Live Saccharomyces cerevisiae could

adsorb 1.9 mg/g Cu (II) and also adsorb lead and zinc (Huang et al.., 1990).

For living biomass, the metal-binding ability of growing cells changes with

an increase of cell age (Kapoor and Viraraghavan, 1995). Although some heavy

metals such as Fe, Cu, Zn, and Mn at low concentration, are essential for

microbial metabolism, many others can not be utilized by microorganisms but

only be accumulated within the polymeric structure (extracellular polymeric

substances (EPS)) of the microorganisms (Cullimore, 1993). Reaching a certain

level, they will impose a highly toxic effect on the living cells (Gadd, 1990).

Therefore, living cells are easily subjected to the toxic effect, resulting in the cell

death. Due to the problems in maintaining active microbial populations under

highly variable conditions of heavy metals, it is not reliable to use the living

biomass system (Matheickal et al., 1996).

Biosorption of metal ions by inactive or dead biomass

Certain types of microbial biomass can passively bind and accumulate

metals even when they are metabolically inactive or killed by physical or chemical

methods (Metheickal et al., 1991; Brady et al., 1994). The nonviable or dead

biomass can be easily stored and used, eliminating the problem of toxicity from

heavy metals. Health hazard, when utilizing potentially pathogenic strains, is also

eliminated. In addition, it does not require the addition of nutrient for cell growth

and the starting-up when they are used in process, resulting in simple process

start-up and control. Furthermore, it can be easily regenerated and reused, and in


56

some cases provide higher capacity (Spinti et al., 1995). Metal bound to the cell

wall is more easily recovered by elution compared with metals accumulated

internally within living cells (Butter et al., 1998). Because of these advantages,

dead biomass is favored when considered as a potential biosorbent to

concentrate and recover heavy metals (Brady et al., 1994).

Inactive Rhizopus arrhizus was found to adsorb a variety of different metal

cations and anions. and the amount of uptake of the cations was directly related to

ionic radii of La, Mn, Cu, Zn, Cd. Ba, Hg, Pb, UO, and Ag (Tobin et al., 1984).

Huang et al. (1988) found that cadmium removal capacity of dead fungal biomass

was the same as live fungi. Ross and Townsley (1986) found that non-growing

biomass of P. spinulosum and A. niger bound considerable amounts of copper,

cadmium and zinc. Non-living Rhizopus nigricans, which was obtained as a

byproduct from a fermentation industry, was shown to be an effective adsorbent

for the removal of lead (Zhang et al., 1998).

Factors affecting biosorption

pH of the aqueous phase will affect the biosorption of metals (Kiff and

Little, 1986; Huang and Huang, 1996). According to the study by Guibal et al.

(1992) on the uranium biosorption by Mucor miehei, pH imposes its influence on

metal or cell wall chemistry. The biosorption capability ofGanoderma lucidum at

pH 6 was much higher than at pH 4 (Matheickal et al., 1991). Tsezos and

Volesky (1981) thought that acid pH in solution will decrease biomass uptake of

metals via competition at the binding site between metal ions and H30\ Kiff and


57

Little (1986) reported on the increase in the biosorption of cadmium on A.

oryzae with an increase in pH. A study by Lewis and Kiff (1988) also showed

that in using Rhizopus arrhizus, its uptake capacity was decreased by acidic

pH, low temperature, and presence of competing cations, and optimum pH was

6 to 9. The biosorption of Ni, Zn, Cd, and Pb by Penicillium digitatum was

found to be highly pH-sensitive and was severely inhibited when pH was below

3 (Galun et al., 1987). Brady et al. (1994) reported that the optimal pH for

biosorption of Zn on Saccharomyces cerevisiae biomass was 7.5 even though

biosorption occurred above pH 4. Ross and Townsley (1986) found that at

lower pH, removal of copper by P. spinulosum was reduced. Rhizopus

nigriccms had significantly low sorption capacity of lead at pH values below 3;

at pH above 4, more lead biosorption was expected to take place (Zhang et al.,

1998). In addition, it was also found that at higher pH, insoluble lead hydroxide

started to precipitate. Tobin and Roux (1998) also reported that at initial pH

values of 5.5 and 7.0, significant precipitation effects occurred in the removal of

chromium by Mucor meihi.

The presence and concentration of organics may hinder the biosorption

process. For example, probably because of binding of chromium with organics

such as proteins, bacteria, or tannins in solution, Saccaromyces cerevisiae

was not effective in removing chromium from tannery wastewater (Brady et

al., 1994). Thus, it can be implied that some biosorbents are applicable to

wastewater with relatively low concentrations of organics.


58

Theoretical Approaches in Heavy Metal Biosorption

Biosorption is a passive non-metabolically mediated process. The toxic

heavy metals in aqueous solution are bound to functional groups in the cell wall of

dead biosorbents. Biosorption may involve different processes, such as ion

exchange, complexation, coordination, electrostatic attraction, or

microprecipitation. Previous studies demonstrated that ion exchange plays a

major role in the surface binding of heavy metal ions by biosorbent. The cell

walls of many microorganisms, including algae, consist mainly of

polysaccharides, proteins, and lipids and therefore offer a host of functional

groups capable of binding to heavy metals (Ting and Lawson, 1989, 1991).

These functional groups include amino, carboxylic, sulfydryl, phosphate, and

thiol groups. They present different affinities and specificities for metal bindings

on the cell surface. The equilibrium amount of a specific metal species can be

determined by the relative affinities of the binding sites for the specific metal and

other metals presented, as well as the residual concentrations of all these metals

remaining in solution. Since a fixed cell of biosorbent offers a finite number of

surface binding sites, the surface adsorption would be expected to show

saturation kinetics with increasing concentrations of metal ions.

Biosorption Process and Mathematical Models

Biosorbents are prepared from the naturally abundant and/or industrial

wastes. Rinsing, protonation, cation conversion, drying and granulation are

common procedures for preparation of biosorbents. Simple cutting and grinding of

the dry biosorbent may yield stable biosorbent particles. Usually, the biosorbent is

packed in a column for continuous removal of heavy metal. Biosorption column


59

operation consists of four steps: biosorbent loading, backwashing, regenerating

and rinsing.

Since the biosorption of heavy metals is always accompanied by the

releasing of roughly equivalent amount of light metals, ion exchange is

recognized as the principal mechanism of metal biosorption. The biosorbent

behaves just as an ion exchanger to make very rapid and efficient metal uptake.

The well-developed and structured knowledge of ion exchange can now be

applied to biosorption. This gives researcher and engineer new tools for

studying, developing, and applying the biosorption process (Yu et al., 1999).

Use of Recombinant bacteria for metal removal

Metal removal by adsorbents from water and wastewater is strongly

influenced by physico-chemical parameters such as ionic strength, pH and the

concentration of competing organic and inorganic compounds. Recombinant

bacteria are being investigated for removing specific metals from

contaminated water. For example a genetically engineered E.coli, which

expresses Hg2+ transport system and metallothionin (a metal binding

protein), was able to selectively accumulate 8 mM Hg2+ /g cell dry weight. The

presence of chelating agents Na+, Mg2+ and Ca2+ did not affect

bioaccumulation (Ahalya et al., 2003).

Objectives

The entire breadth of the literature put forward in this chapter reveal

that there is a great scope for biosorption of heavy toxic metals by

Saccharomyces cerevisiae and the development of the technology for the


60

applicability to the industrial scale. Keeping the advantages of

Saccharomyces cerevisiae over the other organism the present work was

planned with the following objectives.

1. Collection of the microorganism from MTCC and determining the

growth curve for getting better high cell density culture.

2. Standardization of chromium and lead estimation by atomic absorption

spectrophotometer and collection of biomass in three different forms

viz., free cell, dried powder and pretreated.

3. Analysis of biosorption of heavy metals by three forms of yeast cells,

and optimization of pH, temperature, contact time, initial metal

concentration, biomass dosage etc.

4. Evaluation of isotherms, biosorption kinetics and thermodynamic

parameters

5. Optimization of various parameters considering the values obtained in

the lab scale experiments for applicability to large scale industrial

adaptation of the technology by differential evolution approach.

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