Geochemical neutralization

The oxidation-neutralization curve (Fig. 3) gives a long term prediction of AMD generation (see Benzaazoua et al. 2001 for more details). Assuming steady-state geochemical behaviour, the oxidation products (sulfates) would disappear before theneutralizing elements (Ca, Mg, and Mn). 

This book focuses on geochemical approaches in immobilizing, isolating, or neutralizing waste derived from energy production and consumption. 

Since we are interested in the sulfur-carbon chemistry of diagenesis, the relevant geochemical processes are limited to those occurring within the very narrow temperature span of approximately 0 to 60 °C. The reactions must occur under hydrous conditions although in media having at least some capacity to accommodate lipophilic source molecules as a result of the presence of substances such as low grade carbohydrates and small carboxylic acids or their salts. If the sulfur source is a form of reduced sulfur from active microbial reduction of sulfate, the pH will range from near neutral... 

Thus in highly reducing conditions iron can migrate in a wide pH range (from 0 to 6) and precipitates as sediment in the form of oxides and hydroxides only in neutral environments. The acidity of the environment, as a natural geochemical barrier governing the precipitation of iron, is appreciably reduced. Variation in the redox potential as a result of the overall evolution of the atmosphere, hydrosphere, and biosphere plays a large role. 

Thus from the thermodynamic data it follows that if acid or neutral silica-bearing solutions arrived in the depositional basin, the action of the presumed geochemical barriers (gradients of pH, Eh, concentration) could not operate in isothermal conditions of chemogenic deposition of silica, to form cherty or cherty-iron sediments. Only a change in concentration due to evaporation of substantial volumes of water in closed basins could have led to deposition of silica. An easy calculation shows that to deposit chert bands 0.3-0.5 cm thick in that way, a 100-m water layer has to be evaporated. Thus the formation of thick piles of Precambrian iron formations would have required the evaporation of a fantastic amount of water from restricted... 

Similar conditions can be produced when alkaline solutions saturated with silica are mixed with highly acid waters. Another genetic variant inevitably suggests acid thermal waters, in which the solubility of Si02 is a function of temperature (Khitarov, 1953), as the source of the silica. When thermal waters cool, mix with surface waters, and are partially neutralized, ionic-colloidal systems with anomalously high content of monomeric silica can arise. It is obvious that both these variants reflect specific conditions that are not typical of geochemical processes in the weathered layer. 

Chemical models for the speciation of Cu in freshwater (Millero, 1975) predict that free Cu (aq) is less than 1% of the total dissolved Cu and that Cu(C03)i and CuCO are equally important for the average river water. Leckie and Davis (1979) showed that the CuCO complex is the most important one near the neutral pH. At pH values above 8, the dihydroxo-Copper(ll) complex predominates. The chemical form of Cu is critical to the behavior of the element in geochemical and biological processes (Leckie and Davis, 1979). Cupric Cu forms strong complexes with many organic compounds. 

Blodau C. and Peiffer S. (2003) Thermodynamics and organic matter constraints on neutralization processes in sediments of highly acidic waters. Appl. Geochem. 18, 25- 36. 

The ideal environment for bacterial sulfate reduction and sulfide mineral formation is anoxic, non-toxic, and near neutral pH. It is also characterized by adequate organic matter and mesophilic temperatures. In the absence of such an ideal environment, it must be remembered that microoi anisms are diversified and adaptable. For example, bacteria may function in a microenvironment that differs distinctly from the surrounding macroenvironment. Trudinger et al. (1972) concluded that few geochemical factors by themselves can prevent sulfate reduction and sulfide mineralization. 

Separately, based on geochemical modeling, a number of researchers have concluded that the least soluble Np phase in near-neutral, oxidized groundwaters such as occur at Yucca Mountain, is Np(IV) oxide or hydroxide (Wilson and Bruton 1990 Hakanen and Lindberg 1991 Wolery et al. 1995), not a more soluble Np(V) phase such as NaNpOjCO 3.5H2O. 

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