origin and kinds of ion exchange in soil chemistry

origin and kinds of ion exchange in soil chemistry

1. Submitted by G.KAVIYARASAN I M.Sc.(Ag.) Soil Science ID.NO.: 2021520008 TNAU, Coimbatore. Submitted to Dr. S. Thiyageshwari Professor Dept. of SS&AC TNAU, Coimbatore Topic : “Origin of ion exchange in soils and kinds of ion exchange” SAC 503 soil chemistry

2. ION EXCHANGE Reversible process by which cations and anions are exchanged between solid and liquid phase. Between Solid phases – close contact with each other. Exchange of cations – base exchange – H + ion Ion Adsorption – increase in concentration or accumulation of an ion species on solid. Desorption – opposite process – replacement or release of an adsorbed ion.

3. Soil is a heterogeneous, polydisperse system of solid, liquid and gaseous components in various proportions. Solid – primary minerals, clay minerals and hydrous oxides together with organic matter and living organisms – polyphase forming more or less discrete particles or aggregates. Soil solution – medium by which chemical reactions takes place even reactants are not in direct contact. Chemical and physical processes connect with ion exchange include W eathering of minerals, N utrient absorption by plants, S welling and Shrinkage of clay and L eaching of electrolytes.

4. Origin of Ion-Exchange properties of soil

5. 1. Major sites of ion exchange.., Clay and silt fractions (<20microns) and Organic matter. ( Colloidal nature)

6. 2. Soil as amphoteric character Soil has the power to bind both cations and anions. Generally soil carry net negative charge, directly demonstrated by electrophoresis. Existence of positive and negative charge on the same mineral( Theissen ) Demonstrated generally on working with the adsorption of negative and positive gold sols on kaolinite . Whereas adsoption of colloidal gold particles on the edges of the kaolinite plates.

7. 3. Source and origin of negative charges Soil clays carries both negative and positive charges which give rise to cation and anion exchange reactions. pH independent(Permanent) Isomorphous substitutions pH dependant (non-permanents ) Broken Edges Al-OH + OH ==H- == Al- O- + H2O (no charge) (- charge) C-OH + OH ==H- == -C- O- + H20 (no charge) (- charge)

8. I. Permanent charge(pH independent) Due to isomorphous substitution, Major source of negative charge in 2:1 type of clay minerals It is defined as replacement of one ion by another ion of similar radii without affecting crystal structure but in lower positive charge. Ionic radii Al( 0.050 nm), Si( 0.041 nm), Fe 2+ ( 0.064 nm), Mg 2+ ( 0.065nm), Zn 2+ ( 0.075 nm) Silicon can be substituted by Al and Mg can be substituted for Al By this replacement of ions results in the development of negative charge This charge is considered as permanent charge since it does not change with soil pH

9. Octahedral sheet neutral Net negative charge

10. Substitution in tetrahedral sheet Si 2 O 4 SiAlO 4 neutral Tetrahedral sheet +4, +3, -8 (-2*4) negative

11. Substitution in octahedral sheet (OH) 2 Al 2 O 2 (OH) 2 AlMgO 2 neutral negative -2, +3, +2, -4 Octahedral sheet

12. 3 Mg 2+ atoms Charge = 0 2 Al 3+ atoms Charge = 0 Trioctahedral Sheet Dioctahedral Sheet Isomorphous substitution 1 Al 3+ atom, 1 Mg 2+ atom Charge = -1

13. II. pH-dependent charge : Exposed crystal edges Especially important in kaolinite , humus, where no internal charge imbalance H + bound tightly, so the lower the pH , the less exchange there is (i.e., lower nutrient availability )

14. Broken bonds of the edges of the crystal or ionization of hydroxyl groups attached to silicon of broken tetrahedron planes give rise to negative charge In high pH soil, H of hydroxyl group dissociates and surface of clay is left negative with oxygen atoms This type of negative charge is called variable charge or pH dependent charge The magnitude of variable charge varies with pH and type of colloid It is important charge for 1:1 type clay, iron and aluminium oxide clays and organic colloid Charge density and potential are higher on edges and corners and furrows and cavities than flat surface.

15. 4. Sources and origin of positive charge Amphoteric nature of clay fraction requires, however, existence of positive charge 1) Isomorphous substitution- It can also be source of positive charge if the substituting cation has a higher charge than the ions for which it substitutes for eg ., In a trioctahedral sheet there are 3 Mg ions surrounded by oxygen and hydroxyl group and has no charge. When Al substitutes for one Mg ion, net positive charge results 2) Under acid conditions, protonation ( H + ) of ions to OH _ occur and it depends on pH and valence of metal ions Usually significant in hydrous oxides of iron, aluminium, manganese. Gibbsite is positively charged at pH 7.0 or below

16. Charges on soil colloids Colloid type Negative charge Positive charge Humus (O.M.) Silicate clays Oxides of Al and Fe 200 cmol c /kg cmol c /kg 100 cmol c /kg cmol c /kg 4 cmol c /kg 5 cmol c /kg

17. 5. Activity of pH in ion exchange Electric charge & surface charge density are not constant – vary with pH Negative charge grows – positive charge decreases with rising pH due to Increasing ionization of acid groups & Deceasing proton addition to basic groups. Viceversa , if pH decreases.., https://www.intechopen.com/chapters/49766

18. Kinds of Ion Exchange Electric charge on the soil particles is neutralized by an equivalent amount of oppositely charged ions, - E xchangable , Counter- or G egen ions . Held to surface mainly by Coulomb forces .., Van der waals – London forces, Increases strength of bonding of certain ions. Most commonly exchangeable ions are Cations followed by Anions. Two types of Ion Exchange.., Cation Exchange & Anion Exchange

19. Cation Exchange/ Base Exchange Exchange of cations between soils and soil solution. Monovalent cations are most strongly held than divalent – Kaolinite Divalent cations are most strongly held than Monovalent - Montmorillonite H +  > Al +3  > Ca  +2  = Mg +2  > K +  = NH 4 + > Na + Soil fertility - Soil acidity and basicity - Soil physical properties - P urifying or altering Percolating Waters. The plant nutrients like calcium, magnesium and potassium are supplied to plants in large measure from exchangeable forms.

20. Anion Exchange Opposite to Cation exchange.., The positive charges of colloids are due to addition of hydrogen (H + ) in hydroxyl group (OH - ) resulted in net positive charge (OH 2 + ). The relative order of anion exchange is : OH -  > H 2 PO 4 - > SO 4 -  > NO 3 -

21. Reference Firman E. Bear, Chemistry of the Soil, United Book Prints pg. no. 164 https:// www.intechopen.com/chapters/49766 http:// ecoursesonline.iasri.res.in/mod/page/view.php?id=1524