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Measuring Specific Surface Area ofSoils and Soil Minerals by Water

Vapor AdsorptionYusuf Setiawan

Dr. Christopher Amrheins LabSoil Chemistry

Department of Environmental Science

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Introduction

Surface area is related to many physical andchemical properties of soils.

Reactive surfaces in soils vary widely becauseof the differences in mineralogical and organiccomposition, as well as in particle-sizedistribution.

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Soil Texture

SandSilt

Clay

2 mm - 50 m50 -2 m

< 2 m

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Specific Surface Area

Surface area per unit mass of soil

Expressed as square meter per gram soil

(m2

g-1

).

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Implications of Surface Area

Cations Exchange Capacity (CEC)

Water retention and movement

Heavy metal and pesticide adsorption

Nutrients and Irrigation management

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Traditional Methods

N2-gas Measures only the external surface area Requires a specialized instrument

EGME (Ethylene Glycol Monoethyl Ether) Measures total surface area (internal and external) Limited to a single partial pressure (P/Po) equilibrium Samples cannot be used multiple times Time intensive

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Water Vapor Adsorption Method

Soils weight and partial pressure can be measured quickly

Saturated salt solutions control relative humidity

Water adsorption forms monolayer surface area calculated

Water potentiameter measures relative vapor pressure (P/P 0)

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Research Objectives

1. Compare surface area measured from watervapor adsorption compared to the EGMEmethod.

2. Compare surface area of soils saturated withdifferent cations.

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Soils Involved in the Experiment Kaolinite (1:1 clay mineral)

Montmorillonite (2:1 clay mineral)

Grangeville

Holland and Merced soils saturated with various cations: NaCl

KCl MgCl2 CaCl2

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Method Different soil types placed in a vacuum chamber with salts

Saturated salt control different relative humidity:

NaOH (8%) LiCl (11%) CaBr2 (20%) K-Ac3 (24%) CaCl2 (32%) LiNO3 (48%)

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Relative Humidity Chamber

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Decagon WP4C DewpointPotentiameter

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How do we know soils have reachedequilibrium?

Weight of soils does not change.

Water Potential ( ) measurement for soils does notchange

-350

-300

-250

-200

-150

-100

-50

01 2 3 4 5 6 7 8

M a t r i c P o t e n t i a l

( - M

P a )

Time (days)

Kaolinite Matric Potential

8% Humidity

20% Humidity

32% humidity

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Adsorbed Water Weight atDifferent Partial Pressure

Surface area is calculated within the partialpressure (P/Po) of 0.05 to 0.45.

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Linearized Surface Area Curve forWater Vapor Adsorption

y = 334.73x + 12.605R = 0.8887

0

20

40

60

80

100

120

140

160

180

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

1 / [ V a *

( P o

/ P - 1

) ]

Relative Humidity (P/Po)

Kaolinite

Kaolinite

Linear (Kaolinite)

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Comparison of Surface AreaMeasurement

Soil Type Water VaporAdsorption

Surface Area

(m2

g-1

)

EGMESurface Area

(m2

g-1

)

Kaolinite 8 7Montmorillonite 348 305

Grangeville 23 16Merced 163 154Holland 142 108

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Comparison of Surface AreaMeasurement

Soil Type Water VaporAdsorption

Surface Area(m

2g

-1)

EGMESurface Area

(m2

g-1

)

Na+ Merced 108 137K

+Merced 94 112

Mg2+

Merced 124 124

Ca2+

Merced 126 130Na+

Holland 210 102K

+Holland 96 103

Mg2+

Holland 86 92

Ca2+

Holland 57 96

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Conclusion

Water vapor adsorption method iscomparable to the EGME method.

Water vapor method is very easy and quick todetermine surface area of soils.

Surface area measurements of the soilssaturated with different cations showed notrend.

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Acknowledgements

Dr. Christopher Amrhein

Myles Davis

Kearney Foundation