Surface precipitate, mineral-water interface

This book deals only with the chemistry of the mineral-water interface, and so at first glance, the book might appear to have a relatively narrow focus. However, the range of chemical and physical processes considered is actually quite broad, and the general and comprehensive nature of the topics makes this volume unique. The technical papers are organized into physical properties of the mineral-water interface adsorption ion exchange surface spectroscopy dissolution, precipitation, and solid solution formation and transformation reactions at the mineral-water interface. The introductory chapter presents an overview of recent research advances in each of these six areas and discusses important features of each technical paper. Several papers address the complex ways in which some processes are interrelated, for example, the effect of adsorption reactions on the catalysis of electron transfer reactions by mineral surfaces. 

The rates and mechanisms of sorption reactions at the mineral/water interface are critical in determining the mobility, speciation, and bioavailability of metals in aqueous and terrestrial environments. This chapter discusses nonequilibrium aspects of metal sorption at the mineral/water interface, with emphasis on confirmation of slow sorption mechanisms using molecular approaches. It is shown that there is often a continuum between sorption processes, viz, diffusion, sites of varying energy states, and nucleation of secondary phases. For example, recent molecular level in-situ studies have shown that metal adsorption and surface precipitation can occur simultaneously. 

Ni Sorption on Clay Minerals A Case Study. Initial research with Co/clay mineral systems demonstrated the formation of nucleation products using XAFS spectroscopy, but the stmcture was not strictly identified and was referred to as a Co hydroxide-like stmcture (11,12). Thus, the exact mechanism for surface precipitate formation remained unknown. Recent research in our laboratory and elsewhere suggests that during sorption of Ni and Co metal ions, dissolution of the clay mineral or aluminum oxide surface can lead to precipitation of mixed Ni/Al and Co/Al hydroxide phases at the mineral/water interface (14,16,17,67,71). This process could act as a significant sink for metals in soils. The following discussion focuses on some of the recent research of our group on the formation kinetics of mixed cation hydroxide phases, using a combination of macroscopic and molecular approaches (14-17). 

In surface precipitation cations (or anions) which adsorb to the surface of a mineral may form at high surface coverage a precipitate of the cation (anion) with the constituent ions of the mineral. Fig. 6.9 shows schematically the surface precipitation of a cation M2+ to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion, i.e., as the cation is complexed at the surface, a new hydroxide surface is formed. In the model cations at the solid (oxide) water interface are treated as surface species, while those not in contact with the solution phase are treated as solid species forming a solid solution (see Appendix 6.2). The formation of a solid solution implies isomorphic substitution. At low sorbate cation concentrations, surface complexation is the dominant mechanism. As the sorbate concentration increases, the surface complex concentration and the mole fraction of the surface precipitate both increase until the surface sites become saturated. Surface precipitation then becomes the dominant "sorption" (= metal ion incorporation) mechanism. As bulk solution precipitation is approached, the mol fraction of the surface precipitate becomes large. 

Charlet, L. Manceau, A.A. (1992a) X-ray absorption spectioscopic study of the sorption of Cr(III) at the oxide/water interface. II. Adsorption, coprecpitation, and surface precipitation on hydrous ferric oxide. J. Colloid Interface Sd. 148 443-458 Charlet, L. Manceau, A.A. (1992) X-ray absorption spectroscopic study of the sorption of Cr(III) at the oxide-water interface. J. Colloid Interface Sd. 148 425-442 Chatellier, X. Fortin, D. West, M.M. Leppard, G.G. Ferris, F.G. (2001) Effect of the presence of bacterial surfaces during the synthesis of Fe oxides by oxidation of ferrous ions. Fur. J. Mineral. 13 705-714 Cheetham, A.K. Fender, B.E.F. Taylor, R.I. (1971) High temperature neutron diffraction study of Fei. O. J. Phys. C4 2160-2165 Chemical Week (1988) Glidderfs anti rust secret is out." 15 10... 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

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). 

Under environmental conditions, the Fe -H20 interface has a surface layer of corrosion products that develops due to the thermodynamic instability of Fe in the presence of water. Long-term batch and colunm studies have shown that this layer evolves with time into a complex mixture of amorphous iron oxides, iron oxide salts, and other mineral precipitates (2-S). Because this material lies at the metal-water interface, it must, in some manner, mediate the reduction of contaminants by the underlying metal. Understanding the mechanism by which metals reduce contaminants in the presence of a substantial layer of oxides is one of the critical, remaining challenges for researchers in this field. The goal of the following analysis is... 

But within the pH range of natural waters, the dissolution (and precipitation) of carbonate minerals is surface controlled i.e., the rate of dissolution is rate determined by a chemical reaction at the water-mineral interface. Fig. 8.1 gives the data on the dissolution rates of various carbonate minerals in aqueous solutions obtained in careful studies by Chou and Wollast (1989). 

For equilibrium to be reached, all elementary processes must have equal forward and backward rates. This differs from steady-state conditions, where only certain reactions and processes have balanced rates. Thus, at equilibrium, dissolution and precipitation rates of minerals should become necessarily equal. Note also that under equilibrium conditions the net flux of dissolved components at the water-mineral interface is equal to zero and the eventual limitation of the rate by the transport of reactants and products disappear. Close to equilibrium, the reaction kinetics always become entirely controlled by the surface reactions. 

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