PLS 671: Environmental Soil Chemistry

Adsorption Isotherms
Lab #6-11/8/04 

To measure zinc (Zn) sorption on the three soils and soil fractions as a function of concentration at constant temperature.

I.  Experimental Design:

Initial concentrations of 0, 7.64, 15.3, 76.5, 153, and 1.53 × 103 µM Zn (0, 0.5, 1, 5, 10, and 100 ppm) will be reacted with your soil and soil fractions at their native pH.  The Zn stock solution will be 0.0765 M (5,000 ppm), so appropriate volumes of this stock will be added to each centrifuge tube with enough 0.01 M NaCl to equal 36 mL total volume.  Zinc sorption will also be measured on your sand, silt, and clay fractions for the 0, 15.3, 153, and 1.53 × 103 µM Zn treatments.

II.  Procedure:

1.) Weigh out 2 g (in duplicate) aliquots of your air-dried soil into 12 labeled (labeled with the treatments
      described above) 40 mL centrifuge tubes.  Add the calculated amount of Zn stock to achieve the above
       initial concentrations of Zn.  Then add enough 0.01 M NaCl to yield a final volume of 36 mL.  

2.) Weigh 0.20 g of sand, silt, and clay into labeled 40 mL tubes and repeat as above.  

3.) The samples will be equilibrated for 24 h on a shaker. 

4.) Measure equilibrium pH using a calibrated pH meter on all reacted samples. 

5.) The solid and solution will be separated by centrifugation for 15 minutes at 8000 rpm.  If the supernatant is
      not clear, pass the solution through a 0.2µm pore size membrane filter using 25 mm filter holders and 10
      mL syringes into labeled test tubes.

6.) Prepare Zn standards of 0, 1.53 (0.1ppm), 7.64 (0.5ppm), 15.3 (1ppm), 22.95 (1.5ppm), and 30.6
µM Zn.

7.) The equilibrium Zn concentration will be measured using flame atomic absorption spectroscopy (FAAS)
      against your standards.  The amount sorbed will be calculated by difference between solution
      concentration before and after equilibration.

II.  Data Analysis:

1.) Express the amount sorbed on an oven-dried weight basis and plot the data according to the sorption
     equations discussed in lecture. 

2.) Input Zn concentrations and pH into MINEQL+ program and determine any Zn containing solid phase
     which may be controlling solution phase Zn following addition of various levels of Zn to these soils