chemcal properties of material surfaces - m. kosmulski (marcel dekker, 2001) ww

Adsorption phenomena at solid-electrolyte solution interfaces at room temperature and at atmospheric pressure are reviewed in this book with a special emphasis on the mutual relationship

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Adsorption phenomena at solid-electrolyte solution interfaces at room temperature and at atmospheric pressure are reviewed in this book with a special emphasis on the mutual relationship between adsorption and surface charging This relationship is particularly significant for adsorption of inorganic ions on silica, metal oxides and hydroxides, certain salts, e.g silicates, and clay minerals The models of surface

used in different fields, including catalysis, ceramics, corrosion science, environ- mental sciences, geology,' mineral processing, nuclear waste management, and soil

have split the scientists interested in adsorption at solid-solution interfaces into many insulated groups For example, Western papers have rarely been cited in former Soviet Union papers and vice versa even though English translations of the

goals and methods, and specific systems of interest, and often ignore other systems

based on a selective approach They are not necessarily applicable in other systems

existing knowledge and to facilitate the exchange of ideas between different parts of the scientific community

vi Preface

relationship between the zero points obtained by different methods and at different

ill-defined Collections of zero points conlpiled by different authors are compared,

analyzed

Methods used in studies of adsorption of ions, their advantages and

presented The results obtained in simple adsorption systems (with one specifically

complex systems are discussed with many specifically adsorbing species

survey is limited to materials whose solubility is low This does not imply that the solubility is always negligible in the systems of interest

species are broad fields, with their own methodologies, theories, and literature Only selected topics directly related to the main subject of the present book are briefly treated

Many recently published review articles, book chapters, and even entire books are devoted to adsorption of ionic species Usually they cover one adsorbent (or a group of related adsorbents) or specific method(s) Some of these publications were

main source of information

ACKNOWLEDGMENTS

Robert A Osteryoung and Thomas Fanghanel are acknowledged for their

5 , is gratefully acknowledged

References 293

4 STRONGLY ADSORBING SPECIES

I Experimental Methods

TI Small Ions

111 Organic Compounds and their Mixtures

11 Surface Reactions and Speciation

111 Primary Surface Charging

IV Specific Adsorption

Introduction

presentation is confined to materials

composition due to solvation or selective leaching are unavoidable, but this is a part of the adsorption process Adsorbents prepared by grafting or by adsorption of thin films (one or a few molecular layers) of substances whose properties are completely different from those of the support constitute an example of essential

sorption properties of the film and they are widely applied in different fields, but they

materials with external layer at least 10 nm thick are closely related to bulk properties of the coating, and a few examples of such materials will be discussed Solubility is another issue that requires some explanation Materials more

discussed here There is no sharp border between “soluble” and “insoluble”, e.g

1

2 Chapter 1

dissolution rate and/or the equilibrium solubility On the other hand, the solubility

certain complexing and/or redox agents Kinetics of dissolution of silica is related to

many other materials Chemical dissolution of oxides has been reviewed by Blesa et

al [2, 31 It should be emphasized that points of zero charge of relatively soluble materials, e.g BaO and SrO have been reported in the literature

their exchange capacities and diffusion coefficients of particular ions, and the

a perspective, but in some other publications the uptake of ions by gel like materials

terms of the “ion exchange” approach on the one hand and adsorption approach

on the other Finally, in some publications dealing with materials not penetrable

language used in adsorption, e.g “adsorbent in calcium form” is considered as adsorbent with pre-adsorbed calcium Specific approach is required to describe surface charging of and adsorption on clay minerals on the one hand and zeolites on the other

Most materials discussed in this book are electrical insulators, and their surface charge is regulated by sorption processes However, a few oxides show sufficient degree of electronic conductivity that makes it possible to polarize the surface using

an external battery For example, the charging curves of I r 0 2 can be plotted as a

can be modified by polarization by external electric potentials

The “dry” surface chemistry, i.e chemistry of solid-gas interfaces has its own methodology and language A substantial difference between wet and dry surface

surface is in Pact occupied by solvent In spite of an obvious relationship between dry and wet surfaces, only wet surface chemistry will be discussed here, although some quantities (e.g the BET surface area) and relationships involve results obtained for

pressure at room temperature, and sorption of their vapors has been studied Such results, albeit related to sorption of the same species from aqueous solution are beyond the scope of the book

and viewpoints of the authors of the cited publications and of potential readers

is:

i

Introduction 3

is based on concepts of colloid chemistry Many results are compiled in tabular form

adsorbate

Chapter 2, which is not directly related to surface properties presents physical and chemical properties of the materials of interest Not all materials described in

Chapter 2 also shows how many well-defined and potentially interesting materials have not been studied as adsorbents and may stimulate further research The present

exotic oxides”, but a reviewer was against such an intriguing title and finally the title

this idea: there are so many important and well-defined materials whose points of

extensively used as adsorbents crystallographic or thermochemical data are

illustrated in Figs 1.1-1.3, respectively Symbols of elements whose simple or mixed

background: crystallographic and thermochemical data available, gray background: only crystallographic data available

presented below is also used in the next chapters Simple oxides, hydroxides and

electropositive element and then from low to high oxidation state, and then from low to high degree of hydration Then results for mixed oxides (all component oxides

spite of solubility of CO2 in water Namely, basic carbonates are potential products

of reaction of certain (hydr)oxides with atmospheric CO2 Also other sparingly soluble basic salts of water soluble acids can be formed from (hydr)oxides at

unique, but physical properties of the other basic salts are not reported in Chapter 2

explicitly, only formulae of salts for which such data exist are reported

The presentation of adsorption data follows the rule “from the simplest to the

following categories (listed from the simplest to the most complicated)

FIG 1 .I Availability of physical data for oxides; black background: crystallographic and therniochemical data available

I T h 1 Pa 1 U 1NpIPuIAmICrnIBk)Cf) Es I F m ) M d I N o ( L r /

FIG 1.2

only crystallographic data available

Availability of physical data for aluminates; black background: crystallographic and therrnochemical data available, gray background:

Introduction 7

particles on particles This phenomenon plays an important role in transport of toxic

was recently reviewed by Ryan and Elimelech [8]

In most publications a clear distinction is made between sorption (interaction

(particles are formed in the presence of foreign species) Sometimes, however, these

respectively, are used in the title of the paper, abstract, text and figure captions,

coprecipitation was studied Studies reporting only coprecipitation were not taken into account in the literature survey

The quantities, which have been directly measured are referred to rather than those which were derived from the measured data For example when the adsorption

of counterions was assumed to entirely balance the surface charge (measured

counterions”, even if the later was used in the title of the paper, abstract and figure captions

In the studies conducted in aqueous solutions, the presence and thus sorption

repetitions, the studies involving multiple categories of adsorbates are listed only under the most complex one, e.g coadsorption of surfactants and small anions is only listed in the section on surfactants but not in the section on sorption of anions The above classification of adsorbates is relative and subjective, it is practical,

but not generally accepted As a matter of fact, the same adsorbate can belong to

some results (e.g Hoffmeister series) suggest that there is rather a continuum than a

shortcomings of the proposed classification, it is expedient to associate an adsorbate

“specific adsorption”

Chapter 3 presents data on points of zero charge obtained by different methods

in the absence of strongly adsorbing species, usually at low concentrations (on the

halides Also corresponding salts of ammonium and its short-chain tetraalkyl

derivatives are considered as inert electrolytes The definition of the zero point is

region as a whole is electrically neutral, but usually there is some excess of positive or

balanced by counterions (chiefly ions of the inert electrolyte whose sign is opposite to

distribution of the counterions is governed primarily by the electrostatics With

8 Chapter 1

purely electrostatic interaction the adsorption of coions (ions of the inert electrolyte

countercharge is distributed over a thin layer of solution next to the surface, and

a sufficiently thin layer of solution carries net positive or negative charge, despite the entire system is neutral One way of defining the zero points is as the pH, at which certain layer of solution next to the surface has a zero charge The charge of such thin layers cannot be measured directly, so this kind of definition must be based on

assumptions an empirical definition may be used, linking the zero point with the

material

The presence of equal amounts of positively and negatively charged sites more

expedient to assume that the surface charge behaves as smeared out homogeneous

role, especially if the surface shows a patchwise heterogeneity

The materials in the compilation of zero point values are sorted by their formal

approach makes it somewhat difficult to compare results corresponding to given

different phases were used or the crystallographic data were not reported Finally,

product Thus, it would be rather difficult to sort the data (within the same formal chemical formula) by the structure Commercial materials are often characterized by trade names rather than by their crystalline structure

trademark, manufacturer, or otherwise, does not constitute or imply its recommen- dation, or favoring The difference in the point of zero charge PZC between different

anatase Many collections of selected PZC have been published These collections are summarized and briefly discussed after presentation of original data

involving two such oxides or more, and clay minerals, whose crystallographic and thermochemical data are presented in Chapter 2, the zero points of zeolites, clays, and glasses are listed (in this order) after mixed oxides Soils and other complex and ill-defined materials are on the end of the list It should be emphasized that the terms

“soil”, “sediment”, etc have somewhat different meanings in different scientific and

“kaolinite” (clay mineral) are treated as synonyms in some publications The zero

coating) are listed separately from those in which the distribution of components is more uniform

The salts involving relatively soluble acid or base may show very low solubility

A few examples of pH dependent surface charging of such materials are discussed in

sparingly soluble acid and base, that are listed as mixed oxides In the table of zero

Introduction 9

discussed separately, and their zero points are not listed in the table

electrokinetic behavior no zero point has been found, often because the zero point was beyond the experimental range Such results are collected in two separate tables

experimental studies the inert electrolyte concentrations are integer powers of ten,

comparison

The “inert” electrolytes are, indeed indifferent at low concentrations and near

respect to the PZC and insensitive to the nature of these salts However certain

shifts in the isoelectric point IEP and PZC suggest non-electrostatic interactions of these ions and surfaces of materials, especially at high concentrations and far from the PZC These results are shown and briefly discussed next

The compilation of PZC in this book involves results obtained in temperature range 15-40T In some studies the temperature was not controlled or measured (room temperature) or at least the temperature is not reported Detailed discussion

reporting the temperature dependence of the PZC are presented to show the general

polar solvents and water-organic mixtures is not much different from that in aqueous

Strongly adsorbing species are discussed in Chapter 4 Specific adsorption of each class of compounds (small ions, surfactants, polymers) has specific terminology and methodology First, methods used to study specific adsorption of small ions are discussed and the results are presented for cations and anions separately The results are organized according to the adsorbent in the same order as the values of PZC

basically the same conditions There is only one table entry for each set of such data

dealing with single adsorbate for the same type of adsorbent (in terms of formal chemical formula) Some readers may be interested in data for specific adsorbate rather than specific adsorbent, e.g when looking for the best scavenger for certain element or species For their convenience the Tables have indices of entries

organic molecules” used as a title of the next section is again conventional, namely, weak carboxylic acids are discussed in the section on specific adsorption of anions, although at experimental conditions their degree of dissociation is often low In the section on neutral organic molecules the adsorption data for amines, phenols, amino

I O Chapter 1

sorption behavior is similar to that of weak acids and bases On the other hand,

solution containing two or more specifically adsorbing anions Not necessarily does

a real competition occur in such systems; sometimes the presence of other solutes

presence of specifically adsorbing anion(s) or at different concentrations of inert

times ranging from a few minutes to over one year The choice of equilibration time was often based on preliminary studies of the kinetics Some results are presented as

a conclusion of Chapter 4 Studies of adsorption kinetics can be also a source of information about the sorption mechanism

in adsorption modeling appear already in the chapters presenting the experimental

examples are presented in Chapter 6

The phenomena presented in this book were discussed in many reviews For

Chemistry of Ahmitzzunz edited by Sposito reviews the solution and surface chemistry

oxides, oxohydroxides and hydroxides with many references on adsorption of metal

confined to sorption from solution at room temperature, publications on

coprecipitation and adsorption from gas phase or at elevated temperatures are also

adsorption on oxides Schindler [ 191 published a review on similar topic in German Many other reviews related to specific topics are cited in respective chapters

REFERENCES

1 M Lobbus, W Vogelsberger, J Sonnefeld, and A Seidel Langmuir 14: 43864396

2 M A Blesa, P J Morando and A E Regazzoni, Chemical Dissolution of Metal Oxides,

3 J A Salfity, A E Regazzoni and M A Blesa In Interfacial Dynamics (N Kallay, ed.)

4 N Z Misak Adv Colloid Interf Sci 51: 29-135 (1994)

5 0 A Petrii, Electrochem Acta 41: 2307-2312 (1996)

(1 998)

CRC Press, Boca Raton 1994

Marcel Dekker New York 1999, pp 513-540

S Trasatti, Croat Chem Acta 63: 313-329 (1990)

J S Hanson, and D W Fuerstenau, Colloids Surf 26: 133-140 (1987)

J N Ryan, and M Elimelech, Colloids Surf A 107: 1-56 (1996)

T Aoki, and M Munemori Water Res 16: 793-796 (1982)

S Music J Radioanal Nucl Chem 99: 161-170 (1986)

J Lyklema In Adsorption from Solution at Solid Liquid Interface (G.D Parfitt and C

H Rochester, eds.) Academic Press, New York 1983, pp 223-246

G D Parfitt, Pure Appl Chem 48: 415-418 (1976)

J A Schwarz, J Colloid Interf Sci 218: 1-12 (1999)

G A Parks, Rev Mineral 23: 133-175 (1990)

B S Hemingway, and G Sposito In The Environmental Chemistry of Aluminum (G Sposito, ed.), CRC Press 1996 pp 81-1 16

S Goldberg, J A Davis, and J D Hem In The Environmental Chemistry of Aluminum (G Sposito ed.), CRC Press 1996, pp 271-331

G E Brown, V E Henrich, W H Casey D L Clark, C Eggleston, A Felmy, D W

Goodman, M Gratzel, G Maciel, M I McCarthy, K H Nealson, D A Sverjensky, M

F Toney and J M Zachara, Chem Rev 99: 77-174 (1999)

W Stumm Colloids Surf A 73: 1-18 (1993)

P W Schindler and W Stumm In Aquatic Surface Chemistry: Chemical Processes at the Particle-Water Interface (W Stumm, ed.) John Wiley 1987, pp 83-1 10

W Schindler, Oster Chem Z 86: 141 (1985)

2

The surface properties of various phases corresponding to the same chemical

studies performed at room temperature and atmospheric pressure the phases existing

considered

Usually there is no doubt which phase is meant in a publication, but also no

ranked scientific journal and this was supported by three literature references In another recent publication “y-A1(OH)3 (alumina gel, bayerite)” was studied

inconsistent nomenclature are only a tip of the iceberg

12

Physical Properties of Adsorbents 13

crystallographic form or the same name shared by different crystallographic forms

or even different chemical compounds Specific examples of such inconsistencies can

be found in Table 2.1 Probably the most accurate way to distinguish one phase from another would be to specify the space group, the lengths of axes of the elementary cell and the angles between them This, however, is not practiced; sometimes the crystal system is specified

unique, e.g for the trigonal and hexagonal systems it is convenient to use four axes: three of them are in one plane symmetrically spread to 120" and the fourth axis is perpendicular to this plane

Table 2.1 lists adsorbents of interest, i.e their chemical formulas and own

polymorphs) and their crystallographic data: space group number (1-230; groups 1 and 2 belong to triclinic system, 3-15 to monoclinic, 16-74 to orthorhombic, 75-142

system), space group according to Schoenflies, space group according to Hermann- Mauguin (sometimes the group number is unknown, in such instance the crystal-

or c=n, respectively; empty 2 or d cell: data not available; there are also many phases

the "name" cell contains other informations (e.g about thermodynamic stability)

oxohydroxides, and then for mixed oxides (as discussed in Chapter 1) The principle

of organization of Table 2.1 has been described in the Introduction For derivatives

of less common oxides, the compounds having known crystallographic structure are only listed, without specific information on this structure

Reference [I] was the sole source of this data Attempts to find a more up to

date and equally comprehensive compilation failed A relatively recent compilation

[2] covers only rock-forming materials and most of the cited literature dates back to the sixties and early seventies Reference [3] presents a collection of crystallographic data based on older compilations The data from [1-31 match for a vast majority of

136 orthorhonibic

Th7 Th' D3436 C2h6 D44.8

c2h4

c2h3

o h 7 Th7 C2h' C2h' C6h'

1.1152 0.9849

0.7898 0.49 138

0.5710 0.636 0.636 0.47374

I 0580 1.0728 1

0.5623 0.63 12

0.3772 0.31064 0.5421

0.401 0.54052 2.579 0.69 18 0.496 2.976 0.49 1

0.8377 1.510 0.8847 0.438 0.5599 0.366 0.366 0.3 18638 0.3888

0.8717 0.433 0.5498 0.3601

117.67

103.78

99.96 108.6

100.10 109.0

C2h' D6h2 D2h2' C2h' Oh"

D2h16 D6h' Oh'' D6h2 D2h"

14 I /amd P6/mcc P2 /a P2 I /m R-1) P6/nicc Cccm C2/m Ta3d Pnma C2/m P6/mcc Cccm Cccm Ta3d Pnma P6/nicc Ta3d P6/nicc Cccni Ccca T2/m 14/mmm

1 .16207 0.9925 1.728 1.3831 1.434 0.880 1.1723 0.8408

1.429 2.47 12 0.548 0.9726 1.6635

I 7945 1.6552

0.9738 0.9730 1.440 I 1.7807

0.995 2.0296 1.139

1.3038

0.9460 0.9236

0.9285 0.9287

0.53 158

0.9352 0.9335 0.9356

0.5270 0.5360

I 436 0.9297 0.928 0.5121

0.950 0.8044

26

Gallates (attention, the same name is used for salts of 3, 4, 54rihydroxybenzoic acid)