Redox-sensitive elements

Fig. 3. Sample collection deeper into the saturated zone results in more reducing conditions (a) and increase in redox sensitive elements such as Fe (b).

The isotope geochemistry of two redox-sensitive elements, Cr and Se, are compared in Chapter 9 by Johnson and Sullen (2004). Because Cr(VI) is quite soluble in natural waters, and is highly toxic, the signiflcant fractionation in ratios that occurs during redox... 

Uranium is a redox sensitive element, with most naturaiiy occurring uranium either as or Theory wouid indicate that U shouid be preferentiaiiy retained in oxidized phases, such as dissoived whereas shouid be preferentiaiiy... 

In summary, the studies reviewed here use diverse strategies to take advantage of the redox properties of two classes of catechol-quinone compounds present in nature to design new compounds of pharmaceutical interest. In a third class of naturally occurring compounds of complex structure, simplification and removal of the redox-sensitive elements may be key to providing target structures with a novel antiviral character. 

The cycles of trace elements in eutrophic lakes are thus strongly connected, directly and indirectly, to the biological processes and to the cycles of major redox-sensitive elements. 

Elbaz-Poulichet, F., Nagy, A. and Cserny, T. (1997) The distribution of redox sensitive elements (U, As, Sb, V and Mo) along a river-wetland-lake system (Balaton region, Hungary). Aquatic Geochemistry, 3(3), 267-82. 

Kneebone, P.E. and Hering, J.G. (2000) Behavior of arsenic and other redox-sensitive elements in Crowley Lake, CA a reservoir in the Los Angeles aqueduct system. Environmental Science and Technology, 34(20), 4307-12. 

The speciation of redox-sensitive elements (e.g., Cr, Fe, and Mn), redox-sensitive elements that also form organometallic compounds (e.g., As), and redox-sensitive elements that also form organome-tallic and volatile compounds (e.g., Hg) in aquatic systems is discussed in this chapter. Because the scope of this subject is so large, only the dissolved phase in the aquatic system will be considered. 

The mobility of radium is determined by redox-sensitive iron, which readily forms iron oxyhydroxides under oxidizing conditions and thus limits the concentrations of iron and radium because radium is effectively sorbed on iron oxyhydroxide. Redox-sensitive elements are elements that change their oxidation state by electron transfer depending on the relative oxidizing or reducing conditions of the aquatic environment (chapter 1.1.5.2.4 and 0). Thus radium behaves like a redox-sensitive element, even though it only occurs in the divalent form. 

Table 12 shows some redox sensitive elements in the periodic system of the elements, Table 13 depicts standard potentials for some important redox pairs in aqueous systems. 

Table 12 Redox sensitive elements in the PSE (bold = most common oxidation states in natural aqueous systems) (after Emsley 1992, Merkel and Sperling 1996, 1998)...

The unit used for the input of concentrations can be defined with the keyword units. Possible units are mass or moles per liter solution, moles per kg solution or moles per kg water. Concentrations thereby can be given in g, mg, ug (not pg) or mol, mmol, and umol. Temperature (temp) is denoted in °C. The density (density) can be entered in g/cm3, with a default of 0.9998. That information is especially important for highly mineralized waters, like e.g. seawater. To input the measured Eh value a conversion to the pE value is necessary (see chapter 1.1.5.2.2, Eq. 65). If no pE value is given, pE is assumed to be 4 by default. A redox couple (redox) can be defined to calculate the pE value that will be used to model the species distribution of redox sensitive elements if no pE is given. 

A redox couple can be separately defined according to a redox sensitive element (in the example U according to N(5)/N(-3) redox couple) that can be given either as total concentration (like U) or as partial concentrations of the respective species (like Fe). The input enforces a calculation of a redox equilibrium of the redox sensitive elements by means of the given redox couple. In this example the standard pE value for the standard redox couple will not be used for this element (in the example of uranium) to calculate the uranium species. 

If redox sensitive elements (e.g. N03", NH4+ in the case of the seawater analysis) are declared in the input file, a paragraph redox couples will be displayed in the output after description of solution that contains all individual redox couples (in the example N(-3)/N(5)) with their respective redox potentials as pE, and EH value in volts. 

How would you interpret the analysis Have a close look at the redox sensitive elements (which elements do not fit in the general scheme and why ). Present the species distribution of the Ca-, Mg-, Pb- and Zn-species in the form of a pie chart... 

Degradation of organic matter within the aquifer on reduction of redox sensitive elements (Fe, As, V, Cu, Mn, S)... 

In terms of redox sensitive elements it is remarkable that for As, Cu, Fe, N, and U the respective oxidized forms predominated, while Mn predominantly occurs as Mn (+2) and Se as Se (+4). Fig. 20 shows that the Mn oxidation does not start up to a pE > +10, while the analysis on hand has a pE of 6.9. The oxidation of Se starts at lower values so that Se (-2) is already completely oxidized, and there are small quantities of Se(+6). However, the partly reduced form Se(+4) still predominates. 

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. 

Other examples of redox-sensitive elements include heavy elements such as uranium, plutonium, and neptunium, all of which can exist in multiple oxidation states in natural waters. Redox conditions in natural waters are also indirectly important for solute species associated with redox-sensitive elements. For example, dissolution of iron (hydr)oxides under reducing conditions may lead to the solubilization and hence mobilization of associated solid phase species, e.g. arsenate, phosphate (see Sections 3.3.2.1, 3.3.3.2, and 3.3.4.1). 

Bekker A., Holland H. D., Rumble D., Yang W., Wang P.-E., and Coetzee L. L. (2002) MIF of S, oolitic ironstones, redox sensitive elements in shales, and the rise of atmospheric oxygen (abstr.). Geochim. Cosmochim. Acta 66, A64. 

Thomson J., Nixon S., Croudace I. W., Pedersen T. F., Brown L., Cook G. T., and MacKenzie A. B. (2001) Redox-sensitive element uptake in north-east Atlantic Ocean... 

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