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

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

0001-8686/04/$ - see front matter 䊚 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, pHPZC as well as mechanical strength and 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

䊚 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: barkasp@amu.edu.pl(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 w1x.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 w2x.Adsorption at solid–liquid interfaces

is important in technological processes and products

such as corrosion, catalysis, nanoparticle ultracapacitors,

molecular sieves, and semiconductor manufacturing w3x

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 w4,5x.Polymeric reagents are used

exten-sively in the colloidal processing of ceramics w6x

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 w7x.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 w8x, toxic arsenic

w9x, lead w10–13x and chromium w14x 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 w11x.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 w15,16x.Al based compounds are used as coagulants w15–18x.Alumina has also been applied as a catalyst of ozonation w19– 26x and wet air oxidation w27–32x

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 w3x.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 w33x:

Al O 3H O 2 3 2 does not exist gibbsite

Al O H O 2 3 2 diaspore boehmite (bauxite)

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 w33x

The above-mentioned classification is used by

Euro-pean authors.In the USA, the classification is as follows

w33x:

Al O 3H O 2 3 2 gibbsite bayerite nordstrandite

Al O H O 2 3 2 boehmite diaspore –

In 1950, Stumpf et al.reported that apart from

a-Al O2 3 (corundum), another six crystal structures of

alumina occur: g, d, k, h i x-Al O2 3 w33,34x.The

sequence of particular type formation under the thermal

processing of gibbsite, bayerite, boehmite and diaspore

is as follows w35x:

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 w35x: low-temperature aluminas:

Al O Ø2 3 nH O2 (0-n-6) obtained by dehydrating at

tem-peratures not exceeding 600 8C (g-group).This group

belongs to: r, x, h and g-Al O high-temperature2 3

aluminas: nearly anhydrous Al O obtained at tempera-2 3

tures between 900 and 1000 8C (d-group).This group

belongs to:k,u and d-Al O 2 3

All these structures are based on a more or less

close-packed oxygen lattice with aluminum ions in the

octa-hedral and tetraocta-hedral interstices w35x.Low-temperature

aluminas are characterised by cubic close-packed oxygen

lattices; however, high-temperature aluminas are

char-acterised by hexagonal close-packed lattices w36x A

more detailed discussion concerning crystal structures

of alumina was presented elsewhere w37,38x

In terms of catalytic activity, high-temperature

alu-minas are less active than low-temperature alualu-minas

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 w39x

form with the formation of surface hydroxyl groups

w35,39x.At room temperature, alumina adsorbs water as

undissociated molecules bonded with strong hydrogenbonds.At higher temperatures, hydroxyl groups areformed on the surface of alumina and, with an increase

of temperature, are gradually expelled as H O.However,2even at 800–1000 8C and in a vacuum, some tenths of

a percent of water are still retained in the alumina

w35,40,41x

The main two parameters determining the catalyticproperties of alumina are acidity and basicity.Brønstedacidity–basicity is defined as the ability to protonabstraction–acceptation.Lewis acidity–basicity is theability to electron acceptation–abstraction w42x.Chemi-sorption of water on the alumina surface is considered

to be a reaction between Al ion, an acceptor of electronpair (Lewis acid), and hydroxyl ion, its donor (Lewis

base)

Hydroxyl groups formed at alumina surface behave

as Brønsted acid sites.However, the dehydratation oftwo neighbouring OHyions from the surface of aluminacauses the formation of strained oxygen bridge, activeLewis acid sites w43x:

Low-temperature transition aluminas (metastable

phases of low crystallinity characterised by high surfacearea and open porosity w39x) are of great interest due to

their possible usage both as catalysts and adsorbents inwater treatment technology.Al hydroxides are the mainactive 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 temperatureand vapour pressure, active alumina can contain from afew 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 3q in lower

layer ) w44x.

Table 1

Spectral position and assignment for surface hydroxyl groups on transitional aluminas w39x

Both Brønsted and Lewis acid sites are thought to be

the catalytic centres of alumina w43x

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 OHyions can be formed,

giving a square lattice of OHy ions.As a result of

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

OHy can combine at random, but only two-thirds of

the OHy ions can be removed without disturbing the

local order.Further dehydration causes the creation ofsurface defects.The remaining hydroxyl ions coverapproximately 9.6% of the surface Depending on thenumber 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 avery high temperature ()800 8C) when migration of

surface ions is possible.At this high temperature, tons migrate readily on the surface and the gradual loss

pro-of surface area, as well as the slow formation pro-of temperature forms of alumina, indicate that also oxideand aluminium ion migration occur.At this stage ofdehydration, the number of defects on the surfaceincreases considerably.The major defects are two andthree directly adjacent vacancies and two and threedirectly adjacent oxide ions.As a result of dehydrationwith increasing temperature, the Brønsted acid sites,numerous at high water contents, are gradually convertedinto Lewis acid sites w35,41,44–46x

high-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 thatthe (100) crystal face is the only possible termination

of aluminas crystallites and the negligence of the tive spinel nature of aluminas.This suggests that only

defec-AlVI ions would be present in the uppermost layer andthe fully hydrated surface(located on top of equivalent

cations) would be equivalent w39x

3.2.2 Tsyganenko’s model

According to Tsyganenko’s model, the number of thenearest neighbours has a negligible effect on the fre-quency of the OH species.Whereas the number oflattice Al atoms that OH groups are attached to be afactor determining the frequency of surface hydroxylgroups on the alumina surface.According to the model,three forms of surface hydroxyl groups are possible aspresented in Fig.2 and Table 1.In the model, the doublecoordination of Al ions (AlVI and AlIV) in spinel

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 w39,47,48x

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 tohydroxyl group, five hydroxyl groups can be present onthe three possible crystal planes (111, 110, 100) of

alumina(Fig.3, Table 1) w39,46,49x

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 highestfrequency possess the highest basicity (Ib group) and

the OH groups with the lowest frequency are thought toposses the highest acidicity(III group).This correlation,

however, is not always accurate w39x

3.2.4 Busca’s model

The model considers the role of cation vacanciesimposed to the spinel structure by the alumina stoichi-ometry and can be considered as a modification of thepreviously mentioned Knozinger’s model.It takes into¨consideration differences of OH frequency in the case

of OH bounded to AlIV and AlVI ions, as the tion of cation is a main factor determining the OH groupfrequency.The model implies that the free OH bandsare distributed over a much wider spectral range thanconsidered before(Table 1).The possible OH structures

coordina-at the surface of defective spinel transition alumina arepresented in Fig.4.The presence of the cation vacancy

on the surface of alumina determines the multiplicity of

OH bands observed on aluminium oxides w39,50,51x.The vibrational spectrum of surface hydroxyls ofalumina 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 w39x

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 OHy ion or water

mole-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 w52–54x.The

surface hydroxyl groups of hydrous alumina have,

there-fore an amphoteric character.The primary surface charge

density(ss) may be expressed by the following equation

w55x:

The point of zero charge of alumina was assessed tovary from ;7 to ;10 depending on the type of alumina.Some relevant data is presented in Table 2.A detaileddiscussion on point of zero charge of alumina and othermetal oxides was presented by Kosmulski w38,56–60x,Sposito w37,61x and others w62x

In aqueous solution, due to the surface charge ofalumina, an electric double layer is formed as a result

of electrostatic interaction between the charged aluminasurface and ions of an opposite charge present in bulksolution

The surface charge formation and the strong ence of the properties of alumina on the pH value ofthe solution are of crucial importance when discussingalumina’s application as a catalyst or adsorbent in watertreatment technology.This will be discussed below.However, it has to be pointed out that the high catalyticactivity and the high adsorption capacity of alumina inthe process of impurities removal from water will beobtained only when the process is carried out undercertain, optimal for the particular reaction conditions

depend-3.3.2 Models for surface charge formation

The mechanism of charge formation on the surface

of alumina is based on the phenomenon of adsorptionand desorption of protons by active surface centres.Thethree main models: one-pK, two-pK and MUSIC model,

(PZC, point of zero charge, the pH value at which the

net surface charge is zero;sss0), the surface is charged

positively.At a basic medium (pH)pHPZC) the surface

is charged negatively w52x:

Table 2

The summary of the pHPZCof aluminas and hydrated aluminas

Material pHPZC Experimental method Refs.

8.7 Potentiometric acid–base titration w 7x

9.0 Potentiometric acid–base titration w 75x

w 81x Hydroxyl groups Formal charge LogKH

which are used to describe this phenomenon, are

dis-cussed briefly below.Discussion that is more detailed is

presented elsewhere w38,55,76–78x

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 w55,79–82x:H

In one-pK model the surface is assumed to be

mon-ofunctional with surface oxygen groups that undergo

one protonation step w38,55,81–83x:

MUSIC (multi site complexation) model is the most

successful in deriving the surface charging behaviourfrom the properties of the material.In contrast to one-and two-pK models, it considers different types ofsurface groups, which have different protonation con-stants w38x.The MUSIC model is based on Paulingtheory of bond valence.It assumes the presence ofseveral active surface oxygen groups on metal

(hydr)oxides: singly, doubly and triply coordinated with

metal cations of the solid, capable of adsorbing one ortwo protons.The protonation of metal (hydr)oxide

surface groups can be described by the two reactions

wheren 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 nearq

sthe surface

Active surface groups of different metal(hydr)oxides

have different affinities for protons, which can beexplained by differences in the Gibbs free energy levels

of the groups involved.The intrinsic free energy of thereactions Eqs (6) and (7) can be considered to be

composed of local electrostatic contribution and otherunspecified contributions.Following the principle of thistype of approach, the proton association constant can becalculated from the following expression w55x:

logK sAyB n,i (nyyL) (8)

where A, B are constants, andL is the distance between

the metal ion and the adsorbed proton

The calculated proton association constants for aseries of surface groups are presented in Table 3.Onthe 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 protonationreactions of a given surface oxygen will be ‘active’ inthe normally accessible pH range w55,81x

The surface charge density for crystal structure is asfollows w81x:

26 B Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48

{

s sSs N s n( )F nyu q n,2 (nyy1)u n,1

}

where un,1,un,2are surface groups present as a specific

surface species on a specific crystal face defined

accord-ingly to reactions Eqs (6) and (7) as follows:

Ns ( )n – the site density of the specified surface group

on a given surface, defined as the sum of all species

with the same value ofn of this face.

Assuming that only one type of active site group

exists on the surface of metal oxide and when:nys1,

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 w81x:

s ss N F u y s 2 (1yu yu )1 2 (10)

whereu2,u1, and(1yu1yu2) are the fractional surface

coverage of the –OH , SOH and –O, respectively.q 0

2Assuming that only one type of surface group is on

the surface of metal oxide and whenns1,ys1y2, the

MUSIC model is simplified to ‘one-pK’ model.The Eq

(9) can be simplified to the following form w81x:

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 w84,85x, 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 w86x

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 transportcontrolled.If the attachment–detachment step is muchslower than the transport step, the adsorption process isattachment–detachment controlled.If the rates of bothsteps are similar, the adsorption process is controlled byboth mechanisms w86x.The adsorption equilibrium ofions is often formulated by the Langmuir and Freundlichisotherm equations

The Langmuir isotherm describes the dependence ofthe equilibrium surface concentration of an adsorbedmolecule on its gas–liquid phase concentration at con-stant temperature.The Langmuir isotherm is based onthe following assumptions:(1) the solid surface is made

up of a uniform array of energetically identical tion sites; (2) a maximum of one monolayer can be

adsorp-adsorbed; (3) there are no interactions between the

adsorbed molecules.The Langmuir isotherm can beexpressed by the following equation w15,16,42,82x:

where X is the amount of adsorbate adsorbed on 1 g of

alumina(mol), X is the amount necessary to cover thementire surface with a monolayer of adsorbate (mol), C

is equilibrium compound concentration in solution(mol

my 3) and b is adsorption energy constant.

Freudlich isotherm assumes that the heat of adsorptiondecreases exponentially with surface coverage (X) and

can be expressed as follows w15,16,42,82x:

1 yn

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 tophysisorption if the amount physically adsorbed doesnot exceed monolayer coverage.Physical adsorptionnormally proceeds beyond monolayer coverage, and themost commonly used isotherm to describe this situation

is the BET isotherm w15,16,42,82x

The Langmuir and Freundlich isotherms have foundseveral applications mainly because of simplicity andthe necessity of using two parameters only in thecalculations.They have, however, two major drawbacks.

Firstly, the model parameters obtained are usually priate for one set of conditions and cannot be used as aprediction model for another set of conditions.Secondly,these models cannot provide us with a fundamentalunderstanding 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

appro-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

w55,80,86–90x.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 w91x

Several SC models have been proposed: the diffuse

layer model (DLM) w55,92,93x, the basic Stern model

(BSM) w83,93x, the constant capacitance model (CCM)

w92,94,95x, and the triple layer model (TLM) w55,87–

90,96–99x.A detailed discussion on SCMs was

pre-sented by Kosmulski w38x and Sposito w37x.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 w55,80,86–89,96,97x

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 chargesso),

where-as electrolyte ions are where-assumed to adsorb at b-plane

(outer plane characterised by charges sb), which is a

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 ofcounterions is formed near the surface.The closestdistance of approach of the diffuse swarm defines d-plane.The three planes of charge: O-, b- and d-planeare associated with three planes of potential C , C ,0 b

and C and treated as a series of pairs of parallel-platedcapacitors with capacitances C1 and C2 w37,87–

the absolute temperature

Surface outer-sphere complexation reactions for

Mmq and Lly ions can be given by the reactions

w37,88,89x:

28 B Kasprzyk-Hordern / Advances in Colloid and Interface Science 110 (2004) 19–48

KA yŽint s. wAlOH Hxw qxwAyxexpyF c yc yRTŽ o b. ~ (25)

where Cq is the cation and Ay is the anion of the

background electrolyte

Charge balance requires that the sum of the charges

at the O-, b-, and d-plane be equal to zero w37,88,89,91x:

= AlOH yLHy 2 ~yAlOH yA2 ∂ (28)

whereC in the capacitance density, S is the surface area

and a is the suspension density

The mass balance equation for the surface functionalgroup, AlOH is w37x:

3.3.3.1 Interactions with organic molecules.Organic

compounds differ in molecular weight and nature offunctional groups; therefore their sorption mechanismsare diverse.Organic compounds with acidic, basic oramphoteric 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 metalcations, may result in the chemical dissolution of adsor-bents w38x

Organic molecules of molecular weight smaller than

200 do not adsorb on oxide surfaces unless they havefunctional groups such as carboxylic, phenolic-OH, oramino groups which, substituting for the surface hydrox-

yl group, can form complexes with the structural metalions of the oxide surface w103x

Non-ionic, hydrophobic organic chemicals such asalkylbenzenes, chlorobenzenes and polycyclic aromatichydrocarbons interact weakly and non-specifically withmineral surfaces w104,105x.Sorption of these com-pounds on alumina in aqueous solution is difficultbecause water molecules out-compete the non-ionic

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

psystem of the PAHs w104x

The adsorption of chlorophenols on alumina in

aque-ous solutions is weak.Chlorophenols adsorb on metal

oxides (Al O , TiO ) via the phenolate group w106x.2 3 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 w107x

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 w108x

The studies on phenylphosphonate ions adsorption

onto aged g-Al O and boehmite revealed that these2 3

ions undergo adsorption through surface complexes

for-mation, which are most probably monodentately

coor-dinated to the surfaces w95,109x

Several adsorption mechanisms were developed in

order to explain the adsorption of organic molecules

onto hydrous solids based on ligand exchange reaction

w103x, the formation of hydrophobic bonds between the

surface and organic molecules and hydrogen bonding as

an adsorption mechanism w72,110,111x, as discussed

later

Carboxylic acids.The properties of carboxylic acids,

mainly their adsorption affinity towards metal oxides

surfaces, are of great importance in water treatmenttechnology, as these compounds are commonly present

in treated water.They are the main oxidation products, which are resistant to ozone.They are alsobiodegradable.Furthermore, –COOH groups comprise

by-a significby-ant pby-art of nby-aturby-al orgby-anic mby-atter, by-a typicby-alcomponent of natural water

The adsorption of carboxylic acids on alumina (and

other metal oxides such as ZrO2 and TiO2) is very

strong, with adsorption energies much higher than those

of other organic compounds.This particular property ofcarboxylic acids makes alumina an attractive mediumfor the removal of these compounds from treated water.Because most carboxylic acids are weak acids, theirdegrees of dissociation in an aqueous solution andadsorption on alumina are greatly affected by pH w112x.The surface coordination model, or more specificallythe ligand exchange model, based on the assumptionthat anions of the organic acids replace the surfacehydroxo groups of alumina, was used by Kummert andStumm w103x in order to explain the specific interaction

of organic acids with the hydrous oxide.The schematicpresentation of the possible surface coordination reac-tions of the diprotic acid H X2 (e.g salicylic acid, phtalic

acid and catechol) with the surface OH-groups of

g-Al O is presented in Fig.6 w103x.2 3The possible surface coordination reactions are asfollows w103x:

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 2 2 3

Fig.7.Structures of salicylate complexes at alumina surface in ous alumina suspension: (a) bidentate, (b) carboxylate-bonded mon-

aque-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 w113x

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 w114x.The experimental data presented

by Kummert and Stumm w103x 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 w103x.2 3

The tendency of the organic acids to form surface

complexes with Al O2 3 (Eq (32)) is similar to that of

organic ligands to form complexes with Al3qin solution

and catechol) are inner-sphere complexes w103x:

This conclusion was also arrived at by Szekeres et al

w75x for salicylic acid adsorption on g-AlOOH

(boehm-ite).Ainsworth et al.w17,118x, 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 beformed at a low surface coverage (Fig.7a).At the

equilibrium, monodentate phenolate surface complexeswere 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 abidentate inner-sphere complexes formation.In general,the mechanism of salicylate adsorption follows theformation of an outer-sphere complex, subsequent for-

Table 4

The effect of pH value and ionic strength on adsorption capacity of alumina towards benzoic acid w70x

(mmol m ) y 2

acid w S , 9.5 m g 2 y 1 ; particle size distribution:

mation of an inner-sphere monodentate carboxylate

com-plex (accompanied by the loss of H O) and the2

formation of the final product, bidentate complex

w117,118x

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 w70x

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-pHPZC) and negatively charged in a basic medium (pH)

pHPZC).Counterions in the outer layer compensate for

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) w70,112x

Madsen and Blokhus w70x examined the adsorption

capacity of a-Al O and g-AlOOH in respect of benzoic2 3

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 Al2 3

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 w70x.This is, however,

true for lower acids concentrations.For the g-AlOOH

(boehmite) examined by Szekeres et al w75x 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 specificadsorption of salicylate should overcome the non-spe-cific electrostatic effect.The extent of adsorption on anuncharged surface should be, therefore higher at higherionic strength as a result of the higher salicylate activity.The molecular size of the adsorbate and the porosity

of the adsorbent are significant parameters influencingthe adsorption capability of alumina.Small organic acidssuch as salicylic acid are preferentially adsorbed on thewalls of the mesopores with no effect on the charge ofthe external surface.When adsorption inside the pores

is complete, it spreads over the external surface causing

a decrease of the z potential.Polyelectrolytes such aspolyacrylic acid cannot enter the mesopores and areadsorbed on the external surfaces, and only low quanti-ties of them are required to decrease thezpotential w1x

Complexones: The adsorption properties of alumina

towards complexones (a group of polyaminocarboxylic

acids or their salts, which are derivatives of acetic acid) such as:

iminodi-– EDTA(ethylenediaminetetraacetic acid),

– DTPA (diethylenetetraaminehexaacetic acid),

– TTHA(triethylenetetraaminehexaacetic acid),

surface groups and a sorbate molecule) and electrostatic

attraction(electrostatic forces, which arise as a result of

differences in charge between the surface and ionicsolutes).Bowers and Huang w72x 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 AlOHq (type 1, proton

2donor) and AlO (type 3, proton acceptor or electrony

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 isq

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 pHPZC of alumina is 9.0 and most

of the acetic acid groups are unprotonated at the pHvalues where the type 3 group will form.Therefore littleadsorption of most polyacetic acids at high pH valuescan be observed.It is questionable, however, whetherhydrogen 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 beanticipated

Polyelectrolytes and polymers.The adsorption of

polymers and polyelectrolytes onto alumina differs nificantly from that of small molecules mainly becausethese materials have multifunctional groups with differ-ent adsorption potential, varying sizes and conformationsthat influence the adsorption process.For polyelectro-lytes, the major driving force for adsorption is the

sig-Fig.8.Schematic representation of the interaction between (a) Alumina-PAA, (b) Alumina-PVA w6x.

electrostatic attraction w121x.The adsorption of several

polyelectrolytes such as polyacrylic acid (PAA),

poly-vinyl alcohol(PVA), polyvinyl pyrrolidone (PVP) onto

alumina, has been widely studied w1,6,63,65,119,122–

132x

The adsorption of polyelectrolytes on metal oxides

involves several neighbouring sites w1x:

xyy

xsAlOH qRCOO2 ~ sAlOHµ Ž 2.xOOCR∂

(36)

Generally, the acidity of the polyelectrolyte is higher

than the local positive charge (y)x), and the resulting

surface charge is negative w126x.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 neighbouringmolecules.Therefore the orientation of adsorbed mole-cules such as PAA is influenced by two factors: theadsorption affinity of anionic PAA for positively chargedsites of alumina and intermolecular repulsion w1,123x.The conformation of polymer on the alumina surface

is dependent on polymer concentration and changesfrom trains to loops or tails with increasing polymerconcentration w122,123x.When adsorption of two kinds

of polymer takes place, complex phenomena often occur,depending on the combination of polymers and particles

w122x

Another factor that influences the adsorption affinity

of polymers is the pH value of the solution.Theinteraction of PAA with alumina surface is strong due

to the presence of a carboxylic functional erally, carboxyl groups of PAA can act as proton donor

group.Gen-or acceptgroup.Gen-or, and thus adsgroup.Gen-orption may take place byhydrogen bonding between the hydroxylated aluminasurface 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 surfacegroups and possible electrostatic forces should be taken