Acid-promoted dissolution, minerals

Acids are sometimes used ahead of fracturing fluids to dissolve mineral fine particles and allow more rapid injection of the fracturing fluid. When used as the initial stage of a squeeze cementing treatment, the acid-promoted mineral and drilling mud particle dissolution can result in increased entry of the cement slurry into the desired portions of the formation. 

Figure 13.10. Schematic representation of the oxide dissolution processes [exemplified for Fe(III) (hydr)oxides] by acids (H ions), ligands (example oxalate), and reductants (example ascorbate). In each case a surface complex (proton complex, oxalato and ascorbato surface complex) is formed, which influences the bonds of the central Fe ions to O and OH on the surface of the crystalline lattice, in such a way that a slow detachment of a Fe(III) aquo or a ligand complex [in case of reduction an Fe(ll) complex] becomes possible. In each case the original surface structure is reconstituted, so that the dissolution continues (steady-state condition). In the redox reaction with Fe(III), the ascorbate is oxidized to the ascorbate radical A . The principle of proton-promoted and ligand-promoted dissolution is also valid for the dissolution (weathering) of Al-silicate minerals. The structural formulas given are schematic and simplified they should indicate that Fe(III) in the solid phase can be bridged by O and OH.

There is clear evidence that the dissolution of oxide minerals is promoted by the specific sorption of solutes at the mineral-solution interface. Moreover, it has been found that comparatively simple rate laws are obtained if the observed rates are plotted against the concentrations of adsorbed species and surface complexes (Pulfer et al., 1984 Furrer and Stumm, 1986). For example, in the presence of ligands (anions and weak acids) surface chelates are formed that are strong enough to weaken metal-oxygen bonds and thus to promote rates of dissolution proportional to their surface concentrations. Simple rate laws have been also observed with H+—or OH —promoted dissolution of oxides in a manner that can be predicted from knowledge of the oxide composition and the surface concentrations of protons and hydroxyl radicals. 

In the absence of organic ligands, the rate of dissolution of most silicate minerals depends on pH as shown in Fig. 5. In the acid region, we have proton-promoted dissolution ... 

Antioxidants in fruits and vegetables including vitamin C and (3-carotene reduce oxidative stress on bone mineral density, in addition to the potential role of some nutrients such as vitamin C and vitamin K that can promote bone cell and structural formation (Lanham-New 2006). Many fruits and vegetables are rich in potassium citrate and generate basic metabolites to help buffer acids and thereby may offset the need for bone dissolution and potentially preserve bone. Potassium intake was significantly and linearly associated with markers of bone turnover and femoral bone mineral density (Macdonald and others 2005). 

Acid drainage results from the reaction of sulfide minerals with oxygen in the presence of water. As we show in this section, water in the absence of a supply of oxygen gas becomes saturated with respect to a sulfide mineral after only a small amount of the mineral has dissolved. The dissolution reaction in this case (when oxygen gas is not available) causes little change in the water s pH or composition. In a separate effect, it is likely that atmospheric oxygen further promotes acid drainage because of its role in the metabolism of bacteria that catalyze both the dissolution of sulfide minerals and the oxidation of dissolved iron (Nordstrom, 1982). 

Fig. 10.8. Simple biogeochemical model for metal mineral transformations in the mycorhizosphere (the roles of the plant and other microorganisms contributing to the overall process are not shown). (1) Proton-promoted (proton pump, cation-anion antiport, organic anion efflux, dissociation of organic acids) and ligand-promoted (e.g. organic adds) dissolution of metal minerals. (2) Release of anionic (e.g. phosphate) nutrients and metal cations. (3) Nutrient uptake. (4) Intra- and extracellular sequestration of toxic metals biosorption, transport, compartmentation, predpitation etc. (5) Immobilization of metals as oxalates. (6) Binding of soluble metal species to soil constituents, e.g. clay minerals, metal oxides, humic substances.

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