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Crystal morphology and surface reactivity of goethite


Roger James Atkinson B.Sc. (Agric.) Hons.

This thesis is presented for the degree of
Doctor of Philosophy of the University of Western Australia
Department of Soil Science and Plant Nutrition, July 1969

Indexi frontpages.doc
Index to Plates and Figuresv

General Introduction1 pages1-11.doc
Part I: Crystal nucleation in Fe(III) solutions and hydroxide gels.5
1.Fe(III) polymers and hydroxide gels.5
2.Experimental methods8
2.1.Preparation of goethites8
2.2.Electron microscopy10
2.3.X-ray diffractometry11
3.Crystal morphology12 pages12-18.doc
3.1.Morphological types12
3.2.Other morphological features14
3.3.Crystal surfaces17
4.The factors determining crystal morphology19 pages19-32.doc
4.1.The effect of OH/Fe mole ratio19
4.2.The effect of ageing time20
4.3.The effect of pH21
4.4.Other observations23
5.Mechanisms of crystal nucleation26
5.1.Acicular crystal nucleation26
5.2.Twinned crystal nucleation27
5.3.Growth of crystal nuclei31
6.Structure of Fe(III) hydroxide gel35 pages33-45.doc
7.Crystal nucleation in Fe(III) chloride solutions38
8.Growth of goethite in solutions and gels at low pH40
9.Inhibition of crystal growth45
10.Colours of goethite crystals47 pages46-48-fig1-pages49-50.doc
11.Goethite preparations for surface chemistry experiments.49

Part II: Surface chemistry of goethite.51 pages51-62.doc
1.Structure of the goethite crystal surface51
2.Coordination chemistry57
2.1.Fe(III) aquo complexes and phosphato complexes58
2.2.Solubility of iron oxides and Fe(III) phosphate solids61
2.3.Surface charges on Fe(III) solids64 pages63-74.doc
3.Surface charges on goethite: Experimental methods69
3.1.Preparations of goethite69
3.2.Potentiometric titrations70
3.3.Errors in potentiometric titrations72
3.4.Contamination problems73
4.Surface charges on goethite - Results78 pages75-83-tab1.doc
4.1.Surface charge parameters78
4.2.Surface charge parameters and crystal morphology80
4.3.Surface charge parameters and contamination effects82
4.4.Mechanism of charge development reactions84 pages84-94.doc
4.5.Physical significance of surface charge parameters92
5.Phosphate adsorption by goethite: Experimental methods94
6.Fe(III) phosphato complexes on goethite crystal surfaces97 pages95-105.doc
6.1.Phosphate adsorption isotherms97
6.2.Adsorption envelopes99
6.3.Rates of complex formation and base hydrolysis reactions104
6.4.Structure of Fe(III) phosphato complexes106 pages106-114.doc
6.5.The complex formation reaction111
6.6.Thermodynamics of complex formation117

Part III: Kinetics of heterogeneous isotopic exchange reactions.124 pages124-134.doc
1.Effects of surface heterogeneity124
2.General equation for rates of isotopic exchange reactions125
3.Over-equilibration during heterogeneous isotopic exchange131
4.Heterogeneous isotopic exchange described by approximations
to McKay type equations
4.1.Approximations when n ≥ 2132
4.2.Approximations to obtain n = 1139 pages135-145.doc
5.Diffusion models for heterogeneous isotopic exchange143
6.Empirical relationships150 pages146-156.doc
7.An Elovich equation for heterogeneous isotopic exchange155
7.1.The model and the derivation155
7.2.Derivation of the Elovich equation when isotope is initial
present in the solid phase
163 pages157-168.doc
7.3.Dependence of the Elovich equation parameters on reaction
mechanism, concentrations of the exchanging species, pH
and temperature
8.Mechanisms of heterogeneous isotopic exchange reactions168
9.Isotopic exchange between dilute orthophophate solutions and Fe(III)
phosphato complexes on α-FeOOH crystal surfaces: Introduction
170 pages169-180.doc
10.Materials and Methods171
10.1.Goethite preparations171
10.2.Preparation of the equilibrium systems172
10.3.Isotopic exchange173
10.4.Maintaining equilibrium conditions176
10.5.Basic calculations177
11.1.The Elovich equation178
11.2.Interpretation of parameters: Rate laws181 pages181-194.doc
11.3.Interpretation of parameters: Reference states186
11.4.Limits of the Elovich equation: log(1-F) plots190
11.5.Models for heterogeneous isotopic exchange192
11.6.Mechanism of the exchange reaction195 pages195-203.doc
11.7.Mechanism: Structure and bonding in Fe(III) phosphato
11.8.Mechanism: Transition states and acid-base catalysis204 pages204-214.doc
11.9.Origin of surface heterogeneity211
11.10.Hydrolysis of Fe(III) phosphato complexes216 pages215-225.doc

Bibliography226 pages226-236.doc

Index to Plates, Tables and Figures
Part I
Plates 1-8 after p. 11 part1-plates1-8.doc
Plates 9-12, Table 1 and Plates 13-32 after p.18 part1-plates9-18-tab1.doc
Table 2 and Plates 33-44 after p. 45part1-tab2-plates33-44.doc
Fig. 1 after p. 48 pages46-48-fig1-pages49-50.doc
Table 3 and Plates 45-50 after p. 50part1-tab3-plates45-50.doc
Part II
Table 1 and Figs. 1-11 after p. 83 pages75-83-tab1.doc
Figures 12-15 after p. 105 part2-figs12-15.doc
Part III
Table 1 and Figures 1-8 after p. 180 part3-tab1-figs1-8.doc
Figures 9-17 and Table 2 after p. 194 part3-figs9-17-tab2.doc
Figure 18 after p. 203 part3-fig18.doc

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