Sorption Process | BiotechStudies

SORPTION

Sorption is a physical and chemical process by which one substance becomes attached to another.

ABSORPTION: 

The incorporation of a substance in one rotate into another of a different state. E.g. liquids being absorbed by a solid or gases being absorbed by a liquid.

ADSORPTION: 

The physical adherence or bonding of ions and molecules onto the surface of another phase. E.g. reagents adsorbed to a solid catalyst surface.

ION EXCHANGE: 

An exchange of ions between the two electrolytes or between an electrolyte solution and a complex.

MOLECULAR BASIS OF SORPTION

• Sorption has traditionally been divided into two extremes: weak physio-sorption and chemi-Sorption. 
• Sorption can involve strong electrostatic interactions between ions or dipoles and surfaces, including ion exchange type reactions. Sorption can also involve only weak intermolecular force such as vander walls interactions.
• The starting place for Sorption therefore should be a description of the surface.

1- THE OXIDE SURFACE:

- The surface of a solid represents an interface between the gas at solution phase and the mineral crystal.

- The solid surface can be represented as a surface of discrete sites, and individually participate in a reaction resulting in Sorption.

- The dry oxide surface can be described as surface of amphoteric hydroxyl site each capable of both donating and accepting a portion resulting in anionic, donating and accepting a portion resulting in anionic, neutral and cationic sites depending on pH and hydroxyl pK.

- In aqueous system, these charged sites strongly attract water, creating a layer of tightly bound water at the surface then adsorbing organic organic molecule sees “a charged surface of water, any direct association with that surface must first displace that water.

2- SURFACE CHARGE AND THE DOUBLE LAYER:

Surface charge on oxide surfaces primarily originates from three sources- protonatin, deprotonation of surface hydroxyls, isomorphous substitution of aluminium for silicon in layer

a. Variable surface charge:

-At low pH, surface are protonated: H+ ions + OH- ….> OH3+ , cationic.

-At high pH, surface loose their protons – OH---> H+ + OH2+

-when concentration of cationns OH2+ site equal the concentration of anionic –O, surface charge is neutral, that pH is known as ZPC(Zero Point Of Charge).

b. fixed surface charge:

• Isomorphous substitution of Tetra valent silicon by trivalent aluminium in layer result in negative charge fixed charge cannot be changed, concentration is exchanged.

c. The Double Layer:

• The solid surface itself is negatively charged and the charge is constant at constant pH and temp.
• In pure water, the charged layer attract oriented water molecules, forming a highly structured inner layer of vicinal surface water, easily resemble ice than water, low dielectric constant. (6vs 18 for bulk water).
• A second layer of less structured water will form with the opposite orientation, This water dielectric constant is 30.
• Hydrated cations on solution will associate with the surface anionic sites, forming a layer of surface cations.

STERN LAYER

-Anions will loosely associate with the stern layer forming a diffuse negative GOUSY LAYER.

-The thickness of the double layer decreases with increasing ionic strength.

3. SORPTION OF GASES ON SOLID SURFACE:

a. Physio-sorption: 

• It is very rapid and reversible with the equilibrium established quickly as the gas pressure is varied.
• Physio-sorption is generally considered important for vapours below their critical temperature.
• Longer range forces are causing the attraction to occur. Therefore multiple layer of absorbed ions can build up on the surface.

b. Chemio-sorption: (specific adsorption)

• Adsorbed species are chemically bound to the surface creating something like a surface compound e.g. O2 is on a shiny aluminium surface.
• Chemio-sorption is the result of relatively strong binding forces(>30 Kcal/mol).
• Adsorbed species are limited to the formation of a monolayer, because direct interaction to the gas and the surface is required. Once the surface covered physio-sorption can continue.

SORPTION FROM SOLUTION:

• Oxide surface in aqueous system adsorb water molecules via strong electrostatic interactions called HYDROGEN BONDS.
• For the reason Ksp would be expected to be low for the organic compounds higher for polar compounds that can compete for charged sites.
• When hydrophobic molecules associates with soil organic matter, there is no competition with water, associate with soil organic matter there reaction into an organic solvent.

FACTORS AFFECTING SORPTION:

1. SURFACE AREA:

• Adsorption is a surface phenomenon that is directly related to surface. Increase the surface area, adsorption will increase.
• If all were equal, a unit area of clay has the same sorptive capacity as the same unit area of Goethite. Sorption should always be reported on the area basis.
• The Defetion of surface area is hazy and often difficult to measure unambiguously. Micro popres and molecular porosity increases the Sorptive capacity, but are not necessarily measured.

2. MINERAL SURFACE PROPERTIES:

• Surface charge of an oxide mineral surface in aqueous system will change with changing pH as a function of the ZPC of that mineral.
• Charged organic solutes may exchange with other counter ions in double layer, resulting in Physiosorption.

3. ORGANIC CARBON AND THE Koc:

• It has been found that the Sorption of hydrophobic organic compounds is strongly controlled by the presence of soil organic material.
• The Koc then is an expression describing the portioning of a solute between the aqueous and organic phases.

4. SOLUBLITY:

As the solubility of a hydrophobic compound decrease the adsorption coefficient increase, probably from entropy driven interaction with the surface.

5. TEMPERATURE:

• Since adsorption is an exothermic process value of K usually decrease with increasing temperature. A 10% decrease in Ksorp would occur with a temperature rise from 20 to 30 C.
• The probable control is the variation in gamma depending on the magnitude of deltaH (e.g. Schwargenbach it al, 1993)
• For large Apolar compound the delta H is -25KJ/mol.

6. PH

• Only chemicals that tend to ionize are much affected by change in the character of the surface.
• For example: at low pH, humic materials are nearly neutral and more hydrophobic.
• As pH change, surface charge also changes, and the sorption of charged species will be affected.

7. SALINITY:

• An increase in salinity can significantly lower the adsorption coefficient of cations probably due to replacement/ exchange by alkali cations.
• The Adsoption of some acid herbicides increase with greater salinity at pH value above the pKa of acid, pH influences the affect of salinity.
• Increased salinity may also change the interlayer spacing of layer clays as well as the morphology of soil organic matter.