Adsorption Technology in Water Treatment
Worch, Eckhard
Adsorption Technology in Water Treatment
Fundamentals, Processes, and ModelingThe principle of adsorption and the ability of certain solid materials to remove dissolved substances from water have long been known. For about 100 years, adsorption technology has been used to a broader extent for water treatment, and during this time, it has not lost its relevance. On the contrary, new application fields, besides the conventional application in drinking water treatment, have been added in recent decades, such as groundwater remediation or enhanced wastewater treatment. The presented monograph treats the theoretical fundamentals of adsorption technology for water treatment. In particular, it presents the most important basics needed for planning and evaluation of experimental adsorption studies as well as
for process modeling and adsorber design. The intention is to provide general basics, which can be adapted to the respective requirements, rather than specific application examples for selected adsorbents or adsorbates. As a practice-oriented book, it focuses more on the macroscopic processes in the reactors than on the microscopic processes at the molecular level. The book begins with an introduction into basic concepts and an overview of adsorption processes in water treatment, followed by a chapter on adsorbents and their characterization. The main chapters of the book deal with the three constituents of the
practice-related adsorption theory: adsorption equilibria, adsorption kinetics, and adsorption dynamics in fixed-bed columns. Single-solute systems as well as multicomponent systems of known and unknown composition are considered.Aspecial emphasis is given to the competitive adsorption of micropollutants and organic background compounds due to the high relevance formicropollutant removal fromdifferent types
of water. The treatment of engineered processes ends with a chapter on the restoration of the adsorbent capacity by regeneration and reactivation. The contents of the book are completed by an outlook on geosorption processes, which play an important role in seminatural treatment processes such as bank filtration or groundwater recharge. It was in the mid-1970s, at the beginning of my PhD studies, when I was first faced with the theme of adsorption. Although I have broadened my research field during my scientific career, adsorption has always remained in the focus of my interests. I would be pleased if this book, which is based on my long-term experience in the field of adsorption, would help readers to find an easy access to the
fundamentals of this important water treatment process. I would like to thank all those who contributed to this book by some means or other, in particular my PhD students as well as numerous partners in different
adsorption projects.
Eckhard Worch
January 2012
Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Basic concepts and definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Adsorption as a surface process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 Some general thermodynamic considerations . . . . . . . . . . . . . . . . . . . . . . 2
1.1.3 Adsorption versus absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.4 Description of adsorption processes: The structure of the
adsorption theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Engineered adsorption processes in water treatment . . . . . . . . . . . . . . . . 5
1.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.2 Drinking water treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.3 Wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.4 Hybrid processes in water treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3 Natural sorption processes in water treatment . . . . . . . . . . . . . . . . . . . . . 8
2 Adsorbents and adsorbent characterization . . . . . . . . . . . . . . . . . . . . . . . 11
2.1 Introduction and adsorbent classification . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Engineered adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 Activated carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.2 Polymeric adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2.3 Oxidic adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.4 Synthetic zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3 Natural and low-cost adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4 Geosorbents in environmental compartments . . . . . . . . . . . . . . . . . . . . . . 19
2.5 Adsorbent characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.5.1 Densities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.5.2 Porosities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.5.3 External surface area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.5.4 Internal surface area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.5.5 Pore-size distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.5.6 Surface chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3 Adsorption equilibrium I: General aspects and single-solute adsorption . . . 41
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.2 Experimental determination of equilibrium data . . . . . . . . . . . . . . . . . . . 42
3.2.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.2.2 Practical aspects of isotherm determination . . . . . . . . . . . . . . . . . . . . . . . 45
3.3 Isotherm equations for single-solute adsorption . . . . . . . . . . . . . . . . . . . . 47
3.3.1 Classification of single-solute isotherm equations . . . . . . . . . . . . . . . . . . . 47
3.3.2 Irreversible isotherm and one-parameter isotherm . . . . . . . . . . . . . . . . . . 48
3.3.3 Two-parameter isotherms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.3.4 Three-parameter isotherms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.3.5 Isotherm equations with more than three parameters . . . . . . . . . . . . . . . . 58
3.4 Prediction of isotherms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.5 Temperature dependence of adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.6 Slurry adsorber design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.6.1 General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.6.2 Single-stage adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.6.3 Two-stage adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.7 Application of isotherm data in kinetic or breakthrough
curve models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4 Adsorption equilibrium II: Multisolute adsorption . . . . . . . . . . . . . . . . . . 77
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.2 Experimental determination of equilibrium data . . . . . . . . . . . . . . . . . . . 78
4.3 Overview of existing multisolute adsorption models . . . . . . . . . . . . . . . . . 80
4.4 Multisolute isotherm equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.5 The ideal adsorbed solution theory (IAST) . . . . . . . . . . . . . . . . . . . . . . . 84
4.5.1 Basics of the IAST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.5.2 Solution to the IAST for given equilibrium concentrations . . . . . . . . . . . . 88
4.5.3 Solution to the IAST for given initial concentrations . . . . . . . . . . . . . . . . 90
4.6 The pH dependence of adsorption: A special case of
competitive adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.7 Adsorption of natural organic matter (NOM) . . . . . . . . . . . . . . . . . . . . . 98
4.7.1 The significance of NOM in activated carbon adsorption . . . . . . . . . . . . . 98
4.7.2 Modeling of NOM adsorption: The fictive component approach
(adsorption analysis) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.7.3 Competitive adsorption of micropollutants and NOM . . . . . . . . . . . . . . . 104
4.8 Slurry adsorber design for multisolute adsorption . . . . . . . . . . . . . . . . . . . 111
4.8.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.8.2 NOM adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
4.8.3 Competitive adsorption of micropollutants and NOM . . . . . . . . . . . . . . . 113
4.8.4 Nonequilibrium adsorption in slurry reactors . . . . . . . . . . . . . . . . . . . . . . 118
4.9 Special applications of the fictive component approach . . . . . . . . . . . . . . 120
VI Contents
5 Adsorption kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
5.2 Mass transfer mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
5.3 Experimental determination of kinetic curves . . . . . . . . . . . . . . . . . . . . . 124
5.4 Mass transfer models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
5.4.1 General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
5.4.2 Film diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
5.4.3 Surface diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
5.4.4 Pore diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
5.4.5 Combined surface and pore diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
5.4.6 Simplified intraparticle diffusion model (LDF model) . . . . . . . . . . . . . . . 153
5.4.7 Reaction kinetic models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
5.4.8 Adsorption kinetics in multicomponent systems . . . . . . . . . . . . . . . . . . . . 164
5.5 Practical aspects: Slurry adsorber design . . . . . . . . . . . . . . . . . . . . . . . . . 166
6 Adsorption dynamics in fixed-bed adsorbers . . . . . . . . . . . . . . . . . . . . . . 169
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
6.2 Experimental determination of breakthrough curves . . . . . . . . . . . . . . . . 175
6.3 Fixed-bed process parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
6.4 Material balances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
6.4.1 Types of material balances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
6.4.2 Integral material balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
6.4.3 Differential material balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
6.5 Practical aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
6.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
6.5.2 Typical operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
6.5.3 Fixed-bed versus batch adsorber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
6.5.4 Multiple adsorber systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
7 Fixed-bed adsorber design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
7.1 Introduction and model classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
7.2 Scale-up methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
7.2.1 Mass transfer zone (MTZ) model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
7.2.2 Length of unused bed (LUB) model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
7.2.3 Rapid small-scale column test (RSSCT) . . . . . . . . . . . . . . . . . . . . . . . . . . 203
7.3 Equilibrium column model (ECM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
7.4 Complete breakthrough curve models . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
7.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
7.4.2 Homogeneous surface diffusion model (HSDM) . . . . . . . . . . . . . . . . . . . 213
7.4.3 Constant pattern approach to the HSDM (CPHSDM) . . . . . . . . . . . . . . . 217
Contents VII
7.4.4 Linear driving force (LDF) model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
7.4.5 Comparison of HSDM and LDF model . . . . . . . . . . . . . . . . . . . . . . . . . . 224
7.4.6 Simplified breakthrough curve models with analytical solutions . . . . . . . . 226
7.5 Determination of model parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
7.5.1 General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
7.5.2 Single-solute adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
7.5.3 Competitive adsorption in defined multisolute systems . . . . . . . . . . . . . . . 238
7.5.4 Competitive adsorption in complex systems of unknown composition . . . . 238
7.6 Special applications of breakthrough curve models . . . . . . . . . . . . . . . . . . 240
7.6.1 Micropollutant adsorption in presence of natural organic matter . . . . . . . 240
7.6.2 Biologically active carbon filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
8 Desorption and reactivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
8.2 Physicochemical regeneration processes . . . . . . . . . . . . . . . . . . . . . . . . . . 254
8.2.1 Desorption into the gas phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
8.2.2 Desorption into the liquid phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
8.3 Reactivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261
9 Geosorption processes in water treatment . . . . . . . . . . . . . . . . . . . . . . . . 265
9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265
9.2 Experimental determination of geosorption data . . . . . . . . . . . . . . . . . . . 267
9.3 The advection-dispersion equation (ADE) and the
retardation concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
9.4 Simplified method for determination of Rd from
experimental breakthrough curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271
9.5 Breakthrough curve modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
9.5.1 Introduction and model classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
9.5.2 Local equilibrium model (LEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275
9.5.3 Linear driving force (LDF) model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277
9.5.4 Extension of the local equilibrium model . . . . . . . . . . . . . . . . . . . . . . . . . 279
9.6 Combined sorption and biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . 280
9.6.1 General model approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280
9.6.2 Special case: Natural organic matter (NOM) sorption
and biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
9.7 The influence of pH and NOM on geosorption processes . . . . . . . . . . . . . 287
9.7.1 pH-dependent sorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
9.7.2 Influence of NOM on micropollutant sorption . . . . . . . . . . . . . . . . . . . . . 289
9.8 Practical aspects: Prediction of subsurface solute transport . . . . . . . . . . . . 291
9.8.1 General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
VIII Contents
9.8.2 Prediction of sorption coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
9.8.3 Prediction of the dispersivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
10 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
10.1 Conversion of Freundlich coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
10.2 Evaluation of surface diffusion coefficients from experimental data . . . . . 298
10.3 Constant pattern solution to the homogeneous surface diffusion
model (CPHSDM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302
Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327
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