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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: 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 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 [...]... w196x studied the structure and bonding of Cu(II)glutamate complexes at the g-Al2O3 water interface 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 Dissolution of alumina is an important process that influences soil solution chemistry and the geochemistry of natural waters and was studied by several... alumina in the natural aquatic environment, several applications in water treatment technology and also its wide industrial application, the understanding of the chemistry of alumina in water is necessary in order to fully comprehend the mechanisms governing the reactions at solid–liquid interface This is of particular importance in both the understanding of natural aquatic environment processes and. .. 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... purity), pH of the solution, ionic strength, composition of water and the physicochemical properties of organic–inorganic compound Knowledge of the chemistry of alumina in gas phase is widely understood and sometimes inaccurately applied in aqueous phase reactions The main difference between gaseous and aqueous reactions on alumina is the existence of water molecules, which are hard Lewis bases that form... kLi and the surface concentration of metal organic surface complexes w^ MyLix (pH dependant) Kraemer et al w203x 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 applications of Al-compounds in water. .. 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 mechanism of catalytic ozonation on heterogeneous surfaces... fulfil the principle of electroneutrality 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-Al2O3 and g-AlOOH in respect of benzoic acid The results presented in Table 4 indicate that the mineralogical... the inorganic compounds by a ligand-exchange reaction Activated alumina is regenerated by a series of HCl and NaOH solutions w18x 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. .. capacity of alumina towards fluoride strongly depends on pH and is the most efficient at pH 5.0–6.0 w15x The application of alumina for natural organic matter removal from natural and preozonated water was investigated by Lambert and Graham w205,211x Fettig w212x also investigated the adsorption of NOM on alumina Hano et al w213x reported the feasibility of alumina as an adsorbent of phosphorous (the main... concentrations An influence of the competitive adsorption of HQS in the presence of arsenate or fluoride was also investigated The results indicated that HQS-promoted dissolution of alumina is enhanced by hydroxide or fluoride Arsenate 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 . environment and its increasing application in drinking and wastewater purification, the knowledge of the structure of alumina and its possible interactions. environment and its growing application in drinking and wastewater purification, the knowledge of alumina s structure and possible interactions in water are of