Redox mineralization

At intermediate depths (down to 500 m) groundwaters rapidly increase in concentration primarily by the addition of SO4 and Cl. The concentration of bicarbonate ions decreases because of the precipitation of mineral phases such as calcite. Local variations in chemistry and anions may be due to a variety of rock-water interactions or local processes that result in Na-SO4, Na-HC03, and Mg-S04 type waters. The pH begins to rise in this zone and oxygenconsuming reactions and redox mineral controls tend to lower the Eh. The brackish and saline waters found at these intermediate depths have longer residence times. Deep saline waters and brines occur in most locations below depths of 500 m. These fluids are Ca-Na-Cl or Na-Ca-Cl in composition and can have total dissolved loads up to 350 g L. Minor elements such as bromide and strontium can here be thousands of milligrams per liter. 

Anhydrous stannous chloride, a water-soluble white soHd, is the most economical source of stannous tin and is especially important in redox and plating reactions. Preparation of the anhydrous salt may be by direct reaction of chlorine and molten tin, heating tin in hydrogen chloride gas, or reducing stannic chloride solution with tin metal, followed by dehydration. It is soluble in a number of organic solvents (g/100 g solvent at 23°C) acetone 42.7, ethyl alcohol 54.4, methyl isobutyl carbinol 10.45, isopropyl alcohol 9.61, methyl ethyl ketone 9.43 isoamyl acetate 3.76, diethyl ether 0.49, and mineral spirits 0.03 it is insoluble in petroleum naphtha and xylene (2). 

In an oversimplified way, it may be stated that acids of the volcanoes have reacted with the bases of the rocks the compositions of the ocean (which is at the fkst end pokit (pH = 8) of the titration of a strong acid with a carbonate) and the atmosphere (which with its 2 = 10 atm atm is nearly ki equdibrium with the ocean) reflect the proton balance of reaction 1. Oxidation and reduction are accompanied by proton release and proton consumption, respectively. In order to maintain charge balance, the production of electrons, e, must eventually be balanced by the production of. The redox potential of the steady-state system is given by the partial pressure of oxygen (0.2 atm). Furthermore, the dissolution of rocks and the precipitation of minerals are accompanied by consumption and release, respectively. 

Gas emission after core formation the redox state in the iron-containing minerals of the Earth s crust is determined by the ratio of Fe2+ to Fe3+. 

The redox properties of these systems are vitally important for possible abiotic chemical syntheses the main minerals present in young basalt ocean basins, at depths between 300 and 1,300 m, are pyrite (FeS2), pyrrhotite (FeS) and magnetite (Fe3C>4) (the PPM system). 

At greater depths, the redox characteristics are mainly determined by another mineral system fayalite (Fe2Si04), magnetite and quartz (Si02) (theFMQ system). The buffer properties of the two systems can be expressed in terms of the following equations ... 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

If the rTCA cycle were to have functioned on the primeval Earth, all the reaction steps (both redox and non-redox) must have proceeded in high yield. One single metal surface can certainly not drive the whole rTCA cycle more complex mixtures of active mineral surfaces are required (Zang and Martin, 2006). 

Cr(VI).Other remediation processes for Cr(VI) contaminated soils include H2S injection, aqueous Fe(II) injection, and the use of reduced Fe solids. Aqueous-phase Cr(VI)-Fe(II) redox reactions may be significant if Fe2+ concentrations are in equilibrium with relatively soluble, ferric hydroxide-like phases (Tokunaga et al., 2003). The overall interactions involving microbial activity, organic carbon degradation, Fe2+, and mineral surfaces control the net rates of Cr(VI) reactions in soils. 

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