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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 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
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pH dependent surface charge: S-OH + H + ↔ S-OH 2 + protonation (gains protons, attracts anions) S-OH ↔ S-O - + H + deprotonation (loses protons, attracts cations) S-OH + OH - ↔S-O - + H 2 Odeprotonation 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 Acidic conditions
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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
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pH 0, positively charged): S-OH + H + ↔ S-OH 2 + pH > PZC ( p < 0, negatively charged): S-OH ↔ H + + S-O - pH = PZC ( p = 0, uncharged): H + + S-O - ↔ S-OH
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pH below the pH ZPC http://www.gly.uga.edu/schroeder/geol6550/zpcphlow.gif
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pH at the pH ZPC
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pH above the pH ZPC
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Soil components vary in PZC 1.Fe and Al oxides (Oxisols, tropical soils) have high PZC (pH 5-10) 2.Soil organic matter has low PZC (pH<5) 3.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 4.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
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pH for zero point of charge for minerals Mineral pH ZPC Gibbsite5 - 10 Hematite 6 - 7 Goethite 7 – 8 Amorphous Fe(OH) 3 8 - 9 Kaolinite 4 - 5 Montmorillonite 2 - 3 SiO 2 1 - 3 Note that Al and Fe oxides have a high pH ZPC Kaolinite and montmorillonite have low pH ZPC
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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
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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)
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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.
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q Ceq Example of hysteresis during desorption in a batch equilibrium sorption experiment
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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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