Soil Colloids and Cation Exchange Capacity

1. Soil Colloids and Cation Exchange Capacity

2. Soil Colloids • Particles less than 1 or 2 m behave as soil colloids • Total surface area ranges from 10-800 m2·g-1 !!! • Internal and external surfaces have electronegative or • electropositive charges (electronegative charge dominant) • Each micelle adsorbs thousands of hydrated Al3+, Ca2+, H+, • K+, Mg2+ and Na+ ions (enclosed within several H2O molecules) • Cation exchange occurs when ions break away into the soil • solution and are replaced by other ions • Ionic double layer: negatively charged micelle surrounded by • a swarm of cations

4. Crystalline Silicate Clays • Dominant colloid in most soils (not andisols, • oxisols or organic soils) • Crystals layered as in a book • 2-4 sheets of tightly-bonded O, Si and Al atoms • in each layer • Eg. kaolinite, montmorillonite

7. Noncrystalline Silicate Clays • Not organized into crystalline sheets • Both + and – charges; can adsorb anions • such as phosphate • High water-holding capacity • Malleable when wet, but not sticky • Often form in volcanic soils (especially in Andisols) • Eg. allophane and imogolite

8. Iron and aluminium oxides • Found in highly weathered soils • of warm, humid regions (eg. oxisols) • Consist of Fe and Al atoms connected to • oxygen atoms or hydroxyl groups • Some form crystalline sheets (eg. gibbsite and • geothite), but often amorphous • Low plasticity and stickiness

9. Humus • Present in nearly all soils, especially A horizon • Not mineral or crystalline • Consist of chains of C atoms, bonded to H, O & N • Very high water adsorption capacity • Not plastic or sticky • Negatively charged

11. (Singer and Munns, 2002)

12. Phyllosilicates • Tetrahedron: • Two planes of O, • with Si in between • Basic building block • is silicon atom, • connected to 4 O • atoms • Octahedron: • Two planes of O, • with Al or Mg in • between • Basic building block • is Al (or Mg), • connected to six • hydroxyl groups or • O atoms • There are many • layers in each micelle

13. Trioctahedral Sheet Dioctahedral Sheet Isomorphous substitution 1 Al3+ atom, 1 Mg2+ atom Charge = -1 3 Mg2+ atoms Charge = 0 2 Al3+ atoms Charge = 0

14. Isomorphous substitution • Each Mg2+ ion that substitutes for Al3+ causes a negative charge in a dioctahedral sheet • Each Al3+ ion that substitutes for Si4+ causes a • negative charge in a tetrahedral sheet

16. 1:1 Silicate Clay • Each layer contains one tetrahedral and one • octahedral sheet • Eg. Kaolinite, halloysite, nacrite and dickite • Sheets are held together because the apical oxygen • in each tetrahedron also forms the bottom corner of • one or more octahedra in the adjoining sheet

17. Hydroxyl plane is exposed: removal or addition of • hydrogen ions can produce positive or negative • charges (hydroxylated surface also binds with anions) • Hydroxyls of octahedral sheet are alongside • Oxygens of the tetrahedral sheet: hydrogen bonding • results, with no swelling in kaolinites! • Kaolinite useful for roadbeds, building foundations • and ceramics (hardens irreversibly)

18. 2:1 Silicate Clay Each layer contains one octahedral sheet sandwiched between two tetrahedral sheets O on both ends No attraction without cations

19. Expanding 2:1 Silicate Clays • Smectite group: Interlayer expansion may occur as • H2O fills spaces between layers in dry clay • Montmorillonite is a very common smectite • Smectites have a large amount of negative charge • due to isomorphous substitution • Mg2+ often replaces Al3+ in the • octahedral sheet • Al3+ sometimes replaces Si4+ in the • tetrahedral sheet • Weak O:O or O:cation • linkages between layers • leads to plasticity, stickiness, • swelling and a very high • specific surface area

20. (Singer and Munns, 2002)

21. Vermiculite Group (2:1 Expanding Silicate Clay) • Very high negative charge, due to frequent • substitution of of Si4+ ions with Al3+ in the tetrahedral • Sheets • Cation exchange capacity is higher in vermiculites • than in any other clay • Swelling occurs, but less than in smectites due to • strongly adsorbed H2O molecules, Al-hydroxy ions • and cations, which act more as bridges than wedges.

22. Non-Expanding 2:1 Silicate Minerals • Mica Group (illite and glauconite) • Al3+ substituded for 20% of Si4+ in tetrahedral sheets • K+ fits tightly into hexagonal holes between tetrahedral • oxygen groups: virtually eliminates swelling

23. Chlorites are also non-expansive: Mg-dominated hydroxide sheet fits between adjacent 2:1 layers (2:1:1). H-bonded to O atoms between sheets Fe or Mg occupy most octahedral sites

25. Iron and Aluminium Oxides • Modified octahedral sheets with either Fe2+ or Al3+ in • the cation positions • No tetrahedral sheets and no silicon • Lack of isomorphous substitution (little negative charge) • Small charge (+ or -) due to removal or addition of • hydrogen ions from surface hydroxyl groups • Non-expansive and relatively little stickiness, plasticity • and cation absorption

27. Variable Charge (pH-dependent) • Hydrous oxides whether crystalline or amorphous get their charge from surface protonation and deprotonation • >AlO- + H+ >AlOH + H+  AlOH2+ Negative Neutral Positive pH decreasing  • Layer aluminosilicates have a small amount of variable charge because of OH at the edges • All the negative charge on humus is variable • Hydrous oxides are positively charged in some very acid soils and help retain anions

28. Negative charge: • Dissociation of H+ ions, • lack of Al & Si at edge • to associate with O atom • Less Negative to Positive Charge: • As pH increases, more H+ ions bond to • O atoms at the clay surface • Protonation at very low pH (H+ ions attach • to surface OH groups)

30. Less effective cation exchange More effective cation exchange

31. Cation exchange capacity • is highest in soils with: • High humus content • High swelling capacity • High pH

32. Humus • A non-crystalline, organic substance • Very large, organic molecules • 50% C, 40% O, 5% H, 3% N and sometimes S • Structure highly variable • Very large negative charge due to three types • of -OH groups (H+ ions gained or lost) • (i) carboxyl group COOH • (ii) phenolic hydroxyl group(due to • partial decomposition of lignin by • microorganisms) • (iii) alcoholic hydroxyl group

33. State of organic residues one year after incorporation into a soil

34. Humic Substances • Microbes break down complex components • Simpler compounds created; CO2 is released • Synthesize new biomolecules, using C not respired, • as well as N, S & O • Lignin not completely broken down: complex • residual molecules often retain lignin characteristics • Microbes polymerize new, simpler molecules with • one another and with residual molecules • This creates long, complex chains, resistant to • further decomposition • Chains interact with amino compounds • Polymerization process is stimulated by colloidal • clays

37. After one year: • 1/5 to 1/3 of carbon remains in soil • (i) live biomass (5%) • (ii) humic fraction (20%) • (iii) nonhumic fraction (5%) • Humic substances include: • (i) Fulvic acids: lowest molecular weight and • lightest colour (most susceptible to microbes) • (ii) Humic acid (intermediate) • (iii) Humin: highest molecular weight, darkest, • least soluble and most resistant to microbes

38. Humus: Amorphous and colloidal mixture of complex organic substances no longer identifiable as tissues Note: non-humic substances are biomolecules produced by microbes

39. Soil Acidity: Causes and Impacts

43. Soil Acidification • Carbonic acid • Carbon dioxide gas from soil air dissolves in water • Root respiration and soil decomposition provide extra CO2 • CO2 + H2O  H2CO3  HCO3- + H+ • Acids from Biological Metabolism • Microbes break down organic matter, producing organic • acids such as citric acid, carboxylic acids and phenolic acids • RCH2OH… + O2 + H2O  RCOOH  RCOO- + H+ • Accumulation of Organic Matter • (i) Loss of cations by leaching due to soluble humic • complexes combining with non-acid nutrient cations (eg. Ca2+) • (ii) Organic matter is a source of H+ ions

44. Oxidation of Nitrogen (Nitrification) • Nitrogen enters soils as NH4+ • Converted to nitric acid • NH4+ + 2O2 H2O + H+ + H+ + NO3- • Oxidation of Sulphur • Acids in Precipitation • H2SO4 SO42- + 2H+ • HNO3  NO3- + H+ • Plant Uptake of Cations • Plants exude H+ ions or take up anions (eg. SO42-) to • balance off cation uptake

45. Aluminium Toxicity H+ ions adsorbed onto clay surfaces may attack the mineral structure and release Al3+ ions in the process Aluminium is highly toxic to most plants Al promotes hydrolysis of H2O (see Fig. 9.12) Al combines with OH-, leaving H+ ions in the soil solution Tolerant plants secrete organic acids into the soil around the root. Organic acids such as (eg. malate or citrate) are able to chelate the Al that is in the soil solution near the root tip. Al that is bound to organic acids cannot enter the plant root.

47. Acids are neutralized in soils with available bases • Canadian Shield severely affected in central and eastern • Canada

48. H+ + HSO3- sun  H2O H2SO4 2H+ + SO42- SO3 SO2 O2 2O2 2H2O 4NO2 2HNO3 + 2HNO2 4NO 2N20 + O2 H+ + NO3-

49. Acidity of Precipitation in the United States

50. Acidity of Rainfall in New Hampshire