Mineral-solution interface

Nevertheless the potential to be monitored in flotation system would be the mineral potential, not the solution potential. The electrode constracted from the mineral being concentrated should be the most appropriate electrode for f h measurements because the relevant is established at the mineral/solution interface (Woods, 1991). 

Sun Shuiyu, Wang Dianzuo, Long Xiangyun, 1994b. Frontier molecular orbital theory consideration for electron transfer process across sulphide mineral-solution interface. J. BGRIMM, 3(1) 34 - 39 (in Chinese)... 

Techniques for the measurement of surface properties of minerals in gas or vacuum are summarized in Table III. As stated in the previous section, many of these techniques are used indirectly to study processes occurring at mineral/solution interfaces. Methods to study solid/solution interfaces are listed... 

Ions in solution adsorb at mineral/solution interfaces to balance the surface charge and maintain electroneutrality. These ions are known as the counter ions. In contrast to the situation... 

Often, H-bonds are treated implicitly by electrostatic interactions however, for simulations of solutions, clay minerals, and mineral-solution interfaces, explicit consideration of H-bonding should improve results. 

Ba (Dove and Nix, 1997). Rate enhancements in single salt solutions can be predicted by equations that follow the form of Langmuir-type isotherms, whereas in mixed salt solutions, behavior follows a competitive cation-surface interaction model (Dove, 1999). Dove and co-workers argue that the alkali and alkaline earth cations enhance dissolution by modifying characteristics of the solvent at the mineral-solution interface. 

Leckie J. O. (1994) Ternary complex formation at mineral/ solution interfaces. In Binding Models Concerning Natural Organic Substances in Performance Assessment Proceedings of an NEA Workshop. Nuclear Energy Agency, Bad Surzach, Switzerland, pp. 181—211. 

Prasad A., Redden G., and Leckie J. O. (1997) Radionuclide Interactions at Mineral/Solution Interfaces in the Wipp Site Subsurface Environment. Sandia National Laboratories. 

Various chemical surface complexation models have been developed to describe potentiometric titration and metal adsorption data at the oxide—mineral solution interface. Surface complexation models provide molecular descriptions of metal adsorption using an equilibrium approach that defines surface species, chemical reactions, mass balances, and charge balances. Thermodynamic properties such as solid-phase activity coefficients and equilibrium constants are calculated mathematically. The major advancement of the chemical surface complexation models is consideration of charge on both the adsorbate metal ion and the adsorbent surface. In addition, these models can provide insight into the stoichiometry and reactivity of adsorbed species. Application of these models to reference oxide minerals has been extensive, but their use in describing ion adsorption by clay minerals, organic materials, and soils has been more limited. 

Shchiikarcv, A. and Sjoberg, S., XPS with fast-frozen samples A renewed approach to study the real mineral/solution interface. Surf. Sci., 584, 106, 2005. 

There is clear evidence that the dissolution of oxide minerals is promoted by the specific sorption of solutes at the mineral-solution interface. Moreover, it has been found that comparatively simple rate laws are obtained if the observed rates are plotted against the concentrations of adsorbed species and surface complexes (Pulfer et al., 1984 Furrer and Stumm, 1986). For example, in the presence of ligands (anions and weak acids) surface chelates are formed that are strong enough to weaken metal-oxygen bonds and thus to promote rates of dissolution proportional to their surface concentrations. Simple rate laws have been also observed with H+—or OH —promoted dissolution of oxides in a manner that can be predicted from knowledge of the oxide composition and the surface concentrations of protons and hydroxyl radicals. 

In the case of long chain surfactants used as collectors, a signiflcant correlation between their adsorption in the form of aggregates and flotation of minerals with them was established in the 1960 s by Somasundaran et al. (1964). Somasundaran et al. (1976, 1985) developed the relevant dissociation and aggregation equilibria for flotation reagents in the solution as well as at the mineral-solution interface. Importantly, the ion-molecule complexes phenomenon was proposed later by Somasundaran (1976), Hanna and Somasundaran (1976) to account for the flotation maximum exhibited at certain pH values by hydrolyzable surfactants. 

Reagents used in flotation, collectors, frothers, depressants, flocculants and inorganic modifiers can interact with each other in the flotation pulp and at the mineral-solution interface. The chemical equilibria involved in these interactions and the nature of the products will have a significant effect on their adsorption and the resultant flotation processes. 

Understanding of the structure of the adsorbed surfactant and polymer layers at a molecular level is helpful for improving various interfacial processes by manipulating the adsorbed layers for optimum configurational characteristics. Until recently, methods of surface characterization were limited to the measurement of macroscopic properties like adsorption density, zeta-potential and wettability. Such studies, while being helpful to provide an insight into the mechanisms, could not yield any direct information on the nanoscopic characteristics of the adsorbed species. Recently, a number of spectroscopic techniques such as fluorescence, electron spin resonance, infrared and Raman have been successfully applied to probe the microstructure of the adsorbed layers of surfactants and polymers at mineral-solution interfaces. 

Fig. 4.38. Molecular model of interaction of oleate species at mineral-solution interface.

Because several recent papers have reviewed the applications of XAFS spectroscopy to sorption complexes at mineral/solution interfaces (e g., Brown et al. 1999c Brown and Parks 2001), here we list many of the sorption systems that have been studied over the past 15 years using XAFS spectroscopy methods (Appendix — Tables 1 and 2) without detailed discussion of results. The interested reader is directed to the individual papers listed in Tables 1 and 2 (see Appendix) for experimental details and results and to Brown and Parks (2001) for a detailed discussion of many sorption systems of relevance to low temperature geochemistry and environmental science. 

Table 1. Summary of XAFS spectroscopy studies of metal(loid) ion sorption complexes at mineral/solution interfaces in model systems in the absence of complexing ligands1 (arranged in increasing order of atomic number of the sorbate ion). Individual references should be consulted for pH, surface coverages ( ), metal concentration, ionic strength values, background electrolyte type, and other experimental variables. The dominant sorbate geometry or phase listed below should not be generalized beyond the range of variables considered in each study.

The mixed-potential mechanism has two important implications. First, the potential across the mineral/solution interface will be an important parameter in determining flotation recovery. Second, the reaction imparting floatability, the anodic process involving the collector, is amenable to investigation using electrochemical techniques. 

Under experimental conditions, most rock-forming silicate minerals are sufficiently nonreactive that transport of solutes toward, or away from, the dissolving surface does not control the rate. The rate of the overall reaction is controlled by reactions at the interfaces, that is, the second sequential step. In soil, solute transport is sufficiently rapid that silicate weathering is controlled by the reaction kinetics at the mineral-solution interface (Berner 1978). 

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