Ion Exchange 2

1

Ion exchange Ion exchange reaction in soil: According to Lambert Wiklander, ‘ion exchange is the reversible process by which cation and anion are exchanged between solid and liquid phases and between solid phases if in close contact with each other’.0 Ion exchange in soils occurs on surfaces of clay minerals, inorganic compounds, organic matter, and roots. The specific ion associates with these surfaces depend on the kinds of minerals present and the solution composition. In this ion exchange process a cation or anion in the solid phase is exchanged with another cation or anion in the liquid phase. For example, K +NH4+ NH4+ + K+ Colloid solution colloid solution If two solid phases are in contact, exchange of ions may also take place between their surfaces. Ion exchange reactions in soils are very important to plant nutrient availability. Therefore, it is essential that we can understand the nature of the solid constituents and the origin of their surface charge. Based on counter ion, the ion exchange phenomenon can further be divided into i. Cation exchange; and ii. Anion exchange. Processes of ion exchange: Ion exchange may takes place in the following ways: i. Ion exchange between solid and liquid phase. ii. Ion exchange between solid and solid phase iii. Ion exchange between solid phase and root surface. The processes are described below: i. Ion exchange between solid and liquid phase: Ions are strongly adsorbed on the colloidal surfaces. Hence, the colloidal refers to solid particles. When the colloidal surfaces come in contact with solution, ion from solution replace or exchange the equivalent amount of ion from colloidal surface. This ion exchange process/reaction obeys the law of chemical reaction, i.e. the law of mass action. K + NH4+ NH4+ + K+ Colloid Solution Colloid Solution

2

ii.

Ion exchange between solid and solid phase: Ion exchange may take place by direct contact and intermingling of double layers. This can be explained by ‘contact exchange theory’. Kinetic theory of molecules or atoms proposes that each ion adsorbed on the clay particle, due to kinetic energy, occupy a volume named as oscillation volume. When two clay particles are very close come together, then oscillation volume of two ions, adsorbed on either clay, overlaps and exchange of ions takes place without involving of soil solution.

Clay-1

Clay-1

K Na Na K

Clay-2 Clay-2

Fig: Contact exchange of ions by intermingling of double layers. iii. Ion exchange between solid phase and root surface: There are several acid-like compounds such as amino acids, carboxylic acid, etc. in roots. These acids on dissociation produce H+ ions. So, root contains H+ ions. When roots come in close contact with solid phase (clay particles), ion exchange takes place between two phases according to contact exchange theory. The oscillation volumes of H+ ion on root surface and Na+, K+ or any other ion adsorbed on solid phase overlaps each other and ion exchange takes place.

3

Root surface-H+ +

Colloid

Na

colloid

H + Root surface – Na

*Ion exchange reaction is a physico-chemical process: A simple example of ion exchange reaction is as followsH+ Ca²+ Micelle + 2H+ H+ Micelle +Ca²+ Here, Ca²+ is replaced by two H+ ions. By this reaction, we can prove that, i. Ion exchange is a physical process. ii. Ion exchange is purely a chemical process. I. Physical process: The clay particles acquire charges because of isomorphous substitution & pH variation. Cations are adsorbed on colloids or clay surfaces to satisfy negative sites. Adsorption is the concentration of substances on a surface without chemical reaction. It is a physical process. The cations are held on the colloidal or clay surfaces by van der Waals attraction force or weak electrostatic force. The rate of ion exchange increases as the surface area increases. So, ion exchange is a surface phenomenon and no new compounds are formed during ion exchange, one ion adsorbed & another desorbed. ii. Chemical process: a. From the above reaction it is apparent that ion exchange occurs in equivalent proportion, that is, ion exchange reaction maintains a stoichemistry. b. The ion exchange reaction obeys the rule of pure chemical law, i.e. the law of mass action which refers that the rate of a reversible chemical reaction is proportional to the concentration of the reactants. c. As a chemical reaction, the rate & direction of the ion exchange reaction can be manipulated by changing conditions. d. Probably, the most important point in the above respect is the development of charges on colloidal or clay surfaces. Charges are developed due to isomorphous substitution & pH variation. From the above discussion, it is clear that ion exchange holds the characteristics of both physical and chemical process. So, it is conveniently said that ion exchange is a physico-chemical process.

4

• Factors regulating adsorption: There are three factors that regulate the adsorption of ion. There are1. Surface potential 2. Valency, and 3. Hydrodynamic radius of the ion. The more the charges on the colloidal surface cations will be more attracted. That is, the higher the surface potential of a colloid, higher valency cations will be more attracted to the surface. A cation with a higher valency is attracted more to the colloidal surface. Divalent cations are more attracted to the colloidal surface than monovalent cations & trivalent > divalent. An ion having small hydrodynamic radius is more attracted to the colloidal surface (Due to charge density). Thus, having same valency, an ion can be more attracted than another due to variation in hydrodynamic radius. The relative adsorption of ion occurs according to the following series: For monovalent cations: Cs>Rb>K>Na>Li. For divalent cations: Th>La>Ba>Sr>Ca>Mg. Here, Ca gets over the Mg due to its, smaller hydrodynamic radius. Similarly, K+>Na+; Mg++>K+, Ca++>Mg++>K+>Na+. But, there is an apparent exception; the replacing capacity of H+ is very high (higher than the other ions). Then the series could be, H+>Ca++>………………………………………… # Monovalent ‘hydrogen’ in this case behaves more like trivalent lanthanum. *Why does a soil contain more Ca++ than other cations & K+ than Na+? Since clay colloids carry negative charges, cations are attracted to the clay particles. The adsorption reaction depends on the surface potential, valency & hydrodynamic radius of ion occurs among the cations. The relative adsorption of ions occurs according to the following series. (Lyotropic series): H>Ca>Mg>K>Na. The most important factor for the adsorption of an ion is its valency. Divalent ions is general are retained more strongly than are monovalent ions and trivalent ions are retained even more strongly.

5

Moreover, an ion having small HDR is more attracted to the colloidal surface. ---- So, Ca gets preference over Mg due to its smaller HDR. That’s why; a soil contains more Ca++ than Mg++, as well as than other basic cations. ---- As ions with smaller HDR are preferably adsorbed, soil colloid reaction more K+ than Na+, because of K+ having smaller HDR than Na+. *Explain the reason for using NH4OAl(CH3COONH4) solution at pH 7.0 in the determination of CEC of soils. Below pH 6.0, the charge for clay mineral is relatively constant. This charge is considered permanent due to ionic substitution in the crystal unit. But, above pH 6.0(at neutral pH), the charges on the mineral colloid increases slightly because of ionization of hydrogen form exposed hydroxyl groups at crystal edges. These charges are considered as pH dependent charge. Agricultural soil pH ranges between 5.0- 8.5 which are favorable for the dissociation of functional groups and hence, negative charges develop and the CEC reaches its maximum level. The pH 7.0 favors the development of maximum negative charge, as well as the maximum CEC. This is the reason for using NH4OAl solution at pH 7.0 in the determination of CEC of soils. *Why the NH4+ ion is normally used for displacing other cations in the determination of CEC? To determine CEC, the ammonium ion is normally used for displacing other cations from exchange sites for several reasons: 1. It is the most preferred of the cations in the lyotrophic series; such selectivity is coupled with extremely weak hydration, on per with that of the K+ ion enables the NH4 ion to move rapidly to a well-defined number of cation exchange sites. 2. Because it has to be used in concentrated solution for efficient and rapid displacement of basic cations, behind solubility, cheapness of all NH4+-salts is relevant. (As concentrated NH4 is used for rapid displacement, solubility & cheapness of NH4 salts are relevant). (Generally 1M NH4OHAl is used because it is more concentrated than the concentration of cations in soil.)

6 3. A portion of the negative charges in the minerals and

organic parts are pH dependent. NH4OAl, salt of weak acid and weak base, is usually used for this purpose, its pH being adjusted to the value most suited to the investigation. 4. In the final stage of determination of other cations, NH4+ salts interface least in flame and other photometric methods; they are also relatively volatile and can easily be removed from soil extracts if necessary.

• Describe the experimental findings of Thompson and way about ion exchange phenomenon in soil.

Perhaps the most important chemical property of soils is their ability to retain and exchange ions on colloidal surfaces. The first studies of ion exchange reactions were conducted by Thompson and way in the early 1850s. ‘Soil has the capacity to hold ions’- this was established by Thompson and way. The experimental findings of Thompson & way about IER phenomenon in soil are 1. Soil, when leached with a solution of ammonium sulfate [(NH4)2SO4] on a filter paper, release predominantly calcium (Ca) and other cations like magnesium (Mg) and potassium (k) in small amounts, but retain ammonium (NH4+). 2. The total amount of calcium (Ca) and other cations so released is equivalent to that of ammonium (NH4+) retained. NH4+ Clay Clay Ca + 2NH4+ + Ca²+ NH4+ 3. The sulfate concentration remains unchanged. 4. The higher the concentration of (NH4)2SO4, the greater is the release of Ca & other cations. 5. No appreciable effect of temperature is noticeable. 6. The ammonium taken up by the soil is released by treatment with hydrogen, calcium, sodium or other cations. H4N NH4 Na Na collid colloid + 4NH4 (in solution). H4N NH4 Na Na

7

7. On heating strongly, the soil losses its exchanging power.

• Properties of cation exchange reaction (qualitative aspects). 1. Reversibility:

Cation exchange reactions are reversible, or nearly so. Although cations are preferred to various degrees by soil colloids, even strongly adsorbed cations can normally be replaced through proper manipulation of solubility conditions. In fact cation exchange reactions in soil are not the same like pure chemical exchange. An exception to this generalization is the preferential retention of many polyvalent cations (especially the trace metals) by soil organic matter. Such cations, which are thought to be partially covalently bonded which are covalently bonded to organic matter, can be displaced only by other polyvalent cations capable of forming even stronger covalent bonds and their cation exchange reactions will be reversible. Otherwise, these will be slightly irreversible. Other exceptions include cation fixation reactions; cases where large organic cations, such as the pesticides paraquat and diquat, are physically prevented (sterically hindered) from approaching certain interlayer exchange sites and cases where multivalent cations are preferentially adsorbed, because they can simultaneously balance several closely adjacent exchange sites. Na Micelle Micelle Ca + 2Na+ + Ca²+ (in soil) Na

2. Stoichiometry: Cation exchange reactions are approximately stoichiometric, since the amounts exchanged are chemically equivalent. For this reason, the sum of all exchangeable cations present at a specified pH, the cation exchange capacity (CEC), varies only slightly with cation species. As an example, the following exchange reaction can be consideredCaX + 2NH4+ (NH4)2X +Ca²+ Where, X designates an exchanger surface. Two ammonium ions are required to replace a single ammonium ion, in order to preserve thee Stoichiometry of the reaction. [Exchangeable cation composition and CEC values normally are expressed as me/100 g or me/g.]

8

3. Speed: Exchange reactions are rapid. The exchange step itself is virtually instantaneous, if the ions are in close contact. The rate-limiting step often is ion diffusion to or from the colloid surface. This is particularly true under field conditions, where ions may have to move through tortuous pores or through relatively thick, stagnant water films on soil colloid surfaces to reach an exchange site. [The need for diffusion can produce hysteresis (reaction-direction dependence) for some ion exchange reactions.] Under laboratory conditions, samples normally are shaken during exchange reactions, to speed ion movement and to minimize the thickness of stagnant water layers on soil particle surfaces.

4. Mass action: Because of their reversibility, cation exchange reactions can be driven in either the forward or inverse direction by manipulating the relative concentrations of reactants and products. [In the laboratory, common techniques for driving the reactions toward competition include use of high (e.g.1N) concentrations of exchange cations, and maintaining low concentration of product cations by leaching or repeated washings. For example, CaX + 2Na+ (high concentration)

(Na)2X +Ca²+ (low concentration).

This reaction can be manipulated by forming insoluble precipitates, CaX+ Na2CO3 (Na)2X + CaCO3(precipitate) Or, forming volatile gases, NH4X+ NaOH Nax+NH4OH NaX +H2O+NH3(gas) [New compound production is an agreement in chemical reaction, but in ion exchange reaction no new one is produced, just ion rearrangement occurs. For this reason it is not a chemical reaction. But, there energy loss occurs in ion exchange reaction, which is a characteristic of chemical reaction. That’s why law of mass action can be applied to exchange reaction.] 5. Complementary cations: Exchanging one cation for another in the presence of a third (complementary) cation becomes easier as the reaction strength of the third cation increases.

9

For example, replacing calcium by ammonium is easier from a Ca²+-Al³+ soil than from a Ca²+-Na+ soil. The ammonium can replace considerably less aluminum (al) than it can sodium. Thus, more ammonium remains in solution to replace calcium from the Ca²+-Al³+ soil than in Ca++NA+ soil, where more ammonium is utilized to replace Na+. So, rate of ion exchange reaction can vary with the presence of complementary cation. N.B. [Ca-Al & Ca-Na are two systems, replace Ca from Na by NH4+. It’s found that, to replace Ca from Na is difficult than that from Al. Here, Al is a third cation; Ca is in the same amount in the both cases. NH4 is also same, so there is a third one for which the rate of reaction is not same for both of the system, i.e. the amounts of Ca++ released are not same, because most of the NH4 needed to replace Na+. A part of NH4 will replace Na, then its strength will fall short and then it cannot replace the divalent Ca++. But, in the other case, NH4 would not replace Al³+. In that case, more NH4 will remain in soil to replace Ca++ from Ca-OH system. Here, the influence of third cation is applicable.]

Anion effects: The anion accumulated with a replacing cation can affect cation exchange by having the exchange reaction further toward competition, changing the direction of exchange reaction, if the end products arei. more weakly dissociated ii. less soluble, or iii. more volatile An example of (i) is a reaction that forms weakly dissociated H2O. HX + Na+ + OHˉ NaX + H2O This reaction proceeds more completely than the 1st if strongly dissociated HCl had been formed instead HX + Na+ + Clˉ NaX + H +Clˉ The hydrogen on the right of this equation drives the reaction back toward the left. Here, in the first reaction, H2O is weakly dissociated, but HCl, in the second reaction, is highly dissociated. So for the first reaction, the possibility to backward is less, whereas in case of the second reaction the possibility of backward reaction is high due to highly dissociation of the end product HCl. An example of (ii) is the formation insoluble Al (OH)3. Al-X + 3Na+ + 3OHˉ 3Na-X + Al (OH)3(insoluble or slight soluble).

10

This reaction proceeds more completely than one leading to the formation of soluble Al (Cl)3. Al-X + 3Na+ + 3Clˉ 3Na-X + AlCl3 (Al³+ + 3Clˉ) st [ Here, the end product of 1 reaction is insoluble or sparingly soluble. That’s why the chance of reverse reaction is less. On the other hand, AlCl3 is highly soluble. Therefore, for the dissociation of AlCl3, the second reaction has more chance of reserve reaction.] The high concentration of aluminum in solution drives the reaction back toward the left. An example of (iii) is the formation of volatile CO2 H-X-H + CaCO3 Ca-X +H2CO3 Ca-X+H2O+CO2 This reaction proceeds more completely than one leading to the formation of nonvolatile H2SO4. H-X-H + CaSO4 Ca-X + H2SO4 The lose of gaseous CO2 from the 1st reaction removes a reaction product and drives the reaction toward the right. Anions, such as SO4²ˉ, CO3²ˉ and HCO3ˉ can also lower cation adsorption in an exchange reaction by forming complex ions or ion pairs such as CaSO4, MgCO3 or CaHCO3+. They thereby lower the activities of effective concentrations of the respective free cations. Colloidal-specific effects: Soil colloids of high charge density that is high charge or CEC per unit of surface area, i.e. high surface potential, generally have the greatest preference for highly exchanged cations. For example, vermiculite normally retains more calcium than does montmorillonite from a mixed Na+ - Ca²+ solution. Hence, montmorillonite has higher exchangeable sodium percentage (ESP) then vermiculite when the bulk solution Na+ and Ca²+ concentrations are the same. The monovalent cations NH4+ and K+ are exceptions to this generalization because of their unusually strong preference by mica and vermiculite. Partially covalent bonding and/or complex formation may contribute to a similar preference of soils high in organic matter for many polyvalent cations. [In organic matter present, it binds polyvalent cations through covalent bonding and then to replace these. Other strong polyvalents are required. In case of organic colloids, metal-organo-complexes (covalent bonds) are formed. Thus, the reversibility of ion exchange reaction becomes less.

11

Raising soil pH can also change cation selectivity by increasing CEC and thus increasing the preference for polyvalent versus monovalent ions. Valence Dilution: For exchange between cations of unequal valence, dilution of the equilibrium solution favors retention of the more highly charged cation. The dependence of cation exchange on cation valence is called the valence dilution effect. Cation with greater valency has high releasing capacity. This can be shown by the experimental data of the following table. Here, the percentage of calcium replaced by barium remained virtually constant, but that replaced by ammonium decreased with decreasing ammonium concentrations in accordance with the valence dilution effect. Solution added Percent Ca++ replaced by ml N Ba²+ NH4+ 25 0.04 49.7 29.8 100 .01 50.2 20.8 200 .005 50.8 16.6 400 0.0025 52.7 15.2 [More the NH4+ is diluted its replacing capacity will be decreasing. But in case of Ba++ inspite of the dilution replacing capacity will be same.] • Ion exchange reaction in the light of the law of mass action: Ion exchange reaction is a reversible process by which cations & anions are exchanged between solid and liquid phases & between solid phases if in close contact with each other. The law of mass action is- “the rate of chemical reaction at a given temperature is proportional to the active mass of each of the reactants present in the system. The Ion exchange reaction can be described in the light of the law of mass action. For this, the following ion exchange reaction between monovalent ion attached to a colloid & another monovalent ion in soil solution, can be considered: X-Na + K+ X-K + Na+. According to the law, the rate of forward reaction of any instant is proportional to the concentration of the reactants. So, Rate of forward reaction, V1 α [X-Na] [K+] Or, V1 = K1 [X-Na] [K+] Where, K1= velocity co-efficient (constant).

12

Again, the rate of backward reverse reaction, V2α [X-K][Na+] V2 = K2[X-K] [Na+] Where, K2= velocity co-efficient (another constant). At equilibrium, the rate of forward reaction is equal to the rate of reverse reaction, so, V1=V2 K1 [X-Na][K+] =K2[X-K] [Na+] [X-K] [Na+] = K1 = K [X-Na] [K+] K2 Here, K is the equilibrium constant, for the above reaction at a given temperature, which is equal to the ratio of the velocity constant for the forward reaction & backward reaction. • Why the CEC of agricultural soils are greater than AEC? The CEC of a soil is the capacity of clays to adsorb and exchange cations. The AEC is the capacity of clay to adsorb & exchange anions. Cation exchange capacity of a soil is generally more than anion exchange capacity. It is dependent on the charges in electrolyte levels & on soil pH. It is also limited to special types of clays. The clay must be positively charged to be able to adsorb negatively charged ions (anions). But, positive charges on clays occur clay in acid conditions; (i.e. under moderate to extreme acid soil conditions, some silicate clays & iron and aluminum hydrous oxides may exhibit positive net charges; > Al-OH + H+ > Al-OH2+) But, in agricultural soils, pH ranges between 5.0-8.5. There is no constituent at this pH range which can develop positive charge. Only minute quantities of charges develop in certain amino acids. At this pH range, negative charges are dominant in soil colloidal surfaces, which are originated from isomorphous unequivalent ionic substitution and pH related hydroxy groups on the edges & surfaces of the inorganic & organic colloids. Since, normally extreme acid conditions does not prevail in soil, CEC of a soil is generally more than AEC. • Why does CEC vary from soil to soil? Cation exchange capacity is the sum total of the exchangeable cations that a soil (or other substance) can adsorb.

13

The CEC vary from soil to soil because of the several reasons which are as follows: 1. Clay content: Soils that contain high amount of clay minerals have more CEC than soils that contain less clay mineral. 2. Type of clay: CEC of soils also vary with the type of clay they contain. A soil dominated by 1:1- type silicate clay & Fe-Al-oxides with have lower CEC than the soil that contain 2:1- type clays, because expanding type minerals show higher CEC than non-expanding type clay. 3. Organic matter content: The higher is the organic matter content in soil, the higher is the CEC of that soil. CEC also depends on the stage of decomposition of organic matter. A soil containing completely decomposed organic matter, has higher CEC than a soil containing partially decomposed or undecomposed organic matter. 4. Soil pH: At high pH values, negative charges develop and hence the CEC increases. On the other hand, at low pH values, positive charges develop & the CEC is lower. 5. Temperature: The temperature has been found to increase slightly the rate of the rapid ion exchange for the slower exchange. For this reason, in a temperate zone, the soil has high CEC because temperature governs the amount of CEC. 6. Soil Texture: In general, the more the clay in a soil, the more is the CEC of that soil. Finer texture soils tend to have higher CEC than sandy or coarser textured soils. • Factors controlling the CEC of soils: The factors that influence/ control the CEC of soil are as follows: 1. Clay minerals ( amount & type of clay) 2. Soil organic matter (nature & amount of organic matter) 3. Soil pH 4. Size of particles 5. Temperature 6. Soil Texture Clay minerals:

14

CEC of the soil varies with the amount & type of clay minerals present in soil. Amount of clay: Soils that contain high amount of clay minerals have more CEC than soils that contain less clay minerals. For example a soil that has 20% kaolinite has higher CEC than a soil having 10% kaolinite. Type of clay: CEC of soils also vary with the type of clay it contains. A clay soil dominated by 1:1 type silicate clays & Fe-Al-oxides will have lower CEC than the soil containing 2:1 type clays, because expanding type minerals show higher CEC than non expanding type clay. For example, a soil containing 20% montmorillonite has more CEC (80-150 meq/100 g soil) than the soil that contains 20% kaolinite (CEC 3-15 meq/100 g soil), though the amount of clay minerals in the two soils are same. Soil organic matter: CEC depends on the amount & nature of organic matter in soil & on the presence of functional groups in the organic matter. Amount of organic matter: CEC depends on the amount of organic matter present in soil. The higher the organic matter content in soil, the higher is the CEC. For example, soil containing 5% organic matter has higher CEC than a soil containing 2% organic matter. There was a fairly definite increase in the exchange capacity of sandy Florida soils of 2 meq for each 1% increase in organic matter. Nature of organic matter: CEC also depends on the stage of decomposition of organic matter. A soil containing completely decomposed organic matter has higher CEC than a soil containing partially decomposed or undecomposed organic matter. The product of completely decomposed organic matter is humus & its CEC is higher (150-300 meq/100 g soil). The CEC of partially decomposed organic matter is near zero. As a result, CEC of a soil varies with the different stage of decomposition. Presence of functional groups on humus: The reason for the higher CEC of humus is the functional groups on humus particles. These functional groups are –OH, -NH2, -COOH, etc. when the pH of soil solution increases, these functional groups dissociate & form negative charges.

15

OH + OHˉ

NH2 +OHˉ

Oˉ + H2O

NHˉ +H2O

Soil pH: pH also influences the CEC of soils, at very low pH values, the CEC is generally low. Under low pH conditions, only the permanent charges of the 2:1 type clay & a small portion of the pH-dependent charges of organic colloids, allophane, and some 1:1 clays hold exchangeable ions. At high pH, the negative charges on some 1:1 type silicate clays, humus, allophane and even Fe, Al oxides increases; thereby increasing the CEC. At natural or slightly alkaline pH, the CEC reflects most of the pH-dependent charges as well as permanent charge. To obtain a measure of the maximum retentive capacity, the CEC is usually determined at a pH of 7.0 or above. Size of particles: The finer the size of the particles the higher is the specific surface area, and so the higher is the CEC of the soil, and vice-versa. CEC is a surface phenomenon. As a result, CEC increases with the increase in surface area. Clay is a colloidal particle, so its CEC is higher than coarser soil particle. Soil texture: The more clay in a soil, the more CEC is in that soil. Finetextured soils tend to have higher CEC than sandy or coarser textured soils. Different CEC of different textured soils: Sandy soil 0-5 meq/100g soil Fine sandy loam 5-10 meq/100g soil Clay loam 15-20 meq/100g soil Clay 30 meq/100g soil

16

It can be assumed that kaolinite has a cationic exchange capacity of approximately 8 milliequivalents; illite, 30 meqs; montmorillonite, 100 meqs, and humus (colloidal organic matter), 200 meqs per 100 grams of material, on an oven-dry basis. Temperature: The temperature has been found to increase slightly the rate of the rapid ion exchange for the slower exchange. So, ion exchange reaction is accelerated as temperature is increased. • Importance of cation exchange reaction: The properties of ion exchange & the exchange reactions are great fundamental & practical importance in soil science & other fields where clay minerals are studied & used. ion exchange reactions are the most important phenomena in the whole domain of agriculture. In fact, several workers have shown that the capacity of soil to exchange ion is the best single index of soil fertility. The important roles of ion exchange reaction on soil fertility are as follows: 1. Water holding capacity: Soils with high CEC hold more water & better crop growth. On the contrary, soils with low CEC hold small amount of water which is inadequate for better growth of crops. 2. Nutrient availability: Plant nutrient elements are held on soil colloids as exchangeable ions and thus leaching loss of the nutrients is prevented. The mineral ions Ca++, Mg++, K+, Na+, H2PO4ˉ etc are released to the soil solution or adsorbed on the colloidal fraction through the process of ion exchange. Their adsorption by plants and microorganisms are also occurred through ion exchange reaction. 3. Physical property of soil: Soil physical properties can be improved by ion exchange reaction. A soil with low CEC will always have poor structure because it will not hold Ca++ & Mg++ as exchangeable cations. A Ca-saturated soil is granular in structure & porous. Ca-dominated clay ensures good aeration & good drainage. 4. Soil Reaction: CEC influences the soil reaction. Soil reaction means the degree of acidity & alkalinity of a soil usually expressed in pH value. Soils with Ca & Mg have high CEC which increases the soil pH. On the other hand, hydrogen and aluminum containing soils have low pH (4-5). It is well

17

known that pH affects the availability of several nutrients. So ion exchange indirectly influences the availability of plant nutrients. 5. Microbial activity: The predominance of desirable cations such as Ca++ in the exchange complex favorably influences the microbial activity, ammonification & nitrification. 6. Irrigation: CEC of a soil helps to measure irrigation water efficiency of a field. If the CEC of a soil is low, irrigation is needed for several times with small amount of water, because water holding capacity of this soil is low & vice-versa. 7. Efficient use of fertilizer: A soil which has low CEC will hold small amount of fertilizer nutrients than a soil with high CEC, because nutrient ions of the fertilizer are adsorbed by soil colloid by the process of ion exchange. So, the soluble inorganic fertilizers are not washed away from the soil when added to the soil. Thus, the CEC of soil increases the efficient use of fertilizer. 8. Reclaimation of acid soil: Strongly acidic soils are not produce for most crops. When lime is added to neutralize the acidic soils, most of exchangeable H+ & Al³+ ions are neutralized after the pH. H Micelle Micelle + Ca (HCO3) Ca+ 2CO2 +2H2O H This is an ion exchange reaction. So, ion exchange can bring a soil in favorable condition. 9. Reclaimation of Alkali soil: Due to alkaline nature or high pH (>8.5), Na+ ions are adsorbed by soil colloids. If H2SO4 or gypsum (CaSO4 .2H2O) is added, Na+ ions are released into solution which are subjected to drainage. Micelle

Na + H2SO4 Na

Micelle

H + Na2SO4 H

The acidification of soil is accomplished by ion exchange reaction.

18

• Importance of cation exchange phenomenon in agriculture: Cation exchange is an important reaction in soil fertility in causing & correcting soil acidity and alkalinity, in changes of soil physical properties. Cation exchange is very important in soils because of the following relationships: 1. The exchangeable k is a major source of plant K. 2. The exchangeable Mg is often a major source of plant Mg. 3. The amount of lime required to raise the pH of an acidic soil is greater as the CEC is greater. 4. Cation exchange sites hold Ca++, Mg++, K+, Na+ and NH4 ions and show their losses by leaching. 5. Cation exchange sites hold fertilizer K+ and NH4+ and thereby reducing their mobility in soils. 6. Cation exchange sites adsorb many metals, such as Cd++, Zn++, Ni²+, Pb²+, etc, that might be present in waste waters. Adsorption removes them from the percolating water, thereby cleansing the water that drains into groundwater from surface water. • Exchangeable sodium percentage (ESP): The extent to which the adsorption complex of a soil is occupied by sodium. It is expressed as follows: ESP= exchangeable sodium (c mol/kg soil) × 100 Cation exchange capacity (c mol/kg soil) • Base saturation percentage (PBS): The extent to which the adsorption complex of a soil is saturated with exchangeable cations other than hydrogen & aluminium. It is expressed as a percentage of the total cation exchange capacity. The percentage of the CEC that is satisfied by the base forming cations is termed as percentage of base saturation. PBS= exchangeable base forming cations (c mol/kg soil) × 100 CEC (c mol/kg soil) The proportion in any soil of the CEC satisfied by a given cation is termed the percentage saturation for that cation. Thus, if 50% of the CEC were satisfied by Ca++ ions, the exchange complex is said to have a percentage calcium saturation of 50. • Sodium adsorption ratio (SAR):

19

The SAR gives information on the comparative concentrations of Na+, Ca²+ and Mg²+ in soil solutions. It is calculated as follows: SAR= [Na+] . √1/2[Ca²+] + ½[Mg²+] Where, [Na+], [Ca++] & [Mg++] are the concentration (in c mol/kg) of the sodium, calcium & magnesium ions in that soil solution. SAR of a soil extract takes into consideration that adverse effect of sodium is moderated by the presence of calcium and magnesium ions. The SAR is also used to characterize the irrigation water added to these soils. This property is becoming more widely used in characterizing saltaffected soils. High SAR means large amount of Na+ and low SAR indicates larger CA++ & Mg++ content. Soil SAR Normal < 13 to 15 Acid < 13 to 15 Saline < 13 to 15 Saline-sodic < 13 to 15 Sodic < 13 to 15 • CEC of montmorillonite >CEC of kaolinite: The CEC of montmorillonite is greater than that of kaolinite. The reasons are discussed below: # 01. In case of montmorillonite, the negative charges arise mainly from the isomorphous substitution. Isomorphous substitution of Mg²+ for some of the Al³+ ions in the dioctahedral sheet accounts for most of the negative charges for montmorillonite although some substitution of Al³+ for Si4+ have occurred in the tetrahedral sheet. These substitutions account for the high CEC of this clay mineral. On the other hand, there is little or no isomorphous substitution in kaolinite. Owing to the presence of exposed hydroxyl groups, kaolinite has a variable of pH dependent –ve charge, due to dissociation of H+ ion from the exposed OHˉ groups. The CEC of Kaolinite is, therefore, very small & may change with pH. # 02. Montmorillonite is an expanding mineral with the specific surface or total surface area (external & internal) per unit mass of 700-800 m²/g, which accounts for its high capacity to adsorb cations as well as high CEC. The CEC of montmorillonite is 80-150 meq/100 g soil. On the other hand, kaolinite is a non-expanding mineral. The effective surface area of kaolinite is restricted to its outer face or to its external surface area. The total surface area per unit mass of kaolinite is only

20

05-20 m²/g. This accounts for its low capacity to adsorb cations as well as low CEC. The CEC of kaolinite is 03-15 meq/100 g soil. For the above reasons, the CEC of montmorillonite > that of kaolinite. • CEC of montmorillonite> CEC of illite : The CEC of montmorillonite is greater/higher than that of illite, because montmorillonite is a 2:1 type expanding clay mineral. The total surface area per unit mass of montmorillonite is 700-800 m²/g, which accounts for its high capacity to adsorb cations as well as its CEC. Again, isomorphous substitution takes place in montmorillonite; viz. Al³+ for Si4+, Mg²+, Fe++, Ni²+, Li+ for Al³+. Isomorphous substitution of Mg²+ for some of the Al³+ ions in the dioctahedral sheet accounts for most of the negative charges for montmorillonites, although some substitution of Al³+ for Si4+ has occurred in tetrahedral sheets. Thus isomorphous substitution causes high CEC of this clay mineral. The CEC of montmorillonite is 80-150 meq/100g soil. On the other hand, through the illite is a 2:1 type mineral, it is non-expanding mineral. Therefore, its surface area is as low as only 70100 m²/g, which accounts for its low capacity to adsorb cation as well as low CEC. Again, isomorphous substitution takes place in illite. The major source of negative charges of illite is in the tetrahedral sheet where about 20% of the Si-sites are occupied by Al-atoms. This results in a high net negative charge in the tetrahedral sheet. But, to satisfy this charge, K+ ions are strongly attracted in the interlayer space. Thus, K+ ions act as binding agents preventing expansion of the mineral. For this reason, cation adsorption is much less intense in illite (fine-grained mica) than in montmorillonite. The CEC of illite is 10-40 meq/100 g soil. For the above reasons, CEC of montmorillonite> CEC of illite. Kaolinite Non-expanding mineral Montmorillonite fully expanding mineral Vermiculite limited expanding mineral Illite Non-expanding mineral [Origin of positive & negative charges on soil clays/ permanent & pH dependent charges------------ from 2nd year note]. • Origin of negative clays in humus: Negative charges are originated in humus due to dissociation of the following groups:

21

1. Dissociation of carboxyl (-COOH) groups occurring in the polyuronides, aliphatic acids & in the side chain of lignin (at low pH of 4, 5, 6). Humus

Humus

COOH COOˉ+ H+ 2. Dissociation of phenolic hydroxyl groups (OHˉ) in lignin, amino acids & humus (at high pH) Humus

Humus

OH

Oˉ+H+

3. Dissociation of heterocyclic nitrogen/amino group (-NH2) group present in nucleic acid at independent pH values. Humus

Humus

NH2

NHˉ+H+

Charges due to broken edges: In clay minerals, both cation & anion charges are equal in neutral condition. When the clay minerals are broken down physically, unequal charges (positive or negative) are developed/ produced. If the cation charges are more than that of anion, the clay minerals (gibbsite, goethite) show the positive charge & vice-versa. Anion exchange: The anion & molecular retention capacity of most agricultural soils is much smaller than the cation retention capacity. Numerous anionic and non-ionic species are important to today’s agriculture, and their reactions in soils can not be ignored. Common soil anions include Clˉ, HCO3, CO3²ˉNo3ˉ,SO4²ˉ, HPO4²ˉ, H2PO4ˉ, OHˉ and Fˉ. In addition some micronutrients exist as anions, such as H2BO3ˉ and MoO4²ˉ, as do some heavy metals, e.g. CrO4²ˉ and HAsO4²ˉ and also as molecular form such as some pesticides in undissociated form ( DDT; 2, 4, 5-T and 2, 4-D). In humus there is NH2ˉ group. Many anions and all molecular species are related by more complex mechanisms than the single electrostatic attractions involved in most cation adsorption reactions. Anion may be retained by soils through a number of reactions. Some of these are purely electrostatic and are referred to as nonspecific. A variety of non-electrostatic reactions are also possible. These are collectively known as specific adsorption, or chemi-sorption

22

reactions. That is, there are two steps of mechanisms for anion retention in soils: 1. Nonspecific Anion Reactions; and 2. Specific Anion Reactions. Nonspecific Anion Reactions: In this case, adsorption (or retention) is not specific for any anion. An anion, which is more powerful, takes the place first. An anion (approaching a charged surface/ soil solid) is subjected to i. attraction by positively charged sites on the surface, or ii. Repulsion by negative charges. Anion repulsion (negative adsorption): Layer silicates in the clay fraction of soils are normally negatively charged; so, anions tend to repel from the mineral surfaces. Factors affecting anion repulsion include: a. Anion charge & concentration; b. Species of exchangeable cation; c. pH; d. Presence of other anions; and e. Nature & charge of the colloidal surface. Ions commonly exhibiting anion repulsion include Clˉ, NO3ˉ and SO4²ˉ. Anion repulsion expressed in units of milliequivalents repelled per square centimeter of solid surface (me/cm²), increases with anion charge (valence). If the negative charge on a soil colloidal surface remains constant, anions of higher charge are repelled more than anions of lower charge. For example, Mattson found anion repulsion in a Na- montmorillonite suspension to increase in the order: Cl~ NO3ˉ< SO4²ˉ
23

Permanent of the colloids with highly charged and tightly adsorbed anions, such as phosphate, can effectively mask such positive charges. These adsorbed anions present a negative face to anions added subsequently. Anion repulsion is then greater than in the absence of the tightly adsorbed anions. Colloid type, or more correctly, the negative charge on various solids of the soil matrix, has an obvious effect on anion repulsion. The greater the negative charge of the soil solids, the greater the anion repulsion. Montmorillonite soils thus exhibit greater anion repulsion than do kaolinite soils at all pH values. This is especially true at low pH values where kaolinite may develop positive charge. Importance: Anion repulsion has important consequences during solute transport through soils. Since anions are excluded from some of the volume surrounding soil particles, these ions can travel through a soil faster than the water in which the salt was originally dissolved. Electrostatic attraction of anions: Anions approaching possibility charged sites on layer silicate or hydrous oxide minerals are attracted electro statically in the same manner as cations are attracted to negatively charged soil colloids. Anions whose retention in soils can be described by electrostatic considerations alone are said to be non- specifically adsorbed. OHˉ½

OHˉ½

(a)

+ H+ +Clˉ = OHˉ½

OH+½……………Cl

OHˉ½

OH +NO3ˉ

OH+½….Cl

(b)

= OH+½…………..NO3 + Clˉ

Fig: Non-specific anion reactions at a solid/solution interface: a. adsorption

24

b. anion exchange (after Hingston et al; 1967). The figure (a) diagrammatically shows the non-specific adsorption of Clˉ. The dotted line represents electrostatic attraction of a positively charged mineral surface site for the anion. Here, the positive charge which retains Clˉon the mineral surface is the result of protonation of the surface. The figure (b) represents the exchange of one non-specifically adsorbed anion (NO3ˉ) for another Clˉ. The anions Clˉ, NO3ˉ and SO4²ˉare generally considered to be non-specifically adsorbed. The capacity of soils to adsorb anions increases with increasing acidity and is much greater in the kaolinite soil, which has significant pH-dependent charge. At all pH values, the divalent SO4²ˉion is adsorbed to a greater extent than the monovalent Clˉion. The attraction of anion to positively charged colloids depends on the charge strength of the anion: SiO44ˉ>PO4³ˉ>>SO4²ˉ>NO3ˉ≈Clˉ Importance: Chloride, nitrate and sulfate are common and important anions in moist soils and have been studied extensively. Chloride, in particular, is often used as an indicator of NO3ˉmobility in soils, since Clˉ is not subject to the complicating biological reactions characteristic of NO3ˉ. In most other respects, Clˉbehaves in a manner similar to NO3ˉ. Specific anion reactions: A hydrous oxide system is amphoretic, having a negative or positive charge and therefore, either cation or anion exchange capacities, depending upon the pH. These solids (oxides) possess the ability to specifically interact with various anions. This characteristic gives Fe or Al oxide-dominated soils an adsorption capacity for some anions. Infact, ion oxides and other oxides can scavenge arsenate, phosphate, molybdate and similar anions from solutions with high efficiency. OH2+0.5

+1

Fˉ0.5

0

…Clˉ+NaF OH2+0.5

+ NaCl +H2O OH2+0.5 PO4Hˉ1.5

OHˉ0.5

ˉ1 ….Na+ + NaH2PO4²ˉ

OHˉ0.5

(a) ˉ² ..2Na++ H2O

OHˉ0.5

25

Figure: Specific anion reactions at a solid/solution surface: (a) neutralization of positive charge, and (b) ionization of a proton of an adsorbed acid anion (after Hingston et al, 1968). Phosphorus is an essential plant nutrient and is probably the most important example of specifically adsorbed anions. Many soils fix large quantities of phosphorus by converting readily soluble phosphorus to forms less available plants. In terms of ligand exchange or anion penetration theory, phosphorus adsorption on oxide surface can be explained by the following figure: Fe Fe OH O

OH

O

O

P

O OH

Fe

+ 2H2PO4ˉ

Fe

+2OH OH

O

OH

O

O

P

O OH

Fe

Fe

Figure: Representation of H2PO4ˉpenetration into and Fe oxide surface. The figure illustrates that1. Anion exchange: One anion (H2PO4ˉ) is replacing the other one (OHˉ) 2. Ligand exchange: A group (OHˉ) is replaced by another one (H2PO4ˉ). 3. Mechanism of phosphate fixation. 4. Source of OHˉion in soil solution. Molecular retention: A species need not be initially charged to be retained by soils. Molecules in the soil solution may be concentrated to charged species and then adsorbed as cations or anions, or may remain non-ionic and be adsorbed as a consequence of polarity that produces localized charge within the molecule. Certain non-ionic species protonate in acidic solution and become cations subject to adsorption on negatively charged soil solids. B+ H+ BH+ BH+ + M soil BH soil + M+ Where, B is a weakly basic molecule, BH+ is a protonated weakly basic molecule and M+ is a cation.

26

Bailey and white suggested several possibilities for bonding of non-ionic polar molecules to the oxygens of layer silicate surfaces. These include molecular characteristics such as: 1. Chemical character, shape and configuration 2. acidity or basicity 3. water solubility 4. charge distribution 5. polarity 6. size; and 7. polarizability all influence the adsorption of nonionic compounds by soil R-N-H………..O R-C-H…………O Layer silicate O R-C-OH………O O R-C-O………..O R