dasar ilmu tanah ciri-ciri koloid tanah

CIRI-CIRI KOLOID LIAT

&KOLOID ORGANIK

TANAH

SUMBER: myspace.aamu.edu/users/monday.mbila/soils/SPS251Lecture07.ppt

Properties of Clay and Organic Colloids

1. Introduction2. Properties of Colloids3. Types of Colloids4. Structure of Colloids5. Sources of Charge on Colloids6. Reactions of Soil Colloids


Koloid Liat dan Koloid Organik

1. Soil colloids are organic and inorganic matter with very small particle size and a correspondingly large surface area.

2. Organic materials = Koloid humus3. Inorganic materials = Koloid Liat4. Their small size, large surface area, and electrically

charged surface give them the advantage of being highly reactive.

5. Their presence in soil give soil very large surface area.

}Fraksi Koloid


Ciri-ciri Umum Koloid

1. Size- Colloids are generally defined to be less than 2 m in diameter.

2. Surface area- The smaller the particle size in a given mass of soil, the greater the surface area.

3. Surface Charge- Soil colloids carry positive or negative electrostatic charges.


4. Adsorption of ions- Negatively charged soil colloids attract positively charged ions (cations e.g Al3+, Ca2+, Mg2+, K+, Na+, H+, etc) while positively charged colloids attract negatively charged ions (anions e.g NO3

-, SO42-, Cl-, etc).

5. Adsorption of water- Colloids attract and hold large number of water molecules due to the polar nature of water molecules.


Ciri-ciri Umum Koloid

Clay Crystal (Micelle)1. A clay unit has

different layers2. Each Layer is made up

of sheets 3. Each colloid particle

(micelle) attracts thousands of cations to the colloid surface by electrostatic attraction.

4. Some cations will break away from the swarm on the surface and be replaced by other cations of equal charge in a process called “Cation Exchange” process.

5. The cations and anions involved in the process are called exchangeable ions SUMBER:

myspace.aamu.edu/users/monday.mbila/soils/SPS251Lecture07.ppt

1. Crystalline silicate colloids2. Noncrystalline silicate colloids3. Iron and Aluminum Oxide colloids4. Organic Colloids

Soils contain different types of colloids, each with its particular composition, structure, and properties. The four main groups of colloids are as follows:


Kelas Koloid Tanah

1. The predominant type of colloids in most soils is the crystalline silicate type

2. These colloids have a layered structure (like pages of a book) -phyllosilicates.

3. Each layer consists of two to four sheets of closely packed and tightly bonded O2, Si, and Al atoms.

4. Their structure make them negatively charged.

5. Members of this group differ in particle shape, and adsorption of water and ions.

6. Contoh: Kaolinite, Smectite, Mica, dll.

Kaolinite

MicaSUMBER: myspace.aamu.edu/users/monday.mbila/soils/SPS251Lecture07.ppt

Koloid Silikat Kristalin

Single silica tetrahedral structure

Single Al/Mg Octahedral structure

Silica tetrahedral sheet Al octahedral sheet


Struktur Umum Koloid Silikat Kristalin

Figure 1. Silicon tetrahedron structure showing a silicon ion in coordination with four oxygen ions to form a tetrahedral structure.

Figure 3. Aluminum octahedral structure showing aluminum in coordination with six oxygen ions.

Figure 2. Silicon tetrahedron sheet showing one plane of oxygen ions bonded to two silicon ions in two directions to form a sheet of silicon tetrahedrons with unbalanced charges on the apical O ions.

Figure 4. The structure of kaolinite (~0.7 nm thick from the bottom oxygen to the top oxygen) showing apical oxygen ions of a silicon tetrahedral sheet bonded with the octahedral sheet to form a 1:1 layer mineral.

Figure 5. The basic structure of 2:1 clay minerals showing two silicon tetrahedral layers on top and bottom and one aluminum octahedral layer in the middle of the structure

Figure 3. Silicon tetrahedron sheet in figure 2 turned upside down toward figure 3 (the Aluminum octahedral sheet) SUMBER:


Blok-blok Tetra dan Okta saling bersambungan membentuk bidang Si dan Al (Mg) yang berselang seling dengan bidang O2 dan OH.


Susunan lembaran-lembaran dalam liat koloid

Kaolinite (Si4Al4O10(OH)8)- One sheet of tetrahedron bonded to one sheet of octahedron


2:1:1. Chlorite

SmectiteMica (muscovite)

VermiculiteSUMBER:


Koloid Silikat Non-Kristalin

• These clays consist of tightly bonded O2, Si, and Al atoms, but they do not have ordered, crystalline sheets.

– Examples = Allophane, immogolite

– These group of colloids are highly charged, and are formed from volcanic ash.


Oksida Fe dan Al1. These colloids are found in highly weathered

tropical environments.

2. They consist of Fe, Mn, and Al atoms in coordination with O2 atoms.

3. Fe and Al oxides group of colloids consist of crystalline sheets. But there are some members in the group that may not be crystalline.

4. Their net charge range from slightly –ve to moderately +ve.


Struktur Koloid Oksida1. These are octahedral

sheets with either Fe or Al in the cation positions.

2. They do not have tetrahedron sheets, and they do not have Si in their structure.

3. They do not have isormorphous substitution. Eg. Gibbsite [Al(OH)3]shown here.

4. Other examples are Goethite (FeOOH), Hematite [Fe2O3], etc.


Koloid Organik1. These colloids are not

minerals2. They are not crystalline3. They consist of rings and

chains of C atoms bonded to H, O2, and N.

4. Organic colloids have a net –ve charge.

5. Humus particles are the smallest colloids and exhibit very high water adsorbing capacity.

Organic Colloid


StruKtur Koloid Organik1. Consists of large

organic molecules whose chemical composition varies.

2. Structure contains complex series of C chains and ring structures with many functional groups- carboxyl, phenolic, and alcoholic groups.

3. -Ve or +ve charges on the humus colloid develop as H+ ions are either lost or gained by these groups.

Structure of Humic Acid


Sumber Muatan pada Koloid Liat1. Isomorphic Substitution - during weathering, primary minerals

dissolve and recrystalize as secondary minerals • During this process, one element may become substituted for

another element of similar size in the crystal structure without changing the shape of the crystal.

• If the two elements do not have the same ionic charge, then an

unsatisfied net charge remains at that point in the crystal. Common substitutions are Al+3 for Si+4, Mg+2 for Al+3, and Fe+2 for Al+3, each leaving a net negative charge on the crystal. This charge does not depend on pH, thus permanent charge or constant charge.


2. Exposed hydroxyl groups (-OH-_ on the surfaces of clay crystals. This accounts for most of the net negative charge in Kaolinite and some of the charge in Montmorillonite, Vermiculite and Illite.

3. Broken oxygen bonds at the edges of crystals. At the broken edges of crystals, the small Al3+ and Si4+ ions are exposed to weathering and may be lost. The remaining oxygen ions have an unsatisfied net negative charge. This is an important source of charge in all clays. E.g.

>SiOH2+ <------> >SiOH <--------------> >SiO- + H+

>AlOH2+ <------> >AlOH <--------------> >AlO- + H+ SUMBER:


Sumber Muatan pada Koloid Liat

Adsorption (Penjerapan) KationPertukaran Kation• Cation exchange is the exchange of cations between

the soil and the soil solution..– 2 Na+ + Ca-clay <-------------> Na-clay + Ca2+

• Cation Exchange Capacity (CEC)- sum total of exchangeable cations that a soil can absorb

– CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect groundwater from cation contamination.

• Cation Exchange Capacity is a function of:– Type and amount of clay– Humus content


Adsorption (Penjerapan) AnionPertukaran Anion

Some common anions in the soil include: Cl-, HCO3-, CO3

2-, NO3

-, SO42-, HPO4

2-, OH-, F-, H2BO3-, MoO4

2-, etc. There are ways that anions are retained against leaching:

1. Attraction by exposed cations along the edges of clay crystals and exposed cations in humus colloids.

2. Adsorption of H2PO4-, SO4

2-, and MoO42- by Fe, and Al

oxides at low pHTropical soils, with lots of Fe, Al oxides, can have net AEC.


Sifat Tanah yang dipengaruhi oleh Adsorpsi Ion

1. nutrient availability- Exch cations are available for plants.

2. leaching of electrolytes- retention of substances prevents their movement through the soil.

3. soil pH- CEC increases with increase in pH.


RINGKASAN1. The colloids in soils are both organic and inorganic.

2. The size of colloids, structure of colloids (high surface area), and Charges of soil colloids make them the center of chemical and physical activity in soils.

3. The –ve and +ve charge sites they have attract ions and molecules of opposite charge.

4. The replacement of one ion for another on the colloid surface is called cation or anion exchange reaction

5. The total number of –ve colloid charges per unit mass is termed CEC

6. That capacity influences sorption of contaminants, nutrient availability, and pH of soils.


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