Precipitates, Dissolution

Sulphate can also be determined by an exactly similar procedure by precipitation as lead sulphate from a solution containing 50 per cent (by volume) of propan-2-ol (to reduce the solubility of the lead sulphate), separation of the precipitate, dissolution in excess of standard EDTA solution, and back-titration of the excess EDTA with a standard zinc solution using solochrome black as indicator. 

Mercury provides an excellent example of the importance of metal speciation in understanding biogeochemical cycling and the impact of human activities on these cycles. Mercury exists in solid, aqueous, and gaseous phases, and is transported among reservoirs in all these forms. It undergoes precipitation-dissolution, volatilization, complexation, sorption, and biological reactions, all of which alter its mobility and its effect on exposed populations. The effect of all... 

Species may differ by oxidation state for example, manganese(II) and (IV) iron(II) and (III) and chromium(III) and (VI). Oxidation state is influenced by the redox potential. Mobility is affected because oxidation state influences precipitation-dissolution reactions and also toxicity in the case of heavy metals. 

Acid-base, hydrolysis, hydration, neutralization, oxidation-reduction, polymerization, thermal degradation Adsorption-desorption, precipitation-dissolution, immiscible-phase separation, biodegradation, complexation Acid-base, neutralization, oxidation-reduction (most inorganic and some biologically mediated), adsorption-desorption, precipitation-dissolution, complexation Hydrolysis, oxidation-reduction (biodegradation of anthropogenic inorganics), immiscible-phase separation... 

Thus, a substance may be in a solid form or in solution (described by the precipitation-dissolution process), but its toxicity remains unaltered regardless of form. The form or state of a substance, however, influences the transformation and transport processes that can occur. For this reason, partition processes are important to define in a fate assessment. 

Precipitation-dissolution Partly Higher temperatures, pressures, and salinity of the deep-well environment may result in significant differences between reactions in the two environments. 

Precipitation is a phase-partitioning process whereby solids separate from a solution.34 Dissolution involves movement from the solid or gaseous phase to the aqueous phase. Solids dissolve into ions, whereas gases retain their original chemical structure when dissolved. The solubility of a compound (its tendency to dissolve in water or other solutions) is the main property affecting the precipitation-dissolution process. 

Environmental conditions determine in large part the chemical reactions that will occur when waste is injected. For example, precipitation-dissolution reactions are strongly controlled by pH. Thus, iron oxides, which may be dissolved in acidic wastes, may precipitate when injection-zone mixing increases the pH of the waste. Similarly, redox potential (Eh) exerts a strong control on the type of microbiological degradation of wastes. 

Strongly influences precipitation-dissolution reactions. Mixing of solutions with different pH often results in precipitation reactions. See also reservoir matrix below. 

The major processes affecting the geochemical fate of hazardous inorganics are acid-base adsorption-desorption, precipitation-dissolution, complexation, hydrolysis, oxidation-reduction, and catalytic reactions. The significance of these processes to inorganic wastes is discussed only briefly here additional information on individual elements is given in Table 20.16. 

Acid-base equilibrium is very important to inorganic chemical reactions. Adsorption-desorption and precipitation-dissolution reactions are also of major importance in assessing the geochemical fate of deep-well-injected inorganics. Interactions between and among metals in solution and solids in the deep-well environment can be grouped into four types1 2 3 4 ... 

Most of the chemical processes discussed before (acid-base equilibria, precipitation-dissolution, neutralization, complexation, and oxidation-reduction) are interrelated that is, reactions of one type may influence other types of reactions, and consequently must be integrated into aqueous- and solution-geochemistry computer codes. 

Brununer G.W., Tiller K.G., Herms U., Clayton P.M. Adsorption-desorption and/or precipitation-dissolution processes of zinc in soils. Geoderma 1983 31 337-354. 

Robarge W.P. Precipitation/dissolution reactions in soils. In Soil Physical Chemistry, 2nd Sparks D.L., ed. Boca Raton, FL CRC Press, 1999. 

Evaporation concentrates the dissolved constituents of seawater. Because the assumed inflow is twice the assumed outflow, conservative properties—properties that are not affected by precipitation, dissolution, or exchange with the atmosphere—are concentrated by a factor of 2. This increase in concentration changes the balance between the dissolved car-... 

Lichtner, P. C., E. H. Oelkers and H. C. Helgeson, 1986, Interdiffusion with multiple precipitation/dissolution reactions transient model and the steady-state limit. Geochimica et Cosmochimica Acta 50, 1951-1966. 

Steefel, C. I. and A.C. Lasaga, 1994, A coupled model for transport of multiple chemical species and kinetic precipitation/dissolution reactions with application to reactive flow in single phase hydrothermal systems. American Journal of Science 294, 529-592. 

Adsorption influences the reactivity of surfaces. It has been shown that the rates of processes such as precipitation (heterogeneous nucleation and surface precipitation), dissolution of minerals (of importance in the weathering of rocks, in the formation of soils and sediments, and in the corrosion of structures and metals), and in the catalysis and photocatalysis of redox processes, are critically dependent on the properties of the surfaces (surface species and their strucutral identity). 

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). 

Precipitation/dissolution Formation or dissolution of a solid this usually involves hydration... 

The effect of solids on ion speciation is not limited to precipitation/dissolution reactions. Most solid surfaces in seawater possess a net negative charge that enables them to electrostatically attract cations (M ). This attraction can be represented as ... 

Therefore, in the first model, the specific surface area increases with decreasing porosity, while in the second, the opposite relationship is specified. While some attempts have been made to experimentally verify these models in individual rock types (Kieffer et al. 1999 Jove Colon et al. 2004), the data concerning a wide range of rocks and precipitation-dissolution reactions remain limited. 

Dissolution of carbonate minerals does not lead to mineral trapping of C02 (Gunter et al. 1993). However, carbonate dissolution, and other mineral precipitation-dissolution reactions can impact sequestration capacity by altering the permeability of the aquifer near the injection site. 

Interest in trace element speciation studies in natural waters has increased considerably during the last decade. It has become apparent that data on total concentrations of any element rather than on individual well defined chemical entities, are often inadequate to identify transport mechanisms, ultimate fate and toxicity of particular elements to organisms. A study of the different trace metal species and their relative distribution will assist in understanding the chemical processes that take place in the highly reactive estuarine zone and in the open sea. These processes include the rate at which chemical processes take place, the participation in geochemical processes (precipitation/dissolution, adsorption/desorption). 

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