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ENVIRONMENTAL ENGINEERING - II
Unit III: Special Treatment Methods Adsorption Reverse Osmosis Defluoridation
Ion exchange Ultra Filtration
3.1 ADSORPTION
Many industrial wastes contain organics which are refractory and which are difficult or
impossibel to remove by conventional biological treatment processes. Examples are ABS and
some of the heterocyclic organics. These materials can frequently by removed by adsorptiob
on an active-solid surface. The most commonly used adsorbent is activated carbon.
Theory of Adsorption
A solid surface in contanct with a solution tends to accumulate a surface layer of solute
molecules because of the unbalance of surface forces. Chemical adsorption results in the
formation of a monomolecular layer of the adsorbate on the surface through forces of residual
valence of the surface molecules. Physical adsorption results from molecular condensation in
the capillaries of the solid. In general, substances of the highest molecular weight are most
easily adsorbed. There is a rapid formation of an equlibrium interfacial concentration, followed
by slow diffustion into the carbon particles. The overall rate of adsorption is controlled by the
rate of diffusion of the solute molecules within the capillary pores of the carbon particles. The
rate varies reciprocally with the square of the particle diameter, increases with increasing
concentration of solute, increases with increasing temperature and decrease with increasing
molecular weight of the solute. Morris and Weber found the rate of adsorption to vary as the
square root of the time of contanct as shown in below.
The adsorptive capacity of a carbon for a solute will likewise be dependent on both the
carbon and the solute.
Most wastewaters are highly complex and vary widely in the adsorbability of the
compounds present. Molecular structure, solubility, etc., all affect the adsorbability. These
effects are shown below. The relative adsorbability of organics on carbon is shown below.

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Rate of adsorpton of 2-dodecyl benzene sulfonate by Columbia carbon (300C,
75mg/l, 0.273mm diameter). (After Morris and Weber)
Influence of molecular structure and other factors on adsorbability
1. An increasing solubility of the solute in the liquid carrier decreases its adsorbability.
2. Branched chains are usually more adsorbable than straight chains. An increasing length
of the chain decreases solubility.
3. Substituent groups affect adsorbability:
Hydroxyl generally reduces adsorbability. Extent of decrease depends on
structure of host molecule
Amino Effect similar to that of hydroxyl but somewhat greater. Many amino
acids are not adsorbed to any appreciable extent.
Carbonyl effect varies according to host molecule, glyoxylic acid more adsorbable
than acetic but similar increase does not occur when induced into higher
fatty acids
Double bonds variable effects as with carbonyl
Halogens variable effects
Sulfonic usually decreases adsorbability
Nitro often increases adsorbability
4. Generally, strong ionized solutions are not as absorbable as weekly ionized ones; i.e.,
un-dissociated molecules are in general preferentially adsorbed.
5. The amount of hydrolytic adsorption depends on the ability of the hydrolysis to form
an adsorbable acid or base.

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ENVIRONMENTAL ENGINEERING - II Unit – III: Special Treatment Methods – Adsorption – Reverse Osmosis – Defluoridation – Ion exchange – Ultra Filtration 3.1 ADSORPTION Many industrial wastes contain organics which are refractory and which are difficult or impossibel to remove by conventional biological treatment processes. Examples are ABS and some of the heterocyclic organics. These materials can frequently by removed by adsorptiob on an active-solid surface. The most commonly used adsorbent is activated carbon. Theory of Adsorption A solid surface in contanct with a solution tends to accumulate a surface layer of solute molecules because of the unbalance of surface forces. Chemical adsorption results in the formation of a monomolecular layer of the adsorbate on the surface through forces of residual valence of the surface molecules. Physical adsorption results from molecular condensation in the capillaries of the solid. In general, substances of the highest molecular weight are most easily adsorbed. There is a rapid formation of an equlibrium interfacial concentration, followed by slow diffustion into the carbon particles. The overall rate of adsorption is controlled by the rate of diffusion of the solute molecules within the capillary pores of the carbon particles. The rate varies reciprocally with the square of the particle diameter, increases with increasing concentration of solute, increases with increasing temperature and decrease with increasing molecul ...
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