colloidslab - Soil Colloids and Cation Exchange Name...

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Soil Colloids and Cation Exchange Name: Samantha Swanhart Day: Thursday Period: 5-6 Introductory Questions What is a colloid? Indicate the size of a colloid and their importance in soils. Colloids are indicated as the most chemically active fraction of soils. They have the smal- lest particle, therefore have the largest the surface area and reactivity to the surrounding particles and environment. Colloids are either organic (typically as humus) or mineral (clays), and are fur- ther defined by structure as crystalline (defined structure; as in clays) or amorphous (like some oxides of iron and aluminum). Crystalline soil colloids possess a negative charge on the surface, and attracts positively charged ions or particles. Plant nutrients, pesticides and other chemicals are typically positively charged, therefore strongly attracted to these colloids. What is “adsorption”? Identify one property of soils that can be impacted by adsorption of cations at colloid surfaces. Adsorption is the term that describes the retention of cations at a colloid surface. Soil col- loid’s adsorption properties make it possible for inorganic (Ca, Mg, K, Cu) and organic (pesti- cides) cations to accumulate and be controlled by those colloids. Consequently, soil’s acidity and aggregate/disperse potential is influenced by these adsorbed chemicals. Discuss isomorphous substitution and how it occurs in crystalline mineral colloids. Isomorphous substitution occurs when cations exchange during mineral formation in silic- on tetrahedra and/or aluminum octahedra, which results in an electrical charge at the colloid sur- face. For example, when a magnesium ion replaces an aluminum ion in an octahedron, charge decreases from 3+ to 2+, therefore a net negative charge is created on the surface.
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