Exp. 4 Adsorption

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Department of Chemical and Petroleum EngineeringCHME 3706-Physical Chemistry 2 lab

------------------------------------------------------------------------------------------------------------Experiment # 4

Adsorption of Dye by Using Activated Carbon

Objectives: To investigate the adsorption properties of activated carbon by studying the

equilibrium isotherms, adsorption kinetics and operational characteristics of a lab-scale packed-

bed reactor.

IntroductionAdsorption is the collection of a substance onto the surface of the adsorbent solids, whereas

Absorption is the penetration of the collected substance into the solid. Since both of these

frequently occur simultaneously, some choose to call the phenomena sorption. Although both

adsorption and absorption occur in sorption by activated carbon and other solids, the unitoperation referred to as adsorption.

Adsorption consists of using the capacity of an adsorbent to remove certain substances from a

gas or solution. Activated Carbon is an adsorbent that is widely used in water treatment,

advanced wastewater treatment, and the treatment of certain organic industrial wastewater,

because it adsorbs a wide variety of organic compounds and its use is economically feasible.

In water treatment it is used to remove compounds that cause objectionable taste, odor, or

color. It is generally used in granular form in batch, column (both fixed bed and

countercurrent bed), or fluidized-bed operations, fixed bed column being the most common.

Adsorption may be classified as Physical or Chemical adsorption. Physical Adsorption isprimarily due to van der Waals forces and is a reversible occurrence. When the molecular

forces of attraction between the solute and the adsorbent are greater than the forces of

attraction between the solute and the solvent, the solute will be adsorbed by activated surface.

An example of physical adsorption is the adsorption by activated carbon. Activated carbon

has numerous capillaries within the carbon particles, and the surface available for adsorption

includes the surface of the pores in addition to the external surface of the particles. Actually,

the pore surface area greatly exceeds the surface area of the particles, and most of the

adsorption occurs on the pore surfaces. For activated carbon the ratio of the total surface area

to the mass is extremely large. In chemical adsorption, a chemical reaction occurs between

the solid and the adsorbed solute, and the reaction is usually irreversible.

Activated carbon is made from numerous materials such as wood, sawdust, fruit pits and

coconut shells, coal, lignite, and petroleum base residues. When activated carbon particles

are placed in a solution containing an organic solute and the slurry is agitated or mixed to

give adequate contact, the adsorption of the solute occurs. The solute concentration will

decrease from an initial concentration,(Co)to an equilibrium value, (Ce), if the contact time is

sufficient during the slurry tests, it is usually possible to obtain a relationship between the

equilibrium concentration (Ce) and the amount of organic substance adsorbed (X) per unit

mass of activated carbon (m).


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The Freundlich isotherm, which is an empirical formulation, frequently will represent the

adsorption equilibrium over a limited range in solute concentration. One form of the equation

is:

x/m = X= K Ce1/n

Where

x: mass of solute adsorbed

m: mass of adsorbent

X: mass ratio of the solid phase that is, the mass of adsorbed solute per mass of

adsorbent

Ce: equilibrium concentration of solute, mass/volume

K, n: Experimental Constants

The Linear form of Freundlich isotherm can derive from the above equation:

log X = log K + 1/n log Ce

Plotting log X on Y- axis versus log Ceon X- axis the slope of the obtained line represents

1/n and intercept represents log K

One of the most important aspects of the Freundlich isotherm in relation to the feasibility of

using carbon adsorption is the numerical value of n and the value of x/m when C e= Cothe n

value is the same regardless of the units used for the equilibrium concentration. The constant,

K, however, does fluctuate with different units employed for the equilibrium concentrations.

The larger the n value and the x/m value (when Ce= Co), the more economically feasible isthe use of carbon adsorption.

Another Isotherm, which frequently will represent adsorption equilibrium, is the Langmuir

isotherm, which is

x/m = X = aLKLCe/1+KCe

where : aL: mass of adsorbed solute required to saturate completely a unit mass of adsorbent

ACTIVATED CARBON ADSORPTIONISOTHERMS, KINETICS & CONTINUOUS-FLOW OPERATION

BACKGROUND

Adsorption is a unit operation in which surface active materials in true solution are removed

from the solvent by inter-phase transfer to the surfaces of an adsorbent particle. This process is

used in environmental engineering practice for removal of various pollutants such as soluble

organics, dyes, pesticides, lignins, etc., from wastewaters and for removal of colour and taste

and odour-producing substances from natural waters that are to be used as potable water

supplies.


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Adsorption consists of using the capacity of an adsorbent to remove certain substances from a

gas or solution. Activated Carbon is an adsorbent that is widely used in water treatment,

advanced wastewater treatment, and the treatment of certain organic industrial wastewater,

because it adsorbs a wide variety of organic compounds and its use is economically feasible.

It is generally used in granular form in batch, column (both fixed bed and countercurrent

bed), or fluidized-bed operations, fixed bed column being the most common.

APPARATUS

Volumetric flasks, laboratory shaker, balance, glass column, pump, spectrophotometer,

stopwatch, solution holding tank, assorted tubing and glassware.

MATERIALS

1. Granular activated carbon; sieved, washed of fines, and dried to constant weight at 105C.2. Methylene blue.COMMENTS ON EXPERIMENT DESIGN

Each of the adsorption experiments hereinafter described employ a synthetic aqueous solution of

methylene blue as the test solution. Any other surface active material or sorbate such as organic

dyes, phenol, substituted phenols, alkyl benzene sulfonates, or high molecular weight alcohols,

however, may be substituted as the adsorbate. Further, natural waters or wastewaters containing

dissolved organics may also be employed as test solutions for these experiments provided that

they are first filtered to remove particulates. However, if methylene blue is replaced by another

adsorbate, then, it will be necessary to define appropriate analytical techniques for whatever

adsorbate is utilized.

PROCEDURE

A.1. Prepare 1 litre of methylene blue solutions in three beakers at the following

concentrations: I:30, II:30 and III:60 mg/l.

Adsorption Kinetics

2. Place the solutions over magnetic stirrers and start stirring.3. At time zero add 100 mg of granular activated carbon to the first, 250 mg of granular

activated carbon to the second and 250 mg of granular activated carbon to the third

beaker.

4. Take samples at 15 minute time intervals, and continue sampling until the systemequilibrates. If disintegration of the activated carbon should occur, samples may requirecentrifugation or filtration.

5. Determine the remaining methylene blue concentrations, by utilizing the calibrationcurveprepared before.

6. Plot the normalized solution phase concentration (C/Co) vs. time for each beaker.7. Calculate the quantity of methylene blue that was transferred to the surface of the

activated carbon (moles of methylene blue/gram of carbon) for each sample that was

collected. Plot these uptake values vs. time on the same graphs developed in step 6

above.

8. Check whether the order of the rate of adsorption is defined.
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9. Show that the initial rate of adsorption is proportional to methylene blue concentrationand the concentration of granular activated carbon (note that desorption can be neglected

for initial phases of the process).

10. Discuss your observations on the rate of adsorption.B.

1. Prepare a set of eleven 250-ml flasks by placing 100 ml of 125 mg/l ofmethylene blue solution and the following weights of the granular activated

carbon into the flasks: 0, 15, 20, 25, 30, 40, 50, 70, 100, 150, 200 mg.

Adsorption Equilibria

2. Cover the flasks with parafilm and agitate them vigorously on a laboratoryshaker until the system in each flask equilibrates. Then settle or filter the

suspension to remove the carbon.

3. Measure the remaining methylene blue concentrations for each flask.4. Calculate the quantity of methylene blue that was transferred to the surface of

activated carbon for each flask.

5. Plot the data in a standard adsorption isotherm format.6. Determine the Freundlich and Langmuir adsorption constants.7. Discuss the correspondence between the experimental data and the Langmuir

and Freunlich isotherms.

C.1. Place granular activated carbon into the column. Note the dimensions of the

column.

Continuous Flow Systems

2. Prepare approximately 1500 mg/l methylene blue solution.3. Feed the column with stated methylene blue solution at an appropriate rate and

record the time as time zero. Note that the column does not fluidize or expand at

that flow rate if the column is fed upward.4. Sample and determine the concentration of methylene blue in the column effluentuntil complete breakthrough of the column is attained (C/Co=1).

5. Plot the breakthrough curve showing the concentration of methylene blue in thecolumn effluent.

6. Calculate the quantity of methylene blue adsorbed as a function of time bygraphically integrating the area above the curve.

7. What is the capacity of granular activated carbon according to the breakthroughcurve?

8. Compare the capacities predicted from the breakthrough and form the isotherm test.9. Discuss the results of your experiment in detail.


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Methylene Blue Calibration Curve

Exp. 4 Adsorption

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