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									                                       TEXAS A&M UNIVERSITY




                           Lab Report 1
    Sample Evaluation, Pretreatment, Dispersion,
          Size Fractionation, and Texture
                                          Matthew J Keough
                                               9/23/2010




Following the collection of a soil sample, several treatments and analytical techniques were pursued to
evaluate the mineral composition and size fractionation of the soil.
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Introduction:

A natural soil sample contains a mixture of minerals, short-distance ordered phases, organic matters,
and biological organisms. For this series of experiments, a Ships soil was collected from a pre-ground (to
2mm) source, appearing brownish-red and texturally high in clay content. Ships soil comes from
Burleson County and developed in clayey and loamy sediments along the flood plains of the Brazos
River, but is rarely flooded. A diagram of the Ships soil and some surrounding soils is given below in
Figure 1. The Ships soils are very deep and very slowly permeable. Typically, the surface layer is dark
brown clay, while the subsoil is brown clay in the upper part and reddish brown silty clay in the lower
part. These soils are moderately alkaline and calcareous throughout. Cotton, corn, and grain sorghum
are the main crops, with soybeans, alfalfa, and small grains being more minor. Some pasture and
hayland plants include common bermudagrass, improved bermudagrass, and dallisgrass. Ships soil has
some limitations for most urban uses, including flooding, very high potential for shrinking and swelling,
and very slow permeability. This soil was chosen because it is often used in my research laboratory in
conjunction with biochar field tests. A better knowledge of the mineralogy and composition of this soil
would be beneficial to my field of research.

                Figure 1. – Typical pattern of soils in the Ships-Belk and Burleson general soil map unit




Though it would be of utmost significance to analyze a completely undisturbed sample, it is simply not
possible without either disturbing the natural order or obtaining inaccurate results. Therefore a balance
must be met between measuring the various characteristics of the soil while trying to maintain in situ
conditions. Additionally, since soil generally contains a wide range of mineral groups, it is important to
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separate fractions in order to concentrate and better identify individual mineral species of interest.
Some treatments include the removal of cementing materials, size fractionation, cation saturation,
solvation, and heating. Several treatments and analytical techniques were performed to characterize the
soil sample.

Materials and Methods:

Sample Evaluation

To start, a dry sample of the Ships soil was obtained and a sub sample was passed through a 2mm sieve.
This sub sample was then ground with a mortar and pestle to uniform consistency with care taken to not
shatter single unit particles. Three aluminum dishes were labeled and weighed. To each dish, a few
grams of soil sample were added and the weight was recorded. After drying in an oven at 105⁰C
overnight, each dish was weighed again and the average moisture content was computed.

The next step involved testing for cementing and flocculating agents, such as carbonates, sulfides,
manganese oxides, and magnetic minerals. Roughly 0.5 grams of sample was placed into three micro
weighing dishes. To the first dish, several drops of 1N hydrochloric acid were added to test for the
presence of carbonate ions, mainly calcite. Visual observations were recorded and several drops of 30%
hydrogen peroxide were added to the second dish. Vigorous bubbling indicated the presence of
oxidizing/reducing agents like manganese oxides, sulfides, and organic matter. Finally, a magnetic stir
bar was passed over the third dish to test for any minerals exhibiting magnetic properties.

After preliminary testing of cementing and flocculating agents, the soil sample was tested for evaporate
minerals such as halides, sulfates, nitrates, and borates. First, a sub sample of 12.471g (calculated oven
dry weight: 11.968 g) < 2 mm air-dried Ships soil was weighed out and added to a 250 mL centrifuge
bottle. To this, deionized water was added to achieve a ratio of 5:1 and the bottle was placed on a
reciprocating shaker for 30 minutes. Following the shaker, the bottle was centrifuged at 2000 rpm for 10
minutes. The resulting supernatant was relatively clear, and about 20 mL was removed to test electrical
conductivity (EC) and pH. To test for gypsum, roughly 2 mL of supernatant was added to 2 mL of acetone
in a 10 mL test tube and mixed. Any cloudy white precipitate would indicate a presence of gypsum. The
centrifuged sample was saved for future analysis.

Finally, an X-ray diffraction (XRD) analysis was run with the bulk sample to identify major mineral
components. A few grams of sample were placed in a mortar and pestle and crushed to pass through a
140-mesh sieve. The < 140-mesh sample was then transferred to an XRD mount following the front
loading procedure. Special care was taken to fill and level the mount cavity with sample.

Sample Pretreatment

 In order to buffer the remaining centrifuged sample, 50 mL of pH 5 sodium acetate was added to the
250 mL centrifuge bottle. The bottle was shaken periodically and placed in a 90⁰C water bath with the
cap loosely removed for about 30 minutes. Following the hot water bath, the bottle was shaken and
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centrifuged at 2000 rpm for 5 minutes. The supernatant was siphoned off to remove any carbonates in
solution and fresh sodium acetate was added. This procedure was repeated until CO2 bubbling ceased.

Soil organic matter was removed by adding 20 mL of sodium acetate to the sample from which
carbonate minerals have been removed. After gently shaking, the centrifuge bottle was placed into a
larger beaker and 10 mL of 30% hydrogen peroxide was slowly added, watching for excessive
bubbling/frothing. After sitting overnight, the centrifuge bottle was placed in a 70⁰C water bath for 30
minutes then centrifuged at 2000 rpm for 5 minutes. The supernatant was siphoned off and the
centrifuged sample was saved for future analysis.

Size Fractionation

Sample dispersion was initiated by adding 50 mL of pH 10 sodium carbonate dispersion agent to the
sample in which flocculating and cementing materials have been removed. The bottle was shaken and
centrifuged at 2000 rpm for 10 minutes. The clear supernatant was siphoned off and fresh sodium
carbonate was added once again. After another centrifuge cycle, a cloudy supernatant was achieved,
indicating the dispersion of silt and clay fractions.

A 53 µm sieve was placed inside a funnel with a large beaker below to catch any filtrate. The contents of
the centrifuge bottle were passed through the sieve while continuously washing with sodium carbonate.
After all the particles have been washed and transferred onto the sieve, the remaining sand fraction was
collected and dried overnight at 105⁰C.

The remaining silt and clay fractions collected in the large beaker were brought to the 1000 mL mark
and allowed to stand overnight. The <2 µm clay in the upper portion of the beaker was collected into a
4-L beaker. The remaining silt and clay were placed into a labeled centrifuge bottle and attached to an
automated sedimentation-siphoning instrument to separate the clay and silt fractions. Due to the length
of time required for proper sedimentation, the fractionation was allowed to run for several days before
being separated into individual fractions. The silt-containing suspension was washed into a pre-weighed
aluminum dish with deionized water and dried to obtain the silt content.

The remaining clay fraction was collected in a 4 L beaker and a 2 L pitcher, and flocculated with several
grams of NaCl. Any remaining supernatant was siphoned off, and then the contents of the two
containers were mixed. In roughly 250 mL batches, the clay fraction was poured into a centrifuge bottle
along with 10 g of NaCl and run for 5 minutes at 2000 rpm. The clear supernatant was siphoned off and
more of the clay fraction was added/centrifuged until a concentrated sample of the clay fraction was
obtained. Following the fractionation procedure, dialysis of the clay fraction was started to remove
electrolytes and further purify the sample for analysis.

After measuring the volume of suspension, a 25 cm piece of dialysis tubing was cut, tied, and soaked in a
jug of deionized water. Once properly hydrated, the dialysis tubing was expanded and the clay
suspension was slowly poured into the tubing. Any excess particles were washed into the tubing with a
limited amount of distilled water. The other end of the dialysis tubing was tied off and placed into a 4 L
beaker of deionized water for several days.
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Results and Discussion:

Since all calculations are to be based on the oven-dry weight, it is necessary to determine the moisture
content of the soil sample. After obtaining average moisture content from three sub samples, future
calculations can be calibrated to the oven-dry basis without repeating the drying procedure. The
moisture contents of the Ships soil is given below in Table 2.1 and Equation 2.1 shows how these values
were obtained. From the data below, the soil sample has an average moisture content of 4.03%, which
can be used when making future calculations.



                                                                                                     (Eq. 2.1)



                              Table 2.1 Moisture measurement form of Ships soil

Replication    Dish Weight (W1, gram)      Dish + air dry soil (W2,     Dish + oven dry soil (W3,    Moisture (%)
                                           gram)                        gram)
1              1.2646                      3.1475                       3.0757                       3.96
2              1.2873                      3.3502                       3.2701                       4.04
3              1.2981                      3.5081                       3.4211                       4.10
                                           Average                                                   4.03


A preliminary test of cementing and flocculating ions was conducted prior to further analysis.
Hydrochloric acid (1 N) was applied to test for carbonate ions, hydrogen peroxide (30%) was applied to
test for oxidizing/reducing agents, and acetone was applied to test for gypsum. A subjective scale of
reaction is listed in Table 2.2, along with electrical conductivity and pH. It is important to test for
electrical conductivity before pH because the pH electrode contains saturated KCl solution which can
release into the supernatant and increase the observed EC value.

                                Table 2.2 Sample evaluation form of Ships soil

Sample     React with HCl     React with H2O2        Magnetic minerals       Gypsum          EC        pH
           0 = no reaction;   5 = vigorous           0 = none;               5 = abundant
           4                  4                      0                       0               136.1     7.78


The relatively vigorous reaction to HCl and H2O2 indicated a presence of carbonates and
oxidizing/reducing agents, respectively. Depending on the mineral, hydrogen peroxide can function
either as an oxidizer or as a reducer. There were no magnetic minerals or gypsum compounds present in
the Ships soil. Given the reaction to HCl and presence of carbonate ions, it makes sense that the pH
would be slightly basic. According to the electrical conductivity measurements, there were not many
salts present in this soil sample.
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Before beginning the fractionation procedure, a bulk sample was taken to be analyzed by XRD and FTIR.
The XRD pattern is attached at the end of this report as Figure 2 and another XRD pattern with overlying
mineral data is attached as Figure 3. The results of the XRD analysis show a prominent peak at 26.7 on
the 2θ scale, with a d-spacing value of 3.340 angstroms. The next largest peak is at 20.9 on the 2θ scale
with a d-spacing value of 4.248 angstroms. According to these peaks and d-spacing values, quartz is the
dominant mineral in this soil. Plotting other pure mineral patterns on top of the bulk sample pattern
shows calcite and kaolinite as other mineral components. Small amounts of halloysite and albite may
also be present in the soil, however their contents would be much lower than those minerals mentioned
above. According to the FTIR spectrum attached as Figure 4, calcite is identified by its peaks around
1432 cm-1 (CO3 stretching) and 1795 cm-1. Quartz is also identified in this spectrum, as should be
expected, and its notable peaks include 1161 cm-1 (Si-O), 994 cm-1 (Si-O-Si), and 693 cm-1 (Si-O). FTNIR
spectrum is given in Figure 5, and provides deeper analysis into the higher energy vibrations. Kaolinite
can be seen at peaks 5240 cm-1 (stretching/bending vibrations of water), 4533 cm-1, and 7066 cm-1.
Calcite is difficult to see at the near-IR range and may be partially covered by the kaolinite spectrum.
Further interpretation of X-ray diffraction and Fourier Transform Infrared spectra of specific fractions
will be discussed in future laboratory sessions.

Following the soil triangle, the Ships soil is a silty clay or clay soil, depending on the content of missing
fractions. Table 5.1 below shows the oven-dry weights of the sand, silt, and clay fractions. The sand
content was found to be 3.63% (0.4348/11.968 = 0.0363). The silt content was much higher, and found
to be 33.65% (4.0277/11.968 = 0.3365). The clay content was found by more complex methods. After
dialysis, the clay suspension was washed into a container and the weight of suspension was recorded
(129.9055 g). A small drop of the suspension was weighed before and after drying to obtain the content
of clay within the suspension (4.59%). Multiplying the content of clay within the suspension by the
weight of the total suspension gave the total weight of clay in the sample (.0459*129.9055 g = 5.96 g).
The clay content was very high, at 49.80% (5.96/11.968 = 0.4980). It should be noted that a significant
amount of material was inherently lost through the fractionation process, as the sum of the fractions
comes out to only 87.08% of the total sample.

                                     Table 5.1 Weight of each soil fraction

Sample     Dish weight (W1, gram)      Dish + oven dry soil          Weight of soil (W1-W2,
                                       (W2, grams)                   grams)
Sand       1.2848                      1.7196                        0.4348
Silt       1.2925 + 1.2889             5.2669 + 1.3422               4.0277
Clay                                                                 5.9600


After identifying the presence or absence of certain flocculating and cementing agents, it was necessary
to remove any agents that would make fractionation difficult. The sodium acetate treatment worked by
dissolving any remaining carbonate ions, while the hydrogen peroxide removed a great deal of organic
matter. However it should be noted that some organic matter would inherently remain.
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Sample dispersion could be achieved by adding a dilute sodium carbonate (pH 10-10.5) to the soil
sample after pretreatment. Because of its high pH, minerals with variable charges become negatively
charged at the surface or edge, thereby repelling like-charged particles and increasing dispersion. A
dilute sodium carbonate would reduce the electrolyte concentration and increase the double-layer
thickness of the colloids, promoting dispersion. Finally, since sodium is monovalent, more Na+ ions
would be required in the double-layers than polyvalent cations. Such crowding would increase double-
layer thickness as well as dispersion.

In order to calculate the sedimentation of different sized particles, it is important to understand Stokes’
Law. Stokes’ Law is basically a relationship between the force of gravity and the force of buoyancy in a
given solution. Equation 5.6 relates the time it takes for a soil particle to fall a given distance in a
particular fluid.

                                                                                                     (Eq. 5.6)

Where the units of the terms are:

t (settling time): minute
η (viscosity of the fluid): pois, or g/cm•sec
L (distance of the particle traveled): cm
d (diameter of the particle): µm
ρp and ρf (densities of the particle and the fluid respectively): g/cm3

To obtain a concentrated, clean sample of clay for chemical and mineralogical analyses, excess
electrolytes can be removed through dialysis. The clay fraction is poured into a dialysis tubing made of
semi-permeable membrane through which only ions or small molecules can pass. The tubing is then
placed in a beaker of deionized water so that any electrolytes in the tubing can flow through the semi-
permeable membrane into the water while the colloid sized particles stay behind. As this process
requires a highly concentrated solution to transfer into a very dilute solution, the deionized water had to
be changed out of the jugs at least twice a day. This process can be enhanced by a number of methods,
such as increasing the volume of distilled water, however the most practical procedure was selected
based on time and laboratory conditions. The dialysis process is currently underway.

Conclusion:

Since only the weight of the sand fraction is currently known, it is not possible to identify the soil texture
at this time. After obtaining an oven-dry weight of the silt fraction, an estimation of the remaining clay
content can be made. Though originally high in carbonates, oxidizing/reducing agents, and organic
matter, most of these flocculating agents have been removed and the fractions separated into
concentrated portions. Running an XRD or FTIR on the individual fractions will give a better
understanding as to what minerals constitute this Ships soil.
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References:

   1. Soil Mineralogy with Environmental Applications. Edited by Dixon, J. B. and Schulze, D. G. 2002.
      Soil Science Society of America, Inc. Madison, Wisconsin.
   2. Soil Mineralogy Laboratory Manual. Deng, Y., White, G. N., and Dixon, J. B. 2009. Texas A&M
      University, College Station. TX.
   3. Soil Survey of Burleson County, Texas

								
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