Surface precipitation, of metals

If a surface precipitate of metal hydroxy-polymer has formed on an adsorbent, the -pH relationship for the coated adsorbent should resemble closely that observed for particles consisting purely of the hydroxy-polymer or the hydrous oxide of the metal (15). This kind of evidence for Co(ll), La(lII), and Th(lV) precipitation on silica colloids was cited by James and Healy (15). It should be noted, however, that the increase in C toward a maximum value often occurs at pH values well below that required thermodynamically to induce bulk-solution homogeneous precipitation of a metal hydrous oxide (15, 16). If surface precipitation is in the incipient stage under these conditions, it must be a nucleation phenomenon. James and Healy (15) argue that the microscopic electric field at the surface of a charged adsorbent is sufficiently strong to lower the vicinal water activity and induce precipitation at pH values below that required for bulk-solution precipitation of a metal hydrous oxide. 

Several other explanations have been put advanced to explain retention hysteresis, including (1) surface precipitation of metallic cations whose hydroxides, phosphates, or carbonates are sparingly soluble (2) chemical reactions with solid surfaces, including organic surfaces, which form complexes with metallic cations and (3) incorporation into the subsurface organic matter through chemical reactions and biochemical transformation. For the case described by Fig. 5.9 or explanations (1) and (2), the contaminant release always exhibits a hysteresis... 

M.M. Benjamin, Adsorption and surface precipitation of metals on amorphous iron oxy-hydroxide. Environmental Science Technology 17 (1983) 686-692. 

Actually, it is recognized that two different mechanisms may be involved in the above process. One is related to the reaction of a first deposited metal layer with chalcogen molecules diffusing through the double layer at the interface. The other is related to the precipitation of metal ions on the electrode during the reduction of sulfur. In the first case, after a monolayer of the compound has been plated, the deposition proceeds further according to the second mechanism. However, several factors affect the mechanism of the process, hence the corresponding composition and quality of the produced films. These factors are associated mainly to the com-plexation effect of the metal ions by the solvent, probable adsorption of electrolyte anions on the electrode surface, and solvent electrolysis. 

The corrosion of metal surfaces and the precipitation of a metal sulfide by an aqueous acid solution can be prevented by an aldol-amine adduct. Aldol (from acetaldehyde) CH3CH(OH)CH2CHO has been utilized as a H2S scavenger that prevents the precipitation of metal sulfides from aqueous acid solutions. However, when the aldol or an aqueous solution of the aldol is stored, the solution separates quickly into two layers, with all of the aldol concentrated in the bottom layer. The bottom layer is not redispersible in the top layer or in water or acid. In addition, the aldol in the bottom layer has very little activity as a sulfide scavenger. Thus the use of aldol as a H2S scavenger in aqueous acid solutions can result in unsatisfactory results [245,247]. However, the aldol can be reacted with an amine, such as monoethanoleamine (=aminoethanol), to form an aldol-amine adduct to overcome these difficulties. The amine utilized to prepare the aldol-amine adduct must be a primary amine. The aldol-amine adduct preferentially reacts with sulfide ions when they are dissolved in the... 

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. 

In the case of MnO/ ( it is known to reduce on the surface of CNTs and graphene spontaneously to produce Mn02 NPs [182,183]. Solvothermal assisted precipitation of metal oxides can occur in milder solutions [184]. For example, mixed metal oxide NPs of CoFe204 have been deposited on GO from metallic salt precursors via the addition of ethanolamine followed by incubation at 180 °C in a sealed vessel [185]. Mixing GO with Cd2+ in DMSO followed by solvothermal treatment has been shown to both reduce GO to RGO and coat with CdS QDs [186]. 

A.J. (ed.) Aqueous environmental chemistry of metals. Ann Arbor Sci., 193-217 Giammar, D.E. Hering, J.G. (2001) Time scales for sorption-desorption and surface precipitation of uranyl on goethite. Environ. Sci.Techn. 35 3332-3337 Gilkes, R.J. Suddhiprakarn, A. (1979) Biotite... 

Metal reclamation from acid mine drainage and contaminated surface- and groundwater and wastewaters has been extensively studied. Technologies for metal removal from solution are based on the microbial—metal interactions discussed earlier the binding of metal ions to microbial cell surfaces the intracellular uptake of metals the volatilization of metals and the precipitation of metals via complexation with microbially produced ligands. 

Exceeding the limiting current density in practical applications of electrodialysis can affect the efficiency of the process severely by increasing the electrical resistance of the solution and causing water dissociation, which leads to changes of the pH values ofthe solution causing precipitation of metal hydroxide on the membrane surface. 

Beside the reaction of the simple ions with surface groups of the metal oxide, the adsorption process may go through the adsorption of hydrolyzed forms or aqua complexes from the solution, and finally by hydrolysis of such formed surface compounds. When the cation adsorption takes place from the solution nearly saturated by the oxide or hydroxide, then, the process may be accompanied with surface precipitation of the appropriate compound. James and Healy proved, that at edl, the precipitation of the metal oxide or hydroxide may happen at the concentrations lower than the saturation of the bulk, as a result of the lowering of the dielectric constant [138]. 

Oxidative Precipitation. This is a process that describes precipitation of metals, such as Fe2+ or Mn2+, through oxidation. Oxidative precipitation is complex, involving various mechanisms. In general, however, it can be viewed as a two-step process and is demonstrated on Mn2+ below using unbalanced equations. The first step involves a slow reaction that generates a solid surface ... 

The mechanism of hydride formation of ZrMn and TiMn, is related to segregation and preferential oxidation of the Mn on the surface and precipitation of metallic Zr and Ti at the subsurface below the oxidized Mn. The Zr and Ti metallic precipitates catalyze the Hj 2 H reaction. In addition, the Zr and Ti precipitates themselves may form hydrides and spill over atomic H to the intermetallic compound lying below. 

Thermodynamic aspects of the stability of electroless plating solutions have been discussed by Vashkyalis [10]. Electroless plating solutions are thermodynamically unstable and subject to spontaneous decomposition, resulting in precipitation of metal throughout the solution. For the plating solution to be practically useful, the actual occurrence of spontaneous decomposition must be prevented. Thermodynamic conditions which must be met to prevent decomposition can be discussed on the basis of the Gibbs-Thomson equation, which relates the chemical potential of a substance to the curvature of its surface. It follows that the equilibrium potential of a metal particle... 

The adsorption of aqueous Pb(II) has been studied extensively. The following important factors have been studied solution pH [233,234,190,235-239], type of adsorbent [166,171,233,234,190,236,1981 and chemical surface modification [210,223,240], As in the case of many metallic cations, Pb- uptake increases with increasing aqueous solution pH, with a sharp increa.se ( adsorption edge") being observed in a narrow pH range, typically between 3 and 6 [ 171 ], depending on the pHpzc of the carbon used. Adsorption of Pb(II) as a function of solution pH for different initial concentrations is illustrated in Fig. 11. As the pH increases further, there is surface precipitation of the products of hydrolysis of Pb (see Table Al in the Appendix). 

The precipitation of metals onto a silicon surface is essentially a eorrosion process, in which the ionic metal species in the solution are redueed and deposited on the surface simultaneously with oxidation of the silieon atoms. The rate of metal depo-... 

However, the nature, crystallinity (Kinniburg and Jackson, 1976, 1981 McKenzie, 1980), crystal size, and surface charge of metal oxides and mixed metal oxides (e.g., Fe-Al oxides Violante et al., 2003) also play an important role in the sorption selectivity of trace elements in cationic form. McBride (1982) compared the sorption behavior of different Al precipitation products of different crystallinity. The Cu sorption capacity followed tlie order noncrystalline Al-hydroxide > poorly crystalline boehmite > gibbsite. Iron and Mn oxides are... 

A reduction in system pH enhances the solubility of PR, making the precipitation of pyromorphite minerals possible. However, the sorption of Pb decreases sharply as the system pH decreased, producing a sigmoidal function, usually referred to as an adsorption edge, which reflects the affinity of a metal species for a mineral surface (Sposito, 1984). The ability of Pb to form inner-sphere surface complexes is related to the ability of a species in solution to form hydroxides. In fact, it has been shown that surface affinity of metal cations for Fe-oxide and Fe-hydroxide surfaces agrees with their hydrolysis values (Hayes and Katz, 1996). An analogy between solution complexation and surface complexation is represented in the following reactions (Hayes and Katz, 1996) ... 

A boundary layer with a gradient in caustic soda concentration also forms at the surface of the membrane facing the catholyte based on a similar principle, resulting in a caustic soda concentration on the membrane surface which is higher than that in the bulk phase. Since this tends to reduce the current efficiency and electric conductivity of the membrane, it is necessary to minimize the boundary layer thickness or reduce the caustic soda concentration in the bulk phase. It is also essential to purify the brine with ion-exchange resin of high selectivity, in order to prevent precipitation of metal ions as hydroxides in the membrane and the boundary layer (74). 

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