bio so rpt ion

8/7/2019 Bio So Rpt Ion

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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.

bio so rpt ion

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