PROCEDURE FOR REDUCING STRUCTURAL IRON IN SMECTITES USING SODIUM DITHIONITE

by

Joseph W. Stucki and Fabienne Favre

University of Illinois

 

 

 

A.     REFERENCES

 

            Stucki, J.W., D.C. Golden, and C.B. Roth. 1984. Preparation and handling of dithionite-reduced smectite suspensions. Clays and Clay Minerals 32:191-197.

 

B.    REAGENTS

 

NaHCO3 1 M

Na-citrate 0.3 M

Sodium dithionite

 

C.    REDUCTION IN SUSPENSION

 

  1. Transfer about 30 mg (or other appropriate amount) of freeze-dried, Na-saturated, < 2-μm size fraction smectite to a 50 mL polycarbonate (or polyalomar) centrifuge tube.
  2. Prepare a citrate-bicarbonate buffer by mixing 0.3 M Na-citrate with 1.0 M Na-bicarbonate 8:1 v:v.
  3. Add 30 mL of this buffer to the smectite and disperse by shaking gently overnight.
  4. If steps 2 and 3 fail to yield complete dispersal of the clay, see alternate procedure. 
  5. Stopper the reaction tube with a septum cap assembly.
  6. Place the reaction tube into a water bath at 70 oC and insert an O2-free nitrogen gas supply needle into the suspension. Also insert a needle into the head space to serve as a vent. Adjust the gas flow rate so that suspension is not lost through the vent needle.
  7. Remove the cap, add 100 mg of Na2S2O4 salt directly to the suspension, then close immediately.
  8. Place tube back into the water bath and leave for 4 hours for complete reduction (assuming either sample NG-1 or SWa-1 is used). Extent of reduction can be decreased by lessening the time of reaction.
  9. After reduction, remove the tube from the water bath and allow to cool for 1 hr, then remove septum needles.

 

D.    REMOVING EXCESS SALTS

 

  1. Centrifuge for 5 minutes at 5,000 rpm with the SS-34 rotor on the Dupont Sorvall Model RC 5C plus centrifuge in room W-316.
  2. If the supernatant is cloudy, resuspend and double the centrifuge speed.
  3. If the supernatant is clear, decant and discard using the Controlled Atmosphere Liquid Exchanger (CALE) in room W-316. After emptying the tube and before refilling it, vortex the sample to help resuspend it after the new solution is added
  4. Still using the CALE, backfill the sample with about 30 mL of 0.005 M NaCl solution which has been de-oxygenated.
  5. Centrifuge for 30 minutes at 15,000 rpm. Decant and discard the supernatant.
  6. Perform steps 4-5 four times.
  7. Complete resuspension may require brief treatment with the sonic cup horn after backfilling.

 

E.     SATURATION WITH OTHER CATIONS

 

  1. Prepare de-oxygenated solutions of 1 M and 0.005 M chloride salts of the cations of interest, e.g., K, Ca, Cu, and Zn, using the condenser flasks attached to the inert-atmosphere apparatus.
  2. Proceed to wash the reduced sample: first 4 times with the 1 M, then 4 times with the 0.005 M salt solution.
  3. From that point, proceed with the normal procedure for determining cation exchange capacity. Continued use of the inert-atmosphere apparatus depends on whether the final sample needs to be in the reduced state.