Soil Colloidal Chemistry - | University Of Al- Colloidal Chemistry 1 Compiled and Edited by ... Soil Colloids The most ... Good physical properties

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  • Soil Colloidal

    Chemistry

    1

    Compiled and Edited by

    Dr. Syed Ismail,

    Marthwada Agril. University

    Parbhani,MS, India

  • The Colloidal Fraction

    Introduction

    What is a colloid?

    Why this is important in understanding

    soils?

    How can we understand this fraction?

    What are the fundamental basics of this

    fraction?

    2

  • Soil Colloids

    The most chemically active fraction of soils.

    They are very small, less than 2 m in

    diameter

    Shape

    Colloids can be either

    mineral (clays) or organic (humus)

    crystalline (definite structure) or amorphous.

    3

  • Colloids

    Properties imparted to soils

    Static Vs. dynamic properties

    Sand and Silt (no colloids)

    Static and occupy space

    Clay and Humus (colloids)

    Dynamic and very active (charges)

    4

  • Colloids

    Properties imparted to soils by colloids

    Chemical Vs Physical properties

    Chemical

    Sources of ions for plant nutrition

    Source of electro-negativity (CEC)

    Buffering capacity

    Chemical cement agents

    Physical

    Large surface area per unit of mass (cm2/g)

    Plasticity

    5

  • Types of Soil Colloids

    Crystalline silicate clays

    Non-crystalline silicate clays

    Iron and aluminum oxide

    Organic material (Humus)

    6

  • Soil Colloids: Silicate Clays

    Kaolinite Montmorillonite

    7

  • Crystalline Silicate Clays

    What is it?

    Shape: x, y and z.

    Surface area: 2 sources ***

    External Vs Internal

    Composition of crystalline structures:

    Silicon-Oxygen and Aluminum- Hydroxide others

    Silicon Tetrahedral and Aluminum Octahedral sheets

    Si-O Tetrahedral sheet (tetra=four void spaces)

    Al-OH Octahedral sheet (0cta=eight void spaces)

    8

  • Charges

    Isomorphous Substitutions

    Process in which one element substitutes

    another of comparable size in the crystalline

    structure

    Al is slightly larger than Si, consequently Si

    may replace Al!

    IF Al+3 and Si+2 then what?

    9

  • 10

  • Charges

    Permanents

    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)

    11

  • Structures (2D)

    Si Tetrahedral Sheet

    Al Octahedral Sheet

    Al-Si combined

    12

  • Example 1:1 type clay

    Sheets Layer

    Interlayer

    Sheets Layer

    13

  • Clay silicate crystals

    1:1 type KAOLINITE

    2:1 type SMECTITE

    VERMICULITE

    MONTMORILLONITE

    14

  • Clay silicate crystal

    2:1:1 type CHLORITE

    15

  • Clay silicate crystals

    1:1 type KAOLINITE

    4 O and 1 Si

    6 OH and 1 Al

    Hydrogen ion

    STRONG BOND!

    NO WATER and

    NO OTHER ION!

    16

  • 1:1 type clays

    Stable and non expanding clay

    Low total charges

    Relative low specific surface area

    pH dependant charges

    Good physical properties

    Limiting holding capacity for nutrients

    Good for roads, buildings, ceramic and bricks.

    Hexagonal shape

    17

  • Clay silicate crystals

    2:1 type Expanding

    4 O and 1 Si

    Montmorillonite 6 OH and 1 Al

    4 O and 1 Si

    O bonding (WEAK)

    Hydrated

    exchangeable

    cations

    Non Hydrated ions

    18

  • 19

    Smectite (includes montmorillonite)

  • 20

    kaolinite illite

    mont-

    morillonite

    humus

    (fulvic acid)

  • 21

    Sheets and Layers

    Al sheet

    Al sheet

    Al sheet

    Al sheet

    Si sheet

    kaolinite

    Si sheet

    Si sheet

    Si sheet

    illite smectite and

    vermiculite

    1:1 non-expanding 2:1 non-expanding 2:1 expanding

    + +

    +

    +

    + +

    +

    +

    +

    +

    + +

    + +

  • 22

  • 2:1 Clay expanding type

    Expanding clays

    Shrinking and swelling constantly

    Poor physical characteristics

    Abundant charges and surface

    Rich in nutrients

    Good soils for crops if managed properly

    Not affected much by pH

    23

  • Clay silicate crystals

    2:1 type (Fine Mica) Non expanding

    Charges: 20% Al octa-

    by Si

    tetrahedral

    Strong bonding forces

    Large Net Charge Attract ions K+ and NH4

    +

    Fits perfectly in hexahedral

    holes 24

  • Hexahedral holes

    25

  • 26

  • 2:1 Clay non expanding type

    Limited expanding

    Good physical properties

    Medium total charges

    Lower specific surface area than expanding 2:1 clays

    Good soils for crops

    Challenging management for K+ and NH4+

    27

  • Clay silicate crystal

    2:1:1 type

    Fe/Mg instead

    of Al octahedral

    Mg dominated sheet

    CHLORITE Hydrogen

    STRONG

    BOND 28

  • 2:1:1 Clay type

    Non Expanding clays

    Very limited shrinking and swelling

    Good physical characteristics

    Limited charges and surface

    Good soils for crops if managed properly

    Not affected much by pH

    29

  • Distance between UNITS of crystalline

    structures

    1.41 nm Vermiculite

    1.00 nm Micas

    0.71 Kaolinite

    30

  • 31

  • Non silicate Clays

    Alone or mixed with silicate clays

    Organic colloids Humus

    Large molecules (+ and charges)

    Iron and Aluminum oxides

    Modified octahedral sheets with substitutions

    No tetrahedral sheets

    Gibbsite (Al(OH)3) Oxisol and Ultisol

    Goethite (FeOOH) yellow brown color

    Hematite Fe2O3 red color

    32

  • 33

    Clay Minerals Comparison

    Kaolinite Illite Vermiculite Smectite

    Tetrahedral 0 20% Al3+ 10% Al3+ 2.5% Al3+

    Octahedral 0 0 15% Mg2+ 15% Mg2+

    Tetrahedral --- 20% Al3+ 10% Al3+ 2.5% Al3+

    CEC me/100g 3-15 (edges) 30 150 80 - 150

    Shrink-swell Low None Mod to High High

    Interlayer H-bonds Fixed K+ Exch. cations Exch. cations

    Origin Recrystallization

    under intense

    acid weathering

    Early

    alteration of

    micas

    Intermediate

    alteration of

    micas

    Recrystallization

    under moderate

    neutral to

    alkaline weath.

  • Thanks

    34

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