Measuring Specific Surface Area of Soils and Soil Minerals by Water

Vapor Adsorption

Yusuf Setiawan Dr. Christopher Amrhein’s Lab

Soil Chemistry Department of Environmental Science

Introduction

• Surface area is related to many physical and chemical properties of soils.

• Reactive surfaces in soils vary widely because of the differences in mineralogical and organic composition, as well as in particle-size distribution.

Soil Texture Sand Silt Clay 2 mm - 50 µm 50 -2 µm < 2 µm

Specific Surface Area

• Surface area per unit mass of soil

• Expressed as square meter per gram soil (m2 g-1).

Implications of Surface Area

Cations Exchange Capacity (CEC)

Water retention and movement

Heavy metal and pesticide adsorption

Nutrients and Irrigation management

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

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/P0)

Research Objectives

1. Compare surface area measured from water vapor adsorption compared to the EGME method.

2. Compare surface area of soils saturated with

different cations.

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

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%)

Relative Humidity Chamber

Decagon WP4C Dewpoint Potentiameter

How do we know soils have reached equilibrium?

• Weight of soils does not change.

• Water Potential ( Ψ) measurement for soils does not change

-350

-300

-250

-200

-150

-100

-50

01 2 3 4 5 6 7 8

Mat

ric P

oten

tial (

-MPa

)

Time (days)

Kaolinite Matric Potential

8% Humidity

20% Humidity

32% humidity

Adsorbed Water Weight at Different Partial Pressure

Surface area is calculated within the partial

pressure (P/Po) of 0.05 to 0.45.

Linearized Surface Area Curve for Water Vapor Adsorption

y = 334.73x + 12.605 R² = 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)

Comparison of Surface Area Measurement

Soil Type

Water Vapor Adsorption

Surface Area (m2 g-1)

EGME Surface Area

(m2 g-1)

Kaolinite 8 7 Montmorillonite 348 305 Grangeville 23 16 Merced 163 154 Holland 142 108

Comparison of Surface Area Measurement

Soil Type Water Vapor Adsorption

Surface Area (m2 g-1)

EGME Surface Area

(m2 g-1)

Na+ Merced 108 137 K+ Merced 94 112

Mg2+ Merced 124 124 Ca2+ Merced 126 130 Na+ Holland 210 102 K+ Holland 96 103

Mg2+ Holland 86 92 Ca2+ Holland 57 96

Conclusion

• Water vapor adsorption method is comparable to the EGME method.

• Water vapor method is very easy and quick to determine surface area of soils.

• Surface area measurements of the soils saturated with different cations showed no trend.

Acknowledgements

• Dr. Christopher Amrhein

• Myles Davis

• Kearney Foundation