Dissolving metals in solution

FIGURE 22.10 A comparison of a sample (a) containing dissolved metal in solution being analyzed and measured in the conventional way and another sample (b) containing the same metal in the form of nanoparticles being analyzed and measured as a transient pulses of ions. 

Note. For passive metals in solutions free from other oxidising species the presence of dissolved Oj at all pans of the metal s surface is essential to maintain passivity and this can be achieved in certain systems by increasing the velocity of the solution. 

Sorption can significantly diminish the mobility of certain dissolved components in solution, especially those present in minor amounts. Sorption, for example, may retard the spread of radionuclides near a radioactive waste repository or the migration of contaminants away from a polluting landfill (see Chapters 21 and 32). In acid mine drainages, ferric oxide sorbs heavy metals from surface water, helping limit their downstream movement (see Chapter 31). A geochemical model useful in investigating such cases must provide an accurate assessment of the effects of surface reactions. 

Acid drainage is a persistent environmental problem in many mineralized areas. The problem is especially pronounced in areas that host or have hosted mining activity (e.g., Lind and Hem, 1993), but it also occurs naturally in unmined areas. The acid drainage results from weathering of sulfide minerals that oxidize to produce hydrogen ions and contribute dissolved metals to solution (e.g., Blowes et al., 2005). 

In another industrial process, flue dusts from smelting lead and zinc concentrates are boiled in acidified water. Thallium dissolves and is separated from insoluble residues by filtration. Dissolved thallium in solution then is precipitated with zinc. Thallium is extracted from the precipitate by treatment with dilute sulfuric acid which dissolves the metal. The solution may also contain zinc, cadmium, lead, copper, indium, and other impurities in trace amounts. These metals are precipitated with hydrogen sulfide. The pure thallium sulfate solution then is electrolyzed to yield thallium. 

Soluble Zintl ions can also be obtained by electrochemical methods using the respective element as cathode material [34, 60, 61], or through the reaction of the various modifications of the tetrel (Sn and Pb) and pentel (P, As, Sb) elements with dissolved or finely dispersed alkali or alkaline-eatth metals in solution [62] as well as in molten crown-ethers [63]. 

Further evidence has been obtained to support the contention that the active catalysts are metal complexes dissolved in solution. With experiments reported in Table II, the kinetics of oxidation under standard conditions in the presence of various metal salts are compared with the rates of reaction when solid residues have been filtered from solution. The agreement between the rates in Cases 1 and 3 of Table II (where the amount of metal available is dictated by the solubility of metal complexes) shows that solid precipitates play little or no part in catalysis in all the systems studied. The amount of metal in solution has been measured in Cases 2 and 3 metal hydroxide complexes (Case 2) are not as soluble as metal-thiol complexes, and neither is as soluble as metal phthalocyanines (19). The results of experiments involving metal pyrophosphates are particularly interesting, in that it has previously been suggested that cobalt pyrophosphates act as heterogeneous catalysts. The result s in Table II show that this is not true in the present system. 

Electron Transfer from Modified Metals to Dissolved Protein in Solution... 

Generally, pure molten salts are transparent and clear. If a piece of metal is introduced into a pure molten salt, a so-called metal fog is observed streaming from the metal into the melt and coloring it. The color intensity usually increases with increasing concentration of dissolved metal. In most cases the properties of the pure molten salt change when a metal dissolves in it, and a true solution is formed. For example, the solubility is higher if the metals dissolve in their own salts, and several binary systems show a complete miscibility between metal and salt above... 

Copper, being a noble metal, has good resistance to corrosion. A thin adherent film of cuprous oxide and cupric carbonate is formed due to corrosion. Passivation is not a prominent process. The dissolved copper in solution affects the electrode potential such that the increase in velocity of the solution in contact with the metal results in increasing attack of the metal. Thus cuprous oxide is produced under dynamic flow of the solution. The thickness of the oxide film is about 500 nm. 

Figure 12.9. Relationship between total metal in solution in equilibrium with metal-sulfides as a function of pH. The data were generated by MINTEQA2 using an initial concentration of metal 10 2 mol L-1. The data were produced employing an initial equimolar concentration of dissolved metal and dissolved HS- (from Evangelou, 1997, unpublished data, with permission).

Within the ocean, the exchange of material from the dissolved to the suspended particulate state influences the distribution of several elements. This scavenging process removes dissolved metals from solution and accelerates their deposition. The effectiveness of this process is obvious in the depth profiles of metals, especially those of the surface enrichment type. Furthermore, the removal can be expressed in terms of a deepwater scavenging residence time as indicated in Table 10. 

The reduction of various substrates by dissolving metals in alcoholic and aqueous media is a very old procedure in synthetic organic chemistry. In addition to aldehydes, ketones, imines and other unsaturated nitrogen compounds, many other functional groups are reduced under these conditions. Historically, the most common reduction conditions were Na in ethanol, and the reductions were carried out by adding the metal to a solution of the substrate in alcohol and the reaction mixture was heated at reflux for varying periods of time. Other reduction systems included Na-Hg amalgam in water or alcohols and, for easily reduced compounds such as aldehydes and aromatic ketones, Zn-NaOH or Fe-acetic acid have been used. ... 

The dissolving metal reductions are better classified as internal electrolytic reductions in which an electron is transferred from the metal surface (or from the metal in solution) to the substrate. Reduction with low-valent metal ions may also be included in this general class (vide infra). 

A9.7.2.1.2.1 The toxicity of a particular metal in solution, appears to depend primarily on (but is not strictly limited to) the level of dissolved free metal ions. Abiotic factors including alkalinity, ionic strength and pH can influence the toxicity of metals in two ways (i) by influencing the chemical speciation of the metal in water (and hence affecting the availability) and (ii) by influencing the uptake and binding of available metal by biological tissues. 

The etching of Cu metal in solutions of 500 1 DHF is linear with the concentration of dissolved Oj. Etch rates from 2 to 16 nm/min are controllable with O2 partial pressures of 0.05 to 0.65 atm ( 2.5 to 33 ppm O2). The linear behavior indicates that the reaction is limited either by the transport of O2 to the surface or the reaction of O2 and Cu at the surface to form CuO. This DHF 02 chemistry shows great promise for post Cu CMP cleans. 

The freeze-drying method is one of solvent volatilization, but it differs from the common solvent evaporation techniques in that the solvent is sublimed from the solid state. A solution, prepared by means of soluble salts or by dissolving metals in acid, is frozen and the solid solvent, usually water in the form of ice, is sublimed away giving the dried salts. The process gives high surface area powders of excellent chemical homogeneity. 

A long time contact of the dissolved fraction with particulate matter can produce changes in the distribution of chemical forms of heavy metals in solution. Any change in the equilibrium conditions after collection can promote or remove dissolved metals (54) or desorption of adsorbed metals operated by particulate. Biological activity involves photosynthesis and respiration which will change the carbon dioxide content of the water and its pH. All the equilibrium reactions affected by pH will be altered, e.g., the reactions of precipitation, complexation and redox involving heavy metals. 

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