TY  - GEN
AV  - public
TI  - Some kinetic aspects of the mobilization of antimony from natural sources
Y1  - 2011///
ID  - heidok11972
KW  - AuflösungsgeschwindigkeitAntimony 
KW  -  Mobilisation 
KW  -  Minerals 
KW  -  Dissolution Kinetics
A1  - Biver, Marc
UR  - https://archiv.ub.uni-heidelberg.de/volltextserver/11972/
N2  - The aim of the present thesis is to close some gaps in our understanding of the factors that govern the rates of mobilization of antimony from its natural sources, viz. from its most abundant minerals, namely stibnite, the most abundant antimony mineral, and its principal weathering products in the supergene zone, namely senarmontite, valentinite and stibiconite. Antimony may also become enriched in soils and sediments, as a result of anthropogenic or natural contamination, and be mobilized thence; therefore, the specific mobilization of antimony in the form of Sb(V) from sediment was also considered.  The first (introductory) Chapter is meant to give a comprehensive summary of the known geochemistry of antimony along with an outline of the theoretical basis of most of the material presented in the main chapters. In the second Chapter, we study the rate of oxidative dissolution of stibnite in acidic solution and derive a rate law in terms of hydrogen ion concentration and dissolved oxygen, and the activation energy of the process is measured. The effect of Fe3+ and other trivalent cations on the rate, and the formation of elemental sulfur at the dissolving mineral surface are also examined.  In Chapter III, essentially the same study is repeated in basic solution and the observed dissolution rate in a natural system, specifically, the antimony mineralization near Goesdorf (L), is explained on the basis of our findings. Activation energies indicate that the dissolution processes are diffusion controlled in acidic, and surface controlled in basic solution. Another important (new) finding is the promotive effect exerted by metal cations, both in acidic and in basic solution. These are, to our knowledge, the first reported instances of metal-promoted dissolution in the case of a sulfide mineral.  In Chapter IV, we investigate and rationalize the effects of organic ligands which commonly occur in the soil solution on the rate of dissolution of stibnite. Both the type of ligand and the contact time of the solution with the mineral determine whether an enhancement of the Sb mobilization is to be expected. In Chapter V, we turn to the oxide minerals of antimony; we derive rate laws for their proton-promoted dissolution over the environmentally relevant pH range (2 to 11) and measure the activation energies of these processes. These energies are in the range of surface controlled mechanisms, except for valentinite in basic solution, which appears to be transport-controlled, and stibiconite, which dissolves with a negative activation energy in acidic solution. Negative activation energies for mineral dissolution are exceedingly rare though their existence was predicted on theoretical grounds. Stibiconite represents the first reported oxide mineral with a negative dissolution activation energy. The results from this Chapter permit to derive the following sequence of dissolution rates, valid over most of the pH range 2-11: valentinite>senarmontite>stibiconite>stibnite. Geochemical implications of this order of reactivities are discussed.  In Chapter VI, the mobilizing effect of common inorganic anionic aquatic species on antimony bearing sediment, and on pure phases representative of those present in sediments, is examined. The main objective consists in verifying whether carbonate and phosphate, known to mobilize As from sediments and soils, mobilize Sb as well. Phosphate at environmental concentrations has little impact, while that of carbonate, at environmentally relevant concentrations, is very slight, although significant.  The Appendices contain, among others, detailed experimental results that are merely mentioned in the main text, or that are only presented there in graphical form, as well as the development of a UV-photometric method for the determination of elemental sulfur on mineral surfaces. The method is applied to weathered stibnite in Chapters II and III; it is more sensitive, more economic, more environmentally friendly, and easier to carry out than the literature method.
ER  -