Sources. Negative Charge:

Lecture 13

Chapter 5:158-168

Clay Minerals (cont.)

1. Ion Exchange in Soils:

	As a result of negative charges developed by soil colloids ions are absorbed on the surfaces of these colloids in soils.
	The ions absorbed are include Ca2+, Mg2+, K+, Al3+, and Na+.
	In humid regions Ca2+, Al3+ and H+ are by far the most numerous cations absorbed.
	Al3+ and H+ tend to dominate in humid regions.
	In semi arid regions Ca2+, Mg2+, K+, and Na+ tend to dominate.

2. Sources. Negative Charge:

	The main source of charge on clay minerals is isomorphous substitution which confers permanent charge on the surface of most layer silicates.
	Ionization of hydroxyl groups on the surface of other soil colloids and organic matter can result in what is describes as pH dependent charges-mainly due to the dependent on the pH of the soil environment. Unlike permanent charges developed by isomorphous substitution, pH-dependent charges are variable and increase with increasing pH.
	Presence of surface and broken - edge -OH groups gives the kaolinite clay particles their electronegativity and their capacity to absorb cations.
	In most soils there is a combination of constant and variable charge.

3. Cation Exchange :

	Displacement of one cation by another results in the process called cation exchange.
	For example : H+ produced by organic acid.
	Under high rainfall conditions, Ca leached reaction goes to right.
	Under low rainfall conditions, Ca and other soils are not easily leached.
	Reaction doses go to completion and tends to go to the left.

4. Factors Affecting Cation Exchange :

	The charge of the ion. Generally ions with higher valency will exchange for those of lower valency. For example Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺=NH₄⁺ >Na⁺ .
	For ions of same charge, the cation with the smallest hydrated radius is strongly absorbed because it moves close to the site of charge. For examples K with a hydrated radius of 0.532 nm, will exchange for Na , hydration radius of O.790 nm, on the exchange sites.
	The rate of ion exchange in soils is affected by the type and quantity of organic and inorganic colloids. Clay minerals with 1:1 lattice tend to have more rapid rate of exchange than 2:1 clays which have both internal and external exchange sites.

5. Cation Exchange Capacity :

	The cation exchange capacity of soils (CEC) is defined as the sum of positive (+) charges of the adsorbed cations that a soil can adsorb at a specific pH.
	Cation Exchange Capacity (CEC) is expressed as centimoles of positive charge per kilogram (cmol kg^-1) , of oven dry soil..
	Earlier unit was meq per 100 g soils.
	Equivalent weight : Quantity that is chemically equal to 1 gram of H.
	Number of H in equivalent weight is 6.02 x 1023 or Avoagardo's number.
	Milliequivalent is equal to 0.001gm of H.
	Example 6.02 X 1020 charges.
	Total cation exchange capacity of the soil is the total number of exchange sites of both the organic and mineral colloids.

6. Estimating CEC and Exchangeable Cations. (Refer to in text)

7. Table 13.1 Cation Exchange Capacities of Clay Minerals

Adapted From Sparks 1995. Envornmental Chemistry of Soils. Academic Press.

8. Cation Exchange Capacities of Soils

	The CEC of a given soil is determined by the relative amounts of different colloids in that soil and by the CEC of each of these colloids.
	Sandy soils generally have lower CEC than clay soil because coarse textured soils have lower amounts of both clays and organic matter.

Table 13.2

Adapted From Holmgren et. al. (1993). J. Environ. Qual. 22:335-348

9. Importance of Cation Exchange

Cation exchange at negative sites is major retention mechanism for heavy metals, e.g. Cd, Pb and Zn.

10. Measurement of CEC.

	The CEC of soil is usually measured by saturating the soil with an index cation such as Na⁺, removal of the excess salts of the index cation with a dilute solution , and then displacing the Na⁺ with another cation .
	The amount of Na⁺ displaced is then measured and the CEC is calculated.

11. Anion Exchange and Adsorption

	Anion exchange arise from the protonation of hydroxyl groups on the edges of silicate clays and on the surfaces of metal oxide clays.
	Anion exchange is inversely related with pH is greatest in soils dominated by the sesquioxides.
	The anions Cl^-, NO₃^-, and SeO₄^2- and to some extent HS^- ands SO₄²-, HCO₃^-, and CO3- adsorb mainly by ion exchange.
	Borate, phospahate and carboxylate adsorb principally by specific adsorption mechanisms.

12. Metal Cation Adsorption

The ralative affinity of a soil adsorbent to for a a free metal cation with a given valence is positively correlated with the ionic radius.

Cs⁺ > Rb⁺ > K⁺ > Na⁺ > Li⁺

Ba²⁺ > Sr⁺ > Ca²⁺ > Mg²⁺

Hg²⁺ > Cd²⁺ > Zn²⁺

For transition metals the relative adsortion affinities does not conform strictly to ionic radius and tend to follow the following order:

Cu²⁺> Ni²⁺ > CO²⁺ Fe²⁺ > Mn²⁺

Vocabulary

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