JOURNAL OF THE SCIENTIFIC AGRICULTURAL SOCIETY OF FINLAND

Maataloustieteellinen Aikakauskirja

34

Vol. 52:34- 44, 1980

Oxygen as an exchangeable ligand in soil

Erkki aura 1)

Department of Agricultural Chemistry, University of Helsinki 00710 Helsinki 71

Abstract. An attempt was made to clarify the adsorption of oxygen ligands onto
the Al- and Fe-oxides in the soil. In the ligand exchange the central ions Al 3+ and
Fe3+ of the oxides are Lewis acids or electron acceptors and the ligands are Lewis
bases or electron donors. The binding of ligand oxygen by the central ion is primarily
dependent on the nucleophilic strength of the ligand oxygen. The proton is a hard
Lewis acid as are Al3 + and Fe3+

. The pKa-value of the acid corresponding to the
anion shows the ability of ligand oxygen to bind a proton and also the ability to bind
Al3+ and Fe3+ . The greater the nucleophilic strength of the ligand, the more covalent
the nature of the bond between ligand oxygen and the central ion. However, the co-
valent character of this bond does not make the adsorption of a ligand by oxides an
exothermic reaction. The entropy changes in adsorption determine the exchange
equilibrium of the anions. The nucleophilic strength of ligand oxygen also determines
the rate of exchange. If an anion has a high nucleophilic strength, it is rapidly adsorbed
by oxides but the desorption is slow. The theory explains the relationship between the
adsorption of anions to oxides and the pH-value in the equilibrum solution. The theory
also illuminates the factors, which determine the leaching of an anion from the soil and
the uptake rate of an anion from the soil by plants.

Introduction

The oxygen is the most abundant chemical element in the soil. This is
due to the fact that the earth’s crust contains 46.6 % oxygen by weight (Weast
1969). The oxygen plays an important role in the most reactive components
of soil. This chemically active part of the soil consists of soil organic matter,
clay minerals and polymeric Al- and Fe-oxides. The most reactive groups
of the soil organic matter are the carboxyl and phenolic OH groups. These
are able to bind a proton or another cations (Schnitzer 1978). In the clay
minerals the central ions (e. g. Si4+

, Al 3+
, Fe 3+ ) are surrounded by negatively

charged oxygen ions in tetrahedral or octahedral layers. In the amorphous
or crystallized Al- and Fe-oxides the central ions are surrounded by six oxygen
atoms.

x ) Present adress: Institute of Soil Chemistry and Physics, Agricultural Research Centre
31600 Jokioinen.

https://www.c-info.fi/en/info/?token=Y5xeky5_mQWeT6Ho.3QTHxVGpSyKg_TosYQgbHw.vms9mBMgOsQHSARs39pj-WISE1QtBqKXHSOfpYzEZV6qb1gBnAHNlvrUnq8yp_ArasroWwELxHh71MQdVgcz21EZWk6E4Z5CYLIKbmzofvGwpZlgeL_SaQUh43Iv--1Mxgxku2_GtfTfInVKUTmqe6YyrY70Kk7v


35

The polymeric Al- and Fe-oxides of the soil are very porous substances.
In the amorphous form their specific surface area is over 100 m2/g (e. g. Rajan
et al. 1974, Parfitt et al. 1975). The great specific surface shows, that the
amorphous oxides have plenty of pores, which have a diameter in the order of
small molecules (Parfitt 1971). The oxide polymers do not always exist as
separate paricles in soil. The aluminium oxides specially form layers on the
surface of clay particles (Clark 1964). The aluminium and iron oxides can
also be attached to the soil organic matter by various chemical bonds.

The surfaces of polymeric Al- and Fe-oxides are the anion exchangers in
the soil. If the oxygen atom is situated internaly in oxides, it is bound by more
than one Al 3+ (or Fe 3+ )-ion. In this case the oxygen is inactive in the soil.
Whereas on the surface of the oxides, oxygen can be attached to only one
aluminium or iron ion and can also be a part of the structure of the following
anions or ligands: water, hydroxyl, sulphate, phosphate, carbonate, selenite,
molybdate, borate, silicate and the carboxjd and phenolic OH group of organic
matter. These anions are not always coordinated on the surface of oxides.
For example a part of phosphate is blocked in oxides due to the precipitation
reactions of phosphate in the soil. However, primarily the anions that are
adsorbed on the pore surface of the oxides participate in exchange reactions.

The anion exchange in the soil is of great importance e. g. in the plant
nutrition. The purpose of this study was to examine the factors, which effect
the coordination of the ligand oxygen with the central ions of Al- or Fe-oxides.
The factors influencing the rate of exchange were also considered. The em-
phasis was laid on the bonding of oxygen to the central ion. The effect of the
detailed structure of oxides on the coordination was not examined.

Central ions of oxides and ligand exchange

The theory of acids and bases presented by Pearson (1966) seems to be very
useful in the study of ligand exchange in soil: In the ligand exchange the
central ions Al 3+ and Fe 3+ are Lewis acids or electron acceptors and ligands
are Lewis bases or electron donors. The ions Al 3+ and Fe 3+ belong to the
group of hard Lewis acids. These ions have the distinguishing property of
small size and high positive oxidation state. Hard acids prefer to associate
with hard bases. The latter have low polarizability and high electronegativity.
The donor atom is often O, but can also be F or N.

The acidic property of Al 3+ and Fe 3+ in water solution is due to the
fact that the oxygen from six water molecules around the Al 3+ or Fe 3 +

ions are electron donors, which aids the detachment of a proton from the
coordinated water molecule. The electrons of oxygen are shared more strongly
with Fe 3+ than with Al 3+ The acid hydrolysis constant is 5.0 at 25° C for
Al 3+ and 2.2 for Fe 3+ (Hunt 1963, p. 50). Due to the decrease of charge in
hydrolysis, the monomeric AI(H 2 O) SOH or Fe(H 2O) 6OH tend to polymerize.
In water solutions ferric ions polymerize readily above pH 2 and aluminium
ions above pH 5 (e. g. Jackson 1960, Murphy et al. 1975). Polymerization
can be seen as a ligand exchange. The OH-group attached to the central ion
is exchanged by the oxygen which is bound by the other central ion For
example:



36

\ / \ /
- Al -

/ \

\ /
- Ai -

/ \

\ ,
- AlAl OH + HO. O + H 2 O

/ / \

The reduction of Fe 3+ to Fe 2+ decreases greatly the ability of iron to
bind the electrons of the oxygen ligand. Consequently the acid hydrolysis
constant of Fe 2+ at 25° Cis as high as 9.5 (Basolo and Pearson 1967, p. 32).
In reductive conditions the iron oxides tend to break up and Fe 2+ is easily
released from oxides as a monomeric cation.

The chemical bond between the oxygen ligand and central ion of oxides

The binding of oxygen by the central ion is primarily dependent on the
nucleophilic strength of the oxyanion or ligand. The cation H+ is the prototype
hard Lewis acid and any base which binds strongly to the proton will also
bind to other hard acids as well (Pearson 1966) The pKa-value of the acids
corresponding the oxyanions shows the ability of anion oxygen to bind a
proton and also the nucleophilic strength. The higher the pKa-value the more
stable is the bond between oxygen and the central ions Al 3+ and Fe 3 (Edwards
1956, Pearson 1966). Edwards (1956) has shown that the nucleophilic
character of the donor depends on both the basicity of the ligand and also on
the ease with which the donors polarize. The ability of the ligand to coordinate
to the central atom, when it is a hard Lewis acid, is primarily related to the
basicity of the ligand (Edwards and Pearson 1962, Pearson 1966).

Table 1. The pKa-values of acids corresponding to the ligands (Sasaki et al. 1959, Weast 1969).

Ligand Acid pKa (25° C)

h 2 o
no;
hso4
SO;
h 2

po
4

HSe03
MoOj
hco3
HPOj
SeOj
h2bo 3
H3Si04
CO|

pof--4
OH

HaO+ -1.74
hno3
h2 so4
HS04 1.92
H3P04 2.12
H2 Se03 2.46
HMoQ4 4.1
H2 C03 6.37
H2P0 4 7.21
HSeOj 7.31
H3B0 3 9.14 (20° C)
H 4Si04 9.66 (30° C)
HC03 10.25
HPO= 12.67 (18° C)
H 2 O 15.74

The pKa-values of the most usual inorganic ligands are shown in Table 1.
If the pKa-value is high, the anion concentration in the equilibrium solution in
contact with acid soil is low, because the anion ligand tends to be strongly
bound to both Al 3+ (Fe3+ ) and H+

. The anions of polyprotic acids can be



37

bound to two adjacent central ions on the surface of the oxide pores forming
a ring structure (Parfiti et al. 1975, Pareitt and Smart 1978, Rajan 1978).
This binuclear bridging probably increases the stability of the coordination
(Kingston et al. 1974). Since the polymeric Al- and Fe-hydroxides probably
contain plenty of micropores then the size of the ligand will effect the adsorption
of the ligand to the surfaces of the pores. The pKa-value of an acid corresponding
to the ligand reveals something about the character of the bond between the
oxygen and the central ion Al 3+ or Fe 3+

. The greater the nucleophilic
strength of the donor, the more covalent character of the bond. The hydroxyl
oxygen, which has three unshared electron pairs, probably forms both a- and
7r-bond with Fe 3+ and Al 3+-ions (Baran 1971).

The changes of standard enthalpy and entropy in ligand exchange
The covalent character of the strong bond between the oxygen ligand and

the central ion does not make the adsorption of the ligand by oxides an exo-
thermic reaction. When an anion exchanges from the surface of oxides e. g. a
water molecule, energy is consumed for the detachment of H 2O from the
central ion and also for the decresae of the hydration number of the anion.
Often the change of the standard entropy primarily determines the equilibrium
of the ligand exchange in the soil. According to the Van’t Hoff equation the
equilibrium constant K of the chemical reaction is dependent on the tem-
perature as follows;

- AH 0 1
In K h o- constant (1)

R T

where AH°= the change of the standard enthalpy J/mol
R = the gas constant 8.314 J/K mol
T = the absolute temperature K

When an anion A~ exchanges with the ligand water, the equilibrium constant K can be defined
as follows:

AI + H 2O s > a; + (2)

y
X A (H 2 0 1) X A

(A )l XH a0 (A )l XH a 0
(3)

where (A )j = the activity of anion A* in the equilibrium solution
(H 20)! = the activity of water in solution «s 1
XA = the mole fraction of A” on the surface of oxides.

The activity coefficient of A’has the value of 1.
Xh 2 o = the mole fraction of H a O on the surface of oxides.

The activity coefficient of H2O has the value of 1.

If there are no other ligands than A and HaO, it follows that XHaO =1 XA.

Combining this with the equation (3) we get the Langmuir adsorption equation for the
sorption of A":

v K (A')lX A (4)
1 + K (A ) 4

The coefficient K is constant only if all the adsorption sites have the same
standard free energy of adsorption. In reality K is not constant. The appli-



cability of the Langmuir equation can be improved using the equation of the
two adsorption surfaces (Syers et al. 1973). For simplicity the conventional
Langmuir equation is used in this study.

Hartikainen (1979) has studied with several acid soil samples, how the
phosphate concentration in the equilibrium solution in contact with soil
depends on the temperature. This had no effect on the concentration or the
influence was slightly positive or negative. The equation (1) shows that the
change of the standard enthalpy in the adsorption of phosphate is small. Since

AG° = -RT InK (5)
(6)AG0

= AH0
- TAS°

where AG° = the change of free energy J/mol
AS°= the change of entropy J/mol K

the entropy changes determine the equilibrium of phosphate adsorption in acid
soils. The phosphate anion in solution and the central ion on the surface of
Al-and Fe-oxides increase the order of water molecules in the solution. The
attachment of phosphate oxygen to the Al 3+ or Fe 3+ neutralize the charges
of these ions, which increases the disorder of water molecules in the solution.

Biggar and Fireman (1960) have stated that the adsorption of borate to
soil becomes slightly weaker, when the temperature rises. In the studies of
Bingham et al. (1971) the effect of the temperature was to bind borate anions
more strongly, which shows that the reaction was endothermic. Kingston

and Raupach (1967) have studied the sorption of silicate to the Al-hydroxy
polymers and found that adsorption of first silicate layer on the surface of the
oxides was almost independent of the temperature. The measurements of
Reyes and Jurinak (1967) showed no relationship between the temperature
and adsorption of molybdate by hematite, when the concentration of molybdate
was not high in the equilibrium solution.

The nucleophilic strength of ligand oxygen is closely related to the amount
of negative charge associated with ligand oxygen. The higher the pKa-value
of the acid corresponding to the anion ligand is, the more the charge is con-
centrated on the ligand oxygen and the more effectively the ligand oxygen
increases the order of water molecules in the solution (e. g. Fitz 1975) . The
entropies of ligands in water solution are given in Table 2. As Table 2 shows,
the entropies of the ligands are in a negative correlation to the pKa-values of
the acids. The strong adsorption of the anions of weak acids is due to the low
entropy of these anions in the water solution.

Table 2. The entropies of ligands in water solution J/mol K. The entropy of standard ion
H+ is O (Rossini et ai. 1952).

H 2 O 70 NOj 146 SOi 17
HSO~ 127 HPOi -36

OH" -10

38



39

The adsorption of anion and pH

Kingston et al. (1968) have stated that the adsorption of many anions
to Al- and Fe-oxides reaches a maximum where the pH-value is of the same
order as the pKa of the corresponding acid. This result is easily explained by
the theory presented above. When the pH of the equilibrium solution is much
below the pKa-value, the concentration of the anion in solution is low, be-
cause H + competes with Al3+ and Fe 3+ of oxides for anions. If the pH of a
solution is near the pKa, the rise in the pH greatly increases the degree of
dissociation of the acid. When the pH becomes higher than the pKa-value,
the degree of dissociation becomes near 1 and can change only slightly. On the
other hand when the concentration of OH-ions in a solution rises, the adsorption
of OH-groups is increased, but the adsorption of other anions is decreased.

The relationship between the pH and the adsorption of anions can be studied
as follows:

X a = the amount of adsorbed anions mmol/kg of soil (or oxides)
X = the amount of water adsorbed mmol/kgH a o
C

A
= the concentration of anion in the solution mmol/1

X,.„ = the amount of adsorbed OH mmol/kg
OH

C = the concentration of OH in solution mmol/1OH

C
h

= the concentration of hydrogen ion in solution mmol/1
CHA

= the concentration of acid corresponding to the ligand in solution mmol/1
= the fractional surface coverage by anion

Ki
X'

• X' =K, C X'
C X' A 1 A H2O

A HjO
(7)

K 2 —— -X’ = K C X'
C X oh 2 oh h,o

OH H a O
(8)

The constants K! and K 2 show the equilibrium of exchange of anion and
OH with the coordination water. Theoretically the values of K x and K 2 can be
determined using the Langmuir equation.

However, it is very difficult to determine the amount of H 2O and OH
adsorbed on the surface of oxides. The strong nucleophilic character of OH is
seen e. g. from the fact that the Al- and Fe-oxides do not adsorb sulphate,
when the pH is above the zero point of charge (zpc), which for oxides can be
about 8 (Kingston et al. 1972). An approximate idea of the value of K, can
be had by supposing that half of the adsorption sites of H 2O are occupied by
OH, when the pH of the solution is 7. The concentration COH is in this case
10'4 mmol/1 and OOH0 OH has a value of 0.5.

k 2 cohOOH =

1 . K r1 + k 2 '-'OH
■>■ K 2 = 101 1/mmol



If H2O, A’ an OH are the only adsorbing ligands, the number of adsorption sites in the soil
(oxides) according to equations (7) and (8) can be formulated as follows:be formulated as follows:

X' j. K, C X' +K, cn„ X'HgO 1 A H2O 2 0H H 2 O (9)

The combining the equations (7) and (9)

C An _ 1 A"a =

1 + Kj C A + K 2 C0h
(10)

Let Ka be the acid constant corresponding to the anion:

HA > H+ + A

CH/1000 C A/1000Ka =

Cha/1000
(11)

G = Cra “h rnrnol/1
1000 Ka C

C A 1000 Ka + CH

(12)

The last equation was combined with the equation (10) and the curves in
Figure 1 were calculated using the value 104 for the constant K 2. The curves
show that the maximum adsorption of the anions is near the pKa-value. The
rise in the pH has only a negative effect on the adsorption of the anion, whose
pKa is small. If the pKa is high, then adsorption below pH 7is weak. Silicate
is an anion of this kind. The maximum degree of adsorption is near the pKa-
value 9.7 (Kingston et al. 1968). If an anion is polyprotic like for example

Fig. 1. Adsorption of anions
onto oxides and pH of
equilibrium solution. The
curves are calculated using
the equation (10).

40



41

phosphate, the decrease of adsorption with the rise in the pH is less than for an
anion, which dissociates with only one H+

. The dissociation of more than one
hydrogen ion increases the nucleophilic strength of the anion. When the pH
rises above the last pKa-value of the polyprotic anion, the competing effect of
OH becomes clearer. The adsorption decreases more sharply than when the
pH is near the other pKa-values of the polyprotic anion as shown experimentally
by Kingston et al. (1968).

Kinetics of ligand exchange
The rate of the ligand exchange also depends on the nucleophilic strength

of the ligand oxygen. The rate of the exchange of ligand is especially related to
the basicity of the ligand, if the central ion is a hard Lewis acid such as Al 3+

or Fe 3+
. The high pKa shows a high nucleophilic reactivity of oxygen (Edwards

1956, Edwards and Pearson 1962). Correspondingly the detachment of
ligand oxygen becomes slower, when the pKa-value rises. The coordination
water is not tightly bound by the central ions, and the exchange of coordi-
nated water occurs rapidly. The first order rate constant for the removal of
water molecule from the first coordination sphere of Fe 3+ is about 104 s°
(Connick and Stover 1961). The difference between the rates of adsorption
and desorption is distinct in the sorption of an anion, which is tightly' bound
by oxides. For example phosphate is rapidly bound by Al- or Fe-oxides, but
100 % desorption is almost impossible, when ligands, which are not strongly
adsorbed by oxides, are used as exchangers (Kingston et al. 1974). The
micropore structure of hydrous oxides also apparently slows down the desorption
of strongly bound phosphate from acid soil (Aura 1978).

The rate of the exchange of hydroxyl with water is an exception. The
hydroxyl oxygen is very nucleophilic, and easily takes up a proton to form
water. The hydroxyl oxygen does not need to be released from the central ion.
The easily moving proton can be detached from the water in solution and
associated with the OH-group on the surface of the oxides (Fitz 1975 p. 385).

Discussion

The slow desorption of phosphate from aluminium and iron oxides can be
explained by the fact that phosphate can be bound to two adjacent central
ions in the oxides forming a ring structure. This structure probably stabilizes
the bond between phosphate and oxide, but the ring structure does not alone
explain the strong adsorption of phosphate to oxides. For example sulphate
forms the ring structure, when it is bound by Al- and Fe-oxides (Parfitt and
Smart 1978, Rajan 1978), but is desorbed from oxides much more easily than
phosphate. Primarily the nucleophilic strength of the ligand oxygen determines
the bond between anion and Al 3+ or Fe 3+ of oxides.

Both carboxyl and phenolic OH group of soil organic matter bind Al 3+

and Fe 3+ (Schnitzer and Skinner 1965). Since the pKa-value of carboxyl
and phenolic OH groups varies greatly, there are great differences between
carboxyl groups and between phenolic OH groups in the bonding to Al 3+ or



42

Fe 3 +

. Large organic molecules are easily bound to oxides by several carboxyl
and phenolic OH groups, which stabilize the association between organic
molecules and oxides.

The ligands can be grouped as follows. The anions, which are more weakly
adsorbed than water, form the first group. These anions are outside the
coordination sphere of the aluminium and iron and can be bound only by
oxides, if the surface of oxides is positively charged. Nitrate belongs to this
group (Serna et al. 1977). The second group are anions, which are more
strongly bound than water to the oxides, but cannot change the structure of
oxides. Sulphate belongs to this group (Kingston et al. 1972, Rajan 1979).
In the third group are the anions, which are firmly bound by oxides above
zpc making the surface of oxide more negative. Anions of this type can radically
change the structure of oxides, if the anion concentration in equilibrium
solution is high enough. Phosphate (Rajan et al. 1974) and hydroxyl are in
this group. Humus molecules belong to the third group (Schnitzer and
Skinner 1963) as does silicate when the pH is above 7 (Kingston and Raupach

1967). Molybdate and selenite are apparently in the second group.
If an anion is not able to exchange the coordination water from the surface

of oxides, the diffusion of the anion in soil is rapid. The anion is easily carried
in soil by mass flow. The activation energy of diffusion in soil is about the
same as in water solution or about 20 kj/mol (Glastone et al. 1941, p. 401).
When an anion coordinates with the central ions of oxides, the diffusing anion
is repeteadly bound by the oxides. The high nucleophilic strength of the anion
slows down its detachment from the central ions of oxide and decreases the
diffusion coefficient of the anion in soil. For example the diffusion coefficient
of nitrate in soil is about 10'5 cm 2 /s and that of phosphate 10’9 cm 2/s (Nye and
Tinker 1977 p. 82). The dependence of the diffusion coefficient on the tem-
perature is higher for those anions that coordinate with the central ions of
oxides than for those anions that are outside the coordination sphere (see
Aura 1978).

Since the bond between phosphate and Al 3+ or Fe 3+ is strong, the
movement of phosphate in soil by mass flow is slight. The strong adsorption
of anions to oxides reduces the leaching from the cultivated layer. When a
plant takes phosphorus from the soil, a determining factor in the process is
the slow diffusion of phosphate in the soil surrounding the root surface and root
hairs (Nye and Tinker 1977). The uptake of phosphate by mass flow is slight.
Since the diffusion coefficient of phosphate depends strongly on the tempera-
ture, the rise of soil temperature has a considerable positive effect on the
uptake of phosphate from soil.



43

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Ms received September 24, 1979.

SELOSTUS

Happi vaihtuvana ligandina maassa

Erkki Aura 1)

Helsingin yliopiston maanviljelyskemian laitos, 00710 Helsinki 71

Anioninvaihto kuuluu kuten kationinvaihtokin maaperäkemian keskeisimpiin ilmiöihin.
Teoreettisen tarkastelun avulla pyrittiin selvittämään eri happiligandien pidättymistä poly-
meeristen alumiini-ja rautaoksidien pinnoille. Anionin sitoutuminen oksidien Al3+ tai Fe 3+

-

ioneihin riippuu erityisesti ligandihapen kyvystä luovuttaa elektroneja ligandin ja keskusionin
väliseen sidokseen. Koska protoni on kuten Al 3+ tai Fe 3+ tyypillinen elektronien vastaanot-
taja eli Lewisin happo, anionia vastaava happovakio pKa ilmaisee anionin taipumusta sitoutua
oksideihin. Mitä helpommin ligandihappi luovuttaa elektroneja keskusionille, sitä vahvempi
sidos muodostuu hapen ja keskusionin välille. Vahva kovalenttinen sidos ei kuitenkaan mer-
kitse, että anionin adsorptio oksidien pinnoille olisi aina eksoterminen reaktio. Lähinnä entropia-
suhteet määräävät vaihdon tasapainon.

Esitetty teoria selittää anionien pidättymisen ja pH:n välisen yhteyden. Pidättyminen
oksideihin on voimakkainta, kun pH on suunnilleen sama kuin anionia vastaavan hapon pKa-
arvo. Kun maan kanssa tasapainossa olevan liuoksen pH on paljon alle pKa-arvon, anionin
konsentraatio liuoksessa on hyvin pieni, koska H+-ioni kilpailee oksidien Al 3+;n ja Fe 3+:n
kanssa sitoutumisesta anioniin. Kun liuoksen pH on suunnilleen pKa, happamuuden vähenemi-
nen lisää nopeasti anionia vastaavan hapon dissosioituraisastetta. Kun pH on paljon yli pKa-
arvon, anionia vastaava happo on lähes täysin dissoisoitunut. Kuitenkin liuoksen OH-kon-
sentraatio kasvaa edelleen pH:n suuretessa, mikä lisää OH:n pidättymistä oksidien pinnalle,
mutta vähentää muiden anionien adsorptiota.

Ligandihapen taipumus luovuttaa elektroneja keskusionille vaikuttaa myös vaihdon kine-
tiikkaan. Voimakkaasti sitoutuva anioni reagoi nopeasti oksidien pinnan Al3+;n ja Fe 3+:n
kanssa, mutta tällaisten anionien irroittaminen oksideista käyttäen heikosti sitoutuvia ligandeja
on vaikeaa. Esitetyn teorian avulla voidaan selittää useita anionien huuhtoutumiseen tai
kasvin ravinteiden saantiin vaikuttavia tekijöitä.

*) Nykyinen osoite: Maatalouden tutkimuskeskus, maanviljelyskemian ja -fysiikan laitos
31600 Jokioinen.