Chemical of alumina reactions in aqueous solution and its application in water treatment

Advances in Colloid and Interface Science 110 (2004) 19–48
0001-8686/04/$ - see front matter ᮊ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cis.2004.02.002
Chemistry of alumina, reactions in aqueous solution and its application in
water treatment
Barbara Kasprzyk-Hordern*
Department of Water Treatment Technology, Faculty of Chemistry, Adam Mickiewicz University, ul. Drzymaly 24, 60-613 Poznan, Poland
´
{
Abstract
Due to the presence and significance of alumina in the natural aquatic environment and its increasing application in drinking
and wastewater purification, the knowledge of the structure of alumina and its possible interactions with organic and inorganic
compounds in water are of great importance. This is of particular importance in both the understanding of natural aquatic
environment processes and efficient industrial applications. The chemistry of alumina reactions in water is complex. The adsorption
ability of alumina towards organic and inorganic compounds might be influenced by several factors such as: surface characteristics
of the adsorbent (surface area, density, pore volume, porosity, pore size distribution, pH as well as mechanical strength and
PZC
purity), pH of the solution, ionic strength, composition of water and the physicochemical properties of adsorbates. The aim of
this paper is to give a brief review of the properties of alumina and its reactivity with organic and inorganic compounds present
in aqueous solutions. It also summarises the usage of alumina and alumina supported phases in water treatment technology.
ᮊ 2004 Elsevier B.V. All rights reserved.
Keywords: Alumina; Alumina supported phases; Adsorption; Water; Water treatment; Catalytic ozonation; Catalytic wet air oxidation
Contents
1. Introduction 20
2. Classification of alumina 20
3. Physical and chemical properties of alumina 21
3.1. Surface of alumina 21
3.2. Models for the surface hydroxyl groups of alumina 22
3.2.1. Peri’s model 22
3.2.2. Tsyganenko’s model 22
3.2.3. Knozinger’s model 23

¨
3.2.4. Busca’s model 23
3.3. Aqueous interface of alumina 24
3.3.1. Surface charging in solution of indifferent electrolyte 24
3.3.2. Models for surface charge formation 24
3.3.3. Adsorption on alumina 26
3.3.3.1. Interactions with organic molecules 28
3.3.3.1.1. Carboxylic acids 29
3.3.3.1.2. Polyelectrolytes and polymers 32
3.3.3.1.3. Surfactants 37
3.3.3.2. Interaction with inorganic molecules 37
3.3.3.2.1. Anions 37
3.3.3.2.2. Cations 39
3.3.3.3. Dissolution of alumina 41
*Tel.: q48-61-829-3435; fax: q48-61-829-3400.
E-mail address: (B. Kasprzyk-Hordern).
20 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
4. Application of alumina and alumina supported catalysts in water treatment 41
4.1. Adsorption 41
4.2. Catalytic ozonation 42
4.3. Catalytic wet air oxidation 43
5. Concluding remarks 44
Acknowledgements 45
References 45
1. Introduction
The adsorption of molecules at solid–liquid interfaces
and its effects on coagulation, weathering and transport
are directly controlled by numerous properties of the
solid and adsorbate
w

1
x
. Furthermore, colloids play a
crucial role in the aquatic environment in controlling
anionic recycling, transport and stabilising particles,
which all influence the aquatic environment. The mobil-
ity of anions in the aquatic environment is controlled
by adsorption at the solid–liquid interface and by
competition among various anion species for surface
binding sites
w
2
x
. Adsorption at solid–liquid interfaces
is important in technological processes and products
such as corrosion, catalysis, nanoparticle ultracapacitors,
molecular sieves, and semiconductor manufacturing
w
3
x
.
Adsorption of surfactants at the solid–liquid interface is
an important topic in numerous processes ranging from
mineral beneficiation to detergency, including such
applications as wastewater treatment and soil remedia-
tion, dispersion stabilisation in ceramics and enhanced
oil recovery
w
4,5
x

. Polymeric reagents are used exten-
sively in the colloidal processing of ceramics
w
6
x
.
Adsorption of natural organic materials (commonly
present in natural water) such as humic and fulvic acids
is of great importance in environmental, mainly geo-
chemical, processes
w
7
x
. The other important matter is
the fate of contaminants in the environment, which is
strongly influenced by the presence of mineral solids
and colloids both in solid and aqueous phases. The
movement of anthropogenic pollutants in soil, surface
and groundwater and their bioavalibility in natural water
are largely dependent upon their interaction with solid
minerals. The availability of both organic and inorganic
compounds such as biogenic phosphate
w
8
x
, toxic arsenic
w
9
x
, lead

w
10–13
x
and chromium
w
14
x
will strongly
depend on solid–liquid interface reactions. The mobility
of metals will also depend on their speciation and
complexation with natural organic matter. The under-
standing of the adsorption of molecules at solid–liquid
interfaces allows for a prediction of the fate of anthro-
pogenic pollutants in natural water. Knowledge of mech-
anisms governing adsorption processes is, therefore of
great interest both from an environmental (geochemical)
and an industrial point of view.
Most solid phases in natural water contain aluminium
oxides. Alumina plays an important role in regulating
the composition of soil–water, sediment–water, and
other natural water systems
w
11
x
. Active alumina, due
to its high surface area, mechanical strength and thermal
stability has found several applications as an adsorbent
and catalyst. The acid–base properties of alumina are
the main reason for its wide usage. In water treatment
technology, adsorption on several adsorbents such as

active carbon, silica gel and zeolites as well as alumina
is one of the major processes used mainly for the
removal of several organic compounds from water.
These are: dissolved hazardous organic contaminants;
compounds responsible for colour and odour of water;
oxidation and disinfection by-products
w
15,16
x
. Al based
compounds are used as coagulants
w
15–18
x
. Alumina
has also been applied as a catalyst of ozonation
w
19–
26
x
and wet air oxidation
w
27–32
x
.
Due to the presence and importance of alumina in the
natural aquatic environment and its growing application
in drinking and wastewater purification, the knowledge
of alumina’s structure and possible interactions in water
are of great importance. The properties of metal oxide

surfaces in aqueous solution, including surface charging
and sorptive capacity, are determined by the nature of
their surface functional groups, the ability of these
groups to bind protons and adions, and the bonding
requirements of protons and adions. The molecular
structures and compositions of surface functional groups
and adion complexes are of great interest as they
facilitate thermodynamic, mechanistic and kinetic
description of surface reactions
w
3
x
. Because of all the
above reasons, the structure and composition of surface
groups and reactions with organic and inorganic com-
pounds as well as factors controlling these reactions can
be anticipated. The goal of this paper is to give a brief
review of the properties of alumina and reactivity in
aqueous solutions.
2. Classification of alumina
According to Haber (1925) aluminas can be classified
as follows
w
33
x
:
a-group g-group
Al O 3H O
23 2
does not exist gibbsite

Al O H O
23 2
diaspore boehmite (bauxite)
Al O
23
corundum gamma oxide
21B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Aluminum trihydroxide-bayerite, which was not
known in 1925 and, therefore not placed in Haber
classification, should be located in g-group next to
gibbsite
w
33
x
.
The above-mentioned classification is used by Euro-
pean authors. In the USA, the classification is as follows
w
33
x
:
a-group b-group g-group
Al O 3H O
23 2
gibbsite bayerite nordstrandite
Al O H O
23 2
boehmite diaspore –
In 1950, Stumpf et al. reported that apart from a-
Al O (corundum), another six crystal structures of

23
alumina occur:
g
,
d
,
k
,
h
i
x
-Al O
w
33,34
x
. The
23
sequence of particular type formation under the thermal
processing of gibbsite, bayerite, boehmite and diaspore
is as follows
w
35
x
:
Munster (1957) proposed another classification,
¨
which was subsequently modified by Lippens (1961).
The temperature of aluminium hydroxide formation is
the basis of this system of classification. The two groups
of alumina are

w
35
x
: low-temperature aluminas:
Al O ØnHO(0-n-6) obtained by dehydrating at tem-
23 2
peratures not exceeding 600 8C (g-group). This group
belongs to:
r
,
x
,
h
and
g
-Al O . high-temperature
23
aluminas: nearly anhydrous Al O obtained at tempera-
23
tures between 900 and 1000 8C (d-group). This group
belongs to:
k
,
u
and d-Al O .
23
All these structures are based on a more or less close-
packed oxygen lattice with aluminum ions in the octa-
hedral and tetrahedral interstices
w

35
x
. Low-temperature
aluminas are characterised by cubic close-packed oxygen
lattices; however, high-temperature aluminas are char-
acterised by hexagonal close-packed lattices
w
36
x
.A
more detailed discussion concerning crystal structures
of alumina was presented elsewhere
w
37,38
x
.
In terms of catalytic activity, high-temperature alu-
minas are less active than low-temperature aluminas.
This results from not only lower surface area (higher
order and larger particle size) but also the different
population of surface active sites of high-temperature
aluminas when compared to low-temperature ones
w
39
x
.
form with the formation of surface hydroxyl groups
w
35,39
x

. At room temperature, alumina adsorbs water as
undissociated molecules bonded with strong hydrogen
bonds. At higher temperatures, hydroxyl groups are
formed on the surface of alumina and, with an increase
of temperature, are gradually expelled as H O. However,
2
even at 800–1000 8C and in a vacuum, some tenths of
a percent of water are still retained in the alumina
w
35,40,41
x
.
The main two parameters determining the catalytic
properties of alumina are acidity and basicity. Brønsted
acidity–basicity is defined as the ability to proton
abstraction–acceptation. Lewis acidity–basicity is the
ability to electron acceptation–abstraction
w
42
x
. Chemi-
sorption of water on the alumina surface is considered
to be a reaction between Al ion, an acceptor of electron
pair (Lewis acid), and hydroxyl ion, its donor (Lewis
base).
Hydroxyl groups formed at alumina surface behave
as Brønsted acid sites. However, the dehydratation of
two neighbouring OH ions from the surface of alumina
y
causes the formation of strained oxygen bridge, active

Lewis acid sites
w
43
x
:
Low-temperature transition aluminas (metastable
phases of low crystallinity characterised by high surface
area and open porosity
w
39
x
) are of great interest due to
their possible usage both as catalysts and adsorbents in
water treatment technology. Al hydroxides are the main
active species of coagulation.
3. Physical and chemical properties of alumina
3.1. Surface of alumina
Active alumina, depending on the synthesis method,
is contaminated with small amounts of alkali oxides,
iron oxide and sulfate. Depending on the temperature
and vapour pressure, active alumina can contain from a
few tenths to approximately 5% of water. Water, depend-
ing on temperature, yields to physisorption or chemi-
sorption as an undissociated molecule or in dissociated
22 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Fig. 1. Types of isolated hydroxyl ions (q denotes Al in lower
3q
layer) w44x.
Table 1
Spectral position and assignment for surface hydroxyl groups on transitional aluminas w39x

OH band Average Peri’s Tsyganenko’s Knozinger’s
¨
Busca’s
frequency assignment assignment assignment assignment
(cm )
y1
1 3800 A I Ib Al
IV
2 3775 D I Ia -O-Al
IV
3 3745 B II IIb Al
VI
4 3730 E II IIa -O-Al
VI
5 3710 C III III Bridged
6 3690 C III III Bridged
7 3590 H-bonded H-bonded Tribridged
Both Brønsted and Lewis acid sites are thought to be
the catalytic centres of alumina
w
43
x
.
3.2. Models for the surface hydroxyl groups of alumina
3.2.1. Peri’s model
On dry alumina, exposing a (100) plane, the top layer
contains only oxide ions. At lower temperatures, a
completely filled monolayer of OH ions can be formed,
y
giving a square lattice of OH ions. As a result of

y
dehydration, neighbouring hydroxyl groups can react
with each other with the formation of oxygen bridges
and water molecules, which are subsequently desorbed
from alumina surface. During dehydration, adjacent
OH can combine at random, but only two-thirds of
y
the OH ions can be removed without disturbing the
y
local order. Further dehydration causes the creation of
surface defects. The remaining hydroxyl ions cover
approximately 9.6% of the surface. Depending on the
number of neighbouring oxide ions (0–4) with hydroxyl
group, five types of isolated surface hydroxyl groups:
A, B, C, D can be distinguished (Fig. 1, Table 1). The
five isolated bands are observed in the infrared spectra
of dry alumina. Further dehydration and the elimination
of isolated surface hydroxyl groups can occur only at a
very high temperature ()800 8C) when migration of
surface ions is possible. At this high temperature, pro-
tons migrate readily on the surface and the gradual loss
of surface area, as well as the slow formation of high-
temperature forms of alumina, indicate that also oxide
and aluminium ion migration occur. At this stage of
dehydration, the number of defects on the surface
increases considerably. The major defects are two and
three directly adjacent vacancies and two and three
directly adjacent oxide ions. As a result of dehydration
with increasing temperature, the Brønsted acid sites,
numerous at high water contents, are gradually converted

into Lewis acid sites
w
35,41,44–46
x
.
The model, however, valid in principle, does not give
a full description of the structurally complex aluminas.
The main limits of this model are: the assumption that
the (100) crystal face is the only possible termination
of aluminas crystallites and the negligence of the defec-
tive spinel nature of aluminas. This suggests that only
Al ions would be present in the uppermost layer and
VI
the fully hydrated surface (located on top of equivalent
cations) would be equivalent
w
39
x
.
3.2.2. Tsyganenko’s model
According to Tsyganenko’s model, the number of the
nearest neighbours has a negligible effect on the fre-
quency of the OH species. Whereas the number of
lattice Al atoms that OH groups are attached to be a
factor determining the frequency of surface hydroxyl
groups on the alumina surface. According to the model,
three forms of surface hydroxyl groups are possible as
presented in Fig. 2 and Table 1. In the model, the double
coordination of Al ions (Al and Al ) in spinel
VI IV

23B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Fig. 2. The possible surface OH groups: I (terminal),II(bridged), III
(tribridged) w47x.
Fig. 4. Possible OH structures on the surface of defective spinel transition aluminas (h-cation vacancy) w39x.
Fig. 3. Possible surface hydroxyl groups on alumina (s-the net charge at the OH group) w49x.
aluminas is taken into consideration and this is thought
to be responsible for the multiplicity of OH bonds
observed in the infrared spectra of aluminas
w
39,47,48
x
.
3.2.3. Knozinger’s model
¨
Knozinger’s model is the most complete approach to
¨
the understanding of the OH surface groups on alumina.
The basic assumptions are as follows. The termination
of alumina crystallites occurs along three possible crystal
planes (111, 110, 100). The uppermost layer of the
exposed crystal planes reproduces the anion and cation
array typical of the bulk. No reconstruction and ion
migration even at high temperature occurs. The frequen-
cy of hydroxyl groups is imposed by the net electrical
charge at the OH group, which is determined by the
coordination number of both OH group and Al ion
involved. Depending on the coordination properties of
surface anions and the number of Al ions attached to
hydroxyl group, five hydroxyl groups can be present on
the three possible crystal planes (111, 110, 100) of

alumina (Fig. 3, Table 1)
w
39,46,49
x
.
The net charge (Fig. 3) changes the OH stretching
frequency (Table 1) and also changes the acidicity of
the hydroxyl groups. Hydroxyl groups with the highest
frequency possess the highest basicity (Ib group) and
the OH groups with the lowest frequency are thought to
posses the highest acidicity (III group). This correlation,
however, is not always accurate
w
39
x
.
3.2.4. Busca’s model
The model considers the role of cation vacancies
imposed to the spinel structure by the alumina stoichi-
ometry and can be considered as a modification of the
previously mentioned Knozinger’s model. It takes into
¨
consideration differences of OH frequency in the case
of OH bounded to Al and Al ions, as the coordina-
IV VI
tion of cation is a main factor determining the OH group
frequency. The model implies that the free OH bands
are distributed over a much wider spectral range than
considered before (Table 1). The possible OH structures
at the surface of defective spinel transition alumina are

presented in Fig. 4. The presence of the cation vacancy
on the surface of alumina determines the multiplicity of
OH bands observed on aluminium oxides
w
39,50,51
x
.
The vibrational spectrum of surface hydroxyls of
alumina is complex but quite typical. The average
24 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
position of the OH bands in IR spectrum observed for
several transitional aluminas (metastable phases of low
crystallinity characterised by high surface area and open
porosity, which are of practical interest for catalytic
applications) and their adequate model assignment is
proposed in Table 1
w
39
x
.
The discussed models for the surface hydroxyl groups
of alumina concern gas–solid interface. In aqueous
solution, due to the presence of water molecules, greater
complexity of alumina surface groups should be expect-
ed, as the interaction of water molecules with surface
groups of alumina has to be taken into consideration. In
aqueous solution, an electric double layer at the solid–
liquid interface is formed as a result of electrostatic
interaction between the charged alumina surface and
ions of an opposite charge present in bulk solution.

Furthermore, as a result of the solid–liquid interface
interactions, several phenomena might be expected as
discussed below.
3.3. Aqueous interface of alumina
3.3.1. Surface charging in solution of indifferent
electrolyte
The mechanism by which the surface charge is estab-
lished has generally been considered to involve a two-
step process: surface hydratation followed by
dissociation of the surface hydroxide. The hydratation
step may be envisaged as an attempt by the exposed
surface atoms to complete their coordination shell of
nearest neighbours. Both exposed aluminium cations
accomplish this by pulling an OH ion or water mole-
y
cule and the oxygen ions by pulling a proton from the
aqueous phase. In each case, surface hydroxyl groups
will be produced which, in appropriate circumstances,
may ionise as Brønsted acids or bases
w
52–54
x
. The
surface hydroxyl groups of hydrous alumina have, there-
fore an amphoteric character. The primary surface charge
density (
s
) may be expressed by the following equation
s
w

55
x
:
The point of zero charge of alumina was assessed to
vary from ;7to;10 depending on the type of alumina.
Some relevant data is presented in Table 2. A detailed
discussion on point of zero charge of alumina and other
metal oxides was presented by Kosmulski
w
38,56–60
x
,
Sposito
w
37,61
x
and others
w
62
x
.
In aqueous solution, due to the surface charge of
alumina, an electric double layer is formed as a result
of electrostatic interaction between the charged alumina
surface and ions of an opposite charge present in bulk
solution.
The surface charge formation and the strong depend-
ence of the properties of alumina on the pH value of
the solution are of crucial importance when discussing
alumina’s application as a catalyst or adsorbent in water

treatment technology. This will be discussed below.
However, it has to be pointed out that the high catalytic
activity and the high adsorption capacity of alumina in
the process of impurities removal from water will be
obtained only when the process is carried out under
certain, optimal for the particular reaction conditions.
3.3.2. Models for surface charge formation
The mechanism of charge formation on the surface
of alumina is based on the phenomenon of adsorption
and desorption of protons by active surface centres. The
three main models: one-pK, two-pK and MUSIC model,
s sF G qG (1)
qy
Ž.
s HOH
where F is the Faraday constant, is the adsorbedG
q
H
amount of protons and represents the associationG
y
OH
of an OH ion with a surface proton by formation of
water, and is equivalent to proton desorption.
Particles may also become charged by specific adsorp-
tion of ions other than protons.
The properties of the surface of alumina strongly
depend on pH. In an acidic medium, below pH
PZC
(PZC, point of zero charge, the pH value at which the
net surface charge is zero;

s
s0), the surface is charged
s
positively. At a basic medium (pH)pH ) the surface
PZC
is charged negatively
w
52
x
:
25B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Table 2
The summary of the pH of aluminas and hydrated aluminas
PZC
Material pH
PZC
Experimental method Refs.
a-Al O
23
8.4 Electrophoresis w52x
9.2 Electrophoresis w63x
9.1 Electrophoresis w64x
9.2 Electrophoresis w65x
8.6–8.8 Streaming potential w52x
9.4 Streaming potential w52x
9.1 Streaming potential w52x
6.7 Electro-osmosis w52x
7.9 Mass titration w66x
8.5 Mass titration w67x
8.0 Electrokinetic sonic amplitude w68x

9.1 measurement w69x
8.7 – w70x
8.4–9.2 – w71x
g-Al O
23
8.0 Electrophoresis w52x
9.0–9.7 Electrophoresis w72,73x
6.9 Mass titration w66x
8.4 Mass titration w66x
8.0 Mass titration w67x
7.8 Acid–base titration w74x
8.8 – w71x
8.47 – w69x
d-Al O
23
7.30 – w74x
g-AlOOH 9.4 Electrophoresis w52x
8.7 Potentiometric acid–base titration w7x
9.0 Potentiometric acid–base titration w75x
7.7–9.4 – w70x
Al(OH)
3
9.4 Electrophoresis w52x
9.2 Electrophoresis w52x
8.3 Potentiometric acid–base titration w75x
7.7 – w52x
Table 3
Proton association constants for a series of surface groups of alumina
w81x
Hydroxyl groups Formal charge Log K

H
Al–OH y1y2 10.0
Al –O
2
y1 12.3
Al –OH
2
0 y1.5
Al –O
3
y1y22.2
which are used to describe this phenomenon, are dis-
cussed briefly below. Discussion that is more detailed is
presented elsewhere
w
38,55,76–78
x
.
The main assumption of the two-pK model is a
monofunctional surface with the only one type of active
surface oxygen groups that can undergo two protonation
steps, each governed by its own pK value
w
55,79–82
x
:
H
yq 0
SO qH ~SOH K (2)
sH1

0 qq
SOH qH ~SOH K (3)
s2H2
yq q
SO q2H ~SOH K K (4)
s 2 H1 H2
where K is a proton association constant and S is an
H
alumina surface.
In one-pK model the surface is assumed to be mon-
ofunctional with surface oxygen groups that undergo
one protonation step
w
38,55,81–83
x
:
1y2yq 1y2q
SOH qH ~SOH K . (5)
s2H
MUSIC (multi site complexation) model is the most
successful in deriving the surface charging behaviour
from the properties of the material. In contrast to one-
and two-pK models, it considers different types of
surface groups, which have different protonation con-
stants
w
38
x
. The MUSIC model is based on Pauling
theory of bond valence. It assumes the presence of

several active surface oxygen groups on metal
(hydr)oxides: singly, doubly and triply coordinated with
metal cations of the solid, capable of adsorbing one or
two protons. The protonation of metal (hydr)oxide
surface groups can be described by the two reactions
w
81,82
x
:
n*yy2 q n*yy1
() ()
S–O qH ~S –OH K (6)
n s nn,1
n*yy1 q n*y
() ()
S–OH qH ~S –OH K (7)
n s n 2 n,2
where n is the number of metal cations coordinated with
surface O(H),
y
is the bond valence of S–O(H) bond
(the charge of the metal ion divided by its coordination
number), and H is the local proton concentration near
q
s
the surface.
Active surface groups of different metal (hydr)oxides
have different affinities for protons, which can be
explained by differences in the Gibbs free energy levels
of the groups involved. The intrinsic free energy of the

reactions Eqs. (6) and (7) can be considered to be
composed of local electrostatic contribution and other
unspecified contributions. Following the principle of this
type of approach, the proton association constant can be
calculated from the following expression
w
55
x
:
log K sAyB(nyyL)(8)
n,i
where A, B are constants, and L is the distance between
the metal ion and the adsorbed proton.
The calculated proton association constants for a
series of surface groups are presented in Table 3. On
the basis of the calculated proton association constants
of oxo-(K ) and hydroxo-complexes (K ) the conclu-
n,1 n,2
sion can be drawn that only one of the protonation
reactions of a given surface oxygen will be ‘active’ in
the normally accessible pH range
w
55,81
x
.
The surface charge density for crystal structure is as
follows
w
81
x

:
26 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
{
s sS NFnyu q(nyy1)u
ssn n,2 n,1
()
}
q(nyy2)(1yu yu )(9)
n,1 n,2
where
u
,
u
are surface groups present as a specific
n,1 n,2
surface species on a specific crystal face defined accord-
ingly to reactions Eqs. (6) and (7) as follows:
nyy1
()
u sS–OH yN
n,1 n s n
()
ny
()
u sS–OH yN
n,2 n 2 s n
()
N – the site density of the specified surface group
s n
()

on a given surface, defined as the sum of all species
with the same value of n of this face.
Assuming that only one type of active site group
exists on the surface of metal oxide and when: n
y
s1,
the MUSIC model can be simplified to ‘two-pK’ model.
The Eqs. (6) and (7) can be simplified to Eq. (4). The
Eq. (9) can be simplified as follows
w
81
x
:
{}
s sNF u y(1yu yu )(10)
ss212
where
u
,
u
, and (1y
u
y
u
) are the fractional surface
21 1 2
coverage of the –OH , SOH and –O, respectively.
q 0
2
Assuming that only one type of surface group is on

the surface of metal oxide and when ns1,
y
s1y2, the
MUSIC model is simplified to ‘one-pK’ model. The Eq.
(9) can be simplified to the following form
w
81
x
:
s sNF(u y1), (11)
ssH
where
u
is the fractional surface coverage with
H
SOH .
q
2
3.3.3. Adsorption on alumina
Generally, adsorption is the process where matter
dispersed in solution accumulates at an interface on the
adsorbent surface. The adsorption kinetics of any sub-
stance (e.g. small molecule, an ion, a particle, a polymer
or a colloid) can be, therefore described in similar terms.
A generally accepted model of adsorption kinetics,
originally proposed by Baret
w
84,85
x
, consists of two

main steps. The first step is the transport of particles
from bulk solution near to the adsorbent, which can take
place due to one or more contributions such as convec-
tion andyor diffusion. In the second step (attachment
step), the formation of bonds between adsorbate and
adsorbent occurs. An activation energy barrier is the
main factor determining the adsorption rate as it can
decrease the rate of attachment
w
86
x
.
The process of desorption also involves a two-step
reaction: detachment and transport. Both the transport
steps and the attachment–detachment steps proceed
simultaneously. Depending on the rates of the process,
two limiting cases should be taken into consideration.
If the transport step is much slower than the attachment–
detachment step the adsorption process is transport
controlled. If the attachment–detachment step is much
slower than the transport step, the adsorption process is
attachment–detachment controlled. If the rates of both
steps are similar, the adsorption process is controlled by
both mechanisms
w
86
x
. The adsorption equilibrium of
ions is often formulated by the Langmuir and Freundlich
isotherm equations.

The Langmuir isotherm describes the dependence of
the equilibrium surface concentration of an adsorbed
molecule on its gas–liquid phase concentration at con-
stant temperature. The Langmuir isotherm is based on
the following assumptions: (1) the solid surface is made
up of a uniform array of energetically identical adsorp-
tion sites; (2) a maximum of one monolayer can be
adsorbed; (3) there are no interactions between the
adsorbed molecules. The Langmuir isotherm can be
expressed by the following equation
w
15,16,42,82
x
:
XsXbCy(1qbC)(12)
m
where X is the amount of adsorbate adsorbed on1gof
alumina (mol), X is the amount necessary to cover the
m
entire surface with a monolayer of adsorbate (mol), C
is equilibrium compound concentration in solution (mol
m ) and b is adsorption energy constant.
y3
Freudlich isotherm assumes that the heat of adsorption
decreases exponentially with surface coverage (X) and
can be expressed as follows
w
15,16,42,82
x
:

1yn
XskC (13)
where k, n are constants.
The application of the two isotherms mentioned,
which assume monolayer coverage, is generally restrict-
ed to chemisorption. The isotherm can be applied to
physisorption if the amount physically adsorbed does
not exceed monolayer coverage. Physical adsorption
normally proceeds beyond monolayer coverage, and the
most commonly used isotherm to describe this situation
is the BET isotherm
w
15,16,42,82
x
.
The Langmuir and Freundlich isotherms have found
several applications mainly because of simplicity and
the necessity of using two parameters only in the
calculations. They have, however, two major drawbacks.
Firstly, the model parameters obtained are usually appro-
priate for one set of conditions and cannot be used as a
prediction model for another set of conditions. Secondly,
these models cannot provide us with a fundamental
understanding of ion adsorption. Numerous investiga-
tions have been carried out in the past several decades.
Several models such as: the Gouy-Chapman–Stern-
Graham model, the ion-exchange model, the ion-solvent
27B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Fig. 5. Scheme of triple-layer model w89x.
interaction model, the surface complexation models

(SCMs) were successfully applied for the description of
the adsorption of ions on alumina. Among these models,
it has been found that SCMs are the most adequate in
predicting ion adsorption on hydrous alumina
w
55,80,86–90
x
. Surface complexation models combine
the concept of coordination chemistry with those in
electric double-layer theory. SCMs consider the surface
charging (development of electrified interfaces) and ion
adsorption (interfacial distribution of ionic species) as
surface complexation reactions. These reactions are anal-
ogous to the homogeneous phase complexation in addi-
tion to the accounting of the influence of electric
potential developed in the interfacial reactions
w
91
x
.
Several SC models have been proposed: the diffuse
layer model (DLM)
w
55,92,93
x
, the basic Stern model
(BSM)
w
83,93
x

, the constant capacitance model (CCM)
w
92,94,95
x
, and the triple layer model (TLM)
w
55,87–
90,96–99
x
. A detailed discussion on SCMs was pre-
sented by Kosmulski
w
38
x
and Sposito
w
37
x
. The location
of ions adsorbed in a certain layer is strongly dependent
on the relative bonding affinity of ions for the functional
groups of adsorbents. That is the reason why the TLM
model was found to be the most valuable as it is able
to predict adsorption both when ions have lower and
when they have higher affinity with surfaces. The TLM
model, whether in the 1- or 2-pK approaches, is regarded
as a generalised case of other electrostatic models. By
making several assumptions, the TLM can be easily
degenerated into much simpler models such as the CCM
or BSM models

w
55,80,86–89,96,97
x
.
The triple layer model assumes the formation of three
planes of adsorption, to which ions are allocated. Protons
and hydroxides adsorb at the surface or O-plane (inner-
most part, which is characterised by charges
s
), where-
o
as electrolyte ions are assumed to adsorb at b-plane
(outer plane characterised by charges
s
), which is a
b
small distance from the surface (Fig. 5). The adsorption
of the protons and electrolyte ions is assumed to be
responsible for the formation of a net charge at the
surface of hydroxide. To counter the local charge density
at the surface, it is assumed that a diffuse swarm of
counterions is formed near the surface. The closest
distance of approach of the diffuse swarm defines d-
plane. The three planes of charge: O-, b- and d-plane
are associated with three planes of potential C , C ,
0 b
and C and treated as a series of pairs of parallel-plate
d
capacitors with capacitances C and C
w

37,87–
12
89,91,100–102
x
.
Inner-sphere surface complexation reactions in TLM
are presented below. Eqs. (14) and (15) represent the
protonation and deprotonation equilibria
w
37,88,89,91
x
:
qq
AlOH q H ~AlOH
2
q
wx
AlOH
2
wz
x|
K int s exp Fc yRT (14)
Ž.
q o
y~
q
wxwx
AlOH H
yq
AlOH~AlO qH

yq
wxwx
AlO H
wx
K int s exp yFc yRT (15)
Ž.
y o
wx
AlOH
mq my1 q
()
AlOH q M ~AlOM qH
my1 q
()
wxwx
AlOM H
wz
1
x|
K int s exp my1Fc yRT (16)
Ž. Ž .
M o
y~
mq
wxwx
AlOH M
mq my2 q
()
2AlOHqM ~(AlO) M q2H
2

wz
my2 q 2
()
x|
wx
AlO M H
Ž.
2
y~
wz
2
x|
K int s exp my2Fc yRT
(17)
Ž. Ž .
M o
y~
mq
2
wxwx
AlOH M
ly ly1 yy
()
AlOH q L ~AlL qOH
Ÿ
y1 yy
()
wxwx
AlL OH
1

wz
x|
K int s exp y Ÿy1 Fc yRT
(18)
Ž. Ž .
L o
y~
Ÿ
y
wxwx
AlOH L
lyy
ly2 y
Ž.
2AlOH q L ~Al L q2OH
2
y 2
wz
Ÿ
y2 y
Ž.
x|
wx
Al L OH
2
y~
2
wz
x|
K int s exp y Ÿy2 Fc yRT

(19)
Ž. Ž .
L o
y~
Ÿ
y2
wxwx
AlOH L
where M is a metal ion, L is a ligand, K is the ‘intrinsic’
equilibrium constant, R is the ideal gas constant, T is
the absolute temperature.
Surface outer-sphere complexation reactions for
M and L ions can be given by the reactions
mq ly
w
37,88,89
x
:
28 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
mqymqq
AlOH q M ~AlO yM qH
y mqq
wxwx
AlO yMH
1
wz
x|
K int s exp Fmc yc yRT
(20)
Ž. Ž .

M
b
o
y~
mq
wxwx
AlOH M
mqy q
my1
Ž.
AlOHqM qHO~AlO yMOH q2H
2
yq2
my1
Ž.
wx
wx
AlO yMH
2
wx
K int s exp Fmy1 c yc yRT
(21)
Ž. Ž .
Ž.
M
b
o
mq
wxwx
AlOH M

q lyqly
AlOH q H qL ~AlOH yL
2
Ÿ
qy
wx
AlOH yL
2
1
wz
x|
K int s exp F c yŸc yRT
(22)
Ž. Ž .
L o
b
y~
Ÿ
qy
wxwxwx
AlOH H L
q lyqly1 y
()
AlOHq2H qL ~AlOH yLH
2
q
Ÿ
y1y
Ž.
wx

AlOH yLH
2
2
wx
K int s exp F c y Ÿy1 c yRT
(23)
Ž. Ž Ž
L o
b
Ÿ
q 2 y
wxwxwx
AlOH H L
qyqq
AlOHqC ~AlO yC qH
yqq
wxwx
AlO yCH
wz
x|
K int s exp F c yc yRT
(24)
q
Ž. Ž .
C
b
o
y~
q
wxwx

AlOH C
qy qy
AlOHqH qA ~AlOH yA
2
qy
wx
AlOH yA
2
wz
x|
K int s exp F c yc yRT (25)
y
Ž. Ž .
A o
b
y~
qy
w xwxwx
AlOH H A
where C is the cation and A is the anion of the
qy
background electrolyte.
Charge balance requires that the sum of the charges
at the O-, b-, and d-plane be equal to zero
w
37,88,89,91
x
:
s qs qs s0 (26)
O b d

F
qqly
wxw x
s s AlOH q AlOH yL
o22
µ
Sa
q
wz
ly1 y
Ž.
x|
q AlOH yLH
2
y~
wz
my1
Ž.
x|
q my1 AlOM
Ž.
y~
wz
qy
my2
Ž.
x|
wx
q my2 AlO M q AlOH yA
Ž.Ž.

2
2
y~
yymq
wxw x
y AlO q AlO yM
y
wzwz
my1ly1 y
Ž. Ž.
x|x|
q AlO yMOH q ly1 AlL
Ž.
y~y~
ly2 yyq
()
wxw x
y ly2 Al L y AlO yC (27)
Ž.
∂
2
F
y mq
wx
s s m AlO yM q my1
Ž.
µ
b
Sa
y

wz
my1
Ž.
x|
= AlO yMOH
y~
yq qly
wxw x
q AlO yC yl AlOH yL y ly1
Ž.
2
qqy
wz
ly1 y
Ž.
x|
wx
= AlOH yLH y AlOH yA (28)
∂
22
y~
where C in the capacitance density, S is the surface area
and a is the suspension density.
The mass balance equation for the surface functional
group, AlOH is
w
37
x
:
qy

wxwxw xwx
AlOH s AlOH q AlOH q AlO
2
T
wz
wz
my1my2
Ž. Ž.
x|
x|
q AlOM q2 AlO M
Ž.
2
y~
y~
wzw z
ly1 y ly2 y
Ž. Ž.
x|x |
q AlL q 2 Al L
2
y~y ~
y mq
wx
q AlO yM
y
wz
my1
Ž.
x|

q AlO yMOH
y~
q ly
wx
q AlOH yL
2
q
wz
ly1 y
Ž.
x|
q AlOH yLH
2
y~
yq qy
wxw x
q AlO yC q AlOH yA (29)
2
3.3.3.1. Interactions with organic molecules. Organic
compounds differ in molecular weight and nature of
functional groups; therefore their sorption mechanisms
are diverse. Organic compounds with acidic, basic or
amphoteric properties are present in solutions as anions
or cations over a certain pH range. Their sorption will,
therefore be affected by surface charging. Organic com-
pounds, which form very stable complexes with metal
cations, may result in the chemical dissolution of adsor-
bents
w
38

x
.
Organic molecules of molecular weight smaller than
200 do not adsorb on oxide surfaces unless they have
functional groups such as carboxylic, phenolic-OH, or
amino groups which, substituting for the surface hydrox-
yl group, can form complexes with the structural metal
ions of the oxide surface
w
103
x
.
Non-ionic, hydrophobic organic chemicals such as
alkylbenzenes, chlorobenzenes and polycyclic aromatic
hydrocarbons interact weakly and non-specifically with
mineral surfaces
w
104,105
x
. Sorption of these com-
pounds on alumina in aqueous solution is difficult
because water molecules out-compete the non-ionic
29B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
hydrocarbons for sorption to mineral surfaces and the
surface of mineral is coated with at least one layer of
strongly sorbed water that prevents the non-ionic com-
pound from interacting directly with the mineral surface.
Solution pH has no effect on the adsorption of PAHs
(polyaromatic hydrocarbons) on alumina. This is expect-
ed as strictly non-polar, non-ionic hydrocarbons are not

capable of having charge–charge or charge–dipole inter-
actions. It has to be emphasised, however, that a charge-
induced dipole interaction could take place between a
positively charged alumina surface and the electron-rich
p
system of the PAHs
w
104
x
.
The adsorption of chlorophenols on alumina in aque-
ous solutions is weak. Chlorophenols adsorb on metal
oxides (Al O , TiO ) via the phenolate group
w
106
x
.
23 2
The relative adsorption affinity of polyols (2,3-buta-
nediol, glycerol, erythriol, threitol, ribitol, arabinitol,
xylitol, mannitol, dulcitol and sorbitol) is determined by
the number of vicinal hydroxy groups present, the
number of erythro and threo configurations and their
sequencing. Adsorption increases with an increasing
number of vicinal hydroxy groups. Threo-threo sequence
promotes adsorption and its presence is equivalent to
another vicinal hydroxy group, which suggests that
adsorption occurs via a tridentate binding mechanism.
Erythro-erythro sequence has no significant effect
w

107
x
.
The adsorption of N-compounds such as hydroxypir-
idines and quinolines that are known to undergo tauto-
metisation depends mainly on the favoured tautomer
form. Compounds that exist at the hydroxy form in
aqueous solution adsorb on the metal surfaces, while
those that exist in the oxo form do not. The lack of
adsorption of the oxo tautomer is a result of the absence
of favourable electrostatic interactions between the com-
pound and the surface, absence of ligand groups capable
of surface complexation and the presence of strong
intermolecular hydrogen bonding between ligand groups
(carbonyl and amide) and water molecules. The hydroxy
group in the hydroxy tautomer, with or without assis-
tance from the cyclic –N group, is suited to interact
with metal oxide surface via electrostatic forces andyor
surface complexation
w
108
x
.
The studies on phenylphosphonate ions adsorption
onto aged g-Al O and boehmite revealed that these
23
ions undergo adsorption through surface complexes for-
mation, which are most probably monodentately coor-
dinated to the surfaces
w

95,109
x
.
Several adsorption mechanisms were developed in
order to explain the adsorption of organic molecules
onto hydrous solids based on ligand exchange reaction
w
103
x
, the formation of hydrophobic bonds between the
surface and organic molecules and hydrogen bonding as
an adsorption mechanism
w
72,110,111
x
, as discussed
later.
Carboxylic acids. The properties of carboxylic acids,
mainly their adsorption affinity towards metal oxides
surfaces, are of great importance in water treatment
technology, as these compounds are commonly present
in treated water. They are the main oxidation by-
products, which are resistant to ozone. They are also
biodegradable. Furthermore, –COOH groups comprise
a significant part of natural organic matter, a typical
component of natural water.
The adsorption of carboxylic acids on alumina (and
other metal oxides such as ZrO and TiO ) is very
22
strong, with adsorption energies much higher than those

of other organic compounds. This particular property of
carboxylic acids makes alumina an attractive medium
for the removal of these compounds from treated water.
Because most carboxylic acids are weak acids, their
degrees of dissociation in an aqueous solution and
adsorption on alumina are greatly affected by pH
w
112
x
.
The surface coordination model, or more specifically
the ligand exchange model, based on the assumption
that anions of the organic acids replace the surface
hydroxo groups of alumina, was used by Kummert and
Stumm
w
103
x
in order to explain the specific interaction
of organic acids with the hydrous oxide. The schematic
presentation of the possible surface coordination reac-
tions of the diprotic acid H X (e.g. salicylic acid, phtalic
2
acid and catechol) with the surface OH-groups of g-
Al O is presented in Fig. 6
w
103
x
.
23

The possible surface coordination reactions are as
follows
w
103
x
:
qq
'AlOH ~'AlOHqH
2
s qq
wx
µ∂µ ∂
K s 'AlOH H y 'AlOH (30)
a1 2
yq
'AlOH~'AlO qH
s yq
wx
µ∂µ∂
K s 'AlO H y 'AlOH (31)
a2
'AlOHqHX~'AlXHqHO
22
s
wx
µ∂µ∂
K s 'AlXH y 'AlOH H X (32)
12
yq
'AlOHqHX~'AlX qHO

23
s yq
wx w x
µ∂µ∂
K s 'AlX H y 'AlOH H X (33)
22
2'AlOHqHX~'Al Xq2H O
222
n
s
wx
µ∂µ ∂
K s 'Al X y 'AlOH H X 1FnF2 (34)
22 2
where
{}
denotes surface concentrations (mole kg )
y1
and
wx
denotes concentrations ( mole l ).
y1
The nature of the interaction between the carboxylic
groups with hydroxyls on the surface of alumina has
been studied by several groups
w
1,70,75,103,112–
120,154
x
. It is generally believed that both oxygen atoms

30 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Fig. 6. Possible surface coordination reactions of the diprotic acid
H X with the surface OH-groups of g-Al O w103x.
223
Fig. 7. Structures of salicylate complexes at alumina surface in aque-
ous alumina suspension: (a) bidentate, (b) carboxylate-bonded mon-
odentate, (c) phenolate-bonded monodentate, (d) outer-sphere ionic
complex w118x.
in the carboxylic group are anchored on the alumina
surface, but two models have been proposed for the
interaction. The ‘bridging’ model considers that both
oxygen atoms of the carboxylic group are linked to Al–
O sites on the surface through hydrogen bonding
w
113
x
.
Taking the presence of water into consideration, the
formation of hydrogen bonding between carboxylic
groups and the alumina surface might be difficult. The
‘chelating’ model considers that the carboxylic group is
dissociated and forms a bidenate linkage with single
Al–O–H site
w
114
x
. The experimental data presented
by Kummert and Stumm
w
103

x
shows, however, that
only 1:1 surface complexes are formed. No bidentates,
i.e. species Al X, are present on the surface of g-
2
Al O
w
103
x
.
23
The tendency of the organic acids to form surface
complexes with Al O (Eq. (32)) is similar to that of
23
organic ligands to form complexes with Al in solution
3q
w
103
x
:
2q 2q
AlOH qHX~AlXH qHO
22
2q 2q
wxwxwx
K s AlXH y AlOH H X (35)
12
The solute complexes in addition to surface complexes
for diprotic acid H X (e.g. salicylic acid, phtalic acid
2

and catechol) are inner-sphere complexes
w
103
x
:
This conclusion was also arrived at by Szekeres et al.
w
75
x
for salicylic acid adsorption on g-AlOOH (boehm-
ite). Ainsworth et al.
w
17,118
x
, however, identified four
surface complexes of salicylate on the surface of alumina
at pH 2–6 (Fig. 7). Among them are: one outer-sphere
and three mono- and bidentate inner-sphere species.
Bidentate inner-sphere complexes were found to be
formed at a low surface coverage (Fig. 7a). At the
equilibrium, monodentate phenolate surface complexes
were formed (Fig. 7c). The monodentate carboxylate
surface complexes were not as precisely defined (Fig.
7b). Both monodenate inner-sphere complexes and
outer-sphere complexes (Fig. 7d) were found to be the
intermediates of the adsorption process resulting in a
bidentate inner-sphere complexes formation. In general,
the mechanism of salicylate adsorption follows the
formation of an outer-sphere complex, subsequent for-
31B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48

Table 4
The effect of pH value and ionic strength on adsorption capacity of alumina towards benzoic acid w70x
Carboxylic Mineral pH (initial) Electrolyte Surface
acid coverage
(mmol m )
y2
Benzoic a-Al O
23
w3.3, 4.3x 0.0 M NaCl 5.7
acid wS , 9.5 m g ; particle size distribution:
2 y1
BET
3.7 0.1 M NaCl 5.0
d , 0.3 mm, pH , 8.7x
m50% PZC
5.9 0.1 M NaCl 1.3
g-AlOOH w3.3, 4.3x 0.0 M NaCl 2.2
wS , 147.0 m g ; d , 2.7 mm,
2 y1
BET m50%
3.3 0.1 M NaCl 1.5
pH , 7.7–9.4x
PZC
mation of an inner-sphere monodentate carboxylate com-
plex (accompanied by the loss of H O) and the
2
formation of the final product, bidentate complex
w
117,118
x

.
The adsorption capacity of alumina depends on sev-
eral factors such as: the acid–base properties of the
surface hydroxo groups, the specific surface area, the
nature of adsorbates, pH, and ionic strength. Parameters
such as pH and electrolyte content affect both the surface
charge of the solid and the degree of dissociation of
carboxylic acid in the bulk phase
w
70
x
.
In the case of alumina, an amphoteric oxide, the
property of the surface depends strongly on pH. It has
been already emphasised in this paper that alumina
surface is positively charged in an acidic medium (pH-
pH ) and negatively charged in a basic medium (pH)
PZC
pH ). Counterions in the outer layer compensate for
PZC
the surface charges to fulfil the principle of electroneu-
trality. The degree of surface polarisation depends on
the pH of acid or base solutions. Thus, the solution pH
for the treatment of alumina determines its capacity of
counterion exchange (Table 4)
w
70,112
x
.
Madsen and Blokhus

w
70
x
examined the adsorption
capacity of a-Al O and g-AlOOH in respect of benzoic
23
acid. The results presented in Table 4 indicate that the
mineralogical structure of adsorbent strongly affects the
adsorption capacities. According to research, the higher
adsorption capacity of a-Al O is due to a higher Al
23
content when compared to that of g-AlOOH.
Ionic strength is another factor influencing the adsorp-
tion of carboxylic acids on the surface of alumina. As
reported by Madsen and Blokhus, an increase of ionic
strength reduces the maximum adsorption capacity,
which can be explained by the electrostatic shielding of
the surface sites with salt ions
w
70
x
. This is, however,
true for lower acids concentrations. For the g-AlOOH
(boehmite) examined by Szekeres et al.
w
75
x
the amount
of salicylic acid adsorbed on the positively charged
surface (pH 3.0, 6.0) was lower at a higher ionic strength

at lower salicylate concentrations, but higher at a higher
ionic strength at higher salicylate concentrations. It has
been already reported that as a result of high ionic
strength, a charge-screening effect occurs and in conse-
quence, the adsorption of salicylate on the surface of
alumina decreases. This situation, however, takes place
in the case of low acid concentrations in bulk solution.
With increasing salicylate concentration, the specific
adsorption of salicylate should overcome the non-spe-
cific electrostatic effect. The extent of adsorption on an
uncharged surface should be, therefore higher at higher
ionic strength as a result of the higher salicylate activity.
The molecular size of the adsorbate and the porosity
of the adsorbent are significant parameters influencing
the adsorption capability of alumina. Small organic acids
such as salicylic acid are preferentially adsorbed on the
walls of the mesopores with no effect on the charge of
the external surface. When adsorption inside the pores
is complete, it spreads over the external surface causing
a decrease of the
z
potential. Polyelectrolytes such as
polyacrylic acid cannot enter the mesopores and are
adsorbed on the external surfaces, and only low quanti-
ties of them are required to decrease the
z
potential
w
1
x

.
Complexones: The adsorption properties of alumina
towards complexones (a group of polyaminocarboxylic
acids or their salts, which are derivatives of iminodi-
acetic acid) such as:
– EDTA (ethylenediaminetetraacetic acid),
– DTPA (diethylenetetraaminehexaacetic acid),
– TTHA (triethylenetetraaminehexaacetic acid),
– EGTA (
w
ethylenebis(osoxyethylenetrilo)
x
tetraacetic
acid),
– HEDTA (1-(2-hydroxyethyl)ethylenediaminetri-
acetic acid),
– CDTA (trans-1,2-diaminocyclohexanetetraacetic
acid),
was investigated by Bowers and Huang
w
72
x
and
Ryczkowski
w
110,111
x
. The characteristic arrangement
of these compounds is a nitrogen atom connected with
two carboxymethyl groups: –N(CH COOH)

w
109
x
.
22
Generally, the adsorption phenomena of complexones
may proceed according to two basic attractive forces:
specific attraction (chemical bonds, which form between
surface groups and a sorbate molecule) and electrostatic
attraction (electrostatic forces, which arise as a result of
differences in charge between the surface and ionic
solutes). Bowers and Huang
w
72
x
developed a model for
the adsorption of these compounds onto g-alumina using
32 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
hydrogen bonding as the adsorption mechanism. The
alumina surface groups such as AlOH (type 1, proton
q
2
donor) and AlO (type 3, proton acceptor or electron
y
donor) have the stronger hydrogen bond formers, while
AlOH (type 2, electron donor) is relatively weak.
Adsorption of these compounds on alumina increases
with increasing proton concentration due to the forma-
tion of the surface complexes between the polyanions
and the surface hydroxo groups, e.g. AlOH . There is

q
2
a great variety of bonding possibility between alumina
and complexones, which represents rather complex pH
dependent adsorption behaviour. The possible hydrogen
bond interactions between g-alumina surface and the
available organic functional groups are presented in
Table 5.
Among several bonding possibilities presented in
Table 5, any bonding with a type 3 surface group can
be eliminated, since pH of alumina is 9.0 and most
PZC
of the acetic acid groups are unprotonated at the pH
values where the type 3 group will form. Therefore little
adsorption of most polyacetic acids at high pH values
can be observed. It is questionable, however, whether
hydrogen or electrostatic forces are dominant in the case
of complexones adsorption on the surface of alumina.
The presence of water molecules and their contribution
in bonds formation on the surface of alumina should be
anticipated.
Polyelectrolytes and polymers. The adsorption of
polymers and polyelectrolytes onto alumina differs sig-
nificantly from that of small molecules mainly because
these materials have multifunctional groups with differ-
ent adsorption potential, varying sizes and conformations
that influence the adsorption process. For polyelectro-
lytes, the major driving force for adsorption is the
33B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Fig. 8. Schematic representation of the interaction between (a) Alumina-PAA, (b) Alumina-PVA w6x.

electrostatic attraction
w
121
x
. The adsorption of several
polyelectrolytes such as polyacrylic acid (PAA), poly-
vinyl alcohol (PVA), polyvinyl pyrrolidone (PVP) onto
alumina, has been widely studied
w
1,6,63,65,119,122–
132
x
.
The adsorption of polyelectrolytes on metal oxides
involves several neighbouring sites
w
1
x
:
xyy
q yy
xsAlOH qRCOO ~ sAlOH OOCR
µ∂
Ž.
x
22
(36)
Generally, the acidity of the polyelectrolyte is higher
than the local positive charge (y)x), and the resulting
surface charge is negative

w
126
x
. Consequently, the
charged polyelectrolyte that is adsorbed on the surface
of alumina causes the PZC to be reached with very low
adsorbed quantities. A polyelectrolyte such as PAA takes
a highly stretched conformation, which is a result of
repulsion between the dissociated carboxyl groups. This
results in the inability of PAA molecules to enter the
mesopores and adsorption outside these pores occurs.
At a low PAA concentration, molecules are adsorbed on
the alumina surface in flat conformation. For higher
concentration, the adsorbed molecules straighten up as
a result of intermolecular repulsion from neighbouring
molecules. Therefore the orientation of adsorbed mole-
cules such as PAA is influenced by two factors: the
adsorption affinity of anionic PAA for positively charged
sites of alumina and intermolecular repulsion
w
1,123
x
.
The conformation of polymer on the alumina surface
is dependent on polymer concentration and changes
from trains to loops or tails with increasing polymer
concentration
w
122,123
x

. When adsorption of two kinds
of polymer takes place, complex phenomena often occur,
depending on the combination of polymers and particles
w
122
x
.
Another factor that influences the adsorption affinity
of polymers is the pH value of the solution. The
interaction of PAA with alumina surface is strong due
to the presence of a carboxylic functional group. Gen-
erally, carboxyl groups of PAA can act as proton donor
or acceptor, and thus adsorption may take place by
hydrogen bonding between the hydroxylated alumina
surface and the carboxyl groups of the polymer (Fig.
8). Here again, however, the role of water molecules in
the bond formation between PAA and alumina surface
groups and possible electrostatic forces should be taken
34 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Table 6
Adsorption density of PAA on alumina at different pH values w6x
Polymer Mineral pH (initial) Electrolyte Adsorption density
wmg m x
y2
PAA a-Al O
23
3.08–3.20 0.01 M ;1.23
a
Molecular (A-16, Aluminum Company of 7.03–7.37 KNO
3

;0.13
a
size, 5000 America, USA)
wS , 10.29 m g , particle size, 1.15 mmx
2 y1
BET
9.25–9.36 ;0.05
a
Data transferred from figure w6x.
a
Table 7
Maximum adsorbed amount of the polyelectrolytes and their charge density w133x
Mineral Polyelectrolyte pH Electrolyte Adsorption Charge
(initial) density density
(mg m )
y2
(meq g )
y1
d-Al O
23
Polyacrylic acid 5.2 0.01 M 0.70 y3.9
(Degussa)(M , 250 000 g mol )
y1
w
NaCl
wS , 100 m g x
2 y1
BET
Polymethacrylic acid 1.35 y1.5
(M , 1 000 000 g mol )

y1
w
Humic acid 1.92 y1.0
(M )1000 g mol )
y1
w
Gibbsite Polyacrylic acid 5.2 0.01 M 0.42 y3.9
g-Al(OH)
3
(M , 250 000 g mol )
y1
w
NaCl
wS , 29.2 m g x
2 y1
BET
Polymethacrylic acid 0.77 y1.5
(M , 1 000 000 g mol )
y1
w
Humic acid 1.13 y1.0
(M )1000 g mol )
y1
w
into consideration. In the acidic pH range, the adsorption
density is enhanced mainly due to electrical interactions
as the alumina is positively charged and the carboxyl
groups of the PAA molecules (pK ,4.5) are ionised to
a
carboxylate ions. However, at alkaline pH values (pH)

pH of alumina and pK of PAA) , the adsorption of
PZC a
PAA is lowered due to electrostatic repulsive forces
because both polymer and alumina are negatively
charged (Table 6)
w
6,68,123
x
. However, in the case of
acrylic acid—acrylate copolymer, the presence of the
uncharged ester groups on the chain weakens the elec-
trostatic repulsion between the negatively charged car-
boxylates and enables the formation of a thick
adsorption layer even at high pH values
w
125
x
.
The adsorption of polymers such as PVP on alumina
is much lower than PAA mainly due to the hydrophobic
property of PVP ring. However, in the presence of PAA
the adsorption of PVP significantly increases as a result
of the interaction of PVP with PAA adsorbed on alumina
through hydrogen bonding
w
122
x
. The enhancement of
PVP adsorption on alumina was also observed in the
presence of azelaic acid

w
119
x
. PVA, since it is a non-
ionic molecule, interacts with the alumina surface mainly
by hydrogen bonding (Fig. 8) . The adsorption of PVA
is also affected by the pH value. The increase in the
adsorption density of PVA at higher alkaline pH values
can be attributed to the hydroxylation of the alumina
surface, which enhances its hydrogen-bonding capability.
At an acidic pH, the decrease in the PVA adsorption
can be caused by loss of the most active binding sites
for PVA bonding, reduction of the affinity for PVA
bonding, reduction of the affinity of non-ionic com-
pound by a general salting-out effect, the screening
effect of hydrated counterions, or preferential adsorption
of water molecules at those ionised sites
w
6,122
x
.
Generally, the higher the charge density of the poly-
electrolyte, the lower the maximum amount of the
compound adsorbed (Table 7). This is a result of two
phenomena. First of all, smaller amounts of the more
highly charged adsorbate compensate the surface charge
of the adsorbent. And secondly, the conformation of the
polyelectrolyte depends on the charge density. Low-
charged polyelectrolyte has loops and tails in the
adsorbed state. The hydrophobic methyl group of the

polymethacrylic acid and to a greater extent the aromatic
and aliphatic groups of the humic acid additionally
increase the stabilisation of the loops and tails by
hydrophobic interactions. Polymethacrylic acid and
humic acid can, therefore form a denser configuration
on the surface than polyacrylic acid
w
133
x
.
The molecular weight of polymer also influences the
adsorption capacity of alumina. According to Santhiya
et al.
w
63
x
the adsorption density increases with increas-
ing molecular weight of the polymer (Table 8)
w
63,129
x
.
No effect of molecular mass was observed by Vermohlen
¨
et al.
w
133
x
for the low ionic strength. However, with
35B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48

Table 8
Adsorption density of PAA of different molecular weight on alumina
Polymer Mineral pH (initial) Electrolyte Molecular Adsorption Refs.
weight density
(mg m )
y2
PAA a-Al O
23
3.0–3.5 0.01 M 90.000 ;1.57
a
w63x
(A-11, Aluminum KNO
3
50.000 ;1.37
a
Company of America) 5.000 ;1.12
a
wS , 8.76 m g x
2 y1
BET
2.000 ;1.05
a
PAA d-Al O
23
5.2 0.01 NaCl 2.000 ;0.7
a
w133x
(Degussa) 0.1 NaCl –
wS , 100 m g x
2 y1

BET
1 NaCl –
0.01 NaCl 50.000 ;0.7
a
0.1 NaCl ;0.9
a
1 NaCl ;1.2
a
0.01 NaCl 250.000 ;0.7
a
0.1 NaCl ;0.9
a
1 NaCl ;1.6
a
Data transferred from figure w63,133x.
a
high ionic strength (1 M NaCl), a greater amount of
polyelectrolyte with a high molecular mass is adsorbed
on the surface of alumina as presented in Table 8. This
phenomenon is a result of a screening effect that results
in the reduction of repulsion between charged polyelec-
trolyte segments. At low ionic strength, polyelectrolyte
adsorbs in a flat conformation. At higher ionic strength,
however, the repulsion of the negatively charged poly-
electrolyte is lowered and the molecules adsorb in a
coiled conformation. Furthermore, divalent cations such
as Ca enhance this effect as they screen the negative
2q
charges of the polyelectrolytes due to intramolecular
bridges formation between the carboxylic groups (R–

COO –Ca – OOC–R). This phenomenon is more
y 2qy
distinct for polyacrylic acid than polymethacrylic acid,
because of the larger methyl groups, which counteract
the coiling
w
133
x
.
Humic substances: Both NOM and metal oxides are
commonly present in natural water. NOM adsorbs on
metal oxide particles in an aquatic environment and, as
a result, can dominate many surface properties of these
particles. An investigation into the phenomenon of the
adsorption of humic substances (HS, dominant fraction
of NOM) on metal oxides is of great significance in
water treatment and natural water processes. Humic
substances have the properties of weak polyelectrolytes
of a molecular weight of several hundred Daltons or
larger
w
134
x
. These properties allow HS to regulate the
speciation, mobility, and transport of ions, nutrients and
contaminants in soils and aquifers. As they have a high
adsorption affinity towards oxide surfaces, their proper-
ties may be modified by the presence of oxide. There-
fore, the mechanism of HS adsorption onto metal oxide
surfaces is essential

w
135
x
.
The ability of humic substances to form stable com-
plexes with alumina results from the high content of
oxygen-containing functional groups (–COOH, pheno-
lic, enolic and alcoholic OH and _ C_ O). The adsorp-
tion of HS on alumina can take place mainly through
water bridges, electrostatic (Coulombic) attraction, for-
mation of a coordinate link with a single donor group
and formation of a chelate complex. This variety of HS-
alumina bonds formation results from HS complex
structure and randomness of formation. HS consist of
multifunctional aromatic components linked together by
a variety of aliphatic constituents. The main functional
groups are: –COOH, –OH, –C_ O and –NH . Hydro-
2
phobic moieties, such as the long alkyl side chains of
fatty acid residues, provide the amphiphilic character for
humic molecules. The complicated and random structure
of HS does not allow for the proposal of general
mechanism of HS adsorption onto alumina
w
7
x
. Based
on HS solubility in alkaline and acid solutions, they can
be divided into two main fractions of HA (humic acid
soluble in alkaline solutions, precipitated by acidifica-

tion) and FA (fulvic acid dissolved in acidic medium).
These two groups of HS are characterised by different
properties such as: solubility, protonation, binding of
heavy metals and hydrophobic organic pollutants and
adsorption. Both HA and FA possess a high affinity for
hydrous alumina surfaces. However, this affinity is
stronger for HA than for FA at the same pH value
w
136
x
.
Adsorption of HS on the surface of alumina is
influenced by several factors, mainly the pH and ionic
strength, and depends on the charge of both the solute
molecules and the solid particles
w
7,95,135,137
x
. As
presented in Table 9, the adsorption affinity of HS
increases with increasing ionic strength at each pH
studied. At the lowest ionic strength of 1 mmol dm ,
y3
the adsorption of HS molecules decreases with increas-
ing pH. This phenomenon is expected since the HS
molecules are negatively charged at each value, while
36 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Table 9
Adsorption capacity of alumina towards HA w7x
Mineral pH (initial) Electrolyte NaNO

3
Adsorption capacity
a
(mol dm )
y3
(mmol g )
y1
HA Boehmite g-AlOOH 5.0–6.9 0.001 0.143
wS , 107 m g ;
2 y1
BET
0.100 0.222
pH , 8.7x
PZC
0.250 0.606
0.500 0.7
8.3–8.8 0.001 0.107
0.100 0.183
0.250 0.264
0.500 0.33
9.5–9.9 0.001 0.027
0.100 0.077
0.250 0.154
0.500 0.222
Adsorption capacity is related to the number of acidic groups of HA calculated from the total acidity (4.56 mmol g ).
a y1
the surface charge of alumina changes from positive to
negative with increasing pH. The surface charge density
of alumina particles and the dissociation of acidic groups
of humic acid are enhanced by increasing ionic strength

due to the charge screening by counterions. In addition,
electrolytes also induce conformational changes of cross-
linked humic nanoparticles. The size of the humic
macroions decreases with increasing electrolyte concen-
tration due to the charge-screening effect of salts. The
salt effect is much more pronounced at low pH, where
the dissociation of acidic groups is also suppressed, and
an interparticle contraction is often followed by an
aggregation resulting in coagulation of HA
w
7,137
x
.
HA (mainly acidic groups) dissociates, which leads
to the formation of an electric double layer. The HA
particles become more negatively charged with both
increasing pH and ionic strength. Thus, electrostatic
interactions are the main forces that determine HS
interaction with a positively charged alumina surface.
This phenomenon, however, takes place only under
acidic conditions. At a higher pH, where the alumina
surface is uncharged or negatively charged, different
mechanisms of HA polyanions adsorption take place. A
ligand-exchange reaction (surface complexation)
between the surface site 'Al–OH and the dissociated
form (A ) of humic acid can take place
w
7
x
:

y
yy
^ AlyOHqA ~^ AlyAqOH (37)
An anion-exchange reaction of HA with the protonated
sAl–OH sites is another possible mechanism
w
7
x
:
q
2
qy
^ AlyOH «NO qHA
23
qyq y
~^ AlyOH «A qH qNO (38)
23
The two possible mechanisms of HA adsorption on
neutral or negatively charged alumina surface can pro-
ceed simultaneously. An increasing contribution of Eq.
(38) is expected with increasing concentration of HA
w
7,134
x
.
In natural water many inorganic anions might compete
for surface binding sites of alumina with NOM. Anions
such as sulfates, carbonates and phosphates can form
strong inner-sphere complexes. Therefore, the concentra-
tion of the anionic organic species will be dependant on

the relative strength of the different binding reactions.
Nevertheless, in most natural systems, it appears that
the organic anions are the dominant adsorbed species
on hydrous metal oxide surfaces
w
138,139
x
.
According to Edwards
w
140
x
, the acidity of NOM
plays a significant role in the process of NOM adsorption
on metal oxide surfaces. Very strong organic groups
(acid groups that are partially deprotonated at pH-3.0)
represent a significant portion of the total organic acidity
of NOM and they are important in controlling the
formation of surface complexes between organic matter
and oxide surfaces. Weak acids groups in NOM (those
partially protonated at pH 8.0, e.g. benzoic acid) might
be weak carboxylic groups associated with very strong
acids on high molecular weight organic molecules and,
therefore likely to be present on strongly sorbing mole-
cules even if they are not directly involved in complex
formation. The type of organic ligands present in an
organic molecule and the relative strength of their
bonding to oxide surfaces are crucial in NOM adsorption
on oxides. Non-sorbing ligands are incapable of surface
complexation. Weakly sorbing ligands may be sorbed

onto oxide surfaces but the extent of their removal
depends on solution concentration (e.g. benzoic acid).
Strongly sorbing ligands form strong surface complexes
and are nearly completely removed from solution unless
the surface is completely saturated (e.g. phtalic-, sali-
cylic-, and oxalic-acid-type ligands).
37B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Fig. 9. General shape of adsorption isotherm of surfactant on metal
(hydr)oxide.
As discussed above, carboxylic and phenolic func-
tional groups in NOM are responsible for complexes
formation with alumina. An increase of NOM affinity
towards alumina can be obtained (in water treatment
technology) as a result of preozonation. This can be
explained by ozone oxidative properties, which cause
changes in the structure of HS and the formation of
more polar organic compounds of lower molecular
weight than their parent compounds, especially carbox-
ylic acids
w
17
x
.
Surfactants. Adsorption of surfactants at the solid–
liquid interface has been widely investigated
w
4,5,121,132,141–153
x
and was found to depend on the
nature of the surfactant, the solvent and the substrate.

In addition, several properties such as temperature, pH
of the solution and ionic strength affect adsorption
significantly.
A typical Somasundaran–Feurstenau adsorption iso-
therm of ionic surfactants on charged metal oxide
surfaces, as presented in Fig. 9, can be divided into four
regions of different dominant mechanisms.
In region I, at surfactants of low concentrations, where
adsorbate–adsorbate interaction are negligible, the
adsorption takes place mainly due to the existence of
electrostatic interactions between the ionic surfactant
and the oppositely charged solid surface. In region II,
which is characterised by a sharp increase in slope of
the isotherm, the adsorption results from both the elec-
trostatic attraction and the specific interaction between
the surface-active ions and the solid surface. At higher
concentrations, at a particular concentration called HMC
(hemimicelle concentration), lateral interactions among
the adsorbed molecules cause a sharp rise in adsorption.
Surfactant aggregation occurs and monolayer aggregates
(hemimicelles) are formed. In region III (region of
decreasing slope), hemimicelles are present and the
second layer adsorption is beginning to become signifi-
cant. With a further increase of surfactant concentration,
a gradual change from monolayer aggregates to aggre-
gates with reverse orientation is postulated. Bilayer
surfactant aggregates called admicelles are formed. The
maximum surface coverage (region IV) is obtained
when micelle formation (CMC) either in bulk or in
adsorbed monolayer occurs

w
38,121,141,143,145–
147,149,151,155,156
x
.
Non-ionic surfactants, depending on the hydration
properties, adsorb on the alumina surface mainly through
hydrogen bond interactions. Moreover, the lateral inter-
actions are much stronger owing to the absence of
electrostatic repulsion between the surfactant head
groups, which results in a sharp increase in the adsorp-
tion corresponding to solid formation. Non-ionic ethox-
ylated alcohols show strong adsorption affinity for silica
as opposed to other minerals such as alumina. This
phenomenon depends on the nature of the water structure
at the solid–liquid interface, as hydrogen bonding is
relatively weak in comparison with electrostatic and
chemical bonding. The lack of adsorption of some non-
ionic surfactants on alumina is hypothesised to be a
result of the inability of non-ionic molecules to disrupt
the rigid water layer surrounding the substrate
w
121
x
.
The adsorption of non-ionic surfactants can be signif-
icantly enhanced by the presence of ionic surfactants
and vice-versa
w
4,121

x
. Several non-ionic and ionic
surfactants when used in mixtures were found to adsorb
on surfaces on which they do not exhibit much adsorp-
tion themselves. This synergetic effect is assumed to be
a result of both the reduction in charge repulsion leading
to better packing of the ionic surfactants and the increase
of solubilisation of non-ionic surfactants in the hydro-
phobic microdomains formed by the ionic surfactants
w
121
x
. The non-ionic surfactant is strongly co-adsorbed
once hemimicelles of anionic surfactant start to form on
the surface
w
4
x
. Enhanced surface activity of mixtures
of non-ionic surfactants and anionic surfactants has been
demonstrated for many systems such as
w
4,5,148
x
:
– sodium dodecyl sulfonateyoctlyphenolpoly(oxy-
ethylene),
– sodium p-octylbenzenesulfonateydodecyloxyheptae-
thoxyethyl alcohol,
– sodium dodecyl sulfonateydodecyloxyheptaethoxy-

ethyl alcohol,
– tetradecyl trimethyl ammonium chlorideypentade-
cylethoxylated nonyl phenol.
3.3.3.2. Interaction with inorganic molecules.
3.3.3.2.1. Anions. The adsorption of several anionic
inorganic compounds such as arsenite, arsenate, molyb-
date, sulfate, selenate, selenite, phosphate, bromate,
iodate and chlorate on alumina, and especially interac-
tive adsorption have been widely studied
w
2,8,9,37,157–
168
x
. Generally, interaction effects between ions for
38 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
adsorption can be indifferent, competitive, or promotive.
Indifferent and competitive effects are commonly
observed in multi-anion adsorption studies as opposed
to a promotive effect between anions, which has been
rarely reported
w
157
x
.
Anions can adsorb at an oxide–water interface in an
inner- or outer-sphere mode. This distinction is very
important as it determines their chemical behaviour at
the interface. Inner-sphere surface complexes are created
as a result of direct chemical bond formation between
adsorbing anion (Lewis base) and the metal ion at the

surface (Lewis acid). In the case of anions that have
more than one donor atom (e.g. sulfate), possible coor-
dination geometries are monodentate, chelating biden-
tate, or bridging bidentate. Other-sphere surface
complexes are formed as a result of electrostatic attrac-
tion of opposite charges. Hydroxyl groups or water
molecules are known to separate the adsorbed anions
and the surface metal ion centres and act as proton
donors in hydrogen bond arrangements
w
163
x
.
The extent of the interaction effects between anions
for adsorption depends on several factors such as the
relative concentrations, the intrinsic adsorption affinities
of the anions and the pH
w
2,163
x
. Formate, acetate,
CO , SO and SeO are characterised by low to
y 2y 2y
34 4
moderate affinity for adsorption on metal (hydr)oxides.
They are sensitive to competition from high affinity
anions such as PO , oxalate and citrate and the
3y
4
concentration of the background salt. Furthermore, they

can also compete with each other for adsorption
w
157
x
.
As a result of the high sensitivity of moderate affinity
ions, they are known to form an outer-sphere surface
complexes on the surface of metal (hydr)oxides. Con-
tradictory data concerning complexes formation of mod-
erate affinity anions such as SO and SeO were,
2y 2y
44
however, reported. Outer-sphere complexes were report-
ed by Hayes
w
158
x
and Person and Lovgren
w
163
x
.
¨
Wu et al.
w
164,169
x
found that adsorption of MoO
2y
4

is not influenced by SO and SeO . In contrast, the
2y 2y
44
adsorption of SO and SeO was inhibited by
2y 2y
44
MoO . This phenomenon is a result of different com-
2y
4
plexes formation between anions and alumina surface.
Molybdate was found to form strong inner-sphere sur-
face complexes as opposed to sulfate, which was found
to form weak outer-sphere complexes.
Hayes et al.
w
158
x
according to a modified version of
the triple-layer surface complexation model, which
allows analogues of either outer-sphere, ion-pair com-
plexes (placement of the adsorbing ion in the b-plane,
hydroxyl groups or water separate the adsorbed anion
and the surface metal ion centres) or inner-sphere,
surface coordination complexes (placement of the
adsorbing ion in the o-plane, direct chemical bond is
formed) to be specified, classified inorganic ions com-
plexes formed on Al-oxide surfaces. The effect of
changes in ionic strength of the adsorption of inorganic
ions allowed for this classification. Outer-sphere ana-
logues of adsorbed complexes (anions that form weak

bonds with Al-oxide surface) are more sensitive than
inner-sphere complexes (anions that form strong bonds
with Al-oxide surface) to ionic-strength variations since
electrolyte ions are placed in the same plane as the
outer-sphere complexes (b-plane). Inner-sphere com-
plexes, which have no direct dependence on the b-
potential, are less influenced by ionic-strength changes.
Inner-sphere complexes of inorganic anions were pro-
posed by Hug
w
170
x
and Manceau and Cherlet
w
171
x
.
Eggleston et al.
w
172
x
suggested that adsorbates that
behave macroscopically as outer-sphere ( inner-sphere)
support a small (large) population of inner-sphere com-
plexes at any given time, and further, that these inner-
sphere complexes may have short (long) lifetime.
The adsorption of arsenic species: arsenite and arse-
nate on alumina was investigated by Goldberg and
Johnson
w

173
x
. At neutral pH arsenite exists in solution
in the form of H AsO and H AsO since the pK
y
33 24 a
values are high: pK , 9.2 and pK , 12.7. Arsenate can
12
aa
exist in solution as H AsO , H AsO , HAsO ,
y 2y
3424 4
AsO because the pK values are as follows: pK ,2.3
3y 1
4a a
and pK , 6.8 and pK , 11.6.
w
173
x
. The adsorption of
23
aa
arsenite and arsenate on alumina is governed by both
the surface charge of alumina and the form of arsenic
species present in water. That is the reason why the pH
value of aqueous solution is an important factor deter-
mining the process of As-compounds adsorption on
alumina
w
9

x
. Spectroscopic, sorption and electrophoretic
mobility measurements revealed that arsenate adsorbs
on amorphous alumina through the formation of inner-
sphere surface complexes and its adsorption decreases
with increasing pH (apparent sorption maximum occurs
at pH 4). No ionic strength dependence or increasing
adsorption with increasing solution ionic strength occurs
in the case of arsenate adsorption on alumina. Arsenite
forms both inner- and outer-sphere complexes on the
surface of amorphous alumina and its sorption increases
with increasing pH to an adsorption maximum around
pH 8 and decreases with further increase of the pH
value. Ionic strength effects are stronger in the case of
arsenite sorption than that of arsenate, which suggest a
stronger bond formation of arsenate than arsenite
w
173
x
.
As reported by Szczepaniak and Koscielna
w
165
x
and
according to the affinity of the anions to hydrous g-
Al O in an acidic environment, the halogen and oxy-
23
halogen anions can be ordered as:
yy y y y y y y y

ClO -I -ClO -Br -Cl -BrO -IO -IO -F
43 334
The anions (A ) such as I , ClO , Br , Cl , BrO
yyyyyy
33
and IO react with g-Al O according to the mechanism
y
323
of ion pair formation in the outer-coordination sphere
w
165
x
:
39B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Fig. 10. Schematic representation of the various possible surface com-
plexes of carbonate: (a) monodentate; (b) bidentate; (c) bridging-
bidentate. The I and II subscripts are designations for the oxygens
that are coordinated to the metal (OI) and not metal-coordinated (OII)
w162x.
qy q y
^ AlyOHqHA~^ AlyOH yA (39)
2
The anions such as IO and F strongly adsorb on
yy
4
the surface on alumina through the formation of com-
plexes in the inner-coordination sphere
w
165
x

:
yy
^ AlyOHqA ~^ AlyAqOH (40)
Schmitt and Pietrzyk
w
174
x
proposed the following
sequence of anions affinity towards alumina at pH 4.0:
yyy yyyy
CHO -ClO -I -SCN -ClO -Br -NO
232 4 3 3
yy y y
-NO -Cl -BrO -ClO -benzoate
232
y 2y 2yy
-HCO -Cr O -SO -F
227 4
The FTIR spectra of adsorbed species CO at the
2y
3
hydrated g-alumina surface indicates that out of several
possibilities (Fig. 10), only the monodentate inner-
sphere complexed CO anion species is present on the
2y
3
surface in the pH range of 5.2–7.2
w
162
x

.
The FTIR spectra also showed the existence of extra
protonated surface groups associated with the adsorbed
CO . The adsorption reaction and surface groups’
2y
3
protonation can be, therefore expressed with the follow-
ing equations
w
162
x
:
yyyq
wx
^ AlyOHqHCO ~^ AlyOCOO q OH qH
3
(41)
qq
^ AlyOHqH ~^ AlyOH (42)
2
Strong carbonate complexes are also formed on zirconia
surface
w
175
x
.
Since surface protonation plays an important role in
the adsorption of ions on metal (hydr)oxides, enhanced
surface protonation due to CO adsorption may affect
2y

3
the reactivity of Al oxide for adsorption of other anions
as presented below
w
157,162
x
.
Wijnja and Schulthess
w
157
x
indicated that the pres-
ence of certain anions promotes the adsorption of other
anions. Small amounts of CO as well as organic
2y
3
anions such as monocarboxylic organic formate and
acetate anions enhance the adsorption of SO and
2y
4
SeO on Al oxide between pH 6.0 and 8.0. In contrast,
2y
4
the more bulky polycarboxylic organic anions, oxalate
and citrate compete with SO and SeO due to strong
2y 2y
44
adsorption on the oxide surface. The possible mecha-
nism of promotive effect of these ions is the generation
of extra adsorption sites by extra protonated surface

groups that coexist with adsorbed promotive molecules.
The protonation enhancement causes an increase of
adsorption of SO and SeO . This mechanism is only
2y 2y
44
effective for low or moderate affinity anions in the pH
range where the fraction of surface protonation would
be otherwise low in the absence of enhancing anions. It
is worth mentioning that with increasing total anion
adsorption densities, competitive adsorption also begins
to play a role and counteracts with the promotive effect.
The adsorption of inorganic anions onto alumina can
be dependent on organic compounds such as low molec-
ular weight anions of the organic acids. This generally
depends on the type of organic compound. Multicarbox-
ylic ligands such as oxalate, citrate, malate and tartrate
have a relative high adsorption affinity and can compete
with the adsorption of oxyanions on Al hydroxides.
Monocarboxylic organic ligands, such as formate and
acetate, are weaker adsorbates and do not compete so
extensively with oxyanions
w
157
x
.
As reported by Zuyi et al.
w
176
x
, the presence of

humic substances (fulvic acid) influences the sorption
of several anions on alumina. The sorption of I
y
decreases with an increase of both the pH value (pH
3–10) and the concentration of FA, which is a result of
competition for sorption sites between I and FA. In
y
contrast, FA does not affect the adsorption of SeO at
2y
3
pH 3–12 because selenite is characterised by a high
affinity for alumina.
3.3.3.2.2. Cations. The adsorption of metal cations at
the solid–solution interface plays an important role in
determining their fate in the environment, as it is an
important factor that controls their mobility in aquatic
environment. Those metal cations, which are weakly
bound to the surface of metal oxide as outer-sphere
complexes, are more mobile than those present in inner-
sphere complexes. In natural waters, metal speciation
can be influenced by the presence of complex-forming
organic andyor inorganic ligands or simply due to the
fact that metal cation can be extensively hydrolysed at
pH values which are characteristic of soil solutions
w
14,177
x
. Interaction of several inorganic cations, espe-
cially toxic metals such as: Pb, Cr, Zn, Cd, Sr, Co on
metal oxide was discussed in both theoretical and

experimental studies
w
10–14,37,71,73,83,90,98–
100,167,176–198
x
.
Adsorption of metal ions at the oxide–water interface
is strongly pH-dependent, because the properties of both,
40 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
the oxide surface (charge and potential) and the solution
composition (metal ion speciation) change with pH
w
10
x
. There is little or no change in the pH dependence
of adsorption with ionic strength (0.001–1.0 M) for
specifically adsorbed ions like Cu , Pb , Ni and
2q 2q 2
Cd . However, ionic strength can have a remarkable
2q
effect on the adsorption behaviour of alkali earth diva-
lent cations, such as Mg , Ca and Ba . This
2q 2q 2q
suggests a different strength and type of bonding. TLM
model used effectively by Hayes
w
158
x
for inorganic
anions adsorption onto alumina surface can sufficiently

explain the bond formation also in the case of inorganic
cations. The significant influence of ionic strength on
the adsorption process of cations onto alumina shows
that the outer-sphere complex is formed. No ionic
strength influence means strong bonding, specific
adsorption and inner-complex formation
w
178
x
. The for-
mation of inner-sphere complexes of specifically
adsorbed metal ions such as Co , Cu on alumina
2q 2q
surface was often reported
w
177,193
x
.
The adsorption of earth alkaline metal ions such as
Ca , Mg and Ba increases with increasing pH;
2q 2q 2q
limited adsorption is observed at pH and at pH values
PZC
slightly lower than pH . The affinity of earth alkaline
PZC
metal ions for hydrous g-Al O surface is as follows:
23
Mg )Ca )Sr )Ba . This indicates that the sta-
2q 2q 2q 2q
bility of surface complexes decreases with increasing

ionic size
w
11
x
.
The specific binding of Pb on hydrous g-Al O
2q
23
surface in dilute electrolyte might be interpreted by
surface complex formation expressed with the following
equations
w
10
x
:
2qqq
^ AlOHqPb ~^ AlOPb qH (43)
2qq
2^ AlOHqPb ~^ (AlO) Pbq2H (44)
2
The interaction of cations with metal oxide surface at
pH-6 is accompanied by proton displacement with
unhydrolysed cation such as Pb . A specific hypothet-
2q
ical adsorption of hydrolysed PbO or Pb(OH) at
q
2
acidic pH seems not to be probable as an unreasonably
large free energy of adsorption would have to be invoked
w

10
x
. The adsorption of Pb was found to be heteroge-
neous as Pb can form both mono- and bidentate inner-
sphere as well as outer-sphere complexes on alumina
surface
w
12
x
. Bargar et al. confirmed the formation of
both outer- and inner-sphere complexes of Pb on alumina
w
3
x
. It has to be emphasised that mainly bidentate inner-
sphere surface complexes are formed onto different Al-
oxides such as gibbsite, g-Al O and a-Al O
23 23
w
3,10,13,100,181,185
x
. Similarly to many other cations,
Pb or Sr adsorption kinetics are biphasic, that is, an
initially fast reaction to the external surface including
macropores is followed by a slow surface diffusion
along the micropore walls of oxides
w
12,14,184
x
.

The presence of complexing ligands such as chloride
was found to be a significant factor determining the
adsorption of cations on alumina. This is important from
an environmental point of view. Cl did not influence
y
the adsorption of Pb onto alumina surface as Pb forms
2q
inner-sphere complexes as described above. No Pb–
Cl ternary complexes similar to those that predominate
y
at pH 5.0–6.0 on goethite were found. This can be
ascribed to the inability of Cl to bond to surface Al
y
atoms
w
13,181
x
. Similar results were obtained by Kos-
mulski
w
182
x
for Cd adsorption on alumina.
2q
The adsorption affinity of cations such as Ca, Cu,
Cd, Ni, Pb, Zn onto alumina and other mineral solids
in the presence of other complexing agents such as
nitrilotriacetic acid and ethylenediaminetetraacetic acid
has also been studied and was found to increase in their
presence

w
69,165
x
.
Organic matter, e.g. humic substances (fulvic and
humic acids) possessing functional groups which are
available for the interaction with the surface and for the
binding of metal ions at the same time were found,
when being adsorbed on the mineral surface, to enhance
the adsorption affinity of cations for the adsorbent.
Humic substances have a tendency to enhance metal
cation sorption at low pH and reduce metal cation
sorption at high pH. The effect of humic substances on
sorption of inorganic cations as well as anions depends
not only on pH, but also on the nature of the oxide, the
nature of humic substance, fractionation of the humic
substance by sorption, the relative strength of complexes
of both soluble and sorbed humic substances, the extent
of surface coverage by humic substance, the initial
concentration of humic substance and the inorganic
electrolyte composition
w
176,183,199
x
. According to
Zuyi et al.
w
176
x
humic substances (fulvic acid) influ-

ence the sorption of inorganic cations onto oxides
through metal complexation, which may occur via car-
boxylic, phenolic and other functional groups of sorbed
FA. Surface binding of UO and Yb via a FA bridge
2q 3q
between the surface and metal at low pH is responsible
for an increase of cations adsorption on alumina. The
reduction of UO and Yb sorption at pH)8 and
2q 3q
Zn sorption at pH)10 results from the more effective
2q
complexation of the ions with the soluble FA in aqueous
solution. As reported by Floroiu et al.
w
190
x
, the presence
of PAA, a strong complexing agent, results in an increase
of the adsorption of Cd at the pH value of -6 due
2q
to ternary surface complex formation. At pH)6, a
decrease of Cd adsorption on alumina is observed as
2q
a result of an increase of the concentration of soluble
Cd-PAA complexes. Similar results for the trace metal
sorption in the presence of organic ligands were also
observed by Benyahya et al.
w
200
x

. The increase in
sorption of Zn and Cd on alumina is observed at
2q 2q
low pH values (pH-6.5) in the presence of salicylic
acid, which results from ligand-like ternary surface
41B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
complexes formation. These complexes are promoted at
low pH because –AlOH groups can easily exchange
q
2
with salicylic acid. At high pH values (pH)6.5), the
decrease in sorption of Zn and Cd on alumina takes
2q 2q
place as a result of competitive complexation of metal
ions with the dissolved salicylic acid (for which the
complexing ability increases with pH) and the functional
surface groups
w
200
x
. The adsorption of Me-EDTA
(MesCo(II),Co(III),Al(III), Ni, Zn, Cu, Cd, Pb, Ca)
complexes on g- and d-alumina with the formation of
ternary surface complexes was discussed by Nowack et
al.
w
74
x
by the surface complexation model. Fitts et al.
w

196
x
studied the structure and bonding of Cu(II)-
glutamate complexes at the g-Al O –water interface.
23
3.3.3.3. Dissolution of alumina. Dissolution of alumina
should also be taken into consideration when discussing
the properties of alumina in aqueous solution. Dissolu-
tion of alumina is an important process that influences
soil solution chemistry and the geochemistry of natural
waters and was studied by several research groups
w
37,91,201–204
x
. The dissolution rate of metal oxides
strongly depends on the pH of aqueous solution, typi-
cally increases at both acidic pH, below the pH of zero
point of charge of metal oxide and in the alkaline region.
The process of dissolution takes place via the change of
the coordinative partner of the crystal constituents. The
oxide ligands might be replaced by several species such
as: H O H (or H O ), OH or ligands
w
91,201
x
. As
qqy
2, 3
proposed by Furrer and Stumm
w

204
x
, the overall dis-
solution rate is expressed as independent parallel reac-
tion mechanisms and can be presented as the sum of
the rates of proton-promoted, OH -promoted and ligand
y
promoted dissolution
w
203
x
:
R s R q R (45)
q
net H L
i
8
i
where: R is the reaction rate of net dissolution, R
q
net H
is the reaction rate of proton-promoted dissolution and
is the reaction rate of ligand-promotedR
L
i
dissolutionThe effect of various surface species on
dissolution is a function of their surface concentration
w
203
x

:
wx
R sk 'MyL (46)
LL i
ii
where for each ligand i, the rate of ligand-promoted
dissolution is a function of the rate constant and thek
L
i
surface concentration of metal organic surface complex-
es
w
^ MyL
x
(pH dependant).
i
Kraemer et al.
w
203
x
discussed the model of metal
oxide dissolution by taking the synergetic effects of
organic, hydroxide and inorganic ions into consideration.
The process of d-alumina dissolution in the presence of
8-hydroxyquinoline-5-sulfonate (HQS) and salicylate in
the pH range: 3–9 was examined. The greatest effects
were observed at pH values higher than those corre-
sponding to maximal adsorbed ligand concentrations.
An influence of the competitive adsorption of HQS in
the presence of arsenate or fluoride was also investigat-

ed. The results indicated that HQS-promoted dissolution
of alumina is enhanced by hydroxide or fluoride. Arse-
nate adsorbs on the surface of alumina but does not
promote dissolution of alumina.
Several factors influence the dissolution of metal
oxides. The effect of pH value, which affects surface
protonation and deprotonation, has been already men-
tioned. An increase of ionic strength increases dissolu-
tion rate as it increases positive and negative surface
charge at pH below and above pH respectively. In
PZC,
ligand-promoted dissolution, the dissolution rate is
enhanced by inner-sphere monomolecular surface com-
plex formation. Multivalent metal ion adsorption usually
decreases the dissolution rate as a result of both the
blockage of reactive surface sites from interaction with
ligands and protons and a decrease in surface protona-
tion. Acid dissolution is effectively inhibited by binu-
clear inner-sphere surface complexes formation as
opposed to basic dissolution, which is accelerated due
to the surface protonation. Oxoanions such as phosphate,
silicate, arsenate, borate, chromate and sulfate form bi-
or multinuclear inner-sphere surface complexes, which
do not influence the dissolution rate. Bi- or tri-nuclear
surface complexes and bi- or trivalent ligands that form
uncharged surface complexes are the most efficient
passivators. Cations and anions, which adsorb on metal
oxide with the formation of polymers, inhibit dissolution
by crosslinking the surface lattice
w

201
x
.
4. Application of alumina and alumina supported
catalysts in water treatment
The applications of Al-compounds in water treatment
are various. Aluminium sulfate, sodium aluminate or
aluminium chloride are used as coagulants
w
15–18
x
.
Alumina is used mainly as the adsorbent of ionic
compounds. The other possible usages of alumina and
alumina supported catalysts have also recently been
investigated. These are catalytic ozonation and wet air
oxidation processes.
4.1. Adsorption
Adsorption is one of the most important processes in
water treatment technology. It is mainly used for the
removal of colour-causing natural organic compounds
from upland raw waters. Activated carbon is the most
popular adsorbent. However, a significantly greater
adsorption of polar organic compounds, which are typ-
ical constituents of natural waters, might be achieved
42 B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
by adsorbents with a prevalence of polar surface func-
tional groups
w
205

x
. Alumina is a good example.
The typical activated aluminas used in water treatment
are 28=48 mesh (0.3 to 0.6-mm diameter) mixtures of
amorphous and gamma alumina (g-Al O ) prepared by
23
low-temperature (300–600 8C) dehydratation of
Al(OH) . They have surface areas of 50–300 m g .
2 y1
3
The hydroxylated alumina is a subject to protonation
and deprotonation, which removes the inorganic com-
pounds by a ligand-exchange reaction. Activated alu-
mina is regenerated by a series of HCl and NaOH
solutions
w
18
x
.
In water treatment technology, alumina is mainly used
as ion exchanger. Due to its relatively high surface area
and high affinity towards several inorganic anions (see
Section 3.3.3), activated alumina is recommended for
the removal of several inorganic compounds from water.
Activated alumina was found to be the most effective
treatment method (up to 100% efficiency) for the
removal of As(V),Se(VI) and F from water
y
w
15,16,18

x
. The other inorganic compounds such as
barium, cadmium, chromium, lead, mercury, nitrate,
silver and radium were found to be adsorbed on alumina
to a lower extent
w
18
x
.
The most effective adsorption of Se(VI) on alumina
takes place at pH 5–6. However, selenate (Se(VI)) is
so strongly preferred by the negatively charged adsor-
bent than selenite (Se(IV)) that oxidation of selenite
prior to adsorption should be applied. The other impor-
tant matter is interferences by other anions with a high
adsorption affinity towards alumina. The order of pref-
erence of alumina for anions is: OH )HPO )F )
yyy
24
H AsO )HSeO
w
18
x
.
yy
24 3
More than 90% of As(V) is removed from water due
to the adsorption on alumina at pH-7. As(III) is less
strongly adsorbed on alumina
w

18
x
. The adsorption of
As-compounds was discussed in Section 3.3.3.2.1.
Al-SZP (aluminum-loaded Shirasu-zeolite P ; SZP ,
111
Na OØAl O Ø3.3SiO Ø4.3H O to Na OØAl O Ø5.3
223 2 2 223
SiO Ø5.7H O) was found to be an effective adsorbent
22
of As species. Al-SZP is selective towards As(V). The
1
adsorption of As(V) is only slightly dependant on the
initial pH over a wide range of pH: 3–10. According to
Xu et al.
w
206
x
, the presence of arsenite, chloride, nitrate,
sulfate, chromate and acetate ions, as opposed to phosh-
ate, do not affect the adsorption of As(V) on Al-SZP .
1
The adsorption mechanism follows a ligand-exchange
reactions between As(V) ions and the hydroxide groups
present on the surface of Al-SZP
w
206
x
. Red mud,
1

which is a residue of alumina production from the Bayer
process, comprised mainly of Fe O , Al O , SiO and
23 23 2
minor amounts of TiO , CaO and Na O, was found to
22
be another cost-effective adsorbent for arsenic com-
pounds removal from water
w
207–209
x
. Hodi et al.
´
w
210
x
proposed Al O yFe(OH) for the removal of
23 3
arsenic ions from natural water.
Activated alumina is very effective for the removal
of fluoride from water due to a high exchange capacity
for this ion, which is not affected strongly by the
SO or Cl present in water, and it has a relatively
2yy
4
low cost, much lower than the cost of synthetic anion
resin
w
18
x
. The adsorption capacity of alumina towards

fluoride strongly depends on pH and is the most efficient
at pH 5.0–6.0
w
15
x
.
The application of alumina for natural organic matter
removal from natural and preozonated water was inves-
tigated by Lambert and Graham
w
205,211
x
. Fettig
w
212
x
also investigated the adsorption of NOM on alumina.
Hano et al.
w
213
x
reported the feasibility of alumina
as an adsorbent of phosphorous (the main cause of
eutrophication) as a means of the removal of this
compound from river and lake water. The adsorption
capacity of alumina was enhanced 1.7-fold by using
alumina modified with aluminium sulfate. Aluminum
oxide hydroxide was also applied by Tanada et al.
w
214

x
for the removal of phosphate from rivers, lakes and
seawater. No significant competition between phosphate
and other typical water constituents (chloride, nitrate,
hydrogen carbonate, sulfate) for the active surface sites
on aluminium oxide hydroxide was observed.
Microporous alumina pillared montmorillonite and
mesoporous alumina aluminium phosphates were suc-
cessfully applied for the removal of chlorinated phenols
(2,4-chlorophenol, 2,4,6-trichlorophenol and pentachlo-
rophenol) from aqueous solution. The removal of chlo-
rophenols by the adsorbents increased with increasing
chlorine substitution in their molecules
w
215
x
. Alumina
pillared montmorillonite and mesoporous alumina alu-
minium phosphates were also used as the adsorbents of
pesticides such as: atrazine, propazine, prometryne, pro-
pachlor, propanil and molinate. The pillared clays
showed 2–4 times higher adsorption capacity towards
the examined pesticides than mesoporous alumina alu-
minium phosphates. The adsorption of s-triazines onto
mesoporous alumina decreased with the alkyl substitu-
tion of the lateral chains; therefore atrazine was the
most adsorbed compound. Prometryne was adsorbed to
the surface of Al-pillared clays to a higher extent than
other examined pesticides due to its higher basicity. The
removal of propachlor by mesoporous alumina decreased

with the increase of PyAl ratio. The mesoporous alumina
with a PyAl ratio of 0.4 was found to be more efficient
for the removal of molinate and propanil. The amounts
of herbicides decomposed during the process were high-
er for mesoporous alumina than Al-pillared clays
w
216
x
.
4.2. Catalytic ozonation
Several researchers have investigated the possibility
of the application of parent alumina or alumina sup-
ported with metals or metal oxides as catalysts of the
ozonation process
w
217
x
.
43B. Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48
Alumina was shown to be an effective catalyst for
the ozonation of 2-chlorophenol. The highest efficiency
(more than twice as high) of catalytic ozonation when
compared to ozonation alone was observed at neutral
pH. At an acidic pH value, the usage of the Al O yO
23 3
system resulted in an increase of 83.7% of TOC degra-
dation when compared to ozonation alone. Only a 17%
increase of the efficiency of catalytic ozonation was
obtained at a basic pH value, mainly because, under an
alkaline environment, oxidation with ozone is already

strong
w
20
x
. g-alumina was found to be an effective
catalyst for refractory organic compounds such as oxalic,
acetic, salicylic and succinic acid ozonation in water.
However in the presence of PO , the efficiency of
2y
4
catalytic ozonation decreased, due to the adsorption of
PO ions on the alumina surface and the blockage of
2y
4
active surface sites of alumina
w
26
x
.
The TiO yAl O yO system out of three catalysts
2233
studied: TiO ysilicagel, TiO yalumina and TiO yatta-
22 2
pulgyte, was found to enhance humic acid degradation
in water to the highest extent
w
22
x
. The adsorption
phenomenon was found to be an important aspect of

catalytic ozonation and was the highest in the case of
the most efficient catalyst, TiO yAl O
w
22
x
. The TiO y
223 2
Al O yO process was also effective for the removal of
23 3
NOM from the Ebro River
w
22
x
. The efficiency of TOC
removal in the case of the TiO yAl O yO system was
2233
found to be greater when chlorine was also applied.
Moreover, the TiO yAl O yO yCl system caused a
22332
more significant decrease of THM than O yCl
w
23
x
.
32
Cooper and Burch
w
24
x
have shown the significant

influence of Al O , TiO yAl O and Fe O yAl O on
23 223 2323
the efficiency of oxalic acid, chloroethanol and chloro-
phenol removal from water. The modification of the
alumina surface both with TiO and Fe O resulted in a
223
significant increase of the activity of the catalyst with
the exception of oxalic acid degradation, for which
catalytic ozonation in the presence of bare alumina has
been already strong.
Volk et al.
w
21
x
compared the efficiency of three
ozonation systems: O , O yH O and O yTiO fixed on
3322 3 2
alumina beads in the process of fulvic acids removal
from water. The O yTiO system did not provide a
32
significant increase of UV -absorbance reduction as
254
opposed to the DOC reduction, which was shown to be
24% of DOC for the O yTiO system, 18% of DOC for
32
O yH O , and 15% DOC in the case of ozonation alone.
322
The O yTiO system resulted in a BDOC concentration
32
that was 30% lower than with ozonation alone, which

is a result of a higher mineralisation of ozonation by-
products. Moreover, the chlorine demand was minimised
when catalytic ozonation was applied.
Karpel Vel Leitner et al.
w
19,25
x
proposed three new
catalysts (a metal deposited on three different supports)
for ozonation of humic acid, salicylic acid and peptide,
which were as follows: Al O , TiO and clay (mainly
23 2
attapulgite) impregnated with copper (5–10 wt.% Cu).
Ozonation alone was found not to remove these com-
pounds to a great extent from the aqueous solution (12–
15%). However, the usage of catalytic ozonation (at
neutral pH) significantly improved the efficiency of
ozonation (up to 47, 64 and 84% for humic substances,
salicylic acid and peptide, respectively). Udrea and
Bradu
w
218
x
also reported the efficiency of copper–
alumina catalyst in the process of organic compounds
(chloro- and nitrophenols) ozonation in water. Pi et al.
w
219
x
studied the efficiency of CuOyAlO for oxalic aci

3
degradation in aqueous solution.
The catalytic activity of over 20 catalysts (e.g.
Al O , SiO and C impregnated with metals Pt, Pb,
23 2 act
Pd, Ag, Co, Ru, Ir, Rh, Re) was examined by Lin et al.
w
220
x
in the process of formic acid ozonation. The
highest activity was shown by PtyAl O (0.90 molyh
23
g ) and PdyAl O (0.46 molyh g ).
cat 2 3 cat
Beltran et al. investigated the efficiency of Al O ,
23
´
TiO yAl O
w
221
x
and Co O yAl O
w
222
x
catalytic
223 3423
ozonation in the process of oxalic acid removal from
water.
The catalytic activity of alumina supported catalysts

is mainly based on the catalytic decomposition of ozone
and the enhanced generation of hydroxyl radicals. How-
ever, the results obtained from different studies suggest-
ed different ozonation mechanisms. The efficiency of
the catalytic ozonation process depends to a great extent
on the catalyst and its surface properties as well as the
pH of the solution that influences the properties of the
surface active sites and ozone decomposition reactions
in aqueous solutions. Knowledge of alumina interaction
with organicyinorganic molecules in aqueous solution
is, therefore crucial in order to understand the mecha-
nism of catalytic ozonation on heterogeneous surfaces
w
217
x
.
4.3. Catalytic wet air oxidation
Alumina was also found to be an effective catalyst of
wet air oxidation process (WA O ). WAO is known to be
an efficient method for the decomposition of organic
compounds from wastewaters. However, for effective
oxidation, high temperature (200–300 8C) and high
pressure (70–130 bar) have to be applied, which makes
the process economically unfeasible. The introduction
of the catalyst results in both the reduction of oxidation
reaction energy and the reduction of the costs of the
process
w
30
x

.
Alumina supported with copper (5%) was successful-
ly used in the catalytic wet air oxidation (CAO) process
of the treatment of dyeing and printing wastewater from
the textile industry (under the experimental conditions:
200 8C, 2.65 MPa partial oxygen pressure). Over three
times higher COD and TOC removal and two times
higher colour removal of wastewater was obtained after