Question

A) What is the difference between ion exchange and surface adsorption?

B) How is isomorphous substitution in clay minerals related to ion exchange?

C)What is cation exchange capacity (CEC)?

Answer 1

Ans- (a) Ion exchange itself is an adsorption reaction too.
In ion exchange, an ion absorbs to the surface by removing an other ion of the same valence or multiple ions of lower valence. In this way, ion exchange never changes the surface charge.
Specific absorption describes the inner sphere complexing of an ion to the surface without the need for other ions to be removed. In this way, specific sorption is able to alter the surface charge.

(b) During the formation of clay minerals, cations other than Si in the tetrahedral sheet and Al in the octabhedral sheet substitute for these atoms.  Often, they are of lower charge but of similar size in the structure of clay minerals.  This substitution does not change the morphology (shape) of the clay minerals (hence the substituted and unsubstituted forms are isomorphous), but the substitution leaves the clay mineral with a net negative charge that must be satisfied by exchangeable cations.  The substitution also changes the chemical composition of the clay mineral producing stress in the structure.

(c) Cation exchange capacity (CEC) is the totalcapacity of a soil to hold exchangeablecations. CEC is an inherent soil characteristic and is difficult to alter significantly. It influences the soil's ability to hold onto essential nutrients and provides a buffer against soil acidification.