Surfaces solid-water interface

Geckeis H, Klenze R, Kim J1 (1999) Solid-water interface reactions of actinides and homologues sorption onto mineral surfaces. Radiochim Acta 87 13-21... 

Adsorption of (bio)polymers occurs ubiquitously, and among the biopolymers, proteins are most surface active. Wherever and whenever a protein-containing (aqueous) solution is exposed to a (solid) surface, it results in the spontaneous accumulation of protein molecules at the solid-water interface, thereby altering the characteristics of the sorbent surface and, in most cases, of the protein molecules as well (Malmsten 2003). Therefore, the interaction between proteins and interfaces attracts attention from a wide variety of disciplines, ranging from environmental sciences to food processing and medical sciences. 

Adsorption, the accumulation of matter at the solid-water interface, is the basis of most surface-chemical processes. 

At the water-air interface hydrophilic groups are oriented toward the water, hydro-phobic groups are oriented toward air. At solid-water interfaces, the orientation depends on the relative affinities for water and for the solid surface. The hydrophilic groups of amphipathic molecules may - if the hydrophobic tendency is relatively small - interact coordinatively with the functional groups of the solid surface (Ulrich et al., 1988) (see Fig. 4.10). 

The adsorption of humic and fulvic acids on surfaces can be interpreted along the scheme of Fig. 4.9c,d. Because of hydrophobic interaction humic and fulvic acids tend to accumulate at the solid-water interface. At the same time the adsorption is influenced by coordinative interaction, e.g., schematically,... 

Kinetically, the adsorption of humic acids at a solid-water interface is controlled by convection or diffusion to the surface. Even at concentrations as low as 0.1 mg/e near-adsorption equilibrium is attained within 30 minutes. At high surface densities, a relatively slow rearrangement of the adsorbed molecules may cause a slow attainment of an ultimate equilibrium (Ochs, Cosovic and Stumm, in preparation). The humic acids adsorbed to the particles modify the chemical properties of their surfaces, especially their affinities for metal ions (Grauer, 1989). 

A comparison with the reversible interface can be made. The reversible solid electrolyte interface can be used in a similar way to explore the distribution of charge components at solid-water interfaces. As we have seen, the surface charge density, o, (Eqs. (3.1) and (iii) in Example 2.1) can be readily determined experimentally (e.g., from an alkalimetric titration curve). The Lippmann equations can be used as with the polarized electrodes to obtain the differential capacity from... 

As mentioned in the Appendix of Chapter 4, the contact angle 0 increases (cos 8 decreases) with increasing hydrophobic character of the solid surface (ysv < ySL), i.e., extensive adsorption at the air-solid surface and minimum adsorption at the solid water interface is needed. 

Regulation of Trace Elements by the Solid-Water Interface in Surface Waters... 

The solid-water interface, mostly established by the particles in natural waters and soils, plays a commanding role in regulating the concentrations of most dissolved reactive trace elements in soil and natural water systems and in the coupling of various hydrogeochemical cycles (Fig. 1.1). Usually the concentrations of most trace elements (M or mol kg-1) are much larger in solid or surface phases than in the water phase. Thus, the capacity of particles to bind trace elements (ion exchange, adsorption) must be considered in addition to the effect of solute complex formers in influencing the speciation of the trace metals. 

Avena, M.J. Koopal, L.K. (1999) Kinetics of humic acid adsorption on solid-water interfaces. Environ. Sci. Techn. 33 2739-2744 Avnir, D. Jaroniec, M. (1989) An isotherm equation for adsorption on fractal surfaces of... 

Geckeis, H. Rabung, T. 2002. Solid-water interface reactions of polyvalent metal ions at iron oxide-hydroxide surfaces. In Hubbard, A. (ed) Encyclopedia of Surface and Colloid Science. Dekker Inc., 4737-4748. 

Mineral Surfaces. Organic matter is chemically adsorbed (deriva-tized) at the surfaces of clay minerals, zeolites, and related minerals (105) and is at times protected, concentrated, and degraded by contact with the solid surfaces. For example, porphyrins are protected (106), as are optically active amino acids by montmorillonite (107). This may result in part from the position of the organic matter in lattice spaces, as shown by Stevenson and Cheng (108) for proteinaceous substances keyed into hexagonal holes on interlamellar surfaces of expanding lattice clays, or from the fact that there are ordered structures at solid-water interfaces (109). 

In several previous papers, the possible existence of thermal anomalies was suggested on the basis of such properties as the density of water, specific heat, viscosity, dielectric constant, transverse proton spin relaxation time, index of refraction, infrared absorption, and others. Furthermore, based on other published data, we have suggested the existence of kinks in the properties of many aqueous solutions of both electrolytes and nonelectrolytes. Thus, solubility anomalies have been demonstrated repeatedly as have anomalies in such diverse properties as partial molal volumes of the alkali halides, in specific optical rotation for a number of reducing sugars, and in some kinetic data. Anomalies have also been demonstrated in a surface and interfacial properties of aqueous systems ranging from the surface tension of pure water to interfacial tensions (such as between n-hexane or n-decane and water) and in the surface tension and surface potentials of aqueous solutions. Further, anomalies have been observed in solid-water interface properties, such as the zeta potential and other interfacial parameters. 

Stumm, W., Chemistry of the Solid-Water Interface, Wiley, New York, 1992. A gem of a textbook on surface chemistry applied to natural particles by one of the modern-day masters a must read for all serious students. 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

Surface analytical techniques. A variety of spectroscopic methods have been used to characterize the nature of adsorbed species at the solid-water interface in natural and experimental systems (Brown et al, 1999). Surface spectroscopy techniques such as extended X-ray absorption fine structure spectroscopy (EXAFS) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) have been used to characterize complexes of fission products, thorium, uranium, plutonium, and uranium sorbed onto silicates, goethite, clays, and microbes (Chisholm-Brause et al, 1992, 1994 Dent et al, 1992 Combes et al, 1992 Bargar et al, 2000 Brown and Sturchio, 2002). A recent overview of the theory and applications of synchrotron radiation to the analysis of the surfaces of soils, amorphous materials, rocks, and organic matter in low-temperature geochemistry and environmental science can be found in Fenter et al (2002). 

REGULATION OF TRACE ELEMENTS BY THE SOLID-WATER INTERFACE IN SURFACE WATERS... 

Colloids are ubiquitous in natural waters they are present in relatively large concentrations (> 10 cm ) in fresh surface waters, in groundwaters, in oceans, and in interstitial soil and sediment waters. The solid-water interface established by these particles plays a commanding role in regulating the concentrations of most reactive elements and of many pollutants in soil and natural water systems and in the coupling of various hydrochemical cycles. Wells and Goldberg (1994) estimate that the total surface area of the small colloidal (5-200 nm) fraction alone is > 18 m per m of seawater in the upper water column. Processes with colloids are also of importance in technical systems, above all in water technology. 

Sigg, L. (1992b) Regulation of Trace Elements by the Solid-Water Interface in Surface Waters. In Chemistry of the Solid-Water Interface, W. Stumm, Ed., Wiley-Interscience, New York, chap. 11. 

Clearly, first and foremost, more data of higher quality are needed for the thermochemistry of nanoparticles and their composites. Measurements of surface enthalpies, hydration enthalpies, excess heat capacities, and other thermodynamic parameters on well defined chemical systems are needed. The question of apparenf versus true surface properties raised by Diakonov (1998b) needs to be resolved and consistent nomenclature adopted. Surface (solid/gas), interface (solid/solid) and wef (solid/water) parameters each need to be measured and systematized. 

Mam heterogeneous processes such as dissolution of minerals, formation of he solid phase (precipitation, nucleation, crystal growth, and biomineraliza-r.on. redox processes at the solid-water interface (including light-induced reactions), and reductive and oxidative dissolutions are rate-controlled at the surface (and not by transport) (10). Because surfaces can adsorb oxidants and reductants and modify redox intensity, the solid-solution interface can catalyze rumv redox reactions. Surfaces can accelerate many organic reactions such as ester hvdrolysis (11). 

---