Lecture 10 Cation Exchange(1)

Lecture 10 Cation Exchange(1) - Reactions of...

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Unformatted text preview: Reactions of Aluminosilcates Building Blocks for Silicate Clays Silicon Tetrahedron oxygen silicon(Si4+) Aluminum octahedron hydroxide (OH-) Aluminum (Al3+) Tetrahedra and Octahedra Sharing the Oxygens Linkage of thousands of silica tetrahedra and aluminum octahedra O { octahedra { Tetrahedra Si O, OH Al OH 1:1 Mineral 2:1 mineral { octahedra { Tetrahedra { Tetrahedra 1:1 minerals 2:1 minerals Charge Balance Si4+ O2- Al3+ OH- Positive charge Al3+ Si4+ = Negative charge OH- O2- Isomorphous Substitution Substitution of lower-charge cations for higher charge cations during mineral formation. Al3+ for Si4+ in tetrahedra Mg2+ for Al3+ in octahedra The result is a deficit of positive charge or a surplus of negative charge in the mineral structure. Tetrahedral Substitution Al3+ for Si4+ Octahedral Substitution Mg2+ for Al3+ Tetrahedral Substitution Al3+ for Si4+ Octahedral Substitution Mg2+ for Al3+ Charge Na+ Na+ Na+ Na+ Na+ Na+ Na+ Charged Surface Ambient solution concentration High cation concentration --------------Dense swarm of cations K+ Cation Exchange Na+ K+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Which Cations are preferred? K+ K+ Factors Determining Cation Preference 1. Concentration 2. Charge (+1, +2,+3) 3. Size Concentration Soil Solution K+ K+ K+ Na+ K+ K+ K+ K+ K+ K+ K+ Na+ K+ K+ K+ K+ K+ Na+ K+ K+ K+ Na+ K+ K+ Charge Soil Solution Ca+2 Ca+2 Ca +2 Ca+2 Ca+2 K+ K+ Ca+2 Ca+2 K+ K+ K+ Ca+2 K+ Ca +2 K + Ca+2 Ca+2 K+ K+ K+ Ca+2 K+ Ca+2 K+ Size Soil Solution Na+ Stronger+ bond H (H+ preferred) Mineral Cation Exchange Capacity The total quantity of cations a clay can adsorb. Related directly to the amount of Isomorphous substitution Equal to the amount of charge Units are cmolc/kg soil Range: 0 - 180 cmolc/kg What is a Centimole? 1/100 of a mole 1mole = 6.02 x 1023 charges 1cmol = 6.02 x 1021 charges 1 cmol = 6,020,000,000,000,000,000,000 Measuring Cation Exchange Capacity Very High Concentration NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ NH4+ ---------------------Na+ K+ Ca2+ Al3+ H+ Na+ NH4+ ---------------------Mg2+ K+ Na+ Ca2+ Mg+ K+ NH4+ Al3+ ---------------------- NH4+ Soil Ca2+ Mg2+ NH4+ Na+ K+ Ca2+ Mg2+ Na+ K+ beaker Ba +2 Soil Ba 4+ NH 2+ NH4+ NH4+ beaker NH4+ NH4+ The number of ammonium ions = number of charges A clay’s negative sites are saturated With NH4+ and high concentrations of Ba2+ are used to displace them The displaced cations are collected in a beaker and the number of NH cations 4+ is determined. The number of NH4+ cations = 2,000,000 How many exchange sites were on the clay? A clay’s negative sites are saturated with Ca+2, and then high concentrations of Ba2+ are used to displace them The displaced cations are collected in a beaker and the number of Ca cations +2 is determined. The number of Ca+2 cations = 4,000,000 How many exchange sites were on the clay? Ba +2 Soil Ba 4+ NH 2+ NH4+ NH4+ beaker NH4+ NH4+ The number of ammonium ions = number of charges 1Kg of clay whose negative sites are saturated with NH4+. High concentrations of Ba2+ are used to displace them The displaced cations are collected in a Beaker. The volume of solution in the beaker is 1L. The concentration of NH4+ cations in the beaker is equal to 10 cmol/L 10 cmol NH4+ 1 Liter X 1 L of solution = 10 cmol NH4+ 10 cmolc/kg clay = CEC Important Clay Minerals Hydration H O H H O H H O H H O H H O Cl H O H Na - O H + H H H O H Hydration sphere size varies widely H H O O H H H H O H Na O H + Hydroxide (OH-) “Electron greedy” O H + - Kaolinite 1:1 Na+ Limited isomorphous substitution in octahedra (Al3+ for Si4+ ) CEC = 2 – 5 cmolc kg OH + Layers are H-bonded Non-expansible Slightly Negative Adsorption is on external surfaces and edges 2:1 Minerals Smectites: montmorillonite Significant substitution in the octahedra (Al3+ for Si4+) Ca2+ CEC = 80-120 cmolc kg Cations satisfying charge •Layers weakly held together by cations •Highly expansible Mg2+ Na+ Smectites Smectites Abundant water O H H Ca Ca Ca Smectites Smectites Limited water O H H Ca Ca Ca Vermiculite Significant substitution in tetrahedra CEC = 100-180 cmolc kg Source of negative charge Is very close to the adsorbed cations Layers tightly bound Moderately expansible Mg2+ Mg2+ Illite Significant substitution in tetrahedra Source of negative charge Is very close to the adsorbed Cations Potassium fits into cavities on Clay surfaces clamping them shut. Non-expansible CEC = 20-40 cmolc kg K+ K+ Tetrahedra Side View Octahedra Tetrahedra Tetrahedra Top View K+ Clay Layer K K Clay Layer K Minerals Minerals Mineral 1:1 Expansion None CEC (cmol/kg) 2 – 5 tetrahedral substitution Smectite 2:1 High 80 - 120 Octahedral substitution Vermiculite illite 2:1 Limited 100 – 160 2:1 None 20-40 { Kaolinite tetrahedral substitution Clay Layer K K Clay Layer K Where do the minerals occur? Illites Increased weathering Vermiculites Smectites Kaolinite Fe, Al oxides 2:1 1:1 ...
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