One half of the world’s population, about 3 billion people on six continents, lives or works in buildings

constructed of clay - The New York Times

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Clays and Medicines

Kaolin and Pectin (Oral Route)

Kaolin is an adsorbent medicine used to treat diarrhoea. It has the ability to adsorb some of

the bacterial toxins that often cause diarrhoea.

Kaolinite is an ingredient in "Kaopectate, Rolaids, Di-gel, Mylanta, and Maalox."

ANTIBACTERIAL CLAY:

Researchers have discovered several clays that kill—or prevent from growing—bacteria, including antibiotic-resistant strains.

©ISTOCKPHOTO.COM

Geophagy (Eating Clay)

Eating Dirt: It Might Be Good for You

Experts Claim the Habit of Eating Clay May Be Beneficial for Pregnant Women

By MARC LALLANILLAOct. 3, 2005 It melts in your mouth like chocolate, says Ruth Anne T. Joiner, describing her favorite treat.

"The good stuff is real smooth," she adds. "It's just like a piece of candy."

Joiner is describing the delectable taste of dirt -- specifically, clay from the region around her home in Montezuma, Ga.

“about two pounds of "Georgia Grown White Dirt" can

be purchased for a little less than $10”

Properties of Clay Properties of Clay and Organic Colloidsand Organic Colloids

IntroductionProperties of ColloidsTypes of ColloidsStructure of ColloidsSources of Charge on ColloidsReactions of Soil Colloids

Clay and Organic ColloidsClay and Organic Colloids

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

Organic materials =humus colloids Inorganic materials =clay colloids Their small size, large surface area, and electrically

charged surface give them the advantage of being highly reactive.

Their presence in soil give soil very large surface area.

}colloidal fraction

General Properties of Clay General Properties of Clay ColloidsColloids

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.

General Properties of Colloids General Properties of Colloids (contd)(contd)

Clay Crystal (Micelle)Clay Crystal (Micelle)

•A clay unit has different layers

•Each Layer is made up of sheets

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

• 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 Cation ExchangeExchange” process.

• The cations and anions involved in the process are called exchangeable ions

Classes of Soil Classes of Soil ColloidsColloids

1. Crystalline silicate clays (eg. kaolinte, mica, smectites, vermiculites)

2. Noncrystalline silicate clays (eg. Allophane)

3. Iron and Aluminum oxide clays (eg. Oxides)

4. 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:

crystallincrystallinee

Non-crystallineNon-crystalline

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

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

Each layer consists of two to four sheets of closely packed and tightly bonded O2, Si, and Al atoms (making them

negatively charged).

Crystalline colloids differ in particle shape, and adsorption of water and ions.

Examples = Kaolinite, smectite, mica, etc

1. Crystalline silicate 1. Crystalline silicate claysclays

KaoliniteKaolinite

MicaMica

General Structure of Crystalline silicate Clays (2 main building blocks)

Single silica tetrahedral structure

Single Al/Mg Octahedral structure

Silica tetrahedral sheet Al octahedral sheet

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)

Connecting the tetra and octa building blocks to form planes of Si and Al (Mg) ions that alternate with planes of O2 and OH ions.

Arrangement of Sheets in Clays

KaoliniteKaolinite (Si (Si44AlAl44OO1010(OH)(OH)88)-)- One sheet of

tetrahedron bonded to one sheet of octahedron

2:1:1. Chlorite

SmectiteMica (muscovite)

Vermiculite

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.

2. Noncrystalline silicate 2. Noncrystalline silicate colloidscolloids

3. Iron and Aluminum 3. Iron and Aluminum OxidesOxides

These colloids are found in highly weathered tropical environments.

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

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

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

Structure of Oxide colloidsStructure of Oxide colloids

These are octahedral sheets with either Fe or Al in the cation positions.

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

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

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

4. Organic Colloids4. Organic Colloids

These colloids are not minerals

They are not crystalline

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

Organic colloids have a net –ve charge.

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

ColloidColloid

Structure of Organic ColloidStructure of Organic Colloid

Consists of large organic molecules whose chemical composition varies.

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

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

Structure of Humic AcidStructure of Humic Acid

Sources of Charge on Clay Sources of Charge on Clay ColloidsColloids

1. 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 is 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.

Sources of Charge on Clay Sources of Charge on Clay Colloids (contd.)Colloids (contd.)

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

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

Adsorption of CationsAdsorption of Cations

- Cation Exchange 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

- Anion ExchangeSome common anions in the soil include: Cl-,

HCO3-, CO3

2-, NO3-, SO4

2-, HPO42-, 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.

Adsorption of AnionsAdsorption of Anions

nutrient availability- Exch cations are available for plants.

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

soil pH- CEC increases with increase in pH (high OH-).

Soil Properties affected by Adsorption of Soil Properties affected by Adsorption of ionsions

SummarySummary The colloids in soils are both organic and inorganic.

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.

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

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

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

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