Water redox status

Several measures of organic pollutant loading to waters have been developed to indicate the redox status of a system ... 

The main environmental factors that control transformation processes are temperature and redox status. In the subsurface, water temperature may range from 0°C to about 50°C, as a function of climatic conditions and water depth. Generally speaking, contaminant transformations increase with increases in temperature. Wolfe et al. (1990) examined temperature dependence for pesticide transformation in water, for reactions with activation energy as low as lOkcal/mol, in a temperature range of 0 to 50°C. The results corresponded to a 12-fold difference in the half-life. For reactions with an activation energy of 30kcal/mol, a similar temperature increase corresponded to a 2,500-fold difference in the half-life. The Arrhenius equation can be used to describe the temperature effect on the rate of contaminant transformation, k ... 

Speciation is a dynamic process that depends not only on the ligand-metal concentration but on the properties of the aqueous solution in chemical equilibrium with the surrounding solid phase. As a consequence, the estimation of aqueous speciation of contaminant metals should take into account the ion association, pH, redox status, formation-dissolution of the solid phase, adsorption, and ion-exchange reactions. From the environmental point of view, a complexed metal in the subsurface behaves differently than the original compound, in terms of its solubility, retention, persistence, and transport. In general, a complexed metal is more soluble in a water solution, less retained on the solid phase, and more easily transported through the porous medium. 

Therefore, oxidation-reduction processes in nature control the behavior of elements or substances. During oxidation-reduction, the potential for reactions to take effect changes because the redox status of elements changes. A summary of soil-water mineral-ion properties known to be affected by redox chemistry is listed below ... 

In biologically productive lakes which also develop a thermocline in summer, the bottom water may become depleted in oxygen, leading to a change from oxidizing to reducing conditions. As a result, the potential exists for remobilization of nutrients and metals from bottom sediments to the water column, one more example of how alterations in master variables in the hydrological cycle, in this case the redox status, can affect the fate and influence of pollutants. 

Important restraints on the evolution of superior chemical models are the inadequacies in the 1) capability to characterize the organic ligands of natural waters 2) knowledge of redox status of waters, which does not permit realistic computation of redox-controlled speciation 3) available thermodynamic data ... 

Redox Status of Acid Mine Waters Equilibrium or Disequilibrium ... 

Arsenic and selenium demonstrate many similarities in their behavior in the environment. Both are redox sensitive and occur in several oxidation states under different environmental conditions. Both partition preferentially into sulfide minerals and metal oxides and are concentrated naturally in areas of mineralization and geothermal activity. Both elements occur as oxyanions in solution and, depending on redox status, are potentially mobile in the near-neutral to alkaline pH conditions that typify many natural waters. However, there are also some major differences. Selenium is immobile under reducing conditions while the mobility of arsenic is less predictable and depends on a range of other factors. Selenium also appears to partition more strongly with organic matter than arsenic. 

Figure 1 Effect of fermented and green rooibos (Rf Rg) and honeybush (Hf Hg) on the hepatic redox status (reduced to oxidized glutathione ratio) of rats consuming the various herbal teas for 10 weeks. The control group consumed water (Data are from reference 50)...

Considering the limitations of Eh measurement and the common occurrence of homogeneous disequilibrium Q), it would seem prudent to limit the use of Eh to either specific reactions or to qualitative descriptions when discussing natural systems. It has been suggested QS, I2 2Q) that the measurement of dissolved gases such as O2, H2S, NH4, CH4, and H2 may be a preferable method for characterizing the redox status of natural waters. 

A third example of OXC is taken from field observations. In response to the recommendation by Hostcttler (7) that the most reliable characterization of the redox status of a natural water is a complete chemical analysis of all redox-active species, the Illinois State Water Survey (10) collected and analyzed groundwater samples from a pristine aquifer on a monthly basis for one year. Table III lists calculated values of OXj, RDj, and OXC for samples taken from depths of 35,50, and 65 feet. OXC is greatest in the most shallow samples and it decreases with depth. In addition, the lowest values at a constant depth occur in the winter samples and OXC varies little from April to September. This seasonal effect is the result of a decrease in total reductants RDj values for December through February are more than three times those for the spring and summer months. OXj is relatively constant throughout the entire sampling period. It is uncertain whether or not the... 

For a better comprehension of the chemistry of a groundwater system the redox status needs to be well-defined. Until recently, most efforts have relied solely upon Ej.j or pE, intensity factors, as the master variable. However, it is apparent that these intensity factors do not truly represent the redox status of a system because some pertinent redox couples are not electroactive and redox reactions are generally slow and are not at equilibrium. In this paper, the oxidative capacity, a capacity factor, is operationally defined and shown to be a better descriptive parameter of the redox status. Determination of the OXC of an aqueous system allows investigators to cla.ssify the system in terms of well-defined geochemical and microbial parameters. This classification combined with other predictive tools, such as a redox titration, allows one to predict the identity and assess the role of chemical reactions and microbial populations within a specific groundwater system. As such, the capacity factor OXC should be determined in water quality assessment. 

It is useful to evaluate the redox levels that could develop in natural waters and sediments. One might expect to find a more oxidative environment (higher Sh) in systems in equihbrium with the atmosphere. Reducing environments often result from microbial activity in anaerobic systems. It is also useful to consider how the redox status of any system might be measured. 

Reducing environments often result from microbial activity. For example, if oxygen is limiting, some organisms may use sulfate (usually available in natural waters) as an electron acceptor to oxidize organic matter. The redox status would then be a function of the S04 /HS balance. If the [S04 ] = M and [HS ] = W Mand pH 7 ... 

The speciation of Cr(VI) and Cr(lll) is regulated by the oxidation-reduction status of soils and sediments. Masscheleyn et al. (1990) determined the rate of Cr(Vl) redaction as affected by the soil redox status. Soil suspensions amended with Cr(Vl) were eqnilibrated nnder controlled redox conditions (-200, 0, +200, and +500 mV). Figure 12.15 shows the critical soil redox values for Cr(VI) reduction. Cr(VI) reduction occurred at approximately the same redox levels as nitrate. Water-soluble Cr(VI) decreased with a decrease in Eh values from 500 to 300 mV. 

Most leachates can be assumed to have much lower pH values and redox potentials than the ambient groundwater, dispersion therefore causes continual mixing of waters that are different in chemical composition and redox status, and a cycle of changing Eh and pH causes reactions to take place, which in turn stimulate further changes in Eh and pH, and so the cycle continues (Cherry et al, 1984). 

The redox status of an aqueous system is described by the concentrations of the oxidized and reduced chemical species of all components in the chemical system (Scott Morgan 1990). Because of the slowness of oxidation-reduction reactions, natural rock-water systems are often not in redox equilibrium and thus the concept of a system Eh or pE becomes meaningless. Intensity factors such as Eh or pE are not very useful descriptors of the redox state of the system, but capacity factors which reflect the total concentration of redox-sensitive species may be better and more conservative measures of redox state. 

It is also often taken for granted that many of the Earth s subsystems are exposed to free oxygen (O2), leading to a range of one-way reactions of reduced materials (such as organic carbon or metal sulfides) to an oxidized form. As pointed out many times in earlier chapters, the oxidation-reduction status of the planet is the consequence of the dynamic interactions of biogeochemical cycles. As is the case with the acid-base balances, there is considerable sensitivity to perturbations of "redox" conditions, sometimes dramatically as in the case of bodies of water that suddenly become anaerobic because of eutrophication. Another extreme... 

Chromium can exist in several oxidation states from Cr(0), the metallic form, to Cr(Vl). The most stable oxidation states of chromium in the environment are Cr(lll) and Cr(Vl). Besides the elemental metallic form, which is extensively used in alloys, chromium has three important valence forms. The trivalent chromic (Cr(lll)) and the tetravalent dichromate (Cr(Vl)) are the most important forms in the environmental chemistry of soils and waters. The presence of chromium (Cr(Vl)) is of particular importance because in this oxidation state Cr is water soluble and extremely toxic. The solubility and potential toxicity of chromium that enters wetlands and aquatic systems are governed to a large extent by the oxidation-reduction reactions. In addition to the oxidation status of the chromium ions, a variety of soil/sediment biogeochemical processes such as redox reactions, precipitation, sorption, and complexation to organic ligands can determine the fate of chromium entering a wetland environment. 

The mass spectroscopy data of [43] are of special interest because what is measured by this technique is the chemical potential it + rather than the real potential a°jj+. (From a formal perspective, this statement has the status of an extrathermody-namic assumption. - ) Recall from Equation 13.12 that and are related to each other by the surface potential of water. A similar relation holds for the solvation free energy and the work function (Equation 13.15). Applying this relation to the experimental estimates in Table 13.1 yields the = 170 meV value given in the table. 170 meV is within 50 meV consistent with estimates of y from other sources. It should be mentioned that there are good arguments that 4.44 V (or 4.42 V) is not an appropriate value for use in quantum chemistry calculations. " For the conversion of redox potentials computed using implicit solvent models, a value of [7 + (abs) based on the chemical potential rather than the real potential is more consistent (see also [10]). 

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