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Chapter 9 Sorption to organic matter

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    1. Chapter 9 Sorption to organic matter

    3. definitions absorption - sorption (penetration into) a 3D matrix adsorption sorption to a 2D surface Sorbate: the molecule ad- or absorbed Sorbent: the matrix into/onto which the sorbate ad- or absorbs

    4. identical molecules behave very differently, depending on whether they are: in the gas phase (gas) surrounded by water molecules (dissolved) clinging onto the exterior of solids (adsorbed) buried within a solid matrix (absorbed)

    5. sorption affects transport: generally, molecules which are sorbed are less mobile in the environment sorbed molecules are not available for phase transfer processes (air-water exchange, etc) and degradation: sorbed molecules are not bioavailable sorbed molecules usually shielded from UV light (less direct photolysis) sorbed molecules cannot come into contact with indirect photoxidants such as OH rates of other transformation reactions may be very different for sorbed molecules

    6. sorption is a difficult subject because sorbents in the natural environment are complex, and sorption may occur via several different mechanisms

    7. the solid-water distribution coefficient or: the equilibrium constant that wasnt

    8. sorption isotherms describe equilibrium partitioning between sorbed and desorbed phase the sorption isotherm is a plot of the concentration sorbed vs. the concentration desorbed sorption isotherms can have many shapes

    9. sorption isotherms can have many shapes

    10. Equations for sorption isotherms

    11. Freundlich isotherm

    12. Freundlich isotherm shapes

    13. Langmuir isotherm

    14. Langmuir - linearization

    15. In the real world

    16. Dissolved fraction of a compound in a system:

    17. Ways to express the solid/water ratio

    18. Example: 1,4-DMB (Kd = 1 L/kg)

    20. The complex nature of Kd

    21. Recall:

    22. It gets worse:

    23. Sorption of neutral organics to POM

    27. Not all organic carbon is created equal

    28. Soil Organic Matter SOM = Humus Content: ~0 to 5% of most soils Up to 100% of organic soils (histosoils) Higher in moist soils and northern slopes Lower in drier soils and southern slopes Cultivation reduced SOM High surface area and CEC Lots of C and N

    29. table 3.1

    30. Table 3.2

    31. Carbon sequestration Soils sequester carbon in SOM and carbonate minerals About 75% of the terrestrial carbon pool is SOM Declines in the SOC pool are due to: Mineralization of SOC Transport by soil erosion Leaching into subsurface soil or groundwater

    32. Sequestration of Carbon by Soils can be increased via: Changing agricultural practices: No-till agriculture or organic agriculture Limited used of N fertilizer (C released during N fertilizer manufacture) Limited irrigation (fossil fuels burned to power irrigation) Soil restoration

    33. Figure 3.1

    34. Composition of SOM Table 3.3 Major: lignins and proteins Also: hemicellulose, cellulose, ether and alcohol soluble compounds nonhumic substances = juicy carbon that is quickly digested (carbohydrates, proteins, peptides, amino acids, fats, waxes, low MW acids) Most SOM is not water-soluble

    35. Table 3.3

    36. Definitions

    37. Fig 3.3

    38. Humic substances Fig 3.6

    39. structures

    40. Properties of SOM Voids can trap Water Minerals Other organic molecules Hydrophobicity/hydrophilicity Reactivity H-bonding, chelation of metals

    41. Fig 3.8

    42. Conformation and macromolecular structure of HS depend on pH Electrolyte concentration Ionic strength HA and FA concentrations Fig 3.10

    43. Fig 3.10

    44. Functional groups and charge characteristics PZC ~ 3 (pH of zero charge) Up to 80% of CEC in soils is due to SOM Acid functional groups Carbonyls pKa < 5 Quinones also pKa < 5 Phenols pKa < 8 SOM constitutes most of the buffering capacity of soils

    45. Fig 3.13

    49. Solids concentration effect

    50. LFERs for Koc (assuming slope ? 1)

    51. Problem with non linearity

    52. Nonlinear Koc

    53. Effect of T on Kioc

    55. Effect of salinity on Koc

    56. Effect of cosolvents on Koc

    57. Sorption of Neutral Compounds to Dissolved Organic Matter

    58. Relationship between DOC properties and KDOC

    59. Effect of pH, ionic strength, and T on KDOC

    60. LFERs relating KDOC to Kow

    61. PCBs

    62. PCBs

    63. Sorption of acids and bases to NOM

    64. Character of NOM

    65. For weak acids with only one acidic group,

    67. Sorption of bases

    68. Problem 9.1