Interactions surface precipitation

Relaxation studies have shown that the attachment of an ion to a surface is very fast, but the establishment of equilibrium in wel1-dispersed suspensions of colloidal particles is much slower. Adsorption of cations by hydrous oxides may approach equilibrium within a matter of minutes in some systems (39-40). However, cation and anion sorption processes often exhibit a rapid initial stage of adsorption that is followed by a much slower rate of uptake (24,41-43). Several studies of short-term isotopic exchange of phosphate ions between aqueous solutions and oxide surfaces have demonstrated that the kinetics of phosphate desorption are very slow (43-45). Numerous hypotheses have been suggested for this slow attainment of equilibrium including 1) the formation of binuclear complexes on the surface (44) 2) dynamic particle-particle interactions in which an adsorbing ion enhances contact adhesion between particles (43,45-46) 3) diffusion of ions into adsorbents (47) and 4) surface precipitation (48-50). 

Electrostatic vs. Chemical Interactions in Surface Phenomena. There are three phenomena to which these surface equilibrium models are applied regularly (i) adsorption reactions, (ii) electrokinetic phenomena (e.g., colloid stability, electrophoretic mobility), and (iii) chemical reactions at surfaces (precipitation, dissolution, heterogeneous catalysis). 

Filer JM, Mojzsis SJ, Arrhenius G (1997) Carbon isotope evidence for early life discussion. Nature 386 665 Emerson D (2000) Microbial oxidation of Ee(II) and Mn(II) at circumneutral pH. In Environmental metal-microbe interactions. Lovley DR (ed) ASM Press, Washington DC, p 31-52 Ewers WE (1983) Chemical factors in the deposition and diagenesis of banded iron-formation. In Iron formations facts and problems. Trendall AF, Morris RC (eds) Elsevier, Amsterdam, p 491-512 Farley KJ, Dzombak DA, Morel FMM (1985) A surface precipitation model for the sorption of cations on metal oxides. J Colloid Interface Sci 106 226-242... 

Interaction of phosphate solutions with goethite may lead to surface precipitation of phosphates if the concentration of P in solution exceeds the mineral solubility (Jo-nasson et al., 1988). A combined Auger, XPS, scanning SIMS and electron diffraction study showed that after 90 days at 60 °C, crystals of griphite (an Fe hydroxy phosphate) precipitated out of a phosphate solution onto crystals of goethite (Martin et al., 1988). 

The removal of Ra by adsorption has been attributed to ion exchange reactions, electrostatic interactions with potential-determining ions at mineral surfaces, and surface- precipitation with BaSO 4. The adsorptive behavior of Ra2+ is similar to that of other divalent cationic metals in that it decreases with an increase in pH and is subject to competitive interactions with other ions in solution for adsorption sites. In the latter case, Ra is more mobile in groundwater that has a high total dissolved solids (TDS) content. It also appears that the adsorption of Ra + by soils and rocks may not be a completely reversible reaction (Benes et al. 1984, 1985 Landa and Reid 1982). 

The processes of adsorption, precipitation and coprecipitation are difficult to distinguish on that basis from the analysis of the diminution of the ions from the solution, changes of pH and kinetics. Only the spectroscopic investigations of the molecular interactions between adsorbent and adsorbate may help to distinguish a type of the process [146,147]. As an adsorption of the ions, is assumed process of the two-dimensional structure formation, whereas for three-dimensional structures precipitation or surface precipitation takes place. From this reason an AFM method may be useful at investigations of the morphology changes of the adsorbate surface [147]. 

If an electrostatic interaction alone would determine the interaction of precipitating species with the surface of a suspended support, the interaction with precipitating hydrated iron(III) oxide would be small, whereas the interaction with precipitating copper(II) hydroxide would be considerable. Indeed it was observed that hydrated iron(III) oxide, which reacts to form clusters of very small moieties at pH levels of 2, docs not interact significantly with suspended silica. However, the precipitation of copper(II), which also proceeds at a pH level of about 3 with most anions, is not markedly affected by the presence of suspended... 

Surfactant adsorption on saltlike minerals, such as calcite and dolomite, is a more complex process and is less understood than adsorption on oxide surfaces. These minerals are relatively soluble and when in contact with an aqueous medium develop an interfacial region of complex composition (41—43). In addition to the two mentioned mechanisms of adsorption, covalent bonding, salt formation between surfactant and lattice ions at the solid surface, ion exchange of surfactant with lattice ions, and surface precipitation have been suggested as adsorption mechanisms (36, 43—47). The dissolution products of sparingly soluble minerals may interact with the surfactant, precipitate or adsorb at the solid surface, or lead to mineral transformations that affect surface composition and electrochemical properties (46, 48—52). All these factors can be expected to influence surfactant adsorption. 

Ford FG, Kemner KM, Bertsch PM (1999a) Influence of sorbate-sorbent interactions on the crystallization kinetics of nickel- and lead-ferrihydrite coprecipitates. Geochim Cosmochim Acta 63 39-48 Ford FG, Scheinost AC, Scheckel KG, Sparks DL (1999b) The link between clay mineral weathering and the stabilization of nickel surface precipitates. Environ Sci Technol 33 3140-3144 Ford RG, Sparks DL (2000) The nature of Zn precipitates formed in the presence of pyrophyllite. Environ Sci Technol 34 2479-2483... 

As suggested by the above paragraph, other types of phenomena affect the previously described transport mechanisms of the contaminants toward the electrodes. These are the physical-chemical interactions, both between different compounds in the aqueous phase and between these aqueous species and the sohd phases of the soil system. Some of these interactions are precipitation, acid-base, complex formation and redox reactions, adsorption, and ion exchange and surface complexation reactions. 

In the literature there are many attempts to describe the interactions between ions in solution and a mineral surface in contact with them. The resulting interactions can be grouped into various phenomena, such as physisorption, chemisorption, co-precipitation, inclusion, diffusion, surface-precipitation, or even formation of solid solutions. 

Chain architecture also plays a role in determining the adsorption characteristics of copolymers. For instance, if we consider triblock ABA-type copolymers the relative positions of the anchor and buoy blocks become important. When there are two buoy blocks and a central anchor block, the copolymers show diblock AB-type behavior (see Fig. 9 and 10). If, however, there are two anchor blocks and a central buoy block, surface precipitation of the polymer molecule at the particle surface is generally observed. This precipitation (or multilayer formation) process is due to strong interaction between the anchor blocks themselves and manifests itself in the form of an ever-increasing adsorption isotherm (i.e., there is no plateau) of the type shown in Figure 11. When compared with... 

FIGURE 12.1 Transmission electron microscopy image of kaolinite particle showing surface precipitation of Al(OH)3. Mottled appearance resnlts from precipitation of the A1(0H)3 phase on basal surface. (Reprinted from Geochimica et Cosmochimica Acta, 63, Thompson et al., Dynamic interactions of dissolution, surface adsorption, and precipitation in an aging cobalt(II)-clay-water system, 1767-1779. Copyright 1999, with permission from Elsevier.)... 

---