sumber : classes.uleth.ca/200801/geog3080a/ soils %20part%20v%202008. ppt

Sumber: classes.uleth.ca/200801/geog3080a/SOILS%20PART%20V%202008.ppt

KOLOID TANAHdan

KTK

Koloid Tanah

1. Partikel yng ukurannya kurang 1 -2 m bersifat sebagai koloid tanah

2. Total luas permukaannya 10-800 m2·g-1 3. Internal and external surfaces have electronegative or

electropositive charges (electronegative charge dominant)

4. Each micelle adsorbs thousands of hydrated Al3+, Ca2+, H+, K+, Mg2+ and Na+ ions (enclosed within several H2O molecules)

5. Cation exchange occurs when ions break away into the soil solution and are replaced by other ions

6. Ionic double layer: negatively charged micelle surrounded by a swarm of cations.


Liat Silikat Kristalin (Berbentuk Kristal)

1. Dominant colloid in most soils (not andisols, oxisols or organic soils)

2. Crystals layered as in a book

3. 2-4 sheets of tightly-bonded O, Si and Al atomsin each layer

Misalnya: Kaolinit, Montmorillonit.


II. Liat Silikat Non-kristalin

1. Tidak terorganisir sebagai lembaran kristalin

2. Both + and – charges; can adsorb anions such as phosphate

3. Kapasitas menyimpan (menahan) air sangat besar

4. Malleable when wet, but not sticky

5. Often form in volcanic soils (especially in Andisols)

Mis. Allophane dan Imogolite.


III. Oksida-oksida Iron dan Aluminium

1. Ditemukan dalam tanah-tanah yang sangat lapuk di daerah iklim hujid dan hangat (mis.. Oxisols)

2. Terdiri atas atom-atom Fe dan Al yang dihubungkan dnegan atom-atom oksigen atau gugusan hidroksil

3. Some form crystalline sheets (eg. gibbsite and geothite), but often amorphous

4. Low plasticity and stickiness


Humus

1. Ada dalam kebanyakan tanah, terutama Horison A

2. Bukan mineral atau Bukan Kristalin3. Consist of chains of C atoms, bonded to H, O

& N4. Very high water adsorption capacity

5. Not plastic or sticky6. Bermuatan listrik negatif.


Mica

HumicAcid

Kaolinite

(kandite)

Montmorillonite

(smectite)


©2002 Prentice Hall, Inc. Pearson EducationUpper Saddle River, New Jersey 07458

Soils: An Introduction, 5th Editionby Michael J. Singer and Donald N. Munns

Figure 2–11 Summary of aluminosilicate clay structures. (A) Building blocks: Oxygen, OH, or H2O—each 0.3 nm diameter—coordinate around smaller atoms of Si and Al, forming the two basic building blocks: the Si–O tetrahedron and the Al–O, OH octahedron. These units are represented in three ways: as polyhedra, as stick-and-ball drawings showing positions of atom centers and bonds, or as space-fill (sphere-packing) drawings indicating volumes filled by oxygen electron shells. (Parentheses—(Al), (Mg, Fe)—indicate possible isomorphoussubstitutions.) (B) Sheet structures: These are formed by Si–O tetrahedra, each sharing three of their oxygens, or by octahedra sharing all six of their OH or O. Sheets combine to form layers.

(Singer and Munns, 2002)Sumber: classes.uleth.ca/200801/geog3080a/SOILS%20PART%20V%202008.ppt

PhyllosilicatesTetrahedra:• Two planes of O,with Si in between • Basic building blockis silicon atom,connected to 4 Oatoms

Oktahedra:• Two planes of O,with Al or Mg in between• Basic building blockis Al (or Mg), connected to six hydroxyl groups or O atoms.There are many layers in each micelle


3 Mg2+ atomsCharge = 0

2 Al3+ atomsCharge = 0

TrioctahedralSheet

DioctahedralSheet

Isomorphoussubstitution

1 Al3+ atom,1 Mg2+ atomCharge = -1


Substitusi Isomorfik

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


1:1 Silicate ClayEach layer contains one tetrahedral and one octahedral sheet

Eg. Kaolinite, halloysite, nacrite and dickite

• Sheets are held together because the apical oxygenin each tetrahedron also forms the bottom corner of one or more octahedra in the adjoining sheet


• 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 foundationsand ceramics (hardens irreversibly)


2:1 Silicate ClayEach layer contains one octahedral sheet sandwiched

between two tetrahedral sheets.

O on both endsNo attraction without cations


Liat Silikat Tipe 2:1 MengembangKelompok Smectite : Ekspansi (sifat mengembang) antar lapisan dapat terjadi kalau H2O mengisi rongga di antara lapisan dalam liat yang kering

1. Montmorillonit : Liat smectite yang sangat banyak ditemukan

2. Smectites mempunyai banyak muatan negatif karena substitusi isomorfik

3. Mg2+ often replaces Al3+ in the octahedral sheet

4. Al3+ sometimes replaces Si4+ in the tetrahedral sheet

5. Weak O:cation linkages between layers leads to plasticity, stickiness, swelling and a very high specific surface area




Figure 2–11 Continued. (C) Layer structures: The two basic types, 1:1 and 2:1, are shown. Each is represented (left to right) as polyhedral, stick-and-ball, and space-fill drawings, each depicting a side view of two unit layers and the interlayer space between them.

(Singer and Munns, 2002)Sumber: classes.uleth.ca/200801/geog3080a/SOILS%20PART%20V%202008.ppt

Kelompok Vermikulit (Liat Silikat Tipe 2:1 mengembang)

Very high negative charge, due to frequent substitution of of Si4+ ions with Al3+ in the tetrahedral

Sheets

Cation exchange capacity is higher in vermiculitesthan 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.


Mineral Silikat Tipe 2:1 Tidak-mengembang

Kelompok Mica (Illit dan Glauconit):

Al3+ substituded for 20% of Si4+ in tetrahedral sheetsK+ fits tightly into hexagonal holes between tetrahedraloxygen groups: virtually eliminates swelling


Khlorite bersifat Non-expansive (Tidak mengembang):Mg-dominated trioctahedral hydroxide sheet fits between 2:1 layers (2:1:1). H-bonded to O atoms between sheets

Fe or Mg occupy most octahedral sites


Oksida Besi dan Oksida Aluminium

1. Modified octahedral sheets with either Fe2+ or Al3+ in the cation positions

2. No tetrahedral sheets and no silicon

3. Lack of isomorphous substitution (little negative charge)

4. Small charge (+ or -) due to removal or addition of hydrogen ions from surface hydroxyl groups

5. Non-expansive and relatively little stickiness, plasticity and cation absorption


Muatan yang Variable (pH-dependent)1. Hydrous oxides whether crystalline or amorphous get their

charge from surface protonation and deprotonation

2. >AlO- + H+ >AlOH + H+ AlOH2+

Negative Neutral Positive pH decreasing

3. Layer aluminosilicates have a small amount of variable charge because of OH at the edges

4. Semua muatan negatif pada humus bersifat variable5. Oksida-oksida Hydrous bermuatan positive pada tanah-

tanah yang sangat masam dan membantu mengikat anion.


Muatan Negatif:1. Dissociation of H+ ions,

lack of Al & Si at edge to associate with O atom

Muatan Negative menjadi Muatan Positive :1. As pH increases, more H+ ions bond to O

atoms at the clay surface2. Protonation at very low pH (H+ ions attach

to surface OH groups)Sumber: classes.uleth.ca/200801/geog3080a/SOILS%20PART%20V%202008.ppt



Box 2-3 Fixed and Variable Charge


Pertukaran kation lebih

efektif

Pertukaran kation kurang efektif


Kapasitas Tukar Kation

Nilainya sangat tinggi dalam tanah-tanah yang :

1. Kaya humus2. Kapasitas

mengembang snagat tinggi

3. pH nya tinggi


Humus1. Senyawa organik, Non-crystalline2. Molekul organik, ukurannya besar3. 50% C, 40% O, 5% H, 3% N and sometimes S4. Strukturnya sangat variabel.5. 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 bymicroorganisms)

(iii) alcoholic hydroxyl group


Keadaan Bahan organik setelah satu tahun dibenamkan ke dalam tanah


Substansi Humik1. Mikroba mer9ombak komponen yang kompleks2. Senyawa yang lebih sederhana dihasilkan; CO2

dilepaskan3. Sintesis biomolecules baru, dengan menggunakan C, N, S

dan O.

4. Lignin not completely broken down: complex residual molecules often retain lignin characteristics

5. Microbes polymerize new, simpler molecules with one another and with residual molecules

6. This creates long, complex chains, resistant to further decomposition

7. Chains interact with amino compounds8. Polymerization process is stimulated by colloidal clays.


Setelah satu tahun:

• 1/5 hingga 1/3 jumlah Karbon tetap dalam tanah(i) live biomass (5%)(ii) humic fraction (20%)(iii) nonhumic fraction (5%)

Substansi Humik meliputi:

(i) Fulvic acids: lowest molecular weight andlightest colour (most susceptible to microbes)

(ii) Humic acid (intermediate)(iii) Humin: highest molecular weight, darkest,

least soluble and most resistant to microbes


Humus: Amorphous andcolloidal mixture

of complex organicsubstances no

longer identifiableas tissues

Note: non-humic substances are biomolecules

produced by microbesSumber: classes.uleth.ca/200801/geog3080a/SOILS%20PART%20V%202008.ppt

sumber : classes.uleth.ca/200801/geog3080a/ soils %20part%20v%202008. ppt

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