View
62
Download
0
Category
Tags:
Preview:
Citation preview
INTRODUCTION
Environmental pollution has become a key focus of concern for all the nations
worldwide, as not only the developing countries but developed nations as well are
affected by and suffer from it. Pollution has many forms, the air we breathe, the water we
drink, the ground where we cultivate our food crops and even the increasing noise we
hear everyday-all contribute to health problems and lower quality of life.
Among all the environmental pollutions, pollution of water resources is a matter
of great concern. Poor and developing countries are at high risk due to lack of waste
water treatment technologies. Increasing contamination of aquatic sources with large
number of pollutants is not only endangering the aquatic biota but creating a worldwide
shortage of recreational waters (Rai et al., 1998).
The water of aquatic systems gets polluted by domestic activities, mining
activities, municipal wastes, modern agricultural practices, marine dumping, radioactive
wastes, oil spillage, underground storage leakages and industries. But the major culprits
causing the pollution of water resources are different industrial units. Indiscriminate
discharge of toxic chemicals through effluents from a wide range of industries (i.e.
textile, steel, oil, tanneries, canneries, refineries, mines, fertilizers production units,
detergent production units, electroplating units and sugar mills) into water bodies pollutes
these resources and causes hazardous effects on flora and fauna (Singh and Singh, 2000;
Gavrilescu, 2004; Iqbal and Edyvean, 2004; Akar and Tunali, 2005).
I. HEAVY METAL POLLUTION
Among all the pollutants, heavy metals are most dangerous one as these are non –
biodegradable and persist in environment. These enter into the water resources through
both natural and anthropogenic sources. More attention is being given to the potential
health hazards posed by heavy metals. The term heavy metal refers to any metallic
chemical element that has a relatively high density. The density of heavy metals is
usually more than 5.0 g/cm3. Examples of heavy metals include mercury (Hg), cadmium
(Cd), arsenic (As), chromium (Cr), thallium (Tl), lead (Pb), Copper (Cu), Zinc (Zn),
Cobalt (Co), Nickel (Ni), and Iron (Fe). These metals are classified in to three categories:
toxic metals (such as Hg, Cr, Pb, Zn, Cu, Ni, Cd, As, Co, Sn, etc), precious metals (such
as Pd, Pt, Ag, Au, Ru etc.) and radionuclides (such as U, Th, Ra, Am, etc.) (Volesky,
1990; Bishop, 2002). Toxic metals cause toxicity to organisms even at ppm level of
concentration.
Heavy metals are natural components of the earth's crust. To a small extent they
enter our bodies via food, drinking water and air. As trace elements, some of these heavy
metals (e.g. copper, selenium, zinc) are essential to maintain the metabolism of the
human body. However, at higher concentrations they can lead to poisoning. Heavy metal
poisoning could result from drinking-water contamination, high ambient air
concentrations near emission sources, or intake via the food chain.
Heavy metals are dangerous because these tend to bioaccumulate.
Bioaccumulation means an increase in the concentration of a chemical in an organism
over time, compared to its concentration in the environment. Compounds accumulate in
living systems when these are taken up and are stored faster than these are broken down
(metabolized) or excreted.
Heavy metals may enter a water supply through industrial or consumer wastes
releasing heavy metals into streams, lakes, rivers, and groundwater. Unlike organic
pollutants, heavy metals, being non-biodegradable pose a different kind of challenge for
remediation. A well known environmental disaster associated with heavy metals is the
Minamata disease caused by Mercury pollution in Japan.
a) Sources of Heavy Metal Pollution
Heavy metal pollution mainly arises from the effluents of industrial units. Some
of the common industrial units releasing toxic heavy metals into environment are listed in
Table 1. Irrigation by effluents released from paper mills and fertilizer factories are
adding various alkalies, ammonia, cyanides and heavy metals into the water resources
(Singh, 1994; Fazeli et al., 1998). The waste water from the dyes and pigment industries,
film and photography, galvanometry, metal cleaning, electroplating, leather and mining
industries contains considerable amounts of heavy metal ions.
In contrast to herbicides, pesticides and other potential toxicants which undergo
break down, albiet extremely slowly, heavy metals can not be eliminated from a water
body and thus persist in sediments where these are slowly released in to the water.
Table 1: Common industrial units releasing heavy metals into water bodies.
Metal Common sources
Chromium
Chrome plating, petroleum refining, electroplating industry, leather,
tanning, textile manufacturing and pulp processing units. It exists in both
hexavalent and trivalent forms.
Nickel Galvanization, paint and powder, batteries processing units, metal
refining and super phosphate fertilizers.
Lead Petrol based materials, pesticides, leaded gasoline, and mobile batteries.
Copper Electroplating industry, plastic industry, metal refining and industrial
emissions.
Zinc Rubber industries, paints, dyes, wood preservatives and ointments.
Cadmium
Batteries, electroplating industries, phosphate fertilizers, detergents,
refined petroleum products, paint pigments, pesticides, galvanized pipes,
plastics, polyvinyl and copper refineries.
Iron From metal refining, engine parts.
Aluminium Industries preparing insulated wiring, ceramics, automotive parts,
aluminum phosphate and pesticides.
Arsenic Automobile exhaust/industrial dust, wood preservatives and dyes.
Mercury Electric/light bulb, wood preservatives, leather tanning, ointments,
thermometers, adhesives and paints.
b) Health Risks of Heavy Metals
At low concentrations some of the heavy metals stimulate some biological
processes, but at threshold concentration these become toxic. Being nonbiodegradable,
these metals accumulate at various trophic levels through food chain and can cause
human health problems (He et al., 1998). In humans these metals accumulate in living
tissues and thus multiply the danger. Some common harmful effects and health risks of
some heavy metals to human beings are given in Table 2. The health risks of heavy
metals ingestion thus are of wide range. Some metals cause physical discomfort while
others may cause life-threatening illness, damage to vital body system, or cause other
damages. Thus, it is very necessary to control emission of heavy metals into the
environment.
II. CONVENTIONAL TECHNIQUES USED FOR HEAVY METAL
REMOVAL
Several physico-chemical methods like chemical precipitation, electrodialysis,
ion-exchange, ultra-filtration, reverse osmosis etc. are commonly employed for stripping
toxic heavy metals from waste waters (Sandau et al., 1996b; Eccles, 1999; Mehta and
Gaur, 2001a; Ahalya et al., 2003). Brief description of each method is presented below:
a) Reverse Osmosis
It is a process in which heavy metals are separated through 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 (Ozaki et al., 2002).
Table 2: Common health problems caused to humans by heavy metals.
Metal Health Risks
Chromium
Irritant, nausea and vomiting, carcinogen, and cause ulceration. long-term
exposure can cause kidney and liver damage, and damage to circulatory and
nervous tissue.
Nickel
Dermatitis, chronic rhinitis, hypersensitivity reactions in the immune system
which in turn cause hyper allergenic reactions to various substances, Long-term
toxicity may lead to liver necrosis and carcinoma, myocardial infarction,
respiratory illnesses such as asthma, heart and liver damage, skin irritation.
Lead
Effects on the kidneys, gastrointestinal tract, joints and reproductive system,
and acute or chronic damage to the nervous system, anemia, headache, fatigue,
weight loss, cognitive dysfunction and decreased coordination, memory loss,
nerve conductions. The central nervous system is most sensitive to the effects of
lead.
Copper
Biliary obstruction (inability to excrete excess copper), liver disease, renal
dysfunction, fibromyalgia symptoms, muscle and joint pains, depression,
chronic fatigue symptoms, irritability, tumor, anemia, learning disabilities and
behavioral disorders, stuttering, insomnia, niacin deficiency, leukemia, high
blood pressure.
Zinc Nausea and vomiting.
Cadmium
Hypertension or high blood pressure, dulled sense of smell, anemia, joint
soreness, hair loss, dry scaly skin, loss of appetite, decreased production of T-
cells and, therefore, a weakened immune system, kidney diseases and liver
damage, emphysema, cancer and shortened lifespan
Aluminium
Gastrointestinal disturbance, fatigue, headache, poor calcium metabolism,
decreased liver and kidney function, forgetfulness, speech disturbances and
memory loss, weak and aching muscles, seizures, vertigo and loss of balance
Arsenic
Headache, confusion, drowsiness, convulsions, changes in fingernail
pigmentation, vomiting, diarrhea, bloody urine, muscle cramps, convulsions,
gastrointestinal upsets and coma.
Mercury Anxiety, depression, confusion, irritability, insecurity and fatigue.
b) Electrodialysis
In this process, the ionic components (heavy metals) are separated through the use
of semi-permeable ion-selective membranes. Application of an electric potential
between the two electrodes causes the 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 and thus cost involved is high
(Mohammadi et al., 2005).
c) Ultrafiltration
This is a pressure driven membrane operation that uses porous membranes for the
removal of heavy metals. The main disadvantage of this process is the generation of
sludge.
d) 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.
e) Chemical Precipitation
Precipitation of metals is achieved by the addition of coagulants such as alum,
lime, iron salts and other organic polymers. The large amount of sludge (containing toxic
compounds) produced during the process is the main disadvantage.
f) Phytoremediation
Phytoremediation is the use of certain plants to clean up soil, sediment, and water
contaminated with metals. The disadvantages include that it takes a long time for removal
of metals and the regeneration of the plant for further biosorption is difficult.
The above stated methods are effective for removal of metals from water with
high concentration of metals while for low concentrations (ppm level) of metals these
techniques are not very fruitful. These methods also have other several disadvantages,
such as incomplete metal removal, limited tolerance to pH change, expensive equipment
and monitoring system requirements, high reagent or energy requirements and generation
of toxic sludge or other waste products that require disposal (Yan and Viraraghavan,
2001; Aksu et al., 2002; Gavrilescu, 2004; Alimohamadi et al., 2005; Wang and Chen,
2006). Further, these techniques may be ineffective or extremely expensive when metal
concentration in waste water is in the range 1-100 ppm (Mehta and Gaur, 2005).
The Need for Novel Technology
The increasing concern about the contamination of water bodies by heavy metals
has stimulated a large number of researches to find possible ways to remove these toxic
substances from the environment. To overcome some of the limitations of physico-
chemical treatments, there is a need for inexpensive and efficient technology for the
treatment of metal containing wastes so that metal concentration can be reduced to
environmentally acceptable levels (Wilde and Benemann, 1993; Sandau et al., 1996a;
Aksu, 1998; Gavrilescu, 2004). Use of biomass for metal removal/recovery is considered
to be a viable alternative to conventional methods.
III. REMOVAL OF HEAVY METALS THROUGH BIOSORPTION
Biosorption of heavy metals is defined as the ability/use of the biological
materials to remove heavy metals from wastewater through metabolically mediated or
physico-chemical uptake of metal (Fourest and Roux, 1992). The most prominent
features of biosorption are low cost and highly efficient materials to adsorb heavy metals
even when present at very low concentrations (Yu et al., 2001).
a) Removal by Low Cost Sorbents and Industrial Wastes
The use of low cost sorbents has been investigated as a replacement for current
costly methods of removing heavy metals from solutions. The application of anaerobic
granular biomass (Hawari and Mulligan, 2006), peat biomass (Ma and Tobin, 2003),
waste brewery biomass (Marques et al., 2000), husk of Bengal gram (Ahalya et al.,
2005), rice husk and maize cobs (Abdel-Ghani et al., 2007), paper mill sludge (Battaglia
et al., 2003), sewage sludge (Zhai et al., 2004), agro based waste materials (Qaiser et al.,
2007), chitosan (Babel and Kurniawan, 2003), agarose (Pandey et al., 2007), crab shell
and chitin (Barriada et al., 2007), rose waste biomass (Iftikhar et al.,2009), waste sludge
(Slevaraj et al., 2003; Li et al., 2004), tea factory waste biomass (Malkoc et al., 2006) for
heavy metal removal has been explored. But these materials are not easily available at all
the places and all the time.
b) Removal by Plant Biomass
Plant biomass has also been used by various workers for metal removal purposes
(Cimina et al., 2000; Elifantz and Tel-Or, 2002; Li et al., 2004; Shekhar et al., 2004;
Sarin and Pant, 2006; Ahluwalia and Goyal, 2007; Bhatti et al., 2007; Zubair et al., 2008,
Anand Kumar and Mandal, 2009; Romero-Gonzalez et al., 2009).
c) Removal by Microorganisms
The ability of microbial biomass to remove heavy metal ions from polluted aquatic
systems has been reported and has also attracted much interest in recent years. Microorganisms
which have been tested for biosorption of heavy metals include bacteria (Scott and Palmer,
1990; Chang et al., 1997; Nurba et al., 2002; Srinath and Verma, 2002; Ilhan et al., 2004;
Selatnia et al., 2004; Iyer et al., 2005; Shaker and Hussein, 2005; Elangovan et al., 2006;
Srivastava and Thakur, 2007; Rehman et al., 2008; Rajkumar et al., 2009), Yeast (Huang et al.,
1990; Volesky et al., 1993; Goksungur et al., 2005; Seki et al., 2005; Wang and Chen, 2006;
Chergui et al., 2007; Ashwini et al., 2009), fungi (Lewis and Kriff, 1988; Fourest et al., 1994;
Sudha and Abraham, 2001; Dursun et al., 2003; Ahmet et al., 2005; Park et al., 2005; Pal et al.,
2006; Tunali et al., 2006; Liu et al., 2007; Coreno-Alonso et al., 2009; Khambhaty et al.,
2009) and algae (Gupta et al., 2001; Donmez and Aksu, 2002; Terry and Stone, 2002;
Pagnanelli et. al., 2003; Bishnoi and Garima, 2004; Yun, 2004; Loderio et al., 2005; Tuzun et
al., 2005; Vijayaraghavan et al., 2005, 2006; Vilar et al., 2005; Bishnoi et al., 2007; Chojnacka,
2007; Khattar et al., 2007; Doshi et al., 2008; Deng et al., 2009; Gupta and Rastogi, 2009;
Gupta et al., 2010; Lodi et al., 2010) etc. These biosorbents possess metal sequestering
properties and decrease the concentration of heavy metal ions in solution from ppm to ppb
level. They can effectively sequester dissolved metal ions out of dilute complex solutions
quickly and with high efficiency. Therefore biosorption involving microorganisms is an ideal
technique for the treatment of high volume and low metal concentration complex waste waters.
d) Removal by Algae
Of all the microbes, algae are able to take up, accumulate and concentrate heavy
metals in significant amounts from the aqueous solution. Algae have also been
considered to be potential biosorbents because of their easy handling, cheap availability,
relatively high surface area and high binding affinity. Microalgae are often preferred for
bioremediation process owing to their high photosynthetic efficiency coupled with simple
nutritional requirements and these can be easily cultured and grown rapidly in both
industrial and laboratory circumstances (Radway et al., 2001; Tien et al., 2005; Khattar et
al., 2007).
Many studies have demonstrated metal biosorption by seaweeds such as
Sargassum sp. (Esteves et al., 2000; Cruz et al., 2004; de Franca et al., 2006;
Vijayaraghavan et al., 2006; Sheng et al., 2007; Pahlavanzadeh et al., 2010), Laminaria
sp. (Yin et al., 2001; Hashim and Chu, 2004; Lodeiro et al., 2005; Luo et al., 2006) and
Fucus sp. (Herrero et al., 2006), Ulva (Sheng et al., 2004 a, b; Suzuki et al., 2005;
Vijayaraghavan et al., 2005; Kumar et al., 2006), cyanobacteria such as Synechococcus
sp. (Gardea-Torresdey et al., 1998; Satoh et al., 2005), Spirulina sp. (Chojnacka et al.,
2004; Rangsayatorn et al., 2004; Gong et al., 2005; Doshi et al., 2007), Nostoc sp.
(Prashad and Pandey, 2000; El-Sheekh et al., 2005), Lyngbya sp. (Klimmek et al., 2001;
Lee et al., 2004; Kiran and Kaushik, 2008), Oscillatoria sp. (Mohapatra and Gupta
2005), Phormidium sp. (Sadettin and Donmez, 2007), Anacystis sp. (Khattar et al., 2002,
2007). Biosorption by green algae such as Chlorella sp. (Aksu and Acikel, 2000; Mehta
et al., 2002; Fraile et al., 2005; Doshi et al., 2008), Cladophora sp. (Ozer et al., 1994,
2004; Deng et al., 2006), Eklonia sp. (Park et al., 2004), Chaetophora sp. (Andrade et al.,
2005), Sphaeroplea (Rao et al., 2005) has also been reported.
The major advantages of biosorption over conventional treatment methods
include (Kratochvil and Volesky, 1998):
• Low operating cost
• High efficiency
• Minimization of chemical and biological sludge
• No additional nutrient requirement
• Regeneration of biosorbent
• Possibility of valuable metal recovery
• Successful operation over a wide range of pH and temperature
• Resins are hard ligands and are less effective in adsorbing minute quantities of metals
when compared to soft ligands of biological origin
• Microbial biomass required for biosorption may be available as a fermentation waste
product or specifically grown one, using cheap substrates
• Biosorption processes may serve as ‘polishing’ system to existing processes
All these studies have demonstrated that algae have enormous capacity to
bind/accumulate heavy metals. Thus, algae can hopefully be exploited for removal of
heavy metal ions from polluted waste waters and industrial effluents to get fruitful
results. Biosorption capacities of some algae are summarized in Table 3.
Table 3: Reports on heavy metal removal potential of some algae.
Sorption Metal Algae (mg g−1) Reference
Al Laminaria japonica 75.28 Lee et al. (2004)
Spirulina sp. 0.08 Chojnacka et al. (2004)
Co Oscillatoria angustissima 15.32 Mohapatra and Gupta (2005)
Spirulina sp. 0.01 Chojnacka et al. (2004)
Ulva reticulata 45.97 Vijayaraghavan et al. (2005)
Cd Chlorella vulgaris 12.48 Sandau et al. (1996)
L. japonica 146.12 Yin et al. (2001)
L. japonica 125.89 Lee et al. (2004)
Lyngbya taylorii 41.59 Klimmek et al. (2001)
Padina pavonia 123.64 Ofer et al. (2003)
Padina sp. 84.30 Sheng et al. (2004b)
Spirulina platensis 37.09 Rangsayatorn et al. (2004)
Spirulina sp. 11.24 Chojnacka et al. (2004)
S. vulgaris 112.40 Ofer et al. (2003)
Sargassum sp. 120.27 Cruz et al. (2004)
Sargassum sp. 85.42 Sheng et al. (2004b)
Synechococcus sp. PCC 7942 7.19 Gardea-Torresdey et al. (1998)
Cr (III) Spirulina sp. 9.62 Chojnacka et al. (2005)
Synechococcus sp. PCC 7942 5.41 Gardea-Torresdey et al. (1998)
Cr (VI Padina sp. 54.60 Sheng et al. (2004b)
Sargassum sp. 31.72 Sheng et al. (2004b)
Cu C. vulgaris 89.02 Mehta and Gaur (2001a)
C. vulgaris 190.62 Mehta and Gaur (2001b)
C. vulgaris 76.76 Mehta and Gaur (2001c)
C. vulgaris ( acid pretreated 420.63 Mehta et al. (2002)
E. maxima 94.0 Feng and Aldrich (2004)
Microcystis aeruginosa 249.71 Pradhan et al. (1998)
Oscillatoria angustissima 7.63 Mohapatra and Gupta (2005)
Sargassum sp. 45.75 Chaisuksant (2003)
Spirulina sp. 12.45 Chojnacka et al. (2005)
Synechococcus sp. PCC 7942 11.31 Gardea-Torresdey et al. (1998)
Ni Chlorella miniata 0.04 Chong et al. (2000)
C. sorokiniana 0.12 Chong et al. (2000)
C. vulgaris 23.48 Mehta and Gaur (2001a)
C. vulgaris 205.48 Mehta and Gaur (2001b)
C. vulgaris (Acid pretreated) 437.98 Mehta et al. (2002)
Lyngbya taylorii 38.16 Klimmek et al. (2001)
Microcystis aeruginosa 249.98 Pradhan et al. (1998)
Scenedesmus obliquus 30.18 Donmenz et al. (1999)
Spirulina sp. 0.18 Chojnacka et al. (2004)
Synechococcus sp. PCC 7942 3.17 Gardea-Torresdey et al. (1998)
Ulva reticulata 46.51 Vijayaraghavan et al. (2005)
Laminaria japonica 349.09 Lee et al. (2004)
Lyngbya taylorii 304.56 Klimmek et al. (2001)
Spirulina platensis 16.98 Sandau et al. (1996)
Spirulina sp. 0.01 Chojnacka et al. (2004)
Synechococcus sp. PCC 7942 30.45 Gardea-Torresdey et al. (1998)
Zn C. vulgaris 0.26 Chong et al. (2000)
Laminaria japonica 56.87 Lee et al. (2004)
Lyngbya taylorii 32.03 Klimmek et al. (2001)
Microcystis sp. 632.12 Singh et al. (1998)
Oscillatoria anguistissima 21.57 Mohapatra and Gupta (2005)
O. anguistissima 641.28 Ahuja et al. (1999)
Spirulina sp. 0.20 Chojnacka et al. (2004)
IV. METAL TOLERANCE
Presence of some organisms in metal contaminated waters leads to a conclusion
that somehow, these organisms are able to resist against metal toxicity. Several micro-
organisms are able to grow in presence of toxic heavy metals by keeping intracellular
concentrations of toxic forms of metals well below the lethal threshold levels. Baker
(1981) suggested two basic strategies of tolerance to metals
a) Metal exclusion, whereby metal uptake and transport is restricted and
b) Metal accumulation, when there is no such restriction then metals are
accumulated in the cells in a detoxified form.
Several possible mechanisms have been suggested for metal tolerance. These are:
i) Adsorption or metal binding to the surface of cell wall (Lee et al., 2000;
Mehta and Gaur, 2005).
ii) Intracellular compartmentalization of metals in vacuole, cell walls,
polyphosphate bodies (Sufia et al., 1999).
iii) Efflux of heavy metals (Rouch et al., 1989; Nies, 1999, 2003).
iv) Reduced transport (Baker, 1981).
v) Chelation i.e. synthesis of metallothioneins and phytochelatins (Gekeler et al.,
1988; Verma and Singh, 1991; Singh et al., 1992; Gardea –Torresdey et al.,
1998; Marijana and Raspor, 1998).
vi) Secretion of extracellular exudates e.g. polysaccharides and siderophores of
algae also have good metal binding capacity (Plude et al., 1991; Decho and
Herndl, 1995; De Philippis et al., 2001, 2003, 2007). Siderophores are iron
chelating compounds released by the microorganisms. In cyanobacteria,
siderophores are produced as defensive mechanism against heavy metal
pollution.
Metal uptake event includes
i) A passive (rapid uptake) mechanism, where cell surface of a microorganism
functions as an ion exchange site for metal cations.
ii) Active intracellular transport mechanism (slower uptake).
Sandau et al. (1996a) have suggested following types of extracellular sorptive mechanisms:
a) Electrostatic interactions of positively charged heavy metal ions with negatively
charged groups/ligands of the cell walls (adsorption, ion exchange)
b) Microprecipitation
c) Surface complexing
d) Covalent bonds between heavy metals cations and proteins
e) Binding to other polymers
Various intracellular accumulative mechanisms have also been suggested by Sandau et al.
(1996a). These include:
i. Complexation of the accumulated heavy metal ions and reduction in toxicity of
metal concentrations by several species-specific processes (e.g. phytochelatin
reactions, hypertrophying and multiplication of the polyphosphate bodies)
ii. Heavy metal incorporation into vacuoles (demobilization and concentration)
iii. Binding to proteins, lipids, DNA etc.
iv. Efflux of heavy metal complexed substances
v. Membrane reinforcement of cell walls, mitochondria, chloroplasts and nuclei as
heavy metal barriers in adapted cells
These mechanisms, thus provide tolerance to microbes, including algae and
cyanobacteria, towards toxic heavy metal ions. Due to this resistance/tolerance towards
heavy metals, these organisms can be used as indicators and scavengers of heavy metal
pollution (Bilgrammi et al., 1996). Thus, due to significant characteristics of
microorganisms making them tolerant to metal toxicity, various microorganisms can be
exploited to solve the world wide problem of heavy metal pollution.
V. CLASSIFICATION OF METAL BINDING MECHANISMS
Metal biosorption mechanisms vary as there are many ways for the metals to be taken
up by the microbial cells. These mechanisms may be classified following various criteria.
Depending upon whether cellular metabolism is involved or not, biosorption mechanism
can be divided in to
i) Non- metabolism dependent and
ii) Metabolism dependent
During the passive uptake, metal ions are adsorbed on to the cell surface within a
relatively short span of time (few seconds or minutes) and the process is metabolism
independent. In this type of biosorption, metal uptake is through physico-chemical
interaction between the metal ions and the functional groups present on the microbial cell
surface. This is based on physical adsorption, ion exchange and chemical sorption, which
is not dependent on the cellular metabolism.
An initial faster (passive) uptake is followed by a much slower (active) uptake (Bates
et al., 1982; Mehta and Gaur, 2005). Active uptake is metabolism dependent, causing the
transport of metal ions across the cell membrane into the cytoplasm for intracellular
accumulation. This means that this kind of biosorption is possible only with viable cells.
It is often associated with an active defense system of microorganisms, which is
stimulated in the presence of toxic metals.
According to the location where the metal ions are located after their removal
from solution, biosorption can be classified as
i) Extracellular accumulation/ Cell surface sorption
ii) Intracellular accumulation
Volesky (1990) suggested following metal binding mechanisms which are
involved in biosorption process:-
i) Chemisorption by ion exchange, complexation, coordination and chelation
ii) Physical adsorption
iii) Micro- precipitation
These mechanisms may be acting simultaneously to varying degrees depending
on the biosorbent and metal solution in the environment. A systematic presentation of the
relationships between different mechanisms is compiled in Fig. 1. The classification of
bond type (Myers, 1991) was used.
Figure 1: Metal Biosorption mechanisms: bold lines, mechanism probably important in
biosorption; dashed lines, biosorption binding relations of secondary
importance.
Micro precipitation
Physical forces
Adsorption Ion Exchange
Covalent
Electrostatic ion-ion ion -dipole
London-vander waals dipole–dipole-induced
dipole London dispersion
Sorbate/Sorbent
Complexation
Metal/legand
Chemical forces
Solute/Solvent
Binding Sites
Biosorption of metals has been attributed to presence of different types of groups
on the cell. The surface of algal cell wall is composed of macromolecules having an
abundance of charged functional groups, such as hydroxyl, phosphoryl, amino, carboxyl,
sulphydryl, amine, imidazole, sulphate, phosphate, carbohydrate etc. Whether any given
group is important for biosorption of a particular metal ion by a certain biomass depends
on factors such as:
• Quantity of sites in the biosorbent material
• Accessibility of the sites
• Chemical state of the site, i.e. availability
• Affinity between site and metal, i.e. binding strength
Usually, the net charge on cell surface is negative because of the abundance of
carboxylate and phosphate residues (Hamdy, 2000; Andrade et al., 2005; Gong et al.,
2005; Mehta and Gaur, 2005; Apiratikul and Pavasant, 2006; Singh et al., 2007). Since
metal ions in water are generally in cationic form, thus, these are passively adsorbed onto
the cell surface. However, because of the presence of amine and imidazole groups, which
are positively charged when protonated, cell surface may also bind negatively charged
metal complexes. Some binding sites are also present inside the algal cells. Here
intracellular accumulation takes place by binding with cytoplasmic ligands,
phytochelatins, metallothioneins and other intracellular molecules. Thus, algal cells
contain many polyfunctional metal-binding sites for both cationic and anionic metal
complexes. Figure 2 shows the probable sites in an algal cell for the binding of metal
ions.
Each functional group has specific pKa (dissociation constant) (Niu and Volesky,
2000; Volesky, 2007) and it dissociates into corresponding anions and protons at a
specific pH. Structural formulae and pKa values of the binding groups are summarized in
Table 4.
VI. ADSORPTION ISOTHERMS
Biosorption has been studied as a simplified sorption system, usually containing
one heavy metal. This is an appropriate simplification for effective experimentation. In
order to evaluate feasibility and effectiveness of biosorption in waste water treatment, it
is essential to make predictions of the sorption performance (e.g., for facilitating process
design). Extensive studies have also been carried out on biosorption and its dependence
on solution chemistry, ionic competition by other metals, influence of pH and ionic
concentration (Bai and Abraham, 2002). Therefore, it is necessary to develop appropriate
mathematical models of biosorption. Modeling the biosorption-binding equilibrium is a
pre-requisite for understanding and evaluating the feasibility of the biosorption process.
Different adsorption isotherms have been used to quantify and contrast between the
performances of different biosorbents (Davis et al., 2003). Adsorption isotherms are the
way of presentations of amount of solute adsorbed per unit of adsorbent.
The amount of metal M (sorbate) bound per mass of sorbent is called the uptake
(qe). The binding is not only dependent on the sorbent material but also on the
equilibrium concentration (Ce) of the sorbate in the solution and on other parameters such
as pH and equilibrium concentration of other ions in the solution. The relationship
between equilibrium binding and the concentration of ions (at constant temperature) is
Table 4: Major functional groups present on the surface of algal cell wall and involved in
metal binding process.
Binding group Structural formula pKa Ligand atom
Hydroxyl -OH 9.5-13 O Carbonyl (ketone) >C=O - O
Carboxyl -C=O ׀
OH
1.7 - 4.7 O
Sulfydryl (thiol) -SH 8.3-10.8 S Sulfonate O
׀׀ -S=O ׀׀O
1.3 O
Thioether >S - S Amine -NH2 8-11 N
Secondary amine >NH 13 N Amide - C=O
׀ NH2
- N
Imine =NH 11.6-12.6 N Imidazole -C-N-H
>CH ׀׀ H-C-N
6.0 N
Phosphonate OH ׀
-P=O ׀
OH
0.9-2.1
6.1-6.8
O
Phosphodiester > P=O ׀
OH
1.5 O
Phenolic -OH 10 O
depicted in an isotherm plot of qe versus Ce. With increasing metal concentration in
solution its binding increases from zero to the maximum. It is desirable that the sorbent
should possess a high sorption capacity and high affinity for the sorbate species, which is
reflected in a steep slope of the isotherm curve at low equilibrium concentrations.
Table 5 summarizes some of the simple sorption isotherm models that are most
frequently applied. A particular model may not apply to a particular situation, and in
some cases more than one model may explain the biosorption mechanism. There is no
critical reason to use a more complex model if a two-parameter model (such as the
Langmuir and Freundlich isotherm models) can fit the data reasonably well.
The field of biosorption is challenging one. The main objectives are the
elucidation of binding mechanisms, the relative affinity of heavy metals for the biomass
and how both are affected by varying environmental conditions. Ultimately, the goal is
the successful implementation of a remediation programme. The model used to describe
the results should be capable of predicting heavy metal binding at both low and high
concentrations, ideally a model should not only be predictive but should rest on our
understanding of the mechanism of biosorption (Davis et al., 2003). Among all
isotherms, the Langmuir and Freundlich models are most frequently used to describe
metal biosorption (Ledin, 2000).
a) Langmuir Isotherm (Langmuir, 1918)
Langmuir adsorption isotherm has traditionally been used to quantify and
compare the performance of different biosorbents. However, in order to evaluate the
appropriateness of this model, we must look at its underlying assumptions. The Langmuir
Table 5: Adsorption models used frequently for metal removal processes.
Model Equation Advantages Disadvantages
Langmuir
Interpretable parameters
Not Structured: monolayer sorption
Freundlich
Simple expression Not Structured
Combination of Langmuir and Freundlich
Combination of above two
Unnecessarily complicated
Radke and Prausnitz
Simple expression Empirical; Requires three parameters.
Radlich Peterson
Approaches at higher
concentration No significant advantages
Where:
qe = Amount of metal adsorbed (experimental value)
Ce = residual concentration of metal
qmax =Maximum amount of metal ions that can be adsorbed (theoretical value)
b = Biosorption affinity
Kf = Freundlich isotherm constant related to adsorption capacity of biomass
1/n = Freundlich isotherm constant related to adsorption intensity
Cf = Final concentration of metal
β = Metal sorption ability of biomass
a = Model constant
model assumes: (i) a monolayer adsorption of metals on binding sites; (ii) all adsorption
species (metal ions) interact only with a site and not with each other; and (iii) adsorption
energy of all the sites is identical and independent of the presence of adsorbed species on
neighbouring sites. The Langmuir model shows a good fit where passive sorption of
metals on biosorbents prevails (Aksu, 1998). An important point to be noted here is that
most of the above assumptions are not fulfilled in biosorption. Opposite to Langmuir
assumptions, biological surfaces have more than one type of binding sites contributing to
biosorption process, each of which may have different affinity for sorbing heavy metal
ions. This isotherm represents one of the first theoretical treatments of nonlinear sorption
and suggests that uptake occurs on a homogeneous surface by monolayer sorption with
interaction between adsorption molecules.
b) Freundlich Isotherm (Freundlich, 1907)
The Freundlich isotherm is empirical and describes multi-layered adsorption of
metal ions on sorbent surface. In metal biosorption studies, the Freundlich model
describes metal sorption as a function of metal concentration in solution at equilibrium,
without reference to pH or other ions in the same aqueous system. The Freundlich model
provides a more realistic description of metal adsorption by organic matter because it
accounts for sorption to heterogeneous surfaces or surfaces supporting sites of varied
affinity. In contrast to Langmuir model, the Freundlich model does not assume saturation
of metal sorption. The Freundlich model assumes that stronger binding sites on the
biosorbent surface are occupied first and that the binding strength decreases with
increasing degree of site occupation by metal ions. For fitting the model to experimental
data, the Freundlich model generally gives a better fit for higher equilibrium
concentration of metal in solution.
c) Frequently Used Adsorption Models
Frequently used single-component adsorption models, their advantages and
disadvantages (Kuyucak and Volesky, 1989) are given in Table 5.
From the literature survey, it is clear that work has been done on different aspects
of heavy metals in relation to algae. Metal tolerant algal species which adsorb,
accumulate or chelate metal ions can be exploited to solve the problem of heavy metal
pollution. Metal uptake studies have been conducted mainly employing laboratory grown
algal species, using single metal ions. There are not much studies which employed algal
species, naturally growing in polluted water to remove multimetal ions from solution.
Since industrial effluents may contain more than one metal ion, and the algae growing in
metal polluted water may have higher biosorption potential, the present study on
“Evaluation of Heavy Metal Bioremediation Potential of Algae Growing in Polluted
Water” was undertaken so that algal species are identified for their exploitation to remove
multimetal ions from industrial effluents before their discharge into water bodies.
The study was aimed to evaluate the potentialities of algae growing in polluted
waters to scavenge heavy metal ions. Three metals, Cu2+, Cd2+ and Ni2+ were chosen as
these metals are released in abundance in the effluents of dye, electroplating and metal
refining industries.
Recommended