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Mineral Colloids Continued
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Na + K + K + Na + K + K + K + Cation Exchange Na +
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Colloid Soil Solution
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Factors Determining Cation Preference 1. Concentration 2. Charge (+1, +2,+3) 3. Size
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Mineral Cation Exchange Capacity The total quantity of cations a clay can adsorb. Equal to the amount of charge Units are cmol c /kg soil Range: 0 - 180 cmol c /kg Related directly to the amount of Isomorphous substitution
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K HH O HH O HH O HH O +
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1. Kaolinite 1. Has low CEC Reason: limited isomorphous substitution 2. Is non-expanding Reason: layers are “glued” shut by hydrogen bonds 3. Cation adsorption is on the outer mineral surfaces Reason: layers are “glued” shut by hydrogen bonds
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2. Smectite 1. Has high CEC Reason: abundant isomorphous substitution (Mg for Al) 2. Is highly expanding 3. Cation adsorption is in the interlayers of mineral Reason: layers are not “glued” shut by hydrogen bonds Allowing easy movement of water and hydrated cations to the interlayer regions Reason: layers are not “glued” shut by hydrogen bonds Allowing easy movement of water and hydrated cations to the interlayer regions
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3. Vermiculite 1. Has high CEC Reason: abundant isomorphous substitution (Al for Si) 2. Is moderately expanding 3. Cation adsorption is in the interlayers of mineral Reason: layers are not “glued” shut by hydrogen bonds Reason: layers are not “glued” shut by hydrogen bonds, but interlayer cations are very close to the source of the negative charge (the tetrahedra) holding the layers together.
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3. Illite 1. Has low CEC Reason: abundant isomorphous substitution (Al for Si), but potassium ions fit well in cavities on the surface of the interlayers, holding them tightly together. 2. Is non-expanding 3. Cation adsorption is mostly on external mineral surfaces Reason: potassium ions fit well in cavities on the surface of the interlayers, holding them tightly together. Reason: potassium ions fit well in cavities on the surface of the interlayers, holding them tightly together. K K K Clay Layer
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Minerals KaoliniteSmectiteVermiculite illite 1:1 None 2 – 5 2:1 High 80 - 120 2:1 Limited 100 – 160 2:1 None 20-40 Mineral Expansion CEC (cmol/kg) Octahedral substitution tetrahedral substitution { tetrahedral substitution
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Bt horizon E horizon A horizon B horizon (Illuvial) (Elluvial) Bt horizon t = accumulation of silicate clays formed in place or translocated from above Soils and Clays
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Implications of Negative Charge sites. Important Cations: H +, Ca 2+, Mg 2+, Zn 2+, Mn 2+, K +, NH 4 +, Cu 2+ reserve Ca 2+ Mg 2+ Zn 2+ Cu 2+
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Ca 2+ Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Root H+H+ H+H+ H+H+ H+H+
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Flocculation and Dispersion
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Na + Ca 2+ Particles with a Single Charge
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- - - - - - Clay-Sized Particles Na+ - - - Al 3+ DispersedFlocculated
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- - - - - - - - - - - - - - - High cation concentration Ambient concentration
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- - - - - - - - - - - - - - - Ambient solution cation concentration - - - - - - - - - - - - - - -
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Cations satisfying charge on both negatively charged colloids
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Aids in development of soil structure Increases infiltration of water at the soil surface Can increase hydraulic conductivity Increases movement of gases (O 2 ) Aids in root penetration Flocculation
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A Practical Matter for Flocculation
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Wastewater Constituents Pathogenic organisms: bacteria, protozoa, viruses Non-pathogenic bacteria (> 100,000 / ml for sewage) Organic (carbon) particles: feces, food, plant material, humus Soluble Organics (carbon): pesticides, poisons, paints, drugs Soluble Inorganic Chemicals: nutrients (N and P), metals Inorganic particles: sand, silts, clays Organic and inorganic particles Higher turbidity levels are often associated with higher levels of viruses, parasites and bacteria. Turbidity
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Primary Water Treatment
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Primary Treatment Screening, Grit Removal, Primary Settling Bar Screen particles larger than ¼”
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Grit Chamber Large particulates Short time period Sand and gravel esp. storm water V = KD 2
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Primary Settling Tank Longer residence time Chemicals to promote flocculation/coagulation Smaller Particles Suspended solids Solids settle to the tank bottom as “primary sludge” Some remains as suspended solids
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Al 3+ Chemical Flocculation Suspended silts, clays and organic matter are Generally small and negatively charged.
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Primary Treatment Sludge Suspended solids
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Iron and Aluminum Oxides/hydroxides
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Weathering of Rocks and Minerals Rocks Primary Minerals Secondary Minerals chemical physical chemical Quartz Muscovite Feldspars biotite Silicate clays Fe oxides Al oxides Granite Basalt soluble constituents (Ca 2+, Mg 2+, K +, Fe 2+, Al 3+, Si 4+,CO 3 2 -, SO 4 2- )
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Formation Increased Weathering Smectites Kaolinite Fe, Al oxides Hot, wet climates
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Iron Oxides
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Release of metal ions from minerals -SiO-Fe 2+ + 2H + Fe 2+ + 2OH-SI Iron bearing silicate Free iron Chemical weathering
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Fe 2+ Fe 3+ oxygen Fe(OH) 3 water Lack of oxygen (Mobile)
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Redoximorphic Features Fe 2+ Fe 3+
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Iron Oxides Goethite Hematite ColorColor High soil temperature, better drained soils, rapid biomass turnover, high Fe-release rate from rocks most frequently occurring Fe-oxide in soil and has a characteristic yellowish brown color Bo Horizon Bo is an oxic subsurface horizon
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Less weathered, poorer drainage More weathered, better drained Temperate Tropical Water and Temperature Goethite Hematite
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Iron Oxides Can possess negative, positive, zero charge Potential interaction with cations and anions Cl -, F -, Br -, SO 4 2-, NO 3 -, CO 3 2-, PO 4 -3 Anion Exchange
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Aluminum Oxides
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Al 3+ Products of Extreme Weathering Breakdown of Al-O-Si linkages kaolinite Si, Al Removal by leaching water
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Al 3+ Al(OH) 3 Water, crystallization Gibbsite (grayish-white color) The principal aluminum hydroxide in soil
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Products of Extreme Weathering Al(OH) 3 Gibbsite crystalline and octahedral
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Gibbsite Dominated Products of Extreme Weathering kaolinite Environments Highly weathered Less weathered Kaolinite Dominated Gibbsite Al(OH) 3
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Aluminum Oxides Can possess negative, positive, zero charge Potential interaction with cations and anions Cl -, F -, Br -, SO 4 2-, NO 3 -, CO 3 2-, PO 4 -3 Anion Exchange Aluminum has a high affinity for phosphorus
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Al 3+ Chemical Flocculation Suspended silts, clays and organic matter are Generally small and negatively charged.
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Primary Treatment High in Al Suspended solids Aluminum has a high affinity for phosphorus
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Both iron and aluminum oxides are prominent in highly weathered environments Can possess negative, positive, zero charge Both are capable of cation and anion exchange
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Next: Organic Colloids
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