Geochemical and Hydrological Reactivity of Heavy Metals in Soils - Chapter 4 docx

4 Effect of Structural Charges on Proton Adsorption at Clay Surfaces Marcelo J. Avena and Carlos P. De Pauli CONTENTS 4.1 Introduction 4.2 Structure of Clays 4.2.1 The Surface of a Clay Layer 4.2.2 Proton Adsorption 4.2.3 The Intrinsic Component of : The Bond-Valence Principle 4.2.4 The Electrostatic Component of 4.3 Case Study 4.3.1 Modeling Proton Adsorption 4.3.2 Choosing the Model 4.3.3 Application to Montmorillonite 4.3.4 Application to Illite 4.3.5 Application to a Kaolinitic Soil 4.3.6 Differences in Behavior of Clays and Metal Oxides 4.4 Summary and Concluding Remarks 4.5 Acknowledgments 4.6 Appendix: Isolated Layer Model References 4.1 INTRODUCTION Ion adsorption and desorption at the mineral–water interface are important processes in soils, sediments, surface waters, and groundwater. By capturing or releasing ions, mineral surfaces play key roles in soil fertility, soil aggregation, chemical speciation, weathering, and the transport and fate of nutrients and pollutants in the environment. Proton adsorption is a very specific form of ion adsorption. This area is so important K H eff K H eff L1623_FrameBook.book Page 79 Thursday, February 20, 2003 9:36 AM © 2003 by CRC Press LLC that it is usually treated separately from other forms. Most minerals have reactive surface groups that are capable of binding or releasing protons. This leads to the development of electrical charges at the surface and the ability to control the attach- ment of metal complexes, ions different from protons, organic molecules, polymers, microorganisms, and particles. According to Brady et al., 1 understanding proton adsorption is a necessary first step to unraveling the affinity of mineral surfaces for both inorganic and organic species. The main effects of proton adsorption-desorption on the adsorption of metals in general and heavy metals in particular have been recognized for many decades. Protons can be exchanged by metal ions at exchange sites on the mineral surface; desorbing protons can leave negatively charged groups at the surface, which act as Lewis bases that coordinate metal ions; adsorbed protons can form proton bonds between surface groups and metal complexes; and adsorbed protons can also gen- erate positive charges at the surface repelling or attracting respectively positively or negatively charged metal complexes. A good understanding of proton adsorption is essential to learn more about metal adsorption at the mineral–water interface. Most scientific articles on proton adsorption focus discussion on the oxide–water interface. Although it has been intensively studied, proton adsorption at the clay–water interface has been addressed with less detail or was taken as a particular case of adsorption on oxides. This chapter deals mainly with the protonating- deprotonating properties of phyllosilicate clays. It stresses the key role that the presence of structural charges (one of the most important differences between clays and normal metal oxides) plays in determining the adsorption, not only at the basal planes but also at the edges of the particles. A brief description of both the bulk and surface structure of phyllosilicate clays is given, and conventional models for proton adsorption and the electric double layer especially developed for clays are used. The chapter is based on a recent review, 2 which in turn is based on older articles by numerous authors who performed a great deal of work since Pauling 3 introduced the basis for explaining mineral structure and reactivity. Our aims are to provide insight into the main processes that control the proton adsorption at a phyllosilicate surface and to highlight the differences between the behavior of clays and oxides. 4.2 STRUCTURE OF CLAYS Books describing clay structure are numerous. 4–7 Thus, only a brief description is given here. The basic building bricks of phyllosilicate clays are tetrahedrons with Si 4+ in the center and four O 2 − in the corners, and octahedrons with a metal cation Me m+ (usually Al 3+ or Mg 2+ ) in the center and six O 2 − and/or OH − in the corners. The tetrahedrons share oxygens to form hexagonal rings, and the combination of rings lead to the formation of a flat tetrahedral sheet. Similarly, the octahedrons share oxygens to form a flat octahedral sheet. If only two-thirds of the octahedral sites are occupied by cations the sheet is termed octahedral. If all possible sites are occupied the sheet is termed trioctahedral. The tetrahedral and octahedral sheets can be stacked on top of each other to form a phyllosilicate layer. Indeed, the first classification of phyllosilicate clay L1623_FrameBook.book Page 80 Thursday, February 20, 2003 9:36 AM © 2003 by CRC Press LLC minerals is based on the type and number of sheets that form the layer. The super- position of one tetrahedral and one octahedral sheet results in a 1:1 layer. This layer type is represented in soils by the kaolin group, kaolinite being the most common mineral of the group. On the other hand, the superposition of two tetrahedral sheets with one octahedral sheet between them results in a 2:1 layer. There are three clay groups with the 2:1 structure: illitic (mica), vermiculite, and smectite (montmoril- lonite). Schematic representations of sheets, layers, and stacks of layers are given in Figure 4.1. In many phyllosilicate layers there are isomorphic substitutions. These substi- tutions occur when a Si 4+ or Me m+ ion in the ideal phyllosilicate structure is substi- tuted by another cation. Since the valence of the new cation is usually lower than that of Si 4+ or Me m+ , the layer structure has a shortage of positive charges, which is interpreted as a net negative charge. The negative charges originated by isomorphic substitution within the structure of a clay layer are usually called structural charges or permanent charges. Structural charges can be tetrahedral or octahedral, depending on the layer where isomorphic substitution took place. FIGURE 4.1 Schematic representation of sheets, layers, and stacks of layers. L1623_FrameBook.book Page 81 Thursday, February 20, 2003 9:36 AM © 2003 by CRC Press LLC The tetrahedral and octahedral sheets are held together in a 1:1 or 2:1 layer through the sharing of oxygens belonging to the joined faces; very strong bonds keep the sheets together. In addition, layers can associate face-to-face among them to form stacks or platelets (Figure 4.1). Layers having negative structural charges are held together in a platelet by cations intercalated in the interlayer spacing. These cations neutralize the structural charge and serve as an electrostatic binder. In 2:1 layers, when structural charges are located mainly in the tetrahedral sheets, the short distance between interlayer cation and the structural charge sites results in a relatively strong interaction, which impedes complete delamination. This is the case for micas and vermiculites. On the contrary, when structural charges reside mainly in the octahedral sheet the electrostatic interaction is weaker. This is the case for montmorillonite, where the attractive forces are weak enough to allow water to enter the interlayer spacing, produce swelling, and lead to a completely delaminated system under certain conditions. In 1:1 phyllosilicate layers, besides electrostatic interactions there are hydrogen bonds between the octahedral sheet of a layer and the tetrahedral sheet of another layer. Hydrogen bonds are strong enough to keep the layers firmly together and to produce non- delaminating systems, even in the absence of electrostatic attraction between structural charges and interlayer cations. Layers or platelets can also be associated into aggregates or flocs, and in the extreme case of very concentrated solutions, a gel can also be formed. 8–11 This is especially the case of montmorillonite, where edge-to-face interactions can lead to the formation of a gel at concentrations higher than about 4%. From the discussion above, it can be seen how the atomic structure of phyllo- silicate clays plays a key role in determining the final state of clay particles in aqueous media. The presence of structural charges, neutralizing cations, and the capacity of forming hydrogen bonds between different layers produces a system that can be completely delaminated, completely flocculated, or in an intermediate state having flocs mixed with isolated layers. Whether the more stable situation corre- sponds to isolated layers, flocs, or a mixture depends on the type of clay, its concentration, pH, concentration and type of supporting electrolyte, and so on. 4.2.1 T HE S URFACE OF A C LAY L AYER Figure 4.2 shows a drawing of a 2:1 layer with the two tetrahedral sheets sandwiching the octahedral sheet. The layers are very thin and flat — the thickness is only 9.6Å whereas the length and width can be several micrometers. The layers are so thin that a large fraction of atoms is located at the surface. The oxygens at the top of the upper tetrahedral sheet and those at the bottom of the lower tetrahedral sheet are at the surface planes (basal planes) of the layer. The rest of the atoms can be considered in the layer bulk. A simple counting reveals that surface oxygens represent 35 to 40% of the atoms forming part of the solid. This extremely high fraction of surface atoms, as compared to that of bulky oxides explains why reactions at a clay surface may become important. Figure 4.2 is accurate for a complete delaminated system. However, if the solid is formed mainly by thick platelets, such as in micas, vermic- ulites, or illites, the fraction of atoms exposed to the solution is much lower. L1623_FrameBook.book Page 82 Thursday, February 20, 2003 9:36 AM © 2003 by CRC Press LLC When a clay is dispersed in an aqueous solution, surface oxygens or surface hydroxyls become potentially reactive. Their reactivity depends on the type and spatial distribution of the atoms surrounding them. Oxygens at the siloxane surface, for example, are bonded to two Si 4+ , and it is customary to define the species Si 2 - O as a surface group called the siloxane group. 12 Hydroxyls belonging to the basal surface of 1:1 layers are bonded to two Al 3+ , and the formed Al 2 -OH group is sometimes called a gibbsite surface group, 2 because it is the same as the group located at the basal surface of gibbsite. Besides the Si 2 -O and Al 2 -OH groups, the broken edges of the layers contain other surface groups. According to White and Zelazni, 13 there are three main groups at the edges of a phyllosilicate surface: tetrahedral IV Si-OH, octahedral IV Al-OH, and transitional IV Si VI Al-OH groups. The presence of irregularities in the structure, such as steps and ledges, may add other surface groups at the edges. Drawings representing typical basal and edge groups of 2:1 and 1:1 layers are schematized in Figure 4.3. 4.2.2 P ROTON A DSORPTION Proton adsorption on a clay particle is the process of transferring the proton from the solution bulk to the surface of the particle. The term “particle” is used here generically to represent a phyllosilicate layer, platelet, or floc. There are several possible states for adsorbed protons, depending on the aggregation state of the clays (Figure 4.4). In an isolated layer, protons can adsorb either at the basal surface or edge surface. Besides these adsorption modes, protons can be adsorbed in a stack of layers, either between two basal surfaces or at the edge of a layer, but existing in very close contact with a nearby basal surface belonging to another layer. When protons are “absorbed” into a floc, they can be attached to edges, at basal surfaces, to both edges and basal surfaces, or can even be dissolved in the aqueous solution trapped within the floc. An exact mathematical treatment of the proton adsorption process, taking into account all these possible adsorption states, seems to be quite difficult. Thus, a simplified treatment is usually applied. The mathematics of proton adsorption considered here follows that presented by Avena, 2 Avena et al., 14 and Borkovec et al. 15 The treatment is not new, and is written in a general way so that it can be applied to the adsorption of protons to the surface of metal oxides, clays, and other minerals, such as carbonates and sulfurs. Moreover, it can also be used FIGURE 4.2 2:1 Layer showing the oxygen atoms at basal surfaces. Symbols used are the same as in Figure 4.1. Surface atoms Surface atoms Bulk atom L1623_FrameBook.book Page 83 Thursday, February 20, 2003 9:36 AM © 2003 by CRC Press LLC FIGURE 4.3 Phyllosilicate layers showing different surface groups. Symbols used are the same as in Figure 4.1. FIGURE 4.4 Schematic representation of the proton adsorption process on layers, platelets, and flocs. (Reprinted from Ref. 2, p. 109 by courtesy of Marcel Dekker, Inc.) L1623_FrameBook.book Page 84 Thursday, February 20, 2003 9:36 AM © 2003 by CRC Press LLC to describe the adsorption of any other ion at the solid liquid interface if the proper charge of the ion is used in the equations. The binding of a proton ion to a surface group is represented by A x + H + = AH x+1 (4.1) where A x denotes a functional surface group carrying a charge x and AH x+1 is the protonated group. The mass action law of this reaction is (4.2) where Γ AH and Γ A are the surface densities of the protonated group ( AH x+1 in this case) and of the unprotonated group ( A x ), respectively; is the protonation con- stant; and a H,0 is an expression for the proton activity at the location of the adsorption site. a H,0 is defined as (4.3) where Ψ 0 is the smeared-out surface potential and represents the difference in the electrical potential between the surface and the bulk solution, and a H represents the activity of protons in the bulk. The combination of equations 4.2 and 4.3 gives (4.4) In dilute solutions is independent of the electric potential, and is called the intrinsic protonation constant. The left side of equation 4.4 depends on the magnitude of the surface potential, and is called the effective or apparent constant, , (4.5) In logarithmic form, the above equation is (4.6) According to equations 4.5 and 4.6, the effective affinity of a group for protons results from two different contributions: a chemical or intrinsic contribution, given by , and an electrostatic contribution, given by the term containing the surface potential. Any factor or process affecting either or Ψ 0 affects the effective affinity of the reactive group for protons. These factors are discussed in the next sections. K a H AH AH int , = Γ Γ 0 K H int aae HH F RT ,0 0 = − ψ Ke a H FRT AH AH int /− = ψ 0 Γ Γ K H int K H eff KKe H eff H FRT = − int / ψ 0 LogK LogK F RT H eff H =− int . ψ 0 2 303 LogK H int LogK H int L1623_FrameBook.book Page 85 Thursday, February 20, 2003 9:36 AM © 2003 by CRC Press LLC 4.2.3 T HE I NTRINSIC C OMPONENT OF : T HE B OND -V ALENCE P RINCIPLE Atoms and ions located at the surface of a solid are characterized by an imbalance of chemical forces because they usually have a lower coordination number than equivalent atoms in the bulk. The undercoordinated cations are Lewis bases and the undercoordinated anions are Lewis acids, and both are unstable in the presence of water. The tendency to restore the balance of chemical forces drives the reactivity of surface groups. The charge of ions in ionic crystals is neutralized by the surrounding ions of opposite charge. According to Pauling’s principle of electroneutrality, 3 the charge of a cation is compensated by the charge of the surrounding anions and vice versa. Thus, the charge of an anion, for example, is only partially compensated by one surrounding cation, and the magnitude of this partial compensation is given by the bond valence, v , defined as the charge z of a cation divided by its coordination number in the solid, CN : In the case of Al(OH) 3 , for example, where z = 3 and CN = 6, the value v = 0.5 implies that each aluminum atom neutralizes on average half the unit charge of OH − per Al-OH bond. Then, two aluminum atoms need to be coordinated per OH − in order to compensate for the hydroxyl charge and to achieve electroneutrality in the structure. Hiemstra et al. 16,17 applied this concept of local neutralization of charge and geometrical considerations to develop the MUSIC model, which permits estimation of the intrinsic protonation constant of various surface groups. The model was proposed to explain the reactivity of surface groups in metal oxides, but it can also be applied to evaluate the reactivity of groups belonging to clay surfaces. Indeed, Pauling’s concepts, on which the MUSIC model is based, were developed for minerals and clays. The MUSIC model relates the intrinsic affinity for protons of any surface oxygen to the degree of charge neutralization that the surrounding cations achieve. Strictly speaking, the model is only applicable to ionic solids. Bleam 18 re-analyzed the MUSIC model and formulated a similar one: the crystallochemical model, which is based on the bond-valence principle used by crystallographers to predict structure and properties of solids. The crystallochemical model relates the affinity for protons of surface groups to the Lewis basicity of surface oxygens, and is more general than the MUSIC model because it can be also applied to nonionic materials. More recently, Hiemstra et al. 19 combined the MUSIC model and the crystallochemical model, proposing a modified version of MUSIC, including the main concepts of the bond-valence principle. The modified MUSIC model formulates the protonation reactions as K H eff v z CN = L1623_FrameBook.book Page 86 Thursday, February 20, 2003 9:36 AM © 2003 by CRC Press LLC (4.7) (4.8) where Me n − O nv − 2 , Me n − OH nv − 1 and Me n − OH nv 2 are oxo- and hydroxo-surface groups; n is an integer that represents the number of metal ions (Me) bonded to the proto- nating oxygen; and v is the bond valence. Equations 4.7 and 4.8 show that the protonating entity of the group is an oxygen. The model states that the valence V of an oxygen atom in the bulk of a solid is neutralized by j bonds with the surrounding atoms, yielding , where s is the actual bond valence. 19 At the surface, part of the bonds are missing but new bonds, either covalent bonds with protons or hydrogen bonds with water molecules, are formed. At the surface, this usually means that the valence of the surface oxygen is either undersaturated or oversaturated. Therefore, the oxygen will react, tending to restore the equality between V and , which gives a more stable bond arrangement. Mathematically, the model is formulated as (4.9) where is the logarithm of the protonating constant of equations 4.7 and 4.8, and the value 19.8 results from a model calibration using protonation constants of oxo- and hydroxo-solution complexes. 19 To solve equation 4.9, it is necessary to know V (−2) and . This last formula can be expressed as (4.10) where n, m, and i are integers; s Me is the actual bond valence of the Me-O bonds (there are different ways of calculating this bond valence, 19 although as a first approximation the value of v will be used here); s H is the bond valence of the H donating bond and (1−s H ) the bond valence of H accepting bond. The coordination with H donating or accepting bonds is taken into account with appropriate values of s H and (1−s H ). Bleam 18 and Hiemstra, Venema, and Van Riemsdijk et al. 19 used values of s H corresponding to hydrogen bonds between water molecules in pure water. In this case, about 0.2 valence units are transferred per bond. Thus, s H is about 0.8 and (1−s H ) is about 0.2. As a calculation example, Figure 4.5 shows a siloxane group (Si 2 -O) and a protonated siloxane group (Si 2 -OH +1 ) at the basal surface of a layer. In the oxo group, there are two Si-O bonds, each with s Me ≈ v; and two H accepting bonds, each with s H = 0.8. The resulting value for is 2.4 and thus = −7.9. In the case of the hydroxo group, there are two Si-O bonds, and one H donating and one H accepting bond. Therefore, = 1 and = −19.8. The same proce- dure is applied to calculate of other groups. Me O H Me OH n nv n nv −+=− −+ −21 Me OH H Me OH n nv n nv −+=− −+1 2 Vs j =− ∑ − ∑ s j LogK s V Hj int .=− + () ∑ 19 8 LogK H int s j ∑ sns ms i s jMe H H =++− ∑ ()1 s j ∑ LogK H int s j ∑ LogK H int LogK H int L1623_FrameBook.book Page 87 Thursday, February 20, 2003 9:36 AM © 2003 by CRC Press LLC FIGURE 4.5 Clay surface with accepting and donating H bonds. TABLE 4.1 of Different Groups at Clay Surfaces According to Modified MUSIC Model Group S Me ≈≈ ≈≈ νν νν nmiV + Si 2 -O 1 2 0 2 0.4 −7.9 Si 2 -OH 1 2 1 1 1 −19.8 Si-O 1 1 0 3 −0.4 7.92 Si-OH 1 1 1 2 0.2 −4.0 Al-O 0.5 1 0 2 −1.1 21.8 Al-OH 0.5 1 1 1 −0.5 9.9 Al 2 -O 0.5 2 0 1 (2) −0.8 (−0.6) 15.8 (11.9) Al 2 -OH 0.5 2 1 0 (1) −0.2 (0) 4.0 (0.0) IV Si VI Al-O s Al = 0.5, s Si = 1 n Al = 1, n Si = 1 01 (2) −0.3 (−0.1) 5.9 (2.0) IV Si VI Al-OH s Al = 0.5, s Si = 1 n Al = 1, n Si = 1 10 (1) 0.3 (0.5) −5.9 (−9.9) LogK H int s j ∑ LogK H int L1623_FrameBook.book Page 88 Thursday, February 20, 2003 9:36 AM © 2003 by CRC Press LLC [...]... 0. 04 0.03 0.02 2 σ H (C/m ) 0.01 0.00 2 3 4 5 6 7 5 6 7 -0 .01 -0 .02 -0 .03 -0 . 04 -0 .05 pH 0.07 0.06 0.05 2 ∆ q (C/m ) 0. 04 0.03 0.02 0.01 0.00 2 3 4 -0 .01 -0 .02 pH FIGURE 4. 12 Proton and ion adsorption on a kaolinitic soil in LiCl solutions Symbols correspond to experimental data Lines correspond to model predictions Electrolyte concentration: circles and solid line, 0.01 M; triangles and dashed lines,... Lectures, Clay-Water Interface and Its Reological Implications, vol 4, Güven, N and Polastro, R.M., Eds., Clay Mineral Society, Boulder, CO, 1992, p 157 9 Faisandier, K et al., Structural organization of Na- and K-montmorillonite suspensions in response to osmotic and thermal stresses, Clays Clay Miner., 46 , 636, 1998 10 Derrindinger, L and Sposito, G., Flocculation kinetics and cluster morphology in illite/NaCl... Even in the case of an isolated layer, the treatment of the electrostatics is complicated and one must select among different possibilities Consider Figure 4. 13 Drawing A depicts a layer containing an uncharged basal surface, an edge surface with a pH-dependent charge, and structural charges within the clay layer affecting - - -Y - - X FIGURE 4. 13 Three different representations of clay–water interface... Zelazni, L., Analysis and implications of the edge structures of dioctahedral phyllosilicates, Clays Clay Miner., 36, 141 , 1988 14 Avena, M.J., Mariscal, M., and De Pauli, C.P., Proton binding at clay surfaces in aqueous media, in Proceedings of 12th International Clay Conference, Elsevier, in press 15 Borkovec, M., Jönsson, B., and Koper, G.J.M., Ionization processes and proton binding in polyprotic systems:... The clay-water interface, in Proceedings of the International Clay Conference, Denver, Schultz, L.G., van Olphen, H., and Mumpton, F.A., Eds., Clay Mineral Society, Bloomington, IN, 1987, p 247 31 Sondi, I., Biscan, J., and Pravdic, V., Electrokinetics of pure clay minerals revisited, J Colloid Interface Sci., 178, 5 14, 1996 32 Hendershot, W.H and Lavkulich, L.M., Effect of sesquioxide coatings on... one represented by drawing C in Figure 4. 13 was used to model the acid–base properties of clays.2 The treatment of electrostatics in the model is based on previous work by Avena and De Pauli,26 which in turn is based on older studies by Kleijn and Oster43 and Madrid and DiazBarrientos .40 The structure of the clay–water interface according to that model is shown in Figure 4. 14 The structural charges... with protons, and the bondvalence principle seems to predict correctly the acid–base properties of the groups The principle predicts the unreactivity of basal surfaces populated with Si-O2 and Al2-OH groups and the reactivity of basal surfaces populated with Al-OH−1/2, Si-O–1, SiAl-O−1/2 and other groups Thus, the unreactivity of basal surfaces does not need to be assumed a priori when modeling It emerges... curves tend to intersect at pH 3 to 3.5 at ∆q = 0 4. 3.1 MODELING PROTON ADSORPTION Modeling of the clay–water interface began several decades ago The first models were adapted from those used for metal–water and oxide–water interfaces Van Raij and Peech27 carried out pioneering works in applying Gouy-Chapmann and Stern models to kaolinitic soils Since then, many articles in which modeling is performed... appeared in the literature.26 ,40 44 More recently, and due to the development of computers, ab initio methods, and MD and MC simulations have appeared. 24, 25 ,45 ,46 They provide invaluable information at the molecular level but, thus far they can only be applied to extremely small systems such as clusters containing a small number of atoms They cannot be applied to a complex soil system as yet The interest in. .. layer–layer interactions may significantly affect the electrostatic environment of an adsorbing ion Each of the protons represented in Figure 4. 4 is in a particular electrostatic environment and the electric potential is different at each position A very good knowledge of the size and shape of the layers and of the structure of platelets and flocs is required in order to get a good mapping of the electric . Layer 4. 2.2 Proton Adsorption 4. 2.3 The Intrinsic Component of : The Bond-Valence Principle 4. 2 .4 The Electrostatic Component of 4. 3 Case Study 4. 3.1 Modeling Proton Adsorption 4. 3.2 Choosing. unraveling the affinity of mineral surfaces for both inorganic and organic species. The main effects of proton adsorption-desorption on the adsorption of metals in general and heavy metals in particular. solution, and a H represents the activity of protons in the bulk. The combination of equations 4. 2 and 4. 3 gives (4. 4) In dilute solutions is independent of the electric potential, and is