Method for Particle-size Fractionation of Clays

by

Professor Jun Wu

(Laboratory of Joseph W. Stucki, University of Illinois)

In equation (1) (ref. M.L. Jackson (1973) Soil chemical analysis-advanced course, ed.2), T_m is the time for sedimentation in minutes, his the viscosity in poise at the existing temperature, R is the radius in cm of rotation of the top of the sediment in the bottle, S is the radius in cm of rotation of the surface of the suspension in the bottle, N is the rpm, D is the particle diameter in microns, and Ds is the difference in specific gravity between the solvated particle and the suspension liquid.

1. Stir the clay (30 to 50 gram) and shake in about 500 mL of 1 M NaCl solution for 24 hr.

2. Use the SH3000 rotor (Swing Bucket) for the Dupont Sorvall Model RC5C Plus refrigerated centrifuge and a 250-mL centrifuge bottle.

3. Place a mark on the bottle 7 cm from the bottom and place a mark near the bottom corresponding to the top of the sediment (it will be about 0.2 cm from the bottom). These marks correspond to S =10.5 cm and R = 17.3 cm. The viscosity in poise is 0.01111 at 16EC and the difference in specific gravity between the clay (~2.65) and water (0.999) is 1.651. The centrifuging time depends on the amount of sediment, temperature, rpm, and particle size. The time for sedimentation is 9.2 min when you use 500 rpm and wish to separate the <2 mm size fraction (equation 2).

4. To collect the clay particles which are less than 2 mm, decant the supernatant. This supernatant contains the particles (less than 2 mm).

5. When you fractionate 0.5 mm clay particles, centrifuge the supernatant at 2000 rpm for 9.2 min (set to10 min, sometimes the temperature of centrifuge is below 16EC even when you set to 16EC). The particle size of the sediment is between 0.5 to 2 mm. The supernatant has the particles which are less than 0.5 mm. Remember that these parameters are dependent upon your experiment.

6. Wash the clays with 1 M NaCl solution 4 times and rinse with 0.001 M NaCl solution 4 times (or rinse it 2 times and dialyze it)- See Lab book for Na saturation.

If you want to fractionate a purified clay fraction without quartz and iron oxides, after step 5 (skip step 6), follow the procedures beginning with step 7 below:

7. Centrifuge the fractionated sample (step 5) at high speed, such as 12,000 x g (Dupont Model Sorvall RC 5C plus centrifuge with SS-34 rotor, 10,000 rpm) and save the supernatant.

8. Replace the supernatant liquid with fresh 1 M NaCl solution, then shake well again.

9. Repeat Steps 7 and 8 6 to 7 times; and wash the sample several times with pure H₂O to eliminate excess NaCl and to disperse the layers. The first washings in this step consist of a rapid, short agitation followed by short centrifugation (12,000 x g), which removes the largest Fe oxide particles that are settled to the bottom of the centrifuge tubes. As the number of washing cycles increases, the layers become more dispersed, resulting in a suspension or stable gel which still contains some impurities. Any white or brownish-yellow deposit should be removed, then the gel is separated, diluted in pure H₂O and/or 0.5 mM NaCl, and shaken vigorously to redisperse the layers. The centrifugation speed is increased progressively with the number of dispersion-centrifugation cycles, beginning at 10,000 rpm and ending at 15,000 rpm. The dispersion-centrifugation cycles are continued until no change in the color of the deposit is observed.

10. The gel is then diluted with 1 M NaCl to obtain a concentrated suspension, then excess salts are removed by centrifuge washing with 0.5 mM NaCl (see step 6) and the resulting suspension (15 to 25 mg/mL) is kept in a plastic bottle for storage.