Sorption II_April_7-9_2010[1]

Sorption II_April_7-9_2010[1] - Sorption II (Chapter 11...

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Sorption II (Chapter 11 plus) Site-specific sorption to organic matter, clays, and oxides ( continued ) April 7-9, 2010
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General Outline • Clay mineral basics • Exchange (OM and Clay minerals) Other types of aqueous clay-associated interactions • Electron donor-acceptor interactions* • Sorption from air • Chemisorption - irreversible sorption reactions/surface catalyzed transformations * Case Study (Monday April 12 – posted on blackboard)
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C ATION & A NION E XCHANGE W HAT IS EXCHANGE Occurs at the site of negative (cation exchange) and positive (anion exchange) charges, respectively, in soils. Charge balance must be maintained Some charges are pH dependent (organic matter, oxides, and some sites on clays) Some clays have permanent negative charge C ATION AND ANION EXCHANGE are reversible ‘sorption’ processes W HY IS EXCHANGE IMPORTANT nutrition for plants (Ca, K, Mg) buffering ability retention/accumulation/attenuation of contaminants F ACTORS AFFECTING AFFINTIY - Selectivity (trends in valence, charge density) Cation concentrations CATION EXCHANGE CAPACITY (CEC) - capacity of soils to adsorb (exchange) cations, which includes both organic matter an clay mineral domains ANION EXCHANGE CAPACITY (CEC) - capacity of soils to adsorb (exchange) anions, which includes primarily clay mineral domains .
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CEC AND AEC U NITS : – cmol/kg (cmol + /kg or cmol - /kg, respectively); mmol/kg more acceptable to the international system of units. W HERE CAN EXCHANGE OCCUR ON SOILS : Cation exchange – OM (-OH and –COOH sites) – Phyllosilicate clays - inter layer and edge sites – Al and iron oxides Anion exchange – OM (possibly some amine and N-heterocyclic sites, but very few and usually not considered) – Phyllosilicate clays - edge sites only – Al and iron oxides
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Exchange Capacities of Different Minerals The cation and anion exchange capacity of common soil minerals and organic matter. As weathering intensity increases (2:1-->1:1-->oxides/hydroxides), the CEC of clay minerals decreases and the AEC increases. The cation and anion exchange capacities will vary appreciably with soil pH. Soil Component CEC (cmol/kg) AEC (cmol/kg) Illite(2:1) 10-40 1 Smectite (2:1) 90-120 1 Vermiculite(2:1) 100-150 (180) 1 Kaolinite (1:1) 3-15 3-5 Oxides/Hydroxides 0-4 5 Organic Matter 200 - 500 (variable) 0 CEC: 2:1 > 1:1 > oxides/hydroxides AEC: oxides/hydroxides > 1:1 > 2:1
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A1 Oxisol 3 cmol c /kg 0 2 4 6 AEC CEC A2 Oxisol 2 3 2 4 6 A3 Oxisol 2 0 1 2 3 4 DRC Oxisol 3 0 2 4 6 8 2 3 4 5 6 7 8 9 0 4 8 12 16 20 pH 3 4 5 6 7 8 0 10 20 30 40 8R soil K1 Andisol Decreasing pH (addition of H + ) MOH + H + --> MOH 2 + Increasing pH (addition of OH - ) MOH + OH - --> MO - + H 2 O Z ero P oint of C harge ( ZPC ) + = - Organic Matter: RO- + H + --> ROH R-COO- + H + --> R-COOH Oxides, etc.:
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Complexity of the Soil-Water Partition Coefficient (K d ) The apparent distribution of a compound between water and solids (K d ) may be a result of many different types of sorption processes.
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This note was uploaded on 08/28/2011 for the course ESE 544 taught by Professor Lee during the Fall '11 term at Purdue University-West Lafayette.

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Sorption II_April_7-9_2010[1] - Sorption II (Chapter 11...

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