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Thermodynamics of Potassium–Magnesium Exchange inTwo Alfisols of Northern GreeceC. Sinanis a , V. Z. Keramidas b & S. Sakellariadis ba Technological Educational Institute of Krete , Heraclion, Greeceb Department of Hydraulics , Soil Science and Agricultural Engineering, School ofAgriculture, Aristotle University , Thessaloniki, GreecePublished online: 24 Jun 2011.
To cite this article: C. Sinanis , V. Z. Keramidas & S. Sakellariadis (2003) Thermodynamics of Potassium–Magnesium Exchangein Two Alfisols of Northern Greece, Communications in Soil Science and Plant Analysis, 34:3-4, 439-456, DOI: 10.1081/CSS-120017831
To link to this article: http://dx.doi.org/10.1081/CSS-120017831
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Thermodynamics of Potassium–MagnesiumExchange in Two Alfisols of Northern Greece
C. Sinanis,1 V. Z. Keramidas,2,* and S. Sakellariadis2
1Technological Educational Institute of Krete, Heraclion, Greece2Department of Hydraulics, Soil Science and Agricultural Engineering,
School of Agriculture, Aristotle University, Thessaloniki, Greece
ABSTRACT
The exchange behavior of potassium (K) in soils has been extensively
studied relative to a dominant cation, which in most cases was considered
to be calcium (Ca). Magnesium (Mg) was tacitly assumed that existed in
minor quantities or that it had the same exchange behavior with Ca.
However, under certain conditions and practices (Mg-rich soils,
greenhouse crops) Mg can become an important component of the soil
exchange complex participating drastically in the exchange reactions.
Consequently, the exchange behavior of the pair K–Mg has merit and in
this study the thermodynamics of exchange for this pair were evaluated in
two Alfisols with different mineralogical composition of the clay fraction.
The exchange reaction was studied in both directions, namely, Mgsoil !
Ksoil and Ksoil ! Mgsoil:
439
DOI: 10.1081/CSS-120017831 0010-3624 (Print); 1532-2416 (Online)
Copyright q 2003 by Marcel Dekker, Inc. www.dekker.com
*Correspondence: V. Z. Keramidas, Department of Hydraulics, Soil Science and
Agricultural Engineering, School of Agriculture, Aristotle University, Thessaloniki
54124, Greece; E-mail: vkeramid@agro.auth.gr.
COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS
Vol. 34, Nos. 3 & 4, pp. 439–456, 2003
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Comparison of the experimental exchange isotherms to the
non-preference isotherms revealed that both soils exhibited preference for
K relative to Mg. Standard free energies of exchange (DG0) for the reaction
Mgsoil ! Ksoil were negative (25.7 to 26.7 kJ mol21) indicating the
spontaneityof thereactionandthat theformationofaK-soil is favored.Also,
DG0 values for the reaction Ksoil ! Mgsoil were positive (7.3 to
7.5 kJ mol21) which indicated the reluctance of K-soil to be converted to
Mg-soil, thus corroborating the previous findings. Standard enthalpies of
exchange (DH0) were negative (28.7 to 213.5 kJ mol21) for the reaction
Mgsoil ! Ksoil and positive (13.8 to 15.1 kJ mol21) for the backward
reaction suggesting a strong binding of K with some sites of the soils.
Vanselow selectivity coefficients (KV) showed a stronger preference for K
in thesoilwheremica-typemineralsdominated theclayfraction.Selectivity
forK, as judgedby theKV values,was also stronglyK-saturation dependent;
the lower the degree of K saturation the higher the selectivity for K. Under
the conditions of the experiment the exchange reaction of the pair K–Mg
was not reversible but exhibited hysteresis, the magnitude of which was
more pronounced in the soil where mica-type minerals were prevalent in the
clay fraction.
INTRODUCTION
Thermodynamic principles have been used for many years to describe
exchange equilibria on clay and soil surfaces[1 – 4] and excellent reviews on the
subject exist in the literature.[5 – 7] Numerous studies are available where these
principles have been used to obtain exchange coefficients and thermodynamic
parameters for various binary or ternary soil exchange systems. In binary
exchange systems, the cation most frequently studied was potassium due to its
importance as a plant nutrient.[8 – 13] In most of these studies the exchange
behavior of K was evaluated against Ca in an effort to simulate field conditions
of normal soils where Ca is considered to be the dominant cation in soil
solution and on the soil solid phase. Magnesium was tacitly assumed that
existed in minor quantities or that it had the same exchange behavior with Ca.
With respect to the first assumption, however, there are soils of arid and
semiarid regions or soils derived from Mg-rich parent material, in which Mg
can become the second dominant cation. Furthermore, in soils of temperate
regions used for greenhouse crops, heavy dressings of Mg fertilizer and
manure are normally applied. Such a practice can render Mg an important
component of the soil exchange complex participating drastically in the
exchange reactions. With respect to the second assumption, it is reasonable to
expect the same exchange behavior of Ca and Mg because of their similar
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chemical nature. However, considering the subtle differences between them
(ionic size, degree of hydration) and the nature of clay minerals, it is also
reasonable to expect that there might be some differences between them when
they participate in the exchange reaction with K. This, however, needs
experimental proof.
Information on the exchange behavior of K versus Mg is very limited to
the best of the authors’ knowledge.[14,15] In these studies, where the exchange
pairs K–Mg and K–Ca were compared, the results showed that soil
preference for K was increased when Mg was the dominant cation
participating in the exchange reactions.
Therefore, we believed that the exchange behavior of the pair K–Mg
merits attention and it is worthwhile obtaining much more information on the
interaction of these two cations. The objectives of the present work were to
evaluate the exchange behavior of the pair K–Mg in two Alfisols with
different mineralogical composition of the clay fraction, using thermodynamic
principles and deriving thermodynamic parameters characterizing the
exchange process.
MATERIALS AND METHODS
Theoretical Considerations
Considering the following exchange reaction:
MgE2 þ 2K ! 2KE þ Mg ð1Þ
where E denotes the exchanger, the corresponding Vanselow exchange
selectivity coefficient KV, treating the exchanger as an ideal solid solution, is
given by[5]
KV ¼ N2KðCMggMg
Þ=NMgðCKgKÞ2 ð2Þ
where C is concentration of K or Mg in the equilibrium solution in mol kg21, g
is the single ion activity coefficient calculated from the Davies equation[16]
and N the mole fraction for K or Mg on the exchanger, given by the following
equations:
NK ¼ nK=ðnK þ nMgÞ ð3Þ
NMg ¼ nMg=ðnK þ nMgÞ ð4Þ
where n is mol kg21 soil of exchangeable K or Mg.
Potassium–Magnesium Exchange 441
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Exchangeable cations may also be expressed as mol adsorbed charge
kg21 soil (q). Then the total charge on the exchanger phase (Q) is
Q ¼ qMg þ qK ð5Þ
and the equivalent fraction (x̄) of exchangeable K or Mg is given by:
�xK ¼qk
Q�xMg ¼
qMg
Qð6Þ
From the concentration (C) of K or Mg in the equilibrium solution, their
equivalent fraction (x) can also be calculated by the following equations:
xK ¼C
TNxMg ¼
2C
TNð7Þ
where TN is the total normality of the equilibrium solution.
The plot of equivalent fractions of a particular ion on the exchanger phase
versus equivalent fractions in the equilibrium solution results in the exchange
isotherm.
Non-preference exchange isotherms, under constant normality, for the
heterovalent exchange Mg–K, can be constructed using the following
equation:[5]
�xK ¼ 1 þ2
�xKTN
1
x2K
þ1
xK
� �� �212
ð8Þ
The thermodynamic equilibrium constant, Keq, for reaction (1) and treating
the exchanger as a non-ideal solid solution is given by the following equation
using a fraction of unity as the standard state of adsorbed ions:[1]
ln Keq ¼
Z 1
0
ln KV d�xk ð9Þ
The above integral is usually solved with the help of the experimental exchange
isotherm (x̄k vs xk), from which values of KV are estimated at selected values of
x̄k. The equilibrium constant, Keq, may then be calculated by plotting ln KV as a
function of x̄k and determining the area under the curve by applying the
trapezoidal rule. Knowledge of Keq permits the calculation of the DG0, the
standard free energy of the exchange, DH0 the standard enthalpy of exchange,
Sinanis, Keramidas, and Sakellariadis442
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and DS0, the standard entropy of exchange, using the following equations:
DG0 ¼ 2RT ln Keq ð10Þ
d ln Keq=dT ¼ DH0=RT2 ð11Þ
DS0 ¼ ðDH0 2 DG0Þ=T ð12Þ
where R is the universal gas constant and T the absolute temperature.
The above theoretical considerations also apply to the reverse reaction:
2KE þ Mg ! MgE2 þ 2K ð13Þ
Soils and Basic Characterization Analyses
Soil samples were collected from, the Ap horizon (0–20 cm depth) of two
cultivated Alfisols from different locations of northern Greece. Soils of this
Order represent a high percentage of the cultivated soils, are typical of flat
areas of Greece and are used for the establishment of greenhouse crops. Soil 1
is a sandy loam (Typic Haploxeralf) and Soil 2 is a clay loam (Typic
Rhodoxeralf) both developed on limestone (Table 1). The bulk samples were
air-dried and the material that passed through a 2 mm sieve was used for all
subsequent analyses. Organic carbon was determined by the wet oxidation
method of Walkley and Black,[18] CaCO3 volumetrically using a calcimeter
and particle size analysis by the pipette method.[19] Electrical conductivity
was measured in the saturation extract and cation exchange capacity (CEC)
was determined using CH3COONa, 1 N, pH ¼ 8:2; as saturating solution and
CH3COONH4, 1 N, pH ¼ 7; as extracting solution.[20] The mineralogy of clay
fraction of the soils was determined by obtaining X-ray diffractograms of
parallel oriented clay specimens, using a diffractometer Phillips PW, 1830
equipped with a Cu target operated at 45 kV and 30 mA and a graphite crystal
monochromator. Pretreatment of the soils for mineralogical analysis was
performed as described by Kunze and Dixon.[21]
Soil Separates and Their Homoionic Saturation with
Potassium or Magnesium
The general scheme described by Sposito et al.[22] and Fletcher et al.[23]
was followed. Adequate quantities (0.6 and 0.3 kg for Soil 1 and Soil 2,
Potassium–Magnesium Exchange 443
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respectively) of the fine earth of the two soils were mixed vigorously with 1 L
of a 0.5 N NaCl solution and allowed to settle in a 2-L beaker. The supernatant
solution and floatable debris were discarded and the remaining soil material
was mixed with distilled water. After a few minutes of standing, the upper
fraction of the suspension (principally clay and silt) was decanted carefully
and collected. The material remaining in the beaker (principally sand) was
remixed with distilled water. This procedure was repeated several times until
the volume of the collected suspension reached about 10 L. This suspension
was divided into two 5-L samples; one sample for K saturation and the other
for Mg saturation.
Each sample was flocculated by mixing it with 1 L of a 0.1 N solution of
the chloride salts of K or Mg and after standing for 1 h, the supernatant liquid
was discarded. The flocculated soil material of each sample was
homoionically saturated with K or Mg by mixing it with 1 L of 0.1 N
KClO4 or Mg(ClO4)2 solution, shaking for 20 min followed by centrifugation
to remove excess salt solution. This operation was repeated five times after
which the soil separates were mixed again with KClO4 and Mg(ClO4)2
solutions and allowed to settle for 24 h under refrigeration. Since after this
time the supernatant solution was uncolored, indicating a low concentration of
Table 1. Selected chemical, physical, and mineralogical properties of the two soils
studied.
Soil 1
(Typic Haploxeralfa)
Soil 2
(Typic Rhodoxeralf)
pH (1:1 H2O) 7.5b ^ 0.1 6.2 ^ 0.1
CaCO3 (g kg21) 3 ^ 0.1 0
Organic C (g kg21) 9 ^ 1 11 ^ 1
Electrical conductivity of the
saturation extract (dS m21)
0.8 ^ 0.1 0.4 ^ 0.1
Sand (g kg21) 607 ^ 20 337 ^ 20
Silt (g kg21) 225 ^ 20 325 ^ 20
Clay (g kg21) 168 ^ 20 338 ^ 20
CEC (cmolc kg21) 12.7 ^ 1 26.5 ^ 2
Mineral suite of
the clay fractioncMi1, Chl3, Ka4 Sm1, Chl2, Mi3, Ka4
a According to Soil Survey Staff.[17]
b Mean and Standard deviation of duplicate analyses.c Mi ¼ mica, Sm ¼ smectite, Chl ¼ chlorite, Ka ¼ kaolinite. Subscript: 1 ¼ most
abundant; 4 ¼ least abundant.
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soluble organic matter, the suspension was centrifuged, the supernatant liquid
discarded, the soil separates were then re-suspended in 1 L of 0.01 N solution
of the perchlorate salts of the metals and stored under refrigeration. In this way
the soil separates consisted almost entirely of clay and silt and used in the
subsequent exchange experiments. The concentrations of K- and Mg-soil in
the stock suspensions were determined as described by Sposito et al.[22] and
shown in Table 2.
Exchange Experiments
The exchange experiments followed the general scheme of Fletcher
et al.[23] The K- or Mg-soil separates were reacted at 25 ^ 0.28C with
solutions of either Mg(ClO4)2 or KClO4 at eight different concentrations
(treatments) with the total normality maintained at 0.01 in perchlorate. The pH
of all treatments was 6:6 ^ 0:1: Such small variation in pH does not affect the
exchange isotherm.[23] Each treatment was run in triplicate and the same
experiments were repeated at 40 ^ 0.38C. The weight of the stock suspension
used was such that it corresponded to 0.44 g of soil material in the suspension.
The stock suspensions with the reacting solutions were placed in 250 mL
polycarbonate bottles in a thermostated bath, shaken for 30 min and left to
react further for 1 h. The suspensions were then centrifuged in preweighed
polycarbonate tubes and the supernatant equilibrium solution was collected
for the determination of K and Mg. The centrifuge tubes, with the remaining
soil material, were weighed to determine the amount of remaining equilibrium
solution needed to make the necessary corrections for the amount of adsorbed
metals.
Adsorbed metals (K and Mg) were extracted by adding 33 mL of a 1 N
CH3COONH4 solution in the tubes, shaking, centrifuging, and collecting the
supernatant liquid. This procedure was repeated two more times. The amounts
of adsorbed K and Mg (surface excess) were calculated as described by
Sposito et al.[22] In all exchange experiments, K was determined by Flame
Table 2. Concentrations of soil separates in the stock suspension used for the
exchange experiments.
Ksaturation (g kg21 suspension) Mgsaturation (g kg21 suspension)
Soil 1 29.4 ^ 0.05a 32.3 ^ 0.06
Soil 2 22.9 ^ 0.04 32.5 ^ 0.06
a Mean and Standard deviation of triplicate analyses.
Potassium–Magnesium Exchange 445
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Spectroscopy and Mg by Atomic Absorption Spectroscopy using appropriate
standards.
RESULTS AND DISCUSSION
Thermodynamic Parameters of Potassium–Magnesium
Exchange on the Two Soils
The primary laboratory data on the exchange experiments in the two soils at
258Cappear inTables3and4. In theseTables,Crefers toanequilibriummolinity
(mmoles of metal per kilogram of solution) and qm (m ¼ K;Mg) is the absorbed
metal charge in moles (pþ) per kilogram of dry soil separate. There was a trend of
a dependence of total charge, Q, on the charge fraction of the bivalent ion on the
solid phase. Total charge, Q, tended to increase as the fractional saturation with
Mg increased (compare columns 4 and 5 of Tables 3 and 4). This observation
cannot be attributed to experimental imprecision (see the small standard
deviation inTables3and4),but either to the formationofMgOHþ ionpairs[14] or
most likely to the specific character of the exchange reaction of K with bivalent
cations, such as Ca and Mg on 2:1 clay minerals, as explained by other
researchers.[4,24]At lowdegreeoffractionalsaturationwithMg(highdegreeofK
saturation) some trapping of K and/or Mg ions might occur within the 2:1 layers
resultinginasmallmeasuredtotalabsorbedcharge,whereasathighdegreeofMg
saturation the 2:1 layers might open up thus permitting the release and
measurement of K ions (higher total adsorbed charge).
It is to be remembered that in Tables 3 and 4 the degree of K saturation (as
deduced from the values of qk and Q) increases from top to bottom in the
section with the data for the reaction Mgsoil ! Ksoil and decreases from top to
bottom in the section for the reaction Ksoil ! Mgsoil: Comparison, therefore, of
the values of column 3 to the values of the corresponding Vanselow selectivity
coefficients KV (column 6), allows observing the changes of the latter as the
degree of K saturation change (see following section).
Exchange isotherms for the pair K–Mg at 258C for the two soils are
shown in Fig. 1. All isotherms lie considerably above the corresponding non-
preference isotherms (solid line in the Fig. 1) indicating that, under the
conditions of the present experiment, both soils exhibited strong preference
for K relative to Mg irrespective of the direction of the reaction. This general
observation becomes more specific by looking at the calculated thermodyn-
amic parameters shown in Table 5. The equilibrium constants, Keq, for
the reaction Mgsoil ! Ksoil; were greater than unity in both soils (last column)
meaning that the products are favored, in other words, the formation of a K-soil
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Table 3. Experimental data for K–Mg exchange at 258C for Soil 1.
CK
(mmol kg21)
CMg
(mmol kg21)
qK
(mol kg21)
qMg
(mol kg21)
Q
(mol kg21) KV
Mgsoil ! Ksoil
0.21 ^ 0.06a 5.6 ^ 0.3 0.011 ^ 0.002 0.369 ^ 0.005 0.380 ^ 0.03 93.3
0.88 ^ 0.01 5.5 ^ 0.3 0.023 ^ 0.002 0.357 ^ .006 0.380 ^ 0.04 70.4
1.70 ^ 0.18 4.8 ^ 0.2 0.034 ^ 0.001 0.342 ^ 0.006 0.376 ^ 0.04 46.7
3.20 ^ 0.14 4.3 ^ 0.1 0.044 ^ 0.002 0.330 ^ 0.004 0.374 ^ 0.03 50.2
4.90 ^ 0.19 3.2 ^ 0.1 0.084 ^ 0.002 0.288 ^ 0.004 0.372 ^ 0.04 27.9
6.60 ^ 0.15 2.0 ^ 0.1 0.106 ^ 0.003 0.250 ^ 0.005 0.356 ^ 0.03 13.8
8.10 ^ 0.26 1.3 ^ 0.08 0.127 ^ 0.005 0.222 ^ 0.003 0.349 ^ 0.02 8.6
9.10 ^ 0.20 0.7 ^ 0.07 0.158 ^ 0.006 0.191 ^ 0.003 0.349 ^ 0.05 6.8
Ksoil ! Mgsoilb
9.8 ^ 0.18 0.12 ^ 0.02 0.21 ^ 0.004 0.039 ^ 0.004 0.249 ^ 0.02 0.121
9.4 ^ 0.14 0.33 ^ 0.04 0.19 ^ 0.006 0.098 ^ 0.006 0.288 ^ 0.03 0.097
8.9 ^ 0.11 0.64 ^ 0.02 0.16 ^ 0.004 0.133 ^ 0.004 0.293 ^ 0.03 0.089
7.1 ^ 0.12 1.58 ^ 0.01 0.13 ^ 0.002 0.177 ^ 0.004 0.307 ^ 0.02 0.044
5.2 ^ 0.21 2.60 ^ 0.03 0.10 ^ 0.001 0.215 ^ 0.007 0.315 ^ 0.02 0.026
3.4 ^ 0.22 3.60 ^ 0.06 0.07 ^ 0.003 0.248 ^ 0.005 0.318 ^ 0.03 0.017
1.4 ^ 0.15 4.60 ^ 0.05 0.04 ^ 0.001 0.283 ^ 0.004 0.323 ^ 0.02 0.013
a Mean and Standard deviation of triplicate analyses.b One treatment was discarded in this case.
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Table 4. Experimental data for K–Mg exchange at 258C for Soil 2.
CK
(mmol kg21)
CMg
(mmol kg21)
qK
(mol kg21)
qMg
(mol kg21)
Q
(mol kg21) KV
Mgsoil ! Ksoil
0.2 ^ 0.01a 4.94 ^ 0.3 0.011 ^ 0.002 0.458 ^ 0.007 0.468 ^ 0.03 75.3
0.8 ^ 0.02 4.75 ^ 0.4 0.026 ^ 0.002 0.439 ^ 0.005 0.465 ^ 0.03 60.2
1.7 ^ 0.15 4.28 ^ 0.3 0.051 ^ 0.001 0.406 ^ 0.005 0.457 ^ 0.04 59.3
2.6 ^ 0.12 3.62 ^ 0.15 0.078 ^ 0.002 0.384 ^ 0.004 0.462 ^ 0.03 36.7
4.4 ^ 0.15 2.78 ^ 0.21 0.112 ^ 0.003 0.342 ^ 0.005 0.354 ^ 0.05 29.2
6.2 ^ 0.20 1.99 ^ 0.11 0.148 ^ 0.004 0.303 ^ 0.004 0.451 ^ 0.05 20.2
7.3 ^ 0.18 1.28 ^ 0.12 0.182 ^ 0.003 0.258 ^ 0.003 0.440 ^ 0.04 15.9
8.4 ^ 0.25 0.71 ^ 0.05 0.224 ^ 0.004 0.292 ^ 0.003 0.436 ^ 0.04 12.4
Ksoil ! Mgsoil
9.7 ^ 0.21 0.09 ^ 0.01 0.33 ^ 0.003 0.031 ^ 0.004 0.361 ^ 0.02 0.111
9.5 ^ 0.18 0.18 ^ 0.01 0.30 ^ 0.003 0.095 ^ 0.006 0.395 ^ 0.03 0.107
9.1 ^ 0.15 0.38 ^ 0.02 0.26 ^ 0.004 0.156 ^ 0.004 0.416 ^ 0.03 0.089
8.3 ^ 0.12 0.80 ^ 0.02 0.22 ^ 0.004 0.208 ^ 0.003 0.428 ^ 0.05 0.074
7.0 ^ 0.20 1.43 ^ 0.01 0.18 ^ 0.002 0.252 ^ 0.003 0.432 ^ 0.04 0.051
5.2 ^ 0.22 2.35 ^ 0.03 0.14 ^ 0.001 0.302 ^ 0.005 0.441 ^ 0.04 0.036
3.4 ^ 0.20 3.29 ^ 0.03 0.10 ^ 0.002 0.348 ^ 0.004 0.448 ^ .003 0.022
1.8 ^ 0.18 4.20 ^ 0.04 0.06 ^ 0.001 0.391 ^ 0.004 0.445 ^ 0.04 0.015
a Mean and Standard deviation of triplicate analyses.
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from a Mg-soil is favored. This is corroborated by the standard free energy
(DG0) values for the same reaction. These values were negative, 25.7 and
26.7 kJ mol21 for Soil 1 and Soil 2, respectively (Table 5). The negative
values, imply that the reaction is spontaneous and that the driving force for the
formation of a K-soil from a Mg-soil is favored. The spontaneity of the
reaction is also evident from the negative standard enthalpy values (DH0),
which imply that heat is given off (exothermic reaction) as the chemical bonds
between Mg and K with the soil exchange sites are broken and formed,
respectively. Exothermic reactions usually occur at their own accord without
external assistance. DH0 values also convey something about the binding
strength of K to the soil. Negative DH0 values denote stronger binding
within the products, that is, very strong binding of K ions to some sites of
the two soils.
Figure 1. Relationship between the equivalent fraction of K in the equilibrium solution
(xk) and on the solid phase (x̄k), at constant normality 0.01 N; A ¼ soil 1; B ¼ soil 2. Solid
lines represent non-preference isotherms calculated by means of Eq. (8).
Potassium–Magnesium Exchange 449
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One might attempt to compare the two soils with respect to the degree of
K preference, on the basis of the magnitude of the negative DG0 and DH0
values. The low values of DG0 imply that a small amount of energy in the
standard state is obtainable from the reaction; thus the molecular motion of K
ions is restricted when in contact with soil exchange sites, which suggests
formation of strong bonds between K ions and these sites. The less negative
the value of DG0 the stronger the bonds. Also the more negative the value of
DH0 (more heat is given off) the stronger the bonds. Therefore, comparing the
values of DG0 and DH0 (25.7 and 213.5 kJ mol21, respectively) of Soil 1
with illitic mineralogy to those of Soil 2 with montmorillonitic mineralogy
(26.7 and 28.7 kJ mol21) it is evident that Soil 1 exhibits stronger preference
for K than Soil 2.
Standard entropy values (DS0) were negative indicating that the formation
of aK-soil froma Mg-soil produceda state thatwas more ordered in itsmolecular
arrangement. Negative as well as positive entropy changes have been reported in
exchange experiments with soil clays and the pair K–Ca.[4] The direction of
entropy change depended on the type ofclay mineral and whether solution forces
or solid effects prevailed. Because of the similar chemical nature of Ca and Mg
and following the suggestion of Goulding and Talibudeen,[25] it can be argued
that three physical mechanisms contribute to entropy changes during the
exchange reaction Mgsoil ! Ksoil occurring in an aqueous system: 1) on the solid
phase, adsorption of K ions realigns the 2:1 layers in the 001 direction so that the
ditrigonalholes inadjacent tetrahedral sheetscanaccommodateKions, resulting
in a negative entropy change; 2) on the solid phase, adsorption of K ions would
cause a positive entropy change since the randomness of distribution of
exchangeablecationswouldincrease;3) in thesolutionphase, replacingKbyMg
wouldcauseadecrease in theentropy ofhydrationsincewatermoleculesarrange
themselves in a more structured configuration around a bivalent ion compared
Table 5. Calculated values of thermodynamic parameters and
equilibrium constant (Keq) for the K–Mg exchange on the two soils.
DG0
(kJ mol21)
DH0
(kJ mol21)
DS0
(kJ mol21) Keq
Mgsoil ! Ksoil
Soil 1 25.7 213.5 20.026 10.1
Soil 2 26.7 28.7 20.007 14.7
Ksoil ! Mgsoil
Soil 1 7.5 13.8 0.021 0.049
Soil 2 7.3 15.1 0.026 0.053
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to a monovalent ion. The net entropy change would be the algebraic sum of all
entropy changes. Obviously in the present experiment solution effects and the
first mechanism on the solid phase prevailed, resulting in a net negative DS0
value.
All the above drawn conclusions were corroborated by and in
harmony with the thermodynamic parameters calculated for the backward
reaction, namely, Ksoil ! Mgsoil (Table 5). The equilibrium constants, Keq,
for the two soils were less than unity (last column) meaning that the
reactants are favored or that a K-soil is reluctant to be converted to a Mg-
soil. The smaller than unity the equilibrium constant the greater this
reluctance or the stronger the binding strength of K with the soil exchange
sites. So, Soil 1 with illitic mineralogy and a Keq ¼ 0:049 exhibits
stronger preference for K than Soil 2 with montmorillonitic mineralogy
and a Keq ¼ 0:053:Also the positive values of DG0 and DH0 indicate that the reaction is not
spontaneous and needs external assistance to proceed and that the driving
force for the formation of Mg-soil from a K-soil is not favored. Standard
entropy values (DS0) were positive suggesting that the formation of a Mg-soil
from a K-soil produced a state less ordered in its molecular arrangement and
hence resulted in an entropy increase.
Vanselow Selectivity Coefficients
Vanselow selectivity coefficients, KV, were generally greater than unity
for the reaction Mgsoil ! Ksoil in both soils (column 6 of Tables 3 and 4)
providing also evidence that the formation of a K-soil from a Mg-soil is
favored. They were generally less than unity for the reaction Ksoil ! Mgsoil;implying that the formation of a Mg-soil from a K-soil is not favored which is
in harmony with the previous observation. KV values for both reactions and
soils were variable depending on the exchanger composition (compare
columns 3 and 4 to column 6 of Tables 3 and 4).
Dependence of KV on exchanger composition has been reported also by
other researchers in binary exchange experiments with soil clays involving the
pair K–Ca.[25,26] It has been attributed to the existence of adsorption sites with
different selectivity for K or to the fact that the cationic mixture on the
exchanger phase does not behave as an ideal solid–solution mixture. Therefore,
comparing the values of the parameters shown in columns 3 and 6 of Tables 3
and 4, it becomes evident that for the reaction Mgsoil ! Ksoil; KV increased
(stronger preference for K) as the value of qk decreased. Also for the reaction
Ksoil ! Mgsoil;KV values became smaller than unity (stronger preference for K)
Potassium–Magnesium Exchange 451
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as the value of qk decreased. In other words, soils’ preference for K was higher at
low degrees of K saturation for both the forward and backward reactions which
indicates that the high selectivity sites for K are filled first followed by the low
selectivity sites. This becomes more apparent when KV values (expressed as
2 ln KV) for the two soils are plotted as a function of the same fractional K
saturation (an example is given in Fig. 2 for the reaction Mgsoil ! Ksoil for the
two soils). It is seen that the slope of the curves is steep up to an equivalent
fraction of K equal to 0.6 (high selectivity sites are operating) and then levels off
as complete K saturation is approached.
Vanselow selectivity coefficients were generally higher for Soil 1 (illitic)
than for Soil 2 with montmorillonitic mineralogy (compare columns 6 of
Tables 3 and 4). Figure 2 helps to make a comparison between the KV values
of the two soils in a more clear way. It is to be remembered that the KV values
for the reaction Mgsoil ! Ksoil were smaller than unity. The curve for Soil 1
lies above the corresponding curve for Soil 2, which indicates that the KV
value for Soil 1 (illitic) was smaller than the value for Soil 2 for the same
Figure 2. The natural logarithm of the Vanselow selectivity coefficient (KV) as a
function of fractional K saturation for the exchange Ksoil ! Mgsoil in the two soils
studied.
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fractional K saturation. This illustrates the higher preference for K of Soil 1
and it can be attributed to the abundance of micaceos clay minerals in this soil.
Reversibility of Exchange Reaction
A basic tenet for the exchange of cations on soils and clays is the
assumption that exchange reactions are thermodynamically reversible.[5,27]
For the following exchange reaction between K and Mg on a soil clay
MgE2 þ 2KD2KE þ Mg ð14Þ
this assumption implies that exactly the same experimental isotherm is
obtained whether one starts with an exchanger fully saturated with Mg
(forward reaction) or with K (backward reaction). However, there is ample
evidence in the literature that certain reactions on soils and clays are not
thermodynamically reversible but exhibit hysteresis.[24,25,28,29] In most of
these studies the hysteresis was more apparent in binary exchange systems, for
example, K–Ca and when the exchanger was a mica-type mineral. This was
also the case in the present work where the exchange isotherms for the forward
and backward reactions Eq. (14) did not coincide (Fig. 3).
Several mechanisms have been suggested to account for the irreversibility
of many exchange reactions on soils and clays, although none of them gives a
complete explanation of the phenomenon of hysteresis. These mechanisms
Figure 3. Isotherms for the exchange of K–Mg on the two soils showing the
irreversibility of the reaction.
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include failure to reach equilibrium; charge or site heterogeneity at the surface
of the exchanger; differential hydration of the exchanging cations;
dehydration of the exchanger; electrolyte concentration and inaccessibility
of sites caused by domain or quasi-crystal formation.[30] The data obtained
from the present experiment could not lead to a satisfactory explanation about
which mechanism was operating to cause the observed hysteresis, although
domain or quasi-crystal formation was suspected.
The magnitude of hysteresis can be assessed by the area of the loop
between the forward and backward exchange isotherms. From Fig. 3 it is seen
that this area is larger for Soil 1 than for Soil 2. A more quantitative
assessment of the hysteresis can be obtained by determining the product of the
forward and backward equilibrium constants (Keq). The magnitude of the
deviation from unity of this product gives the magnitude of hysteresis.
Calculation of this product from the values of Keq shown in Table 5, gives 0.49
and 0.78 for Soil 1 and Soil 2, respectively. Both, the area of the hysteresis
loop and the magnitude of the deviation from 1.0 of the mentioned product,
indicate that the hysteresis was more pronounced in Soil 1 with micaceous
mineralogy of the clay fraction.
CONCLUSIONS
The thermodynamic study of the forward and backward exchange
reactions of the pair K–Mg on two soils, one illitic and the other
montmorillonitic, revealed the strong preference of the two soils for K in the
presence of Mg. This preference for K was more pronounced in the illitic soil.
This implies that in illitic soils, which are rich in Mg or receive heavy
dressings of Mg fertilizer, the fertilization policy with respect to K should take
into account that K might be strongly bound on the soil colloids and would not
be easily released in the soil solution.
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