Adsorption

Adsorption

Adsorption Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a molecular or atomic film (adsorbate)

ADSORBENT

ADSORBATE

SOLUTION

Adsorption • Consequence of surface energy • Atoms on the surface experience a bond deficiency, because they are not wholly surrounded by other atoms

Types of Adsorption Properties Bonding

Enthalpy Saturation Surface specificity Nature

Physisorption Weak, long range, Van der Waals interactions (London dispersion, dipole-dipole) 5-50 kJ/mol Multi-layer No Reversible

Chemisorption Strong, short range, Chemical bonding involving orbital overlap and charge transfer 40-800 kJ/mol Mono-layer Yes Mostly irreversible

Adsorption Isotherms • Plot of the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant temperature. • Langmuir isotherm (adsorbed layer one molecule thick) • Freundlich isotherm (Heterogeneous adsorbent surface with different adsorption sites) • Brunauer, Emmett and Teller (BET) isotherm (molecules can be adsorbed more than one layer thick)

Langmuir Isotherm • Irving Langmuir (1916) • Assumptions: Uniformity of sites Common Mechanism • Semi-Empirical Formula:

Non interaction Monolayer only

Freundlich Isotherm • Freundlich and Küster (1909) • Empirical formula: • Limitation: Fails at high pressures Q- Mass of adsorbate / mass of adsorbent p- equilibrium pressure of adsorbate c- equilibrium con. Of adsorbate in solution K,n- constants

BET Isotherm • Stephen Brunauer, Paul Hugh Emmett and Edward Teller (1938) • Assumptions: Multilayer No Transmigration Equal Energy Langmuir to each layer

Applications • Activated Carbon Hydrophobic Surface area-500m2/g Waste water treatment Decontaminant in pharmacy • Silica Gel and Zeolites Hydrophilic Polar Drying of process air CO2 and Hydrocarbon removal from natural gas Vapor Adsorption Refrigeration • Protein Adsorption on biomaterials(cells)

Question 1: An adsorption study is set up in a laboratory by adding a known amount of activated carbon to six flasks which contain 200 mL of an industrial waste. An additional flask containing 200 mL of waste but no carbon is run as a blank. Plot the Langmuir isotherm and determine the values of the constants. Flask No. Mass of C (mg)

Volume in Flask (mL) Final COD (mg C/L)

1

804

200

4.7

2

668

200

7.0

3

512

200

9.31

4

393

200

16.6

5

313

200

32.5

6

238

200

62.8

7

0

200

250

Question 2: Calculate the amount of adsorption using Freundlich isotherm of the solute on activated charcoal in which the Slope value n = 0.2 and the distribution coefficient is k = 0.19. The equilibrium concentration of the adsorptive is 0.12.

Given: n = 0.2, k = 0.19, C = 0.12 q = kC^(1/n) = (0.19)(0.12)^(1/0.2) q = 4.7 x 10-6

Question 3: The adsorption of a gas is described by Langmuir adsorption isitherm. Calculate the pressure at which the fractional coverage is (a) 0.1 (b) 0.5 and (c) 0.95

Given, k1=0.9 /kPa at 27oC .Solution: We know that, θ=k1p/1+k1p⇒p=(θ/1−θ)1/k1 Therefore, (a) p=0.1/0.9×1/0.9=0.123 kPa (b) p=0.5/0.5×1/0.9=1.11 kPa (c) p=(0.1/0.05)×1/0.9=21.11 kPa

Question 4: In a solution of 100 mL of 0.5 M acetic acid, 1 g of active charcoal is added, which absorbs acetic acid. It is found that the concentration of acetic acid becomes 0.49M. If surface area of charcoal is 3.01×102 m2, calculate the area occupied by single acetic acid molecule on the surface of charcoal. Solution: The number of moles of acetic acid in 100 mL (before adding charcoal) = (0.5 mol/L)(0.1L) = 0.05 mol.

The number of moles of acetic acid in 100 mL (after adding charcoal) = (0.49 mol/L)(0.1 L) = 0.049 mol.

Question 4: The number of moles of acetic acid adsorbed on the surface of charcoal =0.05−0.049=0.001 mol. The number of molecules of acetic acid adsorbed on the surface of charcoal. =0.001mol×6.02×1023molecules/mol=6.02×1020 molecules Given that the surface area of charcoal, =3.01×102 m2 So the area occupied by single acetic acid molecule on the surface of charcoal is 3.01×102/6.02×1020=5×10−19 m2

Question 5: In Langmuir’s model of adsorption of a gas on a solid surface a. The rate of dissociation of adsorbed molecules from the surface does not depend on the surface covered. b. The adsorption at a single site on the surface may involve multiple molecules at the same time. c. The mass of gas striking a given area of surface is proportional to the pressure of the gas. d. The mass of a gas striking a given area of surface is independent of the pressure of the gas.

Question 6: Which of the following is less than zero during adsorption? a. ∆G b. ∆S c. ∆H d. ∆H and ∆S

Question 7

Question 8: Identify the gas which is readily adsorbed by activated carbon a. H2 b. N2 c. SO2 d. O2

Question 9: Which is not the correct statement in respect of chemisorption? a. b. c. d.

Highly specific adsorption Irreversible adsorption Multilayered adsorption High enthalpy of adsorption

Question 10: Rate of physisorption increase with a. b. c. d.

Decrease in temperature Increase in temperature Decrease in pressure Decrease in surface area