Redox-Potential-pH

Oxidation reactions that take place in aquatic environments can be mediated by direct or indirect photolysis reactions, which depend on the organic chemicals and substrates present. Nonphotolytic oxidation of organic chemicals can occur directly by reactions involving ozone, or via catalytic pathways with certain metals. Abiotic reduction reactions that influence organic chemical transformation in wetlands include Fe and Mn species and sulfides. 

Soil redox potential (Eh) and the pH parameters are closely related. Production of carbon dioxide, an end product of the reduction of oxygen, has considerable influence on the soil s pH. When a reducing wetland soil system becomes oxidized, its pH may decrease drastically due to the oxidation of iron to Fe(lll) and the subsequent hydrolysis of the iron or the oxidation of sulfite to sulfate, which is accompanied by the release of protons. Lowering of the Eh of the soil due to flooding will result in a rise of pH, because many reduction reactions (such as the reduction of sulfate to sulfide, Ee to Fe, and Mn + to Mn +) involve the uptake of protons or the release of hydroxyls. 

The reaction of a redox couple that controls the system will directly cause a change in Eh. pH can affect the redox reactions by determining the concentrations of members of the redox couple in the soil solution. Decrease in soil pH will increase the solubility of trivalent iron and of other oxidized transition metal species, but will have a smaller effect on the solubility of the reduced species of these metals. The redox reactivity of a xenobiotic in soil is dependent on the pH. Altering the pH of the soil can affect redox reactions of toxic organics, just as it affects other pH-dependent reactions such as hydrolysis. 

The reduced forms of cations such as Fe and Mn are much more soluble than other oxidized forms. As a result, in wetland soils, appreciable concentrations of reduced cationic species, such as Fe, can be present in the soil solution. This reduction and consequent dissolution can have a strong influence on the abiotic transformations of toxic organics in the liquid phase through the capacity of metals to catalyze abiotic transformation. Under stronger reduction conditions, sulfate is reduced, producing such S species as sulfides that can also be involved in the degradation of certain toxic organics. 

Hydrolysis reactions are important steps in the degradation of many pesticide compounds. For some toxic organics, hydrolysis reactions are nonbiological and are enhanced in soil that is, hydrolysis reactions in some cases occur more rapidly in soil than in comparable soil-free aqueous systems due to catalysis of the reaction by sorption. 

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Redox potential pH Ionic activities Inert redox electrodes (Pt, Au, glassy carbon, etc.) pH-glass electrode pH-ISFET iridium oxide pH-sensor Electrodes of the first land and M" /M(Hg) electrodes) univalent cation-sensitive glass electrode (alkali metal ions, NHJ) solid membrane ion-selective electrodes (F, halide ions, heavy metal ions) polymer membrane electrodes (F, CN", alkali metal ions, alkaline earth metal ions)... 

Sulfur isotopes can effectively be used to examine important geochemical processes associated with redox changes in sedimentary environments. The speciation of sulfur is strongly affected by redox potential, pH, productivity, microbial sulfate reduction, and iron availability (Berner, 1984). More details are provided on the sulfur cycle in chapter 12. In general, during microbial dissimilatory sulfate reduction there is fractionation of sulfur... 

Table 2.18 Relationships among soil properties (redox potential, pH) and bioavailabiUty of metal ions to plants...

Figure 10. The standard redox potentials (pH 7) of some important redox couples and the free energy changes of processes involving two redox couples (i.e. respiratory processes or Hi/COt methanogenesis) (Fenchel et al, 1998).

Since LAW and H2LAW are present at much lower concentrations than hydrogen sulfide, the redox potential, (pH) of the system is essentially determined by Eq. 21 with [H2S]T = 5x 10 3M. Hence, in analogy to a pH buffer for proton-transfer reactions, the H2S/S(s) couple is used as a redox buffer for electron transfer. 

Meyer, J.S., Davison, W., Sundby, B., Oris, J.T., Lauren, D.J., Forstner, U. et al. (1994) The effects of variable redox potentials, pH, and light on bioavailability in dynamic water-sediment environments, In Hamelink, J., Landrum, P.F., Bergman, H.L. and Benson, W.H. (eds) Bioavailability Physical Chemical and Biological Interactions, Lewis Publ., Boca Raton, FL, USA, Synopsis Chapter, pp. 155-170. 

Dissolution of the chlorides from the corrosion products is an essential part of the conservation process. It is essential that the artefact is immersed in an electrolyte that will not corrode the metal any further, while this dissolution is taking place. Corrosion scientists have developed redox potential - pH diagrams from thermodynamics in order to predict the most stable form of the metal. These diagrams are divided into three zones. Where metal ions are the most stable phase, this is classed as a zone of corrosion. If the metal itself is the most stable species, this is said to be the zone of immunity. The third zone is where solid metal compounds such as oxides, hydroxides, etc, are the most stable and may form a protective layer over the metal surface. This zone is termed passivity and the metal will not corrode as long as this film forms a protective barrier. The thickness of this passive layer may only be approximately 10 nm thick but as long as it covers the entire metal surface, it will prevent further corrosion. 

FIGURE 6.6 Relationships between redox potential (pH = 5), oxygen content, and water-table depth. Each point is an average of 22 monthly collections at each depth ( = 4) in each plot (n = 10) (Megonigal et ah, 1993). 

Iron and manganese transformations are another redox potential-pH regulated process in sediment-water systems, which can affect heavy metal availability (see Chapter 10). The reduced forms of iron and manganese, when oxidized, form amorphous hydrous oxides with large surface... 

The microbial-mediated processes of methylation and demethylation are influenced by factors such as redox potential, pH, sulfate concentration, and microbial activity. In a study conducted by DeLaune et al. (2004), methylation of added Hg in sediment was greater under reduced conditions... 

The redox potential-pH stability diagram (Figure 12.11) indicates that between pH 7 and 8, zinc carbonate (ZnCOj) is formed when the concentration of dissolved carbon dioxide (CO2) is 10 mol L . At low redox values, zinc sulfide is the most stable combination. Zinc precipitation by the hydrous metal oxides of manganese and iron is the principal control mechanism for zinc in wetland soils and freshwater sediments. The occurrence of these oxides as coatings on clay and silt enhances their chemical activity in excess of their total concentration. The uptake and release of the metals is governed by the concentration of other heavy metals, pH, organic and inorganic compounds, clays, and carbonates. 

Redox potential-pH diagrams can be expanded to cover more complex systems when the concentration of all components are known. For instance, chloride, sulfate, phosphate, and other ions may complex with lead under specified redox potential-pH conditions. The forms of lead in complex water systems can be determined where the concentrations and chemistry of all components are known. However, in natural sediment-water systems, the factors affecting lead chemistry may be in a dynamic state, and the chemistry of all the components is not known. Such is the case with interactions between organic matter and metals. 

State, types of functional groups), redox potential, pH, nutrient and carbon availability, contaminant bioavailability and concentration, electron acceptors, temperature, salinity, and microbial consortia and biomass (D Angelo, 2002). Reaction rates can vary over several orders of magnitude depending on these environmental factors. Studies have documented the effects of several of these factors on rates of mineralization of contaminants in wetland substrates. Redox potential, a measure of the electron availability and an indirect measure of the oxygen status, has been used to show certain compounds degrade favorably under aerobic conditions (e.g., naphthalene), others under anaerobic conditions (e.g., DDT), and still others under moderately anaerobic conditions (e.g., polychlorobi-phenyls [PCBs]). 

As(III) may be oxidized to As(V) by S20 in aqueous alkaline solutions/ At pH > 12.0, the observed rate constant is 1.6 0.3 x 10 M s at an ionic strength of 0.1 M. Sustained oscillations in redox potential, pH, and the concentration of dissolved O2 are reported in the Cu(H)-catalyzed reaction betwen K2S2O8 and Na2S203 in a stirred tank reactor. A free radical mechanism involving Cu(I), Cu(II), and the radicals SO4 and S2O3 may be used to account for the dynamic behavior of this sytem. The kinetics and mechanism of the oxidation of thiosulfate coordinated to cobalt(IH) by peroxymonosulfate have been reported.The reaction proceeds via two consecutive nucleophilic additions of the terminal peroxy oxygen atom to the coordinated S20 . 

L. Hayes, Effect of Hydrophobicity of Elastic Protein-based Polymers on Redox Potential. Ph.D. Dissertation, University of Alabama, Birmingham, 1998. 

Transformation of reactive organic substances will result in variations of redox potential, pH-values, as well as concentrations of inorganic ions and dissolved organic carbon the latter two components may enhance complexation of metals. Regarding the subsequent consideration, acid-producing potentials derived from oxidation of sulphides and organic matter are of prime importance. 

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