Affecting redox conditions

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

While not stated explicitly, in this discussion so far, it has been assumed that all the systems were well defined, at equilibrium, and at a constant 25°C. None of these conditions occur in soil in the environment. Soil is not a pure system and, often, all the components affecting redox reactions are not known, defined, or understood, and a host of different redox couples are likely to be present. Unless it is possible to take into account all couples present, it is not possible to describe the exact redox conditions in a soil without measuring it. 

A considerable number of transcription factors have reactive cysteine residues, which enable them to respond to the redox conditions in the cell. Since cadmium perturbs redox homeostasis, it can affect this class of transcription factors. If cadmium can displace the tetra-coordinate zinc atoms in zinc finger-containing transcription factors, it will affect them as well. Many of the pathways involving activation and inactivation of transcription factors involve kinases and phosphatases, themselves under the intricate control of calcium fluxes. It is therefore no surprise that cadmium will exert effects on the activity of transcription factors, the activation of proto-oncogenes, and thereby on gene expression (Figure 20.8i and i ). 

Effects of Flooding and Redox Conditions on OClAIC. Reductive dissolution reactions of the sort indicated in Figures 2.6 and 2.7 will affect the amount of a solute in diffusible forms in the soil and the distribution of the diffusible forms between the soil solid and solution. These processes are discussed in detail in Chapter 3. 1 here exemplify their effects by reference to a study of phosphate diffusion in a soil under different water regimes. 

For all the other halides, Eh-pH conditions have no influence. Boron occurs in water mainly as boric acid H3BO3 and its progressive ionization products at increasing pH. Redox conditions do not affect the speciation state of boron. 

Few examples of studies on cycling of trace elements (other than iron and manganese) at oxic-anoxic interfaces are found in the literature (11-17). Trace element cycling in the water column of a eutrophic lake (Figure 1) is affected by a number of processes related to the redox conditions. 

This chapter discusses the chemical mechanisms influencing the fate of trace elements (arsenic, chromium, and zinc) in a small eutrophic lake with a seasonally anoxic hypolimnion (Lake Greifen). Arsenic and chromium are redox-sensitive trace elements that may be directly involved in redox cycles, whereas zinc is indirectly influenced by the redox conditions. We will illustrate how the seasonal cycles and the variations between oxic and anoxic conditions affect the concentrations and speciation of iron, manganese, arsenic, chromium, and zinc in the water column. The redox processes occurring in the anoxic hypolimnion are discussed in detail. Interactions between major redox species and trace elements are demonstrated. 

The oxidation state of redox-sensitive trace elements such as As(III)/ As(V) and Cr(III)/Cr(VI) is thus affected by the redox conditions, as indicated by the occurrence of major reduced species. Kinetic control of the redox reactions plays an important role. As(III) appears in the anoxic hypolimnion in agreement with the thermodynamic redox sequence together with Fe(II) and sulfide, although the reduction of As(V) is incomplete under these conditions. Whereas the reduced As(III) species can clearly be observed in the... 

Biological, chemical, and physical processes produce distinctive layers, or horizons, in soils. There are numerous types of soil horizons (Birkeland, 1984). However, the most common from the top of a soil profile to the bottom are the O, A, B, C, and R (Table 3.19). Not all of the horizons are present in every soil. Furthermore, the specific characteristics of each horizon are affected by climate, bedrock composition, the types of vegetation, pH, redox conditions, and time (Faure, 1998), 354-355. 

Despite the useful information to study biophysico-chemical processes involving NOM in environmental systems, the SFR concentration depends on various laboratory conditions such as pH and irradiation (Senesi, 1990a Paul et al., 2006), redox conditions, acid hydrolysis, methylation, and temperature (Senesi, 1990a). Moreover, AH and SFR concentration can also be affected by sample humidity and the presence of paramagnetic ions (Novotny and Martin-Neto, 2002 Novotny et al., 2006). 

Some authors claim that liposaccharides can depress the content of TNF-a and increase the activity of superoxide dismutase (SOD) and catalase, thus—via mediators—they can affect the immune system (Can et al. 2003). It has been demonstrated that the NF-p transcription factor, (highly sensitive to the redox potential in its environment), which regulates synthesis of many mediators—cytokines, associated with inflammatory condition and the phenomenon of adhesion of cells— becomes deregulated in old age. Defense functions in such cases (and primarily in arthritis and arthritis-related conditions) are said to be performed by antioxidants (including a-lipoic acid), which can modulate the activity of monocytes and inhibit changes caused by deregulating of the transcription factor NF-kB under the influence of redox conditions in elderly people (Lee and Hughes 2002). 

Oxidation-reduction (redox) reactions, along with hydrolysis and acid-base reactions, account for the vast majority of chemical reactions that occur in aquatic environmental systems. Factors that affect redox kinetics include environmental redox conditions, ionic strength, pH-value, temperature, speciation, and sorption (Tratnyek and Macalady, 2000). Sediment and particulate matter in water bodies may influence greatly the efficacy of abiotic transformations by altering the truly dissolved (i.e., non-sorbed) fraction of the compounds — the only fraction available for reactions (Weber and Wolfe, 1987). Among the possible abiotic transformation pathways, hydrolysis has received the most attention, though only some compound classes are potentially hydrolyzable (e.g., alkyl halides, amides, amines, carbamates, esters, epoxides, and nitriles [Harris, 1990 Peijnenburg, 1991]). Current efforts to incorporate reaction kinetics and pathways for reductive transformations into environmental exposure models are due to the fact that many of them result in reaction products that may be of more concern than the parent compounds (Tratnyek et al., 2003). 

Figure 10.15 Major pathways of the N cycle in sediments (a), as a function of redox conditions in bottom waters and sediments (b). Both diffusive and advective processes strongly control the distribution of O and N compounds which ultimately affect the coupling between nitrification and denitrification. (Modified from Jprgensen and Boudreau, 2001.)...

Redox processes are important for elements which can exist in more than one oxidation state in natural waters, e.g. Fe and Fe, Mn, and Mn. These are termed redox-sensitive elements. The redox conditions in natural waters often affect the mobility of these elements since the inherent solubility of different oxidation states of an element may vary considerably. For example, Mn is soluble whereas Mn is highly insoluble. In oxic systems, Mn is precipitated in the form of oxyhydr-oxides. In anoxic systems, Mn predominates and is able to diffuse along concentration gradients both upwards and downwards in a water column. This behaviour gives rise to the classic concentration profiles observed for Mn (and Fe) at oxic-anoxic interfaces as illustrated in Figure 2. 

U/ U ratios as tracers in springs and small streams, and these have mainly used uranium isotopes to trace the interaction of groundwater with surface water (e.g., Lienert et al., 1994). Lienert et al. (1994) attempted to link changes in uranium behavior to decreases in the anthropogenic inputs of phosphorous, which in turn affects biological activity and redox conditions within waters. As discussed in an earlier section of this chapter, the ratio has... 

Experimental studies. Sorption of radionuclides by colloids is affected by the same solution composition parameters discussed in the previous section on sorption processes. The important parameters include pH, redox conditions, the concentrations of competing cations such as Mg " " and K, and the concentrations of organic ligands and carbonate. The high surface area of colloids leads to relatively high uptake of radionuclides compared to the rock matrix. This means that a substantial fraction of mobile radionuclides could be associated with carrier colloids in some systems. The association of radionuclides with naturally occurring colloids and studies of radionuclide uptake by colloids in laboratory systems give some indication of the potential importance of colloid-facilitated radionuclide transport in the environment as discussed below. 

Surficial processes that affect the redox composition of Earth materials include weathering, drainage, groundwater movement, mechanical mixing and dispersion of rock material, soil formation, the accumulation of organic material and biological processes. There is an almost unlimited number of ways in which these factors can combine to affect the composition of Earth materials and therefore to affect local redox conditions. However, the processes that are most likely to affect redox locally can be simplified. 

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