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| I. Objective: |
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| To measure sorption kinetics of
an important soil anion, phosphate (P), on your soil, using the
stirred-batch method. |
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| Background:
In studies of metal and anion sorption, it is often assumed that
equilibrium is established after 24h. Today, the sorption of P
will be followed as a function of time at one concentration. |
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| I. Experimental Design: | ||
| The P stock solution will be 0.161 M (5000 ppm) and the total volume will be 100 mL in a background of 0.01 M NaNO3. The 125 mL Nalgene bottles will serve as stirred-batch reactors. | ||
| II. Procedure: | ||
| 1.) Weigh out
5.55 g of your air-dried soil into a 125 mL Nalgene bottle. Add a
Teflon coated magnetic stir bar, then 100 mL of 0.01 M NaNO3. Stir to prehydrate the soil while measuring initial pH. |
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| 2.) After
about 10 minutes of stirring, add the calculated amount of P stock to
achieve an initial concentration of 0.32 mM P (10 ppm). Once the P is added, this will be time zero. Place the samples on the shaker at low speed. |
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| 3.) Five mL
samples will be taken at increasing reaction times using a pipette as
shown below: 15 minutes, 30 minutes, 60 minutes, 120 minutes, 20 hours, 24 hours, 48 hours, and 72 hours. Filter the suspensions through a 0.2 µm membrane filter. If you have trouble filtering, the 5 mL subsamples can be centrifuged first at 8000 rpm in 40 mL centrifuge tubes. When pulling the five mL sample, be sure and stir the suspension uniformly on a stir plate and ideally, measure pH at the time of subsampling. Be careful not to break the pH electrode with the stir bar. |
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| 4.) The P
concentration will be measured using a colorimetric procedure on the
platereader. |
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| II. Data Analysis: |
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| 1.) Analyze
the kinetic data. Was 24 h adequate for equilibrium? |
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| 2.) Input
soil solution data, P concentration, and pH into the MINEQL+ program to
determine whether any P-containing solid phases may be controlling P solubility. |
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