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Soil OM is 50-65% C, so we use 57.5% SOM x 0.575 = OC and SOM = OC/0.575

Soil OM is 50-65% C, so we use 57.5% SOM x 0.575 = OC and SOM = OC/0.575 e.g., how much SOM do you have with 2% OC? SOM = 2% ÷ 0.575 = 3.5% or 2% ÷ 0.50 to 0.65 = 4 to 3% OC. Acidic conditions. pH dependent surface charge:

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Soil OM is 50-65% C, so we use 57.5% SOM x 0.575 = OC and SOM = OC/0.575

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  1. Soil OM is 50-65% C, so we use 57.5% SOM x 0.575 = OC and SOM = OC/0.575 e.g., how much SOM do you have with 2% OC? SOM = 2% ÷ 0.575 = 3.5% or 2% ÷ 0.50 to 0.65 = 4 to 3% OC

  2. Acidic conditions pH dependent surface charge: S-OH + H+ ↔ S-OH2+ protonation (gains protons, attracts anions) S-OH ↔ S-O- + H+ deprotonation (loses protons, attracts cations) S-OH + OH- ↔S-O- + H2O deprotonation alkaline conditions (loses protons, attracts cations) pKa’s and Henderson-Hasselbalch eqn tell us whether a compound will be mostly charged (usually negatively) or uncharged at a given pH

  3. Point of Zero Charge PZC  suspension pH at which the particle surface has zero net charge: p = 0 1. When pH < PZC the particle surface is positively charged 2. When pH > PZC the particle surface is negatively charged 3. At PZC, settling of flocs occurs – important in aggregation and retention of ions during irrigation, leaching, etc. * uncharged particles don’t repel each other

  4. pH < PZC (p > 0, positively charged): S-OH + H+ ↔ S-OH2+ pH > PZC (p < 0, negatively charged): S-OH ↔ H+ + S-O- pH = PZC (p = 0, uncharged): H+ + S-O- ↔ S-OH

  5. pH below the pHZPC http://www.gly.uga.edu/schroeder/geol6550/zpcphlow.gif

  6. pH at the pHZPC

  7. pH above the pHZPC

  8. Soil components vary in PZC • Fe and Al oxides (Oxisols, tropical soils) have high PZC (pH 5-10) • Soil organic matter has low PZC (pH<5) • Silicate clays have low PZC (pH 2-5) Interpretation: low PZC = net negative charge over wider soil pH range  more cation adsorption and more CEC High PZC = net positive charge in acid conditions or in lower range of soil pH  more anion adsorption and less CEC • Consider the distribution of soil components in the profile – where would you expect to see more or less anion and cation adsorption? more CEC in Ap or Bt horizons, more AEC in oxide-rich horizons or low OM depths

  9. MineralpHZPC Gibbsite 5 - 10   Hematite  6 - 7 Goethite  7 – 8 Amorphous Fe(OH)3  8 - 9 Kaolinite  4 - 5  Montmorillonite  2 - 3 SiO2  1 - 3 Note that Al and Fe oxides have a high pHZPC Kaolinite and montmorillonite have low pHZPC pH for zero point of charge for minerals

  10. Types of PZC • PZC, p = 0 • Apply electric field, PZC reached when particles flocculate or stop moving • PZNC (N for net), CEC-AEC =0; is + os +d = 0 • Measure Na+ and Cl- sorption with pH; PZNC calculated from intersection point • PZNPC (P for proton), H = 0 (or zero variable charge) • PZSE (SE for salt effect), intersection of two potentiometric titration curves • Most commonly measured

  11. Desorption • removing an ion or molecule from a surface particle and putting it back into solution. • Important for decontamination of soil or sediments and to determine the mobility of contaminants • Hysteresis  apparent irreversibility of sorption (forward and backward reactions did not coincide)

  12. Hysteresis causes: • Experimental error: failure to attain equilibrium during sorption experiments • Chemical or biological transformations not accounted for in sorption study • Trapping of ions or molecules in soil micropores resulting in very slow release •  short term lab sorption experiments may be inadequate to predict behavior over long time periods under field conditions.

  13. q Ceq Example of hysteresis during desorption in a batch equilibrium sorption experiment

  14. Adsorption (open symbols) and desorption (full symbols)isotherms of water at 25 °C on (a) a TiO2 film deposited at 80 °C for 2 h and (b) the same powder after heating at 450 °C http://www.lnqe.uni-hannover.de/projekte/projekte_oekermann.htm#fig4

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