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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
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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
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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.
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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-
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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
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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
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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
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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
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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-
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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
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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).
Industrial applications
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
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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
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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.
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Industrial applications
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
Industrial applications
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).
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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
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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
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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
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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).
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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
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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.
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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
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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
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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.
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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.
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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
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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.
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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
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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,
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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
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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
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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
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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
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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).
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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.
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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
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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
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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
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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
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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
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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.
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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
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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
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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.