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HEAVY METALS:
Sources: mining operations, refining ores, sludgedisposal, fly ash from incinerators, the processing of
radioactive materials, metal plating, or the
manufacture of electrical equipment, paints, alloys,
batteries, pesticides or preservatives.
Major lead pollution is through automobiles and
battery manufacturers.
For zinc and chromium the major application is infertilizer and leather tanning respectively.
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Removal of metal ions from aqueous
stream:
Reverse Osmosis Electrodialysis:
Ultrafiltration:
Ion-Exchange:
Chemical precipitation: Phytoremediation
BIOSORPTION
Biosorption can be defined as the ability of biological materials to
accumulate heavy metals through metabolically mediated or physico-chemical pathways of uptake
Biosorption is a physiochemical process that occurs naturally in
certain biomass which allows it to passively concentrate and bind
contaminants onto its cellular structure.
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Biosorption vs. BioaccumulationMicroorganisms uptake metal either actively (bioaccumulation)
and/or passively (biosorption).
BIOSORPTION
It is a metabolically passive
process i.e. it does not
require energy.
It is more active in deadcells.
It is faster and produces
higher concentration of
metals. Metal affinity is high under
favorable conditions.
Temperature tolerance is
modest It is a reversible rocess
BIOACCUMULATION
It is an active metabolic
process driven by energy
from respiration.
It is active in living cells.
It is a slower process.
Toxicity affects metal uptake
by living cells, but may havea high uptake.
Usually affected by low
temperature.
It is only partially reversible.
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The biosorption process involves a solid phase
(sorbent or biosorbent; biological material) and a
liquid phase (solvent; normally water) containing adissolved species to be sorbed (sorbate, metal ions).
Due to higher affinity of the sorbent for the sorbate
species, the latter is attracted and bound there by
different mechanisms.
The process continues till equilibrium is established
between the amount of solid-bound sorbate species
and its portion remaining in the solution.
The degree of sorbent affinity for the sorbate
determines its distribution between the solid and
liquid phases.
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Biosorbent material Strong biosorbent behaviour of certain micro-
organisms towards metallic ions is a function of the
chemical make-up of the microbial cells. This type of
biosorbent consists of dead and metabolicallyinactive cells.
Some microbes may collect a broad range of metals
while some are specific to certain metals.
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BIOSORBENTS
Bacteria: the walls of bacteria are efficient metalchelators.
Metal binding may be at least a two-stage process ---
first involving interaction between metal ions and
reactive groups followed by inorganic deposition of
increased amounts of metal.
The carboxyl groups of glutamic acid of
peptidoglycan is the major group for metaldeposition.
Eg: Staphylococcus saprophyticus are used for
removal of Cr, Pb and Cd from industrial wastewater.
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Algae : use of algae is becoming more advantageousthan others
Algae have low nutritional requirements as they are
autotrophic and produce huge biomass
And ,compared to bacteria and fungi produce less toxic
materials.
Binding depends on : ionic charge of metal ions, chemicalcomposition of metal soln. and algal sp.
Metabolism-independent accumulation of metals.
many potential binding sites occur in algae cell walls, which
include polysaccharides, cellulose, uronic acid and proteins.
Eg: uptake of Pb by Chlorella vulgaris, Cr (VI) by green algae
spirogyra , Cu by broen seaweed Sargassam sp.
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Fungi andYeast: Fungal materials have high percentage of cell wall
material that shows high metal binding capacity. Metabolism-independent binding of metal ions is
rapid and in large amounts.
A variety of ligands may be involved including,
phosphate, hydroxyl and sulphydryl groups. carboxyl,
amino
Some sp like Aureobasidium pullanans andCladosporium resinae show two phase metal uptake.
Aspergillus nigeris found to remove Pb, Cu and Cd.
Sacchromyces cerevisiae for removal of Pb, Fe,Cu, Cr.
etc.
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Mechanism : The mechanism for biosorption may be complex :
Ion exchange
Chelation
Adsorption by physical forces
Entrapment in inter and intrafibrilliar capillaries and
spaces of the structural polysaccharide network as a
result of the concentration gradient.
Diffusion through cell walls and membranes.
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Mechanism :I ) According to the dependence on the cell's
metabolism, biosorption mechanisms can be
divided into
Metabolism dependent: When transport of metalsoccur across the cell membrane, it leads to
intracellular accumulation, this is dependent on cells
metabolism.
Takes place only with viable cells.
It is often associated with an active defence
system of the microorganism, which reacts in the
presence of toxic metal.
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According to the location where the metal removed
from solution is found, biosorption can be classifiedas
Extra cellular accumulation/ precipitation Cell surface sorption/ precipitation and
Intracellular accumulation.
If toxic metals are present then the method may be
dependent on cells metabolism as the cells produce
certain compounds that favor the precipitation.
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Transport across cell wall or
membrane:
Heavy metal transport across microbial cell
membranes is mediated by the same mechanism
used to convey metabolically important ions such as
potassium, magnesium and sodium.
This method requires cell metabolism.
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Physical adsorption: physical adsorption takes place with the help of van
der Waals' forces.
biomasses of algae, fungi and yeasts ,through
electrostatic interactions between the metal ions in
solutions and cell walls of microbial cells can uptakethe metals.
Ion Exchange:
Cell wall of microorganisms contain polysaccharides,
the bivalent metal ions exchange with the counter
ions of the polysaccharides.For example, the alginates of marine algae occur as salts of K+, Na+, Ca2+, and
Mg2+. These ions can exchange with counter ions such as CO2+, Cu2+, Cd2+
and Zn2
+ resulting in the biosorptive uptake of heavy metals
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Complexation: The metal removal from solution may also take place
by complex formation on the cell surface after theinteraction between the metal and the active groups.
Micro-organisms may also produce organic acids
(e.g., citric, oxalic, gluonic, fumaric, lactic and malic
acids), which chelate toxic metals resulting in the
formation of metallo-organic molecules.
These organic acids help in the solubilisation of metal
compounds and their leaching from their surfaces. Metals may also be biosorbed or complexed by
carboxyl groups found in microbial polysaccharides
and other polymers.
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Use ofRecombinant 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.
Genetically engineered E.coli, which expressesHg2+ transport system and metallothionin (a
metal binding protein) was able to selectively
accumulate 8 mmole Hg2+/g cell dry weight.
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Desorption It is the regeneration of the biosorbant andrecovering the accumulated metals.
This metal recovery process may be destructive
or non-destructive.
Metabolism-independent biosorption is often
reversible and leads to non-destructivedesorption whereas metabolism-dependent
intracellular accumulation is often irreversible,
necessitating destructive recovery
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Regeneration of the biosorbent is done by washingthe metal- laden biosorbent with an appropriate
solution, the type and strength of this solution would
depend on the extent of binding of the deposited
metal.
Dilute solutions of mineral acids like hydrochloric
acid, sulphuric acid, acetic acid and nitric acid can be
used for metal desorption from the biomass.
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The desorption process should:
yield the metals in a concentrated form;
R
estore the biosorbent to close to theoriginal condition for effective reuse
with undiminished metal uptake and
no physical changes or damage to the
biosorbent.
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LIMITATIONS
Early saturation, i.e. once metal interacting sitesare occupied , uptake ceases and desorption is
required.
The potential for biological process improvement
through genetic engg. is limited as cells are not
metabolizing.
There is no potential for biologically altering the
metal valency state.
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EFFECTOF DIFFERENT PARAMETERS :
pH ---- At low pH the positively charged metal ionsare in competition with the protons in the soln. tobind to the negatively charged surface of the active
site of bimass.
So, at low pH metal uptake is less as cell wall remainsassociated with H3O
+ ions, and the metals will face a
repulsive force.
As the pH increases to 3-4 theres a subsequent
increase of negative charge , so uptake increases.
After pH 6 there is a drastic increase in metal uptake
due to formation of metal hydroxides with their
respective metal ions.
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Adsorbent Mass:
As the biomass increases the number of
binding sites for metals also increases.
After some point sorption capacity reaches a
steady state or uptake decreases with biomass
conc. the active sites get blocked and uptake
decreases.
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Contact Time: The rate of sorption is higher at the beginning
due to more biomass which providesincreased contact between metal and
biomass.
As the active sites in the biomass areexhausted uptake is controlled by the rate at
which adsorbate is transferred from exterior
to interior of adsorbant.
Maximum removal time is within the first 45
min.. Equilibrium time is generally set as 4 hrs.
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Initial Metal Ion concentration:
Initial metal concentration provides an important
driving force to overcome all mass transfer resistance
of metal between aqueous and solid phase.
At higher conc. There is an aggregation of adsorbent
particles, such aggregation leads to a decrease in the
total surface area of adsorbent and increase in
diffusion path length.
But after a certain conc. the uptake decreases due to
the blockage of active sites. The initial metal conc. Should be in the range of
200-600 ppm.
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Effect ofTemperature:
Temperature effects a no. of factors for heavy metalbiosorption like :
i. Stability of initial metal ions in soln.
ii. Stability of micro organism-metal complex depending
on the site of biosorption.iii. Effect on microorganism cell wall.
iv. The ionization of chemical moieties in the cell wall.
With increase in temp liquid viscosity decreases ,increasing
the adsorbate diffusion across the external boundarylayer.
It effects the equilibrium capacity of adsorbate depending
on whether process in exothermic or endothermic.
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Langmuir eqn.The rate of attachment to the surface should be
proportional to the driving force times the area.
Driving force is the conc. In the fluid and
Area is amount of bare surface.
Affinity between biomass and diff metals is given by:
q = qmax b Ceq / 1+ b Ceq
Where, q is milligrams of metal accumulated per gram of the
biosorbent material;
Ceq is the equilibrium metal concentration in solution in
mg/L;
qmax is the maximum specific uptake per unit mass of
adsorbent in mg/g and b is the Langmuir constant for ratio of
adsorption and desorption rates.
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Freundlisch eqn.
Linear form :
q = kF Ceq1 /n
Where F and n are constant.q is conc. Of metal ion adsorbed.
Ceq is equilibrium conc. Of metal ions .
These models can be applied at a constant pH.These models are used in literature for modeling of
biosorption equilibrium in the presence of one
metal.