Thermodynamics of Potassium–Magnesium Exchange in Two Alfisols of Northern Greece

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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

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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: [email protected].

COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS

Vol. 34, Nos. 3 & 4, pp. 439–456, 2003

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

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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

Sinanis, Keramidas, and Sakellariadis440



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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,




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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,




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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.




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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).




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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)




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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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Documents

Thermodynamics of Potassium–Magnesium Exchange in Two Alfisols of Northern Greece