Dissolution in acid solutions

Energies of Activation. Low E values (<42 kJ mol-1) usually indicate diffusion-controlled processes whereas higher E values indicate chemical reaction processes (Sparks, 1985, 1986). For example, E values of 6.7-26.4 kJ mol-1 were found for pesticide sorption on soils and soil components (Haque et al., 1968 Leenheer and Ahlrichs, 1971 Khan, 1973) while gibbsite dissolution in acid solutions was characterized by E values ranging from 59 4.3 to 67 0.6 kJ mol-1 (Bloom and Erich, 1987). 

As mentioned earlier, the dissolution of oxides and hydroxides, like feldspars and ferromagnesian minerals, appears to be a surface-controlled reaction. One indication of this is the high E values found by several investigators. Bloom and Erich (1987) obtained E values ranging from 59 4.3 to 67 0.6 kj mol"1 for gibbsite dissolution in acid solutions (pH 1.5-4.0). These values are much higher than for diffusion-controlled reactions reported earlier. 

The mechanism given for dissolution in acid solution applies also in neutral solutions, but with the modification that 0FeOH (assumed to correspond to the condition 6 < 1 above) changes to 0-1. 

In contrast, the presence of these cations has been observed to decrease the rate of dissolution of several feldspars (Sjoberg, 1989 Nesbitt et al, 1991 Stillings and Brantley, 1995) and to decrease the thickness of the leached layers formed on the feldspar surface. Schweda (1990), Nesbitt et al. (1991), and Brantley and Stillings (1996) proposed that feldspar dissolution in acidic solution far from equilibrium is related to competitive adsorption of and other cations on the surface of feldspar. Brantley and Stillings derived a rate equation based on a leached layer and Langmuir adsorption model ... 

Thomas J. E., Skinner W. M., and Smart R. St. C. (2001) A mechanism to explain sudden changes in rates and products for pyrrhotite dissolution in acid solution. Geochim. Cosmochim. Acta 65, 1 — 12. 

The effect of water composition is not unique. Some water components can slow down the process, i.e., play the role of inhibitor, some others, on the contrary, accelerate it, playing the role of catalyst. For instance, alkali and alkali-earth metals usually slow down the process of alumosili-cate dissolution in acid solutions. Similar inhibitor role may also be played by other cations. For instance, AP+ noticeably obstruct dissolution of iron oxides. Inorganic ligands may accelerate and slow down minerals dissolution. A well-known example may be the effect of the phosphoric acid. 

Assuming that only tin is dissolving (platinum dissolution in acidic solution is negligible), the anodic Tafel equation is ... 

The data from these three experiments indicate that reductive dissolution occurs in ammonium EDTA solutions. Although some magnetite may directly dissolve, releasing ferric ions that subsequently reduce on the bare steel, reduction of ferric iron in the scale by base metal or N2H4 appears to predominate. The accelerative effect of reductive dissolution in acidic solution is frequently attributed to potential changes... 

The increased activity of platinum - molybdenum alloys is evidently due to a change in the general course of the reaction [193, 240]. Addition of molybdenum (on account of its ready dissolution in acidic solutions and the possibility of change in the valence state of the ions) can lead to formation of a redox system (Mo " "— Mo ) near the electrode, and this leads to a change in the course of the reaction. This point of view is confirmed by the fact that oxidation of methanol and formaldehyde is accelerated by molybdate ions [245]. Shropshire [245] proposed the following scheme to explain this effect ... 

Chitosan can dissolute in acidic solutions, so it is hard to recycle from acidic solution. It is thus necessary to stabilize it chemically for the recycling in acidic solutions. Several processes have been developed to stabilize chitosan, such as suspending cross-linked technique, emulsification technique and spray drying technique [10]. Through stabilization, chitosan can be prepared as microspheres, easily separated from solutions. To get new or improved properties, chitosan-based microspheres have been researched recently. In addition, to expend the range of chitosan applications, chitosan-based microspheres can be used as functional materials with modified properties, such as adsorption resins, magnetic resins and nanocomposites, etc. The preparation and application of different kinds of chitosan-based microspheres are reviewed in this paper. 

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

Vitreous siUca is relatively inert to attack from most acids for temperatures up to 100°C. The weight loss data in acid solutions are summarized in Table 3. The main exceptions are phosphoric acid, which causes some corrosion above approximately 150°C, and hydrofluoric acid, which reacts readily at room temperature (91). This latter dissolution proceeds as follows ... 

In acidic solutions, equiUbtium is achieved more slowly. Polymerization of smaller species appears to occur sequentially a given polymer species first increases in size and then disappears, presumably because of its inclusion in higher order polymers. Depolymerization of siUcate species appears to be rapid, because crystalline Na2Si02 and Na2H2Si04 8H2O yield equivalent distributions of siUcate species in water upon dissolution. 

Acid Leaching. DHect acid leaching for vanadium recovery is used mainly for vanadium—uranium ores and less extensively for processing spent catalyst, fly ash, and boiler residues. Although 20 in spent catalysts dissolves readily in acid solutions, the dissolution of vanadium from ores and... 

The most important outcome of this theory is that the rate of dissolution should be potentially greater as the pH increases, which is in conflict with simple concepts of corrosion kinetics. However, the theory has been proved to be applicable to many systems, and BonhoeflFer and Heusler found that iron in sulphuric acid corroded at a greater rate with increase in pH, whilst Kabanov etal. found that it corroded faster in alkaline solution than in acid solution for the same electrode potential. 

The dissolution of copper and of brasses in acid solutions has been studied by several authors". Various substances have inhibiting effects on the rate of attack of copper or brasses in nitric acid" and in hydrochloric acid". ... 

The alloying elements molybdenum and copper do not, by themselves, enhance passivity of nickel in acid solutions, but instead ennoble the metal. This means that, in practice, these alloying elements confer benefit in precisely those circumstances where chromium does not, viz. hydrogen-evolving acidic solutions, by reducing the rate of anodic dissolution. In more oxidising media the anodic activity increases, and, since binary Ni-Mo and Ni-Cu alloys do not passivate in acidic solutions, they are generally unsuitable in such media. 

Adsorbed species may also accelerate the rate of anodic dissolution of metals, as indicated by a decrease in Tafel slope for the reaction. Thus the presence of hydrogen sulphide in acid solutions stimulates the corrosion of iron, and decreases the Tafel slope The reaction path through... 

In general, corrosion of metal is always accompanied by dissolution of a metal and reduction of an oxidant such as a proton in acidic solution and dissolved oxygen in a neutral solution. That is, metal corrosion is not a single electrode reaction, but a complex reaction composed of the oxidation of metal atoms and the reduction of oxidants. 

Trigonal, metallic selenium has been investigated as photoelectrode for solar energy conversion, due to its semiconducting properties. The photoelectrochemistry of the element has been studied in some detail by Gissler [35], A photodecomposition reaction of Se into hydrogen selenide was observed in acidic solutions. Only redox couples with a relatively anodic standard potential could prevent dissolution of Se crystal. 

Dremlyuzhenko SG, Voloshchuk AG, Zakharuk Zl, Yurijchuk IN (2008) Thermodynamic evaluation and potentiometric study of Cdi xMnxTe and Cdi xZnx Te dissolution in acid and alkaline solutions. Inorg Mater 44 21-29... 

It is found that the dissolution of zinc sulfides occurs more rapidly when they are in contact with copper sulfide or iron sulfide than when the sulfides of these types are absent. This enhancement is brought about by the formation of a galvanic cell. When two sulfide minerals are in contact, the condition for dissolution in acidic medium of one of the sulfides is that it should be anodic to the other sulfide in contact. This is illustrated schematically in Figure 5.3 (A). Thus, pyrite behaves cathodically towards several other sulfide minerals such as zinc sulfide, lead sulfide and copper sulfide. Consequently, pyrite enhances the dissolution of the other sulfide minerals while these minerals themselves understandably retard the dissolution of pyrite. This explains generally the different leaching behavior of an ore from different locations. The ore may have different mineralogical composition. A particle of sphalerite (ZnS) in contact with a pyrite particle in an aerated acid solution is the right system combination for the sphalerite to dissolve anodically. The situation is presented below ... 

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