Maataloustieteellinen Aikakauskirja
Vol. 60: 73—79, 1988

Determination of soil specific surface area by water vapor adsorption
111 Comparison of surface areas determined by water vapor and nitrogen
gas adsorption

RAINA NISKANEN' and VÄINÖ MÄNTYLAHTI 2

1 Department of Agricultural Chemistry, University of Helsinki,
SF-00710 Helsinki, Finland

2 Viljavuuspalvelu Oy (Soil Analysis Service Ltd.), Vellikellontie 4,
SF-00410 Helsinki, Finland

Abstract. The specific surface areas of ten soil samples (clay content 1—72 %, organic
carbon content 0.8—11.5 %) were determined by water vapor and nitrogen gas adsorption.
The surface areas obtained by application of the BET equation to water vapor sorption at p/p0

0.12—0.42 (21—195 mVg) were, on the average, 80 % of the areas determined by water vapor
sorption at p/p0 0.20, range 27—229 mVg. A BET water monolayer coverage was formed on
the soil surface at p/p0 0.12—0.20. The BET water area correlated closely with the one-point
water area (p/p Q 0.20). The surface area determined by nitrogen gas adsorption ranged from
0.3 to 21 mVg and did not correlate closely with water areas. The water surface areas were
closely related to soil organic carbon content, while the nitrogen area was primarily related
to soil clay content.

Index words: gas adsorption, BET equation, monolayer coverage, relative humidity, mineral soils

Introduction

The specific surface area of soils and clays
is conventionally determined by low-tempera-
ture adsorption isotherms of non-polar gases
like nitrogen. Adsorption of polar molecules
like water is also used for estimation of soil
surface area. In addition to application of
water adsorption isotherms, one-point methods
are developed. According to Quirk (1955),
surface areas determined by water vapor
adsorption are only approximations to the

areas determined by nitrogen adsorption. In
this connection, areas determined by one-
point adsorption of water at a relative
humidity (p/p0) of 20 % are as useful as
areas determined by water vapor adsorption
isotherms. The disagreement of surface areas
determined by water vapor and nitrogen
adsorption is explained to be due to the fact
that only the external area is estimated by
nitrogen adsorption, while water vapor
adsorption estimates the total surface area

73

JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND

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(van Olphen 1975). The previous study (Nis-
kanen and Mäntylahti 1987 a) showed that
soil surface areas determined by water vapor
adsorption at p/p0

20 % were closely related
to soil clay and organic carbon content. The
aim of this investigation was to study water
vapor sorption on soil and to compare surface
areas determined by the aid of water vapor
sorption isotherms and by water vapor sorp-
tion at p/pG 0.20 with areas determined by
nitrogen gas adsorption.

Material and methods

The material consisted of ten soil samples
which were air-dried and ground to pass a
2-mm sieve. The particle-size distribution of
the inorganic matter was determined by the
pipette method (Elonen 1971) and the or-
ganic carbon content by a modified (Graham
1948) Alten wet combustion method.

For the determination of water vapor
adsorption-desorption isotherms, 1 g of soil
in a tared weighing bottle was equilibrated at
20 °C in a desiccator over solutions con-
trolling relative humidities (p/p0) 0.0—0.97
(Table 1). After equilibration for 2 weeks the
soil + weighing bottle was weighed and
thereafter placed over solution controlling the
higher (adsorption) or lower (desorption)
relative humidity. In the end, soil was dried
for 4 hours at 105 °C (Niskanen and Mänty-

lähti 1987 b). The experiment was carried
out in duplicate.

The soil specific surface area was estimated
by the aid of soil water content at p/p0

0.20

Table 1. Solutions used for controlling water vapor
pressure (Gal 1967, Anon. 1984).

Relative humidity,
p/p0 , at 20 °C

Solution

0.0h2 so4
0.12H2 S04-H2 0 mixture (density 1.56)

Saturated CH,COOK 0.20
0.42» Zn(N0 3)2 .6H 20

» NaNOz 0.66
0.88» K 2Cr0 4

» k 2 so4 0.97

(desorption) (Niskanen and Mäntylahti 1987 a)
and also calculated on the basis of water vapor
desorption isotherms (p/p0 0.12—0.42) by
the BET equation (Brunauer et al. 1938).
The cross-sectional area of 0.106 nm2 (Gal
1967)was assigned for a water molecule. Con-
sidering the water monolayer on soil surface
complete, the water content of 1 % of dry soil
corresponds to the surface area of 35.45 mVg
dry soil.

The nitrogen BET areas of the samples were
measured with a Micromeritics Surface Area
Analyzer MIC-2200. The analysis is based on
nitrogen gas adsorption at low temperature
(—196 °C). At the temperature of liquid
nitrogen, the point is measured where the mo-
nomolecular layer is adsorbed on the sample.
A close-packed arrangement of nitrogen
molecules on the surface gives a mean cov-
erage of 0.162 nm2 per N 2 molecule (Gregg
and Sing 1982).

Results and discussion

Water vapor sorption isotherms (Fig. 1)
showed that the equilibrium soil wetness at a
given relative humidity was greater in the
desorption than in adsorption of water. This
hysteresis may be attributed to the geometric
non-uniformity of pores, the contact-angle
effect, by which the contact angle and the
radius of curvature is greater in an advancing
meniscus than in a receding one, entrapped air
decreasing the water content of newly wetted
soil as well as swelling and shrinking (Hillel
1971). In physical adsorption, usually a
hysteresis loop is observed between the
adsorption and the desorption isotherms. If
the loop does not close at monolayer coverage
and below, different values are obtained for
the areas derived from the adsorption and
desorption isotherm. According to van Olphen

(1975), the desorption isotherm representing
equilibrium more closely, it would be pre-
ferable to calculate areas using desorption
isotherms.

The sigmoidal form of water sorption

74



isotherms indicates multimolecular adsorption
of a physical nature. The amount of water,
w, which will sorb onto soil surface at rela-
tive humidity p/p„, at any temperature T, is
dependent on the interaction energy between
water and soil and the coverage of the surface.
The BET equation (Brunauhr et al. 1938),
derived for the adsorption of non-polar
molecules on solid surfaces, has been applied
also to water vapor sorption on soils (e.g.
Orchiston 1953, Quirk 1955, Puri and Mu-
rari 1963, Dechnik and Stawinski 1970,
Karathanasis and Hajek 1982, Nieminen and
Kellomäki 1982, Nieminen 1985). The main
fundamental assumption in the derivation of
this equation is that the first layer of gas
molecules is attracted to the surface with an
energy greater than that of the subsequent
layers. The energy of the first layer for a given
gas is characteristic of the solid. The heat of

subsequent sorption is simply the heat of con-
densation of the gas with respect to its own
liquid phase. The BET equation is written as

P/Po _ 1P/Po +

w (I—p/p0) w m C wm C
where w m is the quantity of water forming a
monolayer and C is a constant related to the
average heat of monolayer adsorption.

In the application of the BET equation to

water sorption, is plotted against
w (I—p/pD

)

p/pQ
. The BET plots of water vapor sorption

(desorption) on experimental soils are given
in Fig. 2 which shows that the BET equation
was obeyed only at low relative humidity. The
plots can be considered in broad outline linear
at p/p Q

0.12—0.42. In general, the BET
equation is most useful at p/p Q 0.05—0.45
(Mortland and Kemper 1965).

Fig. I. Water vapor adsorption-desorption isotherms of experimental soils.

75



The quantities of water forming a mono-
layer, wm , ranging from 0.59 to 5.50 °7o of
dry soil were calculated on the basis of the
BET equation (p/p0 0.12—0.42) by the
method of least squares. Division of the water
sorption values by wm gives the thickness of
the water molecule layer at different relative
humidities (Fig. 3). The water monolayer
coverage was formed on the surface of ex-
perimental soils at p/p0 0.12—0.20, soils
were covered by a two-molecule layer at p/p0

0.64—0.77 (Fig. 3). Coarse soils were covered
by a monolayer at a lower p/p0

than clay
soils. The number of molecule layers at p/p0

0.20 was 1.00—1.45. The results were of the
same magnitude as those obtained by Niemi-
nen and Kellomäki (1982) who studied water
adsorption on the fine fractions of 90 till
samples. In this material, the monolayer
coverage was formed, on the average, at
p/p0 0.11, range 0.03—0.39.

The specific surface areas of experimental
soils calculated on the basis of BET monolayer
water adsorption, wm, are given in Table 2.
The BET water areas were to some extent
lower than areas calculated by water adsorp-

Fig. 2. BET plots for water vapor sorption on experi-
mental soils.

Fig. 3. Thickness of the water molecule layer adsorbed on soil at different relative humidities

76



Table 2. Surface area of soil samples.

Sample Locality Sampling Org. C, Particle-size Surface area, mVg dry soil
depth, % distribution by adsorption of

cm (pm), %

nitrogen water vapor
<2 2—20 >2O at p/p0 0.20 BET-area

1 Imatra 20—40 11.5 72 17 11 20.85 229.36 194.74
2 Viikki o—2o 5.3 58 14 28 10.01 102.10 87.75
3 Viikki 20—40 0.8 39 6 55 14.80 27.65 27.45
4 Imatra o—2o 1.1 28 41 31 6.5472.32 58.68
5 Hyvinkää 20—40 1.3 19 47 34 8.7741.12 30.91
6 Tohmajärvi o—2o 3.8 4 10 86 2.2260.62 44.92
7 » 30—50 1.1 3 17 80 3.6231.91 22.06
8 Hyvinkää o—2o 1.7 3 2 95 1.3426.94 21.04
9 Viikki o—2o 3.0 2 2 96 0.2728.36 21.61

10 Vaala 20—40 1.3 I 3 96 0.7535.10 27.57

tion at p/p0 0.20 (Table 2). On the average,
the BET areas were 80 % of the areas deter-
mined by sorption at p/p0 0.20. The rela-
tionship between water areas was as follows:
H2O-area (p/p0 0.20) (mVg) = 3.36 + 1.16
H2O-area (BET) (mVg) (r = 0.999***, n =

10).
The application of the BET equation to the

sorption of N 2 is a recognized method for
determination of surface area of soils and
clays. Weakly adsorbed nitrogen will not
penetrate the interlayer surfaces of clay
minerals. Because a sample has to be »de-
gassed» of any sorbed molecules before
determination of surface area, all adsorbed
water is lost. This causes any expanding clay
lattice to collapse and the non-polar nitrogen
gas cannot subsequently enter interlayer areas.
Water, which is polar, is strongly adsorbed
and it penetrates the interlayers leading to high-
er values for surface area. The nitrogen area
will yield a measure of the external surface
area, whereas the water area will give infor-
mation about the total surface area, internal
and external included (van Olphen 1975).

The nitrogen BET areas of experimental
soils were in general much lower than the
water areas (Table 2). The only exception was
the deeper layer soil No. 3 the N2-area of
which was about half the water areas. The
relationship between N - and water areas was
not very close: N2-area (mVg) = 1.64 +

0.08 H2O-area (p/p0 0.20) (mVg) (r = 0.74*,
n = 10) and N2-area (mVg) = 1.74 + 0.10
H2 O-area (BET) (mVg) (r = 0.77**, n =

10).
In a larger soil material (n = 60), 84 % of

the variation in surface area determined by
water adsorption at p/p0 0.20 was explained
by organic carbon and clay content (Niska-
nen and Mäntylahti 1987 a). In this small
material the correlation coefficient between
water area (p/pQ 0.20) and organic carbon
content was r = o.94*** (n = 10) and that
between water area and clay content r =

o.Bl**. The correlation coefficient between
N2 -area and clay content was r = o.92***
and that between N2 -area and organic carbon
content r = 0.63*.

H 2 O-areas were closely related to the
organic carbon content, while N2 -areas
seemed to be related primarily to the soil clay
content. According to Dobrzanski et al.
(1971), the N2-area correlates positively with
clay content and negatively with humus con-
tent. The values of the external surface areas
are lower in the soil with organic substances
and higher after their removal, while the total
surface area is higher in the presence of
organic substances than in their absence
(Dobrzanski et al. 1972). The negative in-
fluence of the organic substances on the ex-
ternal surface area is greater, with a higher
content of clay, because organic substances

77



block micropores. This trend is observable
also in the present material, for instance, when
soils Nos 2 and 3 are compared (Table 2). Soil
No. 2 with higher organic carbon and clay
content had a lower N2 -area and higher H2O-
- than soil No. 3 with lower clay and car-
bon content.

Adsorption of polar molecules like water
provides an experimentally simpler method
for determination of surface area than N -

adsorption. Polar molecules offer the advan-
tage of exploring both the external and in-
ternal area. The disadvantage of polar mole-
cules is their interaction with the surface and

between themselves. Water molecules are
attracted to the bare exchangeable cations and
cluster round them, which implies overlap-
ping of the monolayer and multilayer process-
es (Greenland and Mott 1978). In agri-
cultural soils the contribution of both organic
matter and clay content is a major advantage.
Another advantage is that it makes unne-
cessary the degassing of soil which can affect
soil properties.

Acknowledgement. The authors wish to thank Dr. Pert-
ti Nieminen, Tampere University of Technology, for
analysis of surface area by nitrogen gas adsorption.

References

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Dechnik, I. & Stawinski, J. 1970. Determination of the
total surface area of soils on the basis ofone measure-
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Dobrzanski, 8., Stawinski, J. & Walczak, R. 1971. The
availability of the method of thermal desorption of
nitrogen for estimation of the surface area of soil
material. Polish J. Soil Sci. 4: 81—87.

—, Dechnik, 1. & Stawinski, J. 1972. Correlation
between the soil surface-area and humus compounds
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Elonen, P, 1971. Particle-size analysis of soil. Acta Agr.
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Gal, S. 1967. Die Methodik der Wasserdampf-Sorp-
tionsmessungen. Berlin, 139 p.

Graham, E. 1948. Determination of soil organic matter
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Greenland, D. J. & Mott, C.J .B. 1978. Surfaces of soil
particles. The chemistry of soil constituents (eds. Green-
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Gregg, S.J. & Sing, K.S.W. 1982. Adsorption, surface
area and porosity. 2nd ed., London, 303 p.

Hillel, D. 1971. Soil and water. Physical principles and
processes. 288 p. New York.

Karathanasis, A.D. & Hajek, B.F. 1982. Quantitative
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Mortland, M.M. & Kemper, W.D. 1965. Specific
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Niskanen, R. & Mäntylahti, V. 1987 a. Determination
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II Dependence of soil specific surface area on clay and
organic carbon content. J. Agric. Sci. Finl. 59: 67—72.
& Mäntylahti, V. 1987 b. Determination of soil

specific surface area by water vapor adsorption. I
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Ms received October 16, 1987

78



SELOSTUS

Maan ominaispinta-alan määrittäminen
vesihöyryn adsorption avulla
111 Vesihöyryn ja typpikaasun adsorption
avulla määritettyjen ominaispinla-alojen
vertailu

Raina Niskanen 1 ja Väinö Mäntylahti 2

' Maanviljelyskemian laitos, Helsingin yliopisto,
00710 Helsinki
2 Viljavuuspalvelu Oy, Vellikellontie 4,
00410 Helsinki

Kymmenen maanäytteen (savespitoisuus 1—72 %, or-
gaanisen hiilen pitoisuus 0.8—11.5 %) ominaispinta-ala
määritettiin vesihöyryn ja typpikaasun adsorption avul-
la. Vesihöyryn adsorptiosta 12—42 % suhteellisessa kos-
teudessa BET-yhtälön avulla lasketut ominaispinta-alat
(21 —195 mVg) olivat keskimäärin 80 % ominaispinta-
aloista, jotkasaatiin 20 % suhteellisessa kosteudessa ta-
pahtuneen vesihöyryn adsorption perusteella (27 —

229 mVg). BET-yhtälön avulla laskettu yhden molekyy-
lin paksuinen vesikerros muodostui maan pinnalle

12—20 % suhteellisessa kosteudessa. BET-yhlälön avulla
lasketun ja 20 % suhteellisessa kosteudessa määritetyn
ominaispinta-alan välinen korrelaatio oli tiukka. Typpi-
kaasun adsorption avulla määritetyt ominaispinta-alat
(0.3 —21 mVg) eivät kovin kiinteästi korreloineet vesihöy-
ryn avulla määritettyjenpinta-alojen kanssa. Vesihöyryn
adsorption avulla määritetyt pinta-alat riippuivat voimak-
kaasti maan orgaanisen hiilen pitoisuudestakun taas typ-
pikaasun adsorption avulla määritetyt pinta-alat olivat
ensisijaisesti riippuvaisia saveksen pitoisuudesta.

79