To CLASS PAGE:
|To measure sorption kinetics of
an important soil anion, phosphate (P), on your soil, using the
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.
|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.|
|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.
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
|3.) Five mL
samples will be taken at increasing reaction times using a pipette as
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.
|4.) The P
concentration will be measured using a colorimetric procedure on the
|II. Data Analysis:
the kinetic data. Was 24 h adequate for equilibrium?
soil solution data, P concentration, and pH into the MINEQL+ program to
determine whether any
P-containing solid phases may be controlling P solubility.