The Science Behind Clay Mineralogy and Regulations

1
Hydraulic conductivity decline from wastewater
2
Infiltration rate decline from wastewater
Sandy Soil
3
Smectite (swelling 21 mineral)
Grain coating
(Borchardt, 1986)
4
Smectite low hydraulic conductivity micropore
network
(Borchardt, 1986)
5
11 Clay MineralsCEC 7 meq/100 gnon-expanding
6
11 Clay Minerals
7
21 Clay MineralsCEC 40 meq/100 gnon-expanding
8
21 Clay Minerals (expanding)CEC 100 200
meq/100 g (vermiculite) 70 120 meq/100 g
(smectite)
9
21 Clay Minerals
10
Laboratory Methods

• Direct methods ()
• XRD (all crystalline minerals)
• Infrared spectroscopy (mainly kaolinite)
• Thermal analysis (DTA) mainly kaolinite,
  gibbsite)
• Indirect methods
• Atterberg Tests
• COLE
• Bulk Density/Ksat
• CEC

11
X-ray Diffraction (XRD)

• Identifies minerals based on their crystal
  structure (repeating planes of atoms), and
  expansion and contraction of structure following
  chemical and heat treatments.

12
X-ray diffractometer - for crystalline minerals
when greater than 5 to 10 concentration
100,000
13
soil
X-ray diffraction pattern
14
X-ray Diffraction (XRD)

• Pros and Cons
• Direct measurement of minerals
• Cost (175 - 300 for XRD cost for
  time-consuming clay separation)
• Time (2-3 day turnaround for XRD at commercial
  lab, excluding separation time)
• Detection limit (approx 5-10) exactly where
  cutoff is in taxonomy
• Semi-quantitative (approx. 10-20)

15
X-ray Diffraction (XRD)

• Identifies minerals based on their crystal
  structure, expansion and contraction of structure
  after chemical and heat treatments.
• Define the results
• Picture of Diffractogram
• Mineral ID
• Quantify results
• Pros and Cons
• Direct measurement of minerals
• Cost
• Time
• Sensitivity (approx 10) exactly where cut is
  in taxonomy
• Semi-quantitative (approx. 10-20)

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Apparent CEC

• Cation Exchange Capacity (CEC) definition Sum
  of exchangeable bases (Na, K, Ca2, Mg2) plus
  acidity (H, Al3) at a specific pH.
  (from Soil Sci. Soc. Am., 1997).
• However, direct extraction of soil cations cannot
  distinguish between exchangeable cations and
  cations dissolved from soluble salts.

17
Apparent CEC (contd)

• More accurate to first saturate soil with a
  single index cation (NH4, Na, or Ba2), then
  displace and measure the amount of that cation
  adsorbed).
• In practice, CEC is operationally defined by the
  procedure used and tailored to the purpose for
  which the results will be applied (measured CEC
  depends on the method used).

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CEC Method
NH4OAC Solution pH 7
clay platelet
water
NH4
NH4
NH4
Ca2
NH4
NH4
NH4
NH4
NH4
Na
NH4
NH4
NH4
NH4
NH4
Soil pH
NH4
Al3
NH4
NH4
NH4
NH4
NH4
NH4
NH4
NH4
NH4
Mg2
NH4
NH4
NH4
NH4
NH4
K
NH4
NH4
NH4
NH4
Na
NH4
NH4
NH4
NH4
H
NH4
NH4
NH4
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H2O or alcohol
water
clay platelet
NH4
NH4
Ca2
NH4
NH4
NH4
NH4
Na
NH4
NH4
NH4
NH4
NH4
Al3
pH 7
NH4
NH4
NH4
NH4
NH4
NH4
NH4
Mg2
Na
NH4
NH4
NH4
NH4
NH4
NH4
H
NH4
K
NH4
NH4
NH4
NH4
NH4
NH4
20
NaOAC Solution pH 7
clay platelet
water
Na
NH4
Na
Na
Na
Na
NH4
Na
Na
Na
NH4
Na
Na
NH4
Na
Na
pH 7
Na
NH4
Na
Na
NH4
Na
Na
Na
Na
Na
NH4
Na
Na
NH4
Na
Na
Na
NH4
Na
Na
Na
Na
Na
NH4
Na
Na
Na
Na
Na
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water
clay platelet
Na
Na
Na
Na
NH4
Na
NH4
Na
Na
Na
Na
Na
Na
Na
NH4
Na
NH4
pH 7
Na
Na
Na
Na
Na
Na
NH4
NH4
Na
Na
Na
Na
Na
Na
NH4
Na
NH4
Na
NH4
Na
Na
Na
NH4
Na
Na
Na
Measure displaced ammonium in the water (e.g.,
Kjeldahl)
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Apparent CEC Methods
Soil Taxonomy Method
EPA Method 9081

• Wash soil with pH-7 NH4OAc solution to NH4
  saturate the exchanger phase.

Wash soil with pH-8.2 NaOAc solution to Na
saturate the exchanger phase.
Rinse out dissolved Na.
Rinse out dissolved NH4.
Displace (w/ NH4) and measure exchangeable Na.
Displace (w/ Na) and measure exchangeable NH4.
Divide soil CEC by clay.
Divide soil CEC by clay.
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Apparent CEC (contd)

• Because EPA Method 9081 is a pH 8.2 method
  instead of a pH 7.0 method, it is NOT appropriate
  for apparent CEC measurements tied a cutoff of
  16.3 meq/100 g as used in soil taxonomy.

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CEC increases with increasing pH

• (Data compiled by S. W. Buol)

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Soil minerals have a range of CEC(chemical
compositions vary)
(typical CEC range in meq / 100 g mineral)

• kaolinite 2 15
• smectite 70 120
• vermiculite 100 200
• mica 10 40
• (organic matter) 150 300
• (Sparks, 1995)

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Natural variation in chemical composition and CEC
of 47 soil smectites (Borchardt, 1989)
Increasing octahedral Al vs. Fe
CEC 110 ? 23 Range 47 162 meq / 100 g
Increasing tetrahedral charge vs. octahedral
charge
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Basis for apparent CEC cutoff of 16.3 meq / 100 g
for 10 smectite

• Based in taxonomy to define kandic horizons
• Assumes kaolinite and smectite are dominant clay
  minerals
• Average CEC values used
• kaolinite 7 meq / 100 g
• smectite 100 meq / 100 g
• (90 kaol. x 7) (10 smect. x 100) 16.3
  meq/100g

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Range of apparent CEC for range of mineral CEC

• Low end (kaolinite 2 smectite 70 meq/100 g)
• (90 kaol. x 2) (10 smect. x 70) 9.8
  meq/100g
• High end (kaolinite 14 smectite 120 meq/100 g)
• (90 kaol. x 15) (10 smect. x 120) 25.5
  meq/100g

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Apparent CEC

• Pros and cons
• Reproducible and precise
• Relatively cheap (50)
• Based on the actual mineral properties that
  relate to water movement.
• Should not be used in soils lt 35 clay
• Should not be used on soils gt0.5 OC
• 16.3 meq/100g acec is based on ideal soil sample
  containing only smectite and kaolinite
• Most soil labs are not set up to run the correct
  test

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Assumptions in apparent CEC cutoff of 16.3 meq /
100 g

• (originally used in taxonomy to
• Show high end calculation
• 16.3 is mid point.
• Show range for different methods and pH.