Mineral/particle solution interfaces

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

Several different approaches can be used to model the interaction of solutes with reactive mineral surfaces. The conceptual approaches differ in the degree to which they account for observed or postulated solution and surface reactions. Whatever the approach, the description of interactions at the particle/solution interface must inevitably take into account the effect of pH on solute adsorption. 

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). 

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. 

Clearly, it is important that there be a large contact angle at the solid particle-solution-air interface. Some minerals, such as graphite and sulfur, are naturally hydrophobic, but even with these it has been advantageous to add materials to the system that will adsorb to give a hydrophobic film on the solid surface. (Effects can be complicated—sulfur notability oscillates with the number of preadsoibed monolayers of hydrocarbons such as n-heptane [76].) The use of surface modifiers or collectors is, of course, essential in the case of naturally hydrophilic minerals such as silica. 

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

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. 

Adsorption isotherms are habitually obtained using the solution depletion method, which consists of comparing the solute concentrations before and after the attainment of adsorption equilibrium. Electrokinetic or zeta potentials are determined by two techniques microelectrophoresis [12,14,17] and streaming potential [13,58,59]. The former is employed to measure the mobility of small particles of chemically pure adsorbents, whereas the latter is adopted to investigate the electrophoretic behaviour of less pure coarser mineral particles. A correlation between the adsorption and electrophoretic results is usually examined with the aim of sheding light on the mechanism by means of which the surfactants are adsorbed at the solution-solid interface. This implies the necessity of maintaining the same experimental conditions in both experiments. For this purpose, the same initial operational procedure is applied. 

Another aspect of the impurity doping which will be common in mineral particles is illustrated by the effect of Nb(V) on anatase. This ion substitutes isomorphical-ly for Ti(IV) and as an n dopant creates a Shottky barrier at the interface. This assists injection of an electron from a reducing solute into the conduction band (1J[). The flat band is also shifted cathodically. It has recently been claimed that the relative inefficiency of haematite as a photoanode is a function of low mobility of carriers and that this problem may be... 

As depicted in Figure 4.5, the surfactant bilayer created at the solid/aqueous solution interface provides hydrophobic loci for the solubilization of monomer or radicals these can polymerize further on the modified inorganic surface and effectively coat the sohd particles according to an emulsion-like polymerization reaction. Three steps are involved in the coating reaction. In the first step, the emulsifier adsorbs onto the mineral surface, forming micelle-like aggregates. In a second step, the monomer is solubihzed in the adsorbed micelles. Finally, the polymerization takes place as a conventional emulsion polymerization in the monomer-sweUed admiceUes (Fig. 4.6). 

Following this step there is continued dissolution, which removes whatever hyperfine particles may have resulted during sample preparation. After removing these, further dissolution breaks down the outer surface of the residual layer at the same rate that alkalis are replaced by hydrogen at the interface between fresh mineral surfaces and the residual layer. This releases all constituents to the solution. Release is now stoichiometric, based on solution chemistry and surface morphological results. Thus, the reaction is surface-controlled (Velbel, 1985). 

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... 

Adsorption of organic solutes at the surface of suspended particles, that is, the mineral water interface, can be also characterized by specific coordinalive... 

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