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Role of Adsorbents

In Catalytic Reaction: Heterogeneous catalysisCatalysis: The phenomenon of change in the rate ofreaction with the help of a catalyst

Adsorption or Contact Theory: Due to the closeproximity of adsorbed reactants the molecules reactfaster and the product formed desorbed due to

decreased affinity with the adsorbent surfaceStepI : Diffusion of reactants to surface of catalyst

Step II: Adsorption of reactant molecules at the surface of

the catalystStepIII: Chemical reaction at the surface

StepIV: Desorption of products from the surface

StepV: Diffusion of products away from the surface


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Role of Adsorbent(Catalyst)

Finely divided state of catalyst is more efficient as it

increases the surface areaRough surface enhances the activity as the roughsurface posses more active sites like cracks, corners

and peaksPromoters increase the rate of the reaction by eitherincrease active sites or by increase the lattice space

Catalytic poison destroys the activity of catalyst as itpreferentially gets adsorbed st the active sitesCatalyst is specific as different adsorbents will pos-

sess different affinities for various adsorbate


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Role of adsorbent in ion-exchange adsorptionThe process of releasing the cation or anion and adsorbing

another like ionSynthetic resins are used as ion-exchange resin or ion-exchanger in ion-exchange processInsoluble cross-linked, long-chain org polymers with the

functional gp responsible for ion-exchanging propertiesThey posses one adsorbed ion on itThey release this ion and adsorbs another like ionTYPES

Cation exchange resin (R-H+):with acidic functional gp(-COOH, -SO3H, etc) ex. Styrene-divinyl benzene

Anion Exchange Resin (R+OH-)With basic functional gps(-NH

2

, =NH as hydrochloride)

Styrene-divinyl benzene contain amino or quaternaryammonium or quarternary phosphonium or sulphonium gp as anintegral part of the resin matrix


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Applications of Ion- exchangeAdsorption

Softening of Water

Zeolite Process

Deionization or Demineralization of water

Mixed bed Deioniser

Electrical Demineralization of waterMedical use: Ex. Dialysis

Ion-Exchange Chromatography


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Zeolite Process Zeolite: (Ze) HydratedSodiumaluminiumsili-

cate

Cation Exchanger

Exchange reversiblyits sodium ions forhardness producing

ions, Ca2+

, Mg2+

etcNa2Ze + Ca(HCO3)2

CaZe+ 2NaHCO3

Regeneration: CaZe +NaCl

Na2Ze + CaCl2

C i E h


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Cation Exchanger Capable of exchanging their cation (H+) with other cations

present in the water.

Cation exchange resin is represented as R-H+ Resins containing acidic fn group(- COO-H+, -SO3-H+) are

capable of exchanging their H+ions with other cations of hardwater.

Mainly styrene (divinyl) benzene copolymers which onsulphonation or carboxylation become cabable to exchange theirhydrogen ions with cations in water

Process: The hard water is first passed through a cation exchange column,

which removes all the cations like Calcium, Magnesium from itand equivalent amount of H+ ions are released.

M

+

+ R

-

H

+

RM + H

+

CaCl2+2 RH R2Ca + 2HCl

NaCl + RH RNa + HClRegeneration:when the cation exchange resin is exhausted, it can

be regenerated by passing a solution of dil. HCl or H2SO4:RNa + HCl RH + NaCl


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Anion Exchanger

Resins containing basic functional groups (NH2) or quaternaryammonium groups are capable of exchanging their OH-ions withother anions of hard water.

Anion exchange resin is represented as R+OH-

Anion exchange resin Styrene-divinyl benzene or amine-formaldehyde copolymers which contain amino or quarternary ammor quarternary phosphonium or quarternary sulphonium gp asintegral part of resin matrix

When water is then passed through an anion exchange column, which

removes all the anions like chlorides, sulphates etc. and equivalentamount of OH- ions are released from the column.

X- + ROH RX + OH -

SO42-+ 2ROH R2SO4+ 2OH

-

RegenerationThe exhausted anion exchanger dil.NaOH can be used. RCl + NaOH ROH + NaCl

Net result on passing through both cation and anion exchanger:H++ OH- H2O (Pure water)


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Softening of water As the metallic ions (cations) in the water affix themselves to the

exchange material, the latter releases its hydrogen ions on a

chemically equivalent basis. Because of the relatively high concentration of hydrogen ions, thesolution is very acid. deionization process is just half complete.

While the positive metallic ions have been removed, the water now

contains positive hydrogen ions, and the anions originally in the rawwater. The partially treated water now flows through a second unit, this time

an anion exchange material normally consists of replaceable hydroxylanions and fixed irreplaceable cations.

The negative ions in solution (the anions) are absorbed into the anionexchange material. Released in their place are hydroxyl anions.

Contains the positive hydrogen ions released in the initial exchangeplus the negative hydroxyl ions released in the second exchange.

They have combined (positive to negative) to produce watermolecules.

H++ OH- H2O

The result of this two-stage ion exchange process is completely freefrom cations and anions. This water is known as Demineral water ordeionised water.


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Ion Exchange Process / Demineralization

process / de-ionisation process

Advantages:

1) Used to soften highly acidic or alkaline water

2) Produces water of very low hardness(2ppm)

Disadvantages:

1) The equipment is costly and expensive

2) If water contains turbidity, then the output of the

process is reduced. The turbidity must be below10ppm.If it is more it has to be removed by coagulationand filtration.


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Mixed-bed Deioniser

Deionizers are categorized as mixed-bed, containing both cation and anionresin in a single vessel, or dual-bed,where each resin type is in a separate

vesselMixed-bed deionisers: The cation-exchange and anion-exchange

resins are intimately mixed andcontained in a single pressure vessel.

The two resins are mixed byagitation with compressed air, sothat the whole bed can be regard asan infinite number of anion and

cation exchangers in.Contd------

Mi d b d D i i


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Mixed-bed Deioniser

Regeneration:

Seperation of both ion exchanger: When the resins areexhausted the mixed bed is back washed by forcing water in theupward direction as the lighter anion exchanger get displaced toform an upper layer above the heavier cation exchanger

Anion exchanger is regenerated by passing caustic soda from the

top The lower cation exchange resin is regenerated by acid from thebottom

The two beds are then rinsed thoroughly, mixed again by forcingcompressed air

Advantages: More convenient to use More efficient removal of dissolved salts- Dissolved salts can be

reduced to less than 1ppm

Limitation: More expensive to regenerate Mixed-bed plants are more sensitive to impurities in the water

supply and involve a more complicated regeneration process.

Mixed-bed deionizers are normally used to polish the water tohigher levels of purity after it has been initially treated by either atwo-bed deionizer or a reverse osmosis unit.


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Electrical demineralization of water

Under the influence of the strong direct current field applied, chargedions are pulled off the resin and drawn towards the respective,oppositely-charged electrodes

Ion exchange resins are supported on a paper or a fibre and used as

membranes.Ion-selective membranes

Cation selective membrane permits only cations not anions because thepresence of charge fixed (inside the membrane) fn gps such as RSO3

- or

RCOO-reject anions Anion selective membrane permits only anion because of positively

charged fixed ions R4N+

The rate of diffusion of ion is increased by passing electric current

through the electrode placed near the membrane Contd------


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Electro Dialysis (ED)

Role of Adsorbent in Chromatography


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Role of Adsorbent in Chromatography Based on the differences in the adsorption

coefficients of substances on solid Process:StepI: Separation of various compounds

When the solution containing different solutes(mobilephase-adsorbate) is poured down a column, filled withfinely divided adsorbent(stationary phase-adsorbent)partial separation takes place due to the difference inadsorption coefficient

The separation is improved by passing suitable

solvent(developer) slowly through the column A number of horizontal bands of different colours are

produced in the column

Chromatogram: The banded column of the adsorbedsubstance contd------


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StepII: Identification f compounds

Colored compounds produce colored rings, zones orbands

Colorless compounds are observed by eitherexposing UV light on the column or by sprayingsuitable chemical reagent in order to make them

colored.StepIII: Separation and estimation

The various zones are dissolved separately in

suitable solvents and estimated The process of recovery of various substances is

elution and the solvent is called eluent.

contd------



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Ion Exchange Chromatography

Charged substances are separated by differentialadsorption on adsorbents carry an opposite charge

Example : Separation of proteins

At strong acidic pH, proteins are present as cations

as a result of dissociation of carboxylic acid groupand protonation of amino group

At pH values above 12, they present as anions

because the amino groups exist as free base Adsorption and desorption of proteins on charged

stationary phase is brought by changing pH.


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Advantages of Chromatography

Separation of a mixture of similar molecules is

possible only in adsorption chromatography

Large volume of samples can be separated

Applications of chromatography

Quantitative separation of two or more substances

Purification of substances Identification of product



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Activated carbon

Activated carbon, also called activatedcharcoalor activated coal, is extremely

porous carbon which had a very largesurface area available for adsorption orchemical reactions.

It was first used as an adsorbent in gasmasks in world war I..

Preparation and Properties


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Preparation and PropertiesActivated carbon is produced from carbonaceous source

materials like nutshells, wood and coal.Preparation1) Direct Process: Heating carbon at 850C in a current of su-perheated steam

2) Briquetting process: Powdered carbon is mixed withwood, tar and caustic soda ,compressed in small briquettesand then heated in steam at 800C.

3) Chemical Process: Wood is impregnated with the solutionof zinc chloride or phosphoric acid and then carbonized.PropertiesActivated carbon has enormous surface area of the order of1000 m2 / g.Activated carbon is able to adsorb practically any volatile or-ganic compounds(VOC) at lower temperature and can be re-

leased on heating to a high temperature

Classification Based on the size of particle


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Classification -Based on the size of particlePowdered activated carbon (PAC)Diameter of less than 200meshUsed in purification of liquidsSaw dust and lignite are raw materialsGranular activated carbon (GAC)

Diameter of greater than 0.1nmGranular activated carbon are fine granules has a relative-ly larger particle size and presents a smaller external sur-face has large internal surface area due to porosity.Produced from coconut shells, fruit pits , charcoal and bri-quetted coalDiffusion of the adsorbate is an important factor.These carbons are preferred for all adsorption of gases andvapours as their rate of diffusion are faster.Granulated carbons are used for water treatment, de-

odourisation and separation of components of flow system.GAC can be either in the granular form or extruded form


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Classification-contd

Impregnated carbonPorous carbons containing several types of inorgan-ic impregnant such as iodine, silver, cation such as

Al, Mn, Zn, Fe, Li, C or application in air pollutioncontrol especially in museums and galleriesPolymers coated carbon

This is a process by which a porous carbon can becoated with a bio-compatible polymer to give asmooth and permeable coat without blocking thepores


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Types of Different Activated Carbon

PAC

GAC Exrtuded GAC


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Adsorption in GAC

GAC uses the physical adsorptionprocess whereby attractive

van der Waals forces pull the so-lute out of solution and onto itssurface

Adsorption proceeds through 3basic steps Substances adsorb to the exterior

of the carbon granules

Substances move into the carbonpores

Substances adsorb to the interior

walls of the carbon

Role of Activated carbon


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In air pollution abatement

Activated carbon has very high surface area and it ad-sorb odorous, gaseous, and liquid contaminants form-ing a strong chemical bond or attraction.

In producing high vacua - For this partly evacuated appa-

ratus is connected to a vessel containing activated carbon,cooled in liquid air. At this temperature carbon adsorbsresidual air very effectively.

In gas masks or respirators - it adsorbs poisonous or foul

smelling and other harmful gases and vapour more readilythan it adsorbs air.. Thus the air getfiltered on passing throthe gas mask before breathing.

Remove offensive odor from the air (deodorizer) in air-conditioning process inlarge restaurants, auditoriums andin refrigerators, in shoe insole

Role in Activated carbon In air pollution abate-


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Role in Activated carbon In air pollution abate-ment-Contd

An air purifierremoves contaminants from air and particularly bene-ficial to allergy sufferers and asthmatics, and at reducingsecond-hand tobacco smoke. In room air purifiers, the activatedcarbon is often combined with zeolite.

In gas purification, activated carbon is used to remove un-desirable or noxious components like Benzene. It also selectively removes residual Hydrogen Sulphide, Napthalene, Cyanogencompounds, Resins, Gums and Nitrogen Oxides.

Purification of feed or exhaust air from unhealthy or annoy-ing substances like Hydrogen Sulphide and Carbon Disulphide

For the fine purification Carbon Dioxide In the gas phase application the carbon is used to trap any

undesirable gases such as Xenon, Krypton in the Nuclear Pow-er Plants

Role of Activated Carbon in indoor air


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pollution -Volatile Organic Compounds

Volatile Organic Compounds(VOCs) are organic chemicalcompounds that have high enough vapor pressures undernormal conditions to significantly vaporize and enter the at-

mosphere. VOC's are often used in paint, carpet backing, plastics, cos-

metics and wood preservatives, contribute tosick building syndrome because of their high vapour pressure

Air purifiers are used to reduce the concentration of these airbornecontaminants and are especially useful for people who suffer fromallergies and asthma.

Activated carbon because of porous nature absorb volatilechemicals and changing contaminants from a gaseous phase to asolid phase,

VOC can be recovered later by desorption

In abatement of water pollution The fresh water available on the earth is only 2%. Hence proper


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y p pmaintenance,conservation and use of water resources is requiredto avoid chances of water scarcity forfuture generation.

It is therefore necessary to treat water and reuse wastewater, soas to increase and improve the quality of water and make it foruse.

Activated carbon is used in water treatment process.

Theory: It is both hydrophobic and oleophilic (ie) it hates water but has affinity

for oil and other chemicals. When the flow condition is suitable, dis-solved chemicals in water flowing over the surface of carbon get ad-

sorbed on the surface while the water passes over. Activated carbons used in waste water treatment are Granular Activat-

ed carbon(GAC), Extruded solid carbon black (CB) and powdered Ac-tivated carbon (PAC).

GAC is used because of its porous structure and high surface area. Itis cheaper and can be reused For drinking water, the solid carbonblock filters are most efficient and effective.

GACs can be regenerated in a furnace by heating it in the presence of

limited amount of water vapour, flue gas and oxygen. Some carbon (5- 10%) is destroyed during regeneration process.

Activated Carbon in Domestic Water Treatment


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Activated carbon (AC) filters have been used in home water purifica-tion systems primarily to remove taste and odor.

AC is most effective at removing organic compounds such as volatileorganic compounds, pesticides and benzene. It can also remove somemetals, chlorine and radon.

GAC (granular activated carbon), can be used after coagulation and

sedimentation as a layer, or used after sand filtration to remove organ-ics from the water.

PAC (powdered activated carbon), is more commonly used than GACto control taste and odor in drinking water treatment. PAC can be

added directly to the water prior to coagulation or just sand filtration. Purification of drinking water (5-20 grams of powdered activated car-

bon can purify about one thousand litres of water in an hour)The PACadsorbs contaminants and is then removed by sedimentation or filtra-

tion In soft drink industries and in breweries, the potable water used is ster-

ilized with chlorine. The excess chlorine and its odour, dissolved organic contaminants, the trihalomethanes and phenolics are removed by

treatment with activated carbon

Removal metals from waste water solution


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Removal metals from waste water solution In purifying waste water, most of the contamination in the waste wa-

ter is broken down biologically by micro organisms. Toxic contaminants in the water can upset the biology badly, unless

the toxins are made harmless through adsorption onto activated car-bon.

PAC

Primary water

treatment filtration &sedimentation insettling tank

coagulants

Sand filtrationGAC

GAC for filtration&adsorption

Flocculation and Settling

BiologicalTreatmentDisinfection/chlorination

Disposal into water bodies


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Industrial effluent treatment

In industrial contamination of ground water reserves, acti-vated carbon is made use to reduce total organic halogens(TOX) and adsorbable organic halogens (AOX) including

chloroform, tetrachloroethylene and trichloroethane tomeet environmental legislation

Recovery of fuels from synthesis gases like Motor fuel Hy-drocarbons, Polymers, Ethylene and etc.

Activated bituminous C is used for the reduction of highchlorine content of water

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