Interface mineral/particle, properties

Many important processes in the environment occur at boundaries. Here we use the term boundary in a fairly general manner for surfaces at which properties of a system change extensively or, as in the case of interfaces, even discontinuously. Interface boundaries are characterized by a discontinuity of certain parameters such as density and chemical composition. Examples of interface boundaries are the air-water interface of surface waters (ocean, lakes, rivers), the sediment-water interface in lakes and oceans, the surface of an oil droplet, the surface of an algal cell or a mineral particle suspended in water. 

The above processes involve separation based either on bulk properties (for example, size, density, shape, etc.) directly or by subtle control of the chemistry of the narrow interfacial region between the mineral particle and the aqueous solution in which it is suspended. In the processing of certain ores, such as those of uranium, gold or oxidized copper, chemical alteration of the minerals may be required to recover the valuable metals. These techniques are not discussed here, except to include those aspects which are directly related to surfaces and interfaces. 

Table IV Techniques for Measurement of Properties of Mineral/ Particle Solution Interfaces...

Recent advances in the development of non-invasive, in situ spectroscopic scanned-probe and microscopy techniques have been applied successfully to study mineral particles in aqueous suspension (Hawthorne, 1988 Hochella and White, 1990). In situ spectroscopic methods often utilise molecular probes that have diagnostic properties sensitive to changes in short-range molecular environments. At the particle-solution interface, the molecular environment around a probe species is perturbed, and the diagnostic properties of the probe, which can be either optical or magnetic, then report back on surface molecular structure. Examples of in situ probe approaches that have been used fruitfully include electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spin-probe studies perturbed vibrational probe (Raman and Fourier-transform IR) studies and X-ray absorption (Hawthorne, 1988 Hochella and White, 1990 Charletand Manceau, 1993 Johnston et al., 1993). 

Reactions across the interface between colloid crystals and the soil liquid phase may also suppress the availability of nutrient elements to plants. The effectiveness of these interfacial reactions in supporting optimum plant growth ultimately depends on the arrangements of ions in the surfaces and subsurfaces of the mineral crystals. For this reason much of this volume is devoted to the arrangement of ions in crystalline mineral particles commonly occuring in soils and the properties that these particles contribute to soil systems. 

The wetting properties of the particles play a crucial role in flotation. We have already discussed the equilibrium position of a particle in the water-air interface (Section 7.2.2). The higher the contact angle the more stably a particle is attached to the bubble (Eq. 7.19) and the more likely it will be incorporated into the froth. Some minerals naturally have a hydrophobic surface and thus a high flotation efficiency. For other minerals surfactants are used to improve the separation. These are called collectors, which adsorb selectively on the mineral and render its surface hydrophobic. Activators support the collectors. Depressants reduce the collector s effect. Frothing agents increase the stability of the foam. 

A prototypical example of a molecular probe used extensively to study the mineral adsorbent-solution interface is the ESR spin-probe, Cu2+ (Sposito, 1993), whose spectroscopic properties are sensitive to changes in coordination environment. Since water does not interfere significantly with Cu11 ESR spectra, they may be recorded in situ for colloidal suspensions. Detailed, molecular-level information about coordination and orientation of both inner- and outer-sphere Cu2+ surface complexes has resulted from ESR studies of both phyllosilicates and metal oxyhydroxides. In addition, ESR techniques have been combined with closely related spectroscopic methods, like electron-spin-echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR), to provide complementary information about transition metal ion behaviour at mineral surfaces (Sposito, 1993). The level of sophistication and sensitivity of these kinds of surface speciation studies is increasing continually, such that the heterogeneous colloidal particles in soils can be investigated ever more accurately. 

Many of the important chemical reactions controlling arsenic partitioning between solid and liquid phases in aquifers occur at particle-water interfaces. Several spectroscopic methods exist to monitor the electronic, vibrational, and other properties of atoms or molecules localized in the interfacial region. These methods provide information on valence, local coordination, protonation, and other properties that is difficult to obtain by other means. This chapter synthesizes recent infrared, x-ray photoelectron, and x-ray absorption spectroscopic studies of arsenic speciation in natural and synthetic solid phases. The local coordination of arsenic in sulfide minerals, in arsenate and arsenite precipitates, in secondary sulfates and carbonates, adsorbed on iron, manganese, and aluminium hydrous oxides, and adsorbed on aluminosilicate clay minerals is summarized. The chapter concludes with a discussion of the implications of these studies (conducted primarily in model systems) for arsenic speciation in aquifer sediments. 

Physicochemical models of partitioning at the solid-water interface, such as that used here to model ion exchange, require detailed knowledge about the particles. The surface properties of the mineral phases present, as well as equilibrium constants for ion binding to both fixed and variable charge sites associated with each phase, are required. These data requirements and the uncertainty about modeling sorption in mixtures of minerals (e.g., 48-50) make such models difficult to apply to complex natural systems. This is especially the case for modeling solute transport in soil-water systems, which... 

Planar polymer films were recently mineralized with calcium phosphate [267], Using the Langmuir monolayer technique, it was possible to control the particle growth by the polymer film properties at the air-water interface and the subphase parameters (pH, ion strength). Small changes it the growth conditions resulted in various particle shapes and dimensions. Such examples of controlled biomimetic mineralization are indeed very motivating for further studies of crystallization processes in synthetic membranes. 

However, as we noted early in this chapter, numerous assumptions are employed in the field applications of surface complexation models. Davis et al. (1998) noted that surface complexation models are mainly developed from well-controlled laboratory experiments. It is unclear how the models can be applied to soil and sediments where the double layers of the heterogenous particles may interact and the competitive adsorption of many different ions can cause significant changes in the electrical properties of mineral-water interfaces. 

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