PLS 671: Environmental Soil Chemistry

“Sorption Kinetics of Phosphate” 
Lab #7-11/15/04 

I.  Objective:

To measure sorption kinetics of an important soil anion, phosphate (P), on your soil, using the stirred-batch method.

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.

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.   

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.  

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.

4.) The P concentration will be measured using a colorimetric procedure on the platereader. 

II.  Data Analysis:

1.) Analyze the kinetic data.  Was 24 h adequate for equilibrium? 

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.