Preface ........................................................ ix
1. Introduction ................................................ 1
2. Humic substances - a brief review ........................... 4
2.1. Natural organic matter and humic substances ........... 4
2.2. Isolation and classification of humic substances ...... 6
2.3. Formation and decomposition of humic substances ...... 10
2.4. Chemical and physical properties of humic
substances ........................................... 13
2.5. Environmental concentrations of humic substances ..... 24
2.6. Humic substances - accident or design? ............... 30
3. Environmental solution and surface chemistry ............... 32
3.1. Solutions and solutes ................................ 32
3.2. Natural particulate matter ........................... 34
3.3. Physico-chemical interactions in environmental
aqueous systems ...................................... 37
3.4. Equilibrium and kinetics ............................. 44
3.5. Chemical speciation .................................. 45
3.6. Calculation of equilibrium concentrations ............ 46
4. Proton dissociation from weak acids ........................ 52
4.1. Acids and bases ...................................... 52
4.2. Buffering ............................................ 55
4.3. Kinetics ............................................. 55
4.4. Diprotic acids ....................................... 57
4.5. Extension to higher polyprotic acids ................. 59
4.6. Electrostatic interactions among sites ............... 61
4.7. Proton dissociation from well-defined polymers ....... 73
4.8. Proton dissociation from humic substances ............ 76
5. Metal-ligand interactions .................................. 77
5.1. Coordination ......................................... 77
5.2. Chemical equilibria involving metal ions, protons
and simple weak acid ligands ......................... 86
5.3. Multisite ligands ................................... 98
5.4. Electrostatic interactions ........................... 99
5.5. Results with well-defined macromolecules ............. 99
6. Methods for measuring cation binding by humic
substances ................................................ 103
6.1. The humic sample .................................... 103
6.2. Determination of proton binding by potentiometry .... 104
6.3. Analytical determination of acid group contents ..... 108
6.4. Direct measurement of equilibrium metal binding -
principles .......................................... 110
6.5. Separation methods to quantify equilibrium metal
binding ............................................. 113
6.6. Competition methods ................................. 119
6.7. Electrochemical techniques .......................... 121
6.8. Spectroscopic methods ............................... 123
6.9. Measurement of the kinetics of metal - humic
interactions ........................................ 126
7. Quantitative results with isolated humic substances ....... 128
7.1. Proton dissociation ................................. 128
7.2. Equilibrium binding of metal ions ................... 136
7.3. Kinetics of metal ion binding ....................... 151
8. Cation binding sites in humic substances .................. 157
8.1. Proton-dissociating groups .......................... 157
8.2. Binding sites for metals information from binding
studies ............................................. 159
8.3. Information from spectroscopy ....................... 162
8.4. Viscometry .......................................... 169
8.5. Summary ............................................. 169
9. Parameterised models of cation-humic interactions ......... 171
9.1. Overview and philosophy ............................. 171
9.2. Models that describe the binding of a single
cation .............................................. 173
9.3. Simpler models that include competition ............. 183
9.4. The site heterogeneity/polyelectrolyte models of
Marinsky and colleagues ............................. 192
9.5. Modelling electrostatic effects in humic
substances .......................................... 194
9.6. Humic Ion-Binding Models V and VI ................... 201
9.7. The NICA and NICCA models ........................... 206
9.8. Summary ............................................. 209
10. Applications of comprehensive parameterised
models ..................................................... 210
10.1. Interactions with protons ........................... 210
10.2. Binding of single metal cations interpreted
with Model V ........................................ 216
10.3. Binding of single metal cations interpreted
with Model VI ....................................... 220
10.4. Application of the NICCA model ...................... 229
10.5. Metal binding as a function of ionic strength ....... 235
10.6. Non-specific binding ................................ 240
10.7. Competition between metals .......................... 240
10.8. Proton-metal exchange ............................... 246
10.9. Comparison of the NICCA-Donnan model and Model VI ... 249
10.10.Applications of the models to field situations ...... 251
11. Predictive modelling ...................................... 253
11.1. Electrostatic interactions .......................... 253
11.2. Binding sites ....................................... 254
11.3. Prospects for predictive modelling .................. 260
12. Cation-humic binding and other physico-chemical
processes ................................................. 262
12.1. The conformation of humic matter .................... 262
12.2. Aggregation of humic substances ..................... 266
12.3. Adsorption of humic substances by mineral
surfaces ............................................ 270
12.4. Binding of organic cations by humic substances ...... 278
12.5. Colloid stability ................................... 278
12.6. Dissolution of minerals ............................. 282
12.7. Formation of mineral precipitates ................... 285
12.8. Other processes ..................................... 287
12.9. Concluding remarks .................................. 287
13. Cation binding by humic substances in natural waters ...... 288
13.1. Chemical speciation calculations .................... 288
13.2. Interactions with major ions and protons ............ 290
13.3. Interactions of humic substances with major
cations ............................................. 301
13.4. Competition ......................................... 308
13.5. Interactions of heavy metals with humic substances
in natural waters ................................... 315
13.6. Modelling heavy metal speciation in natural
waters .............................................. 320
13.7. Interactions with metallic radionuclides ............ 331
13.8. Binding by dissolved humic matter compared
to adsorption by suspended particulates ............. 331
14. Cation binding by humic substances in soils and
sediments ................................................. 334
14.1. Components of the soil system ....................... 335
14.2. Sorption of major cations by organic-rich acid
soils ............................................... 339
14.3. Sorption of major cations by mineral soils .......... 347
14.4. Sorption of trace cations by soil solids ............ 352
14.5. Dissolved organic matter (DOM) in soil solution ..... 359
14.6. Colloids in soil and aquifer porewaters ............. 371
14.7. Cation-humic interactions in sediments .............. 371
15. Research needs ............................................ 380
15.1. Research needs for isolated humic materials ......... 380
15.2. Research needs in field studies ..................... 385
15.3. Cation-humic interactions in catchments ............. 387
References .................................................... 391
Index ......................................................... 422
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