Combined Size and Density Fractionation of Soils for Investigations of Organo-Mineral Interactions

This method is an interesting approach to study organo-minerals interactions because it fractionates soil organic matter according to its association with different minerals. It is a physical fractionation process, which means it doesn't alter the chemistry of the fractions. As a result, analysis of the fraction tells us something about the natural composition of organo-mineral complexes.

This method is really useful as we try to learn more about the ability of different soils to sequester carbon away from the atmosphere. To separate the free light fraction, weigh five to eight grams of air dried sieved soil in a 50 milliliter conical centrifuge tube. Record the mass of soil with four significant figures.

Using a graduated cylinder, add 35 to 40 milliliters of sodium polytungstate or SPT. With a density of 1.62 grams per cubic centimeter, centrifuge for 90 minutes at 2500 times gravity in a swinging bucket centrifuge to afford a clear separation between the free light fraction and the pellet. Pour all floating and suspended material into a 250 milliliter polycarbonate centrifuge bottle.

Ensure that the pallet remains firmly lodged at the bottom of the tube. Rinse the material adhering to the wall of the tube into the same polycarbonate bottle. Use a squirt bottle filled with deionized water holding the tube almost upside down above the polycarbonate bottle.

Setup a vacuum ultrafiltration line with 0.45 micron filters. Slightly moisten the filters and apply vacuum before tightening the funnels to avoid tears. Slowly pour the content of the polycarbonate bottle into the funnel of a vacuum filtration unit.

Rinse out any residue left in the polycarbonate bottle into the filtration unit. Do not allow the solution to pull over a depth greater than approximately one centimeter over the filter. Add more than 10 milliliters of deionized water to the filtering unit at least three times to ensure that all traces of SPT are removed.

Make sure to rinse the side of the filtration funnel. After releasing the vacuum on the filtration line, remove the funnel from the filtration unit. Recover the material adhering to the sides into a labeled aluminum bowl using a squirt bottle filled with deionized water.

Carefully lift the filter with tweezers and rinse the material present into the same aluminum bowl using a squirt bottle filled with deionized water. Dry the boat contents at a maximum of 65 degrees Celsius to constant weight. After at least 48 hours, cool the bowl contents in a desiccator containing fresh desiccant for at least 30 minutes.

Gently scrape the material off the aluminum bowl with a plastic spatula. Record the mass of the free light fraction with four significant figures, then place the sample in a storage vial. To perform release of the occluded light fraction, add 35 to 40 milliliters of SPT with a density of 1.62 gram per cubic centimeter to the centrifuge tube containing the pellet from the free light fraction extraction.

Resuspend the pellet. Insert the ultrasonic probe at two centimeters below the surface of the solution and place the tube in iced water to prevent bulk solution heating. Sonicate the sample using the necessary time to reach the target energy of 280 joules per milliliter.

To separate the occluded light fraction, centrifuge for 90 minutes at 2500 times gravity in the swinging-bucket centrifuge. Pour all floating and suspended materials into a 250 milliliter polycarbonate centrifuge bottle. Ensure that the pellet remains firmly lodged at the bottom of the tube.

Rinse the occluded light organic material adhering to the all of the tube into the same polycarbonate bottle as before. Add refrigerated deionized water to the tube containing the pellet up to the 40 milliliter mark. Sonicate at 75 joules per milliliter Leave the content to settlement for the time required for material finer than eight microns to settle.

Then pipette out the supernatant down to the 10 milliliter mark into a 50 milliliter centrifuge tube. Place the tubes containing the less than eight micron fraction as well as those containing the greater than eight micron fraction into the oven at 45 degrees Celsius to evaporate the remaining excess liquid. To perform density fractionation, add 35 to 40 milliliters of SPT with the density of 2.78 grams per cubic centimeter to each size fraction.

Resuspend the pellet and centrifuge for 90 minutes at 2500 times gravity. Then, pour all floating and suspended materials into a 250 milliliter polycarbonate centrifuge bottle, ensuring that the pallet remains firmly lodged at the bottom of the tube. It may be difficult to get a clear separation between the floating material and the pellet.

In this case, you can try aspiration of the supernatant down to a set level. Now transfer the pellets into 250 milliliter polycarbonate bottles using a squirt bottle filled with deionized water. Add deionized water to the polycarbonate bottles to lower the density without exceeding the 200 milliliter mark.

Do this for all fractions. Following centrifugation of the bottles for 20 minutes at 5000 times gravity, decant the supernatant into the used SPT jar for recycling. After performing the washing procedure as described in the text protocol, collect the heavy fraction in an aluminum bowl, the heavy fraction is the pellet and fine particles adhering to the side of the bottles.

Dry the heavy fraction at a maximum of 105 degrees Celsius to constant weight. Store the heavy fraction in the same way as the light fraction. The distribution of material mass between fractions, showed strong differences between sites.

As initially indicated through bulk sample mineralogy as assessed by powder X-ray diffraction, at site one dominated by primary silicates such as quartz and feldspars, most of the materials are recovered in heavy fraction one which was designed to concentrate coarse silicates. Site two showed a greater percentage of phyllosilicates mostly kaolinite during mineralogical analysis. Accordingly, heavy fraction three which was designed to concentrate fine silicates had more materials at site two than at site one.

Finally, site three was the richest in oxides and also showed the greatest amount of material in heavy fraction two which was designed to concentrate coarse oxides. Overall, the data indicates that the method was successful in physically fractionating bulk samples into their main mineralogical components. As for any physical separation scheme, it is important to tailor your fractionation parameter to your specific objectives.

Make sure to have a test run and to work with several replicates to check reproducibility. Even though Sodium polytungstate is less toxic than previously used and solution, it remains an irritant and is harmful if swallowed or inhaled. Avoid its release into the environment.

Following the isolation, the different fractions can be analyzed for mineralogy as well as organic matter content and composition. This gives us information about both the mineral and the organic matter partner in the organo-mineral association. This method may shed new light on organic matter degradability.

Indeed apparent recalcitrants may not be so much a property of a given organic compound but of its affinity for different minerals.