Predominance field diagrams

Fig. 15.2 Predominance Field Diagram for arsenic under the condition of manganese nodule formation at the bottom of the deep sea (after Glasby and Schulz 1999). The calculations in this diagram were performed with the model PHREEQC. The partial diagram on the left only represents the predominant ranges of the aquatic species. The diagram on the right covers the ranges of several minerals which are, under the given conditions, supersaturated (SI > 0).

Eh-pH diagram showing the predominance fields for oxidized (upper right) and reduced (lower left) forms of selected redox-active species. Note that the curves represent totals for each species i.e., further speciation is not shown. Curves are drawn from variations on Equation (18) for 25°C, using values of E° from Table 16.6 and additional constants from various sources (5, 8, 23). Dashed diagonal lines are for the H2/H20 (lower) and H20 / 02 (upper) couples and together they enclose the conditions over which water is stable. 

The diagram s vertical boundaries (Fig. 15, number 1-5) are reactions that describe a dissolution in water (hydrolysis) independent of the EH value. The boundaries of the respective predominance fields are calculated via the equilibrium constants for the conversion of the species at each side of the boundary line into each other. 

The tabulated thermodynamic data can be used to develop the pe-pH diagrams for arsenic, shown in Figures 2 and 3, that summarize the predominance fields for aqueous species and the mineral stability fields, respectively. These diagrams also help to focus the discussion on environmentally relevant geochemical processes. 

A system with two variables can be graphically illustrated either three-dimensionaUy or by projecting into a plane defined by axes on which the values of variables are plotted. So-called diagrams of predominating existence area (stability field diagrams) are obtained. 

Fig. 8. E/2-pH diagrams showing predominance fields of aqueous species and minerals at 25°C. Iron species and minerals are shown in (A) uranium species and minerals are illustrated by (B). The boundaries of the fields were drawn for a total dissolved Fe concentration of 0.01 ppm, a total dissolved carbon concentration of 10 ppm, a total sulfur concentration of 10 ppm, and a total dissolved uranium concentration of 0.001 ppm.

Figure 1.93. /02-pH diagram with the stability fields of aqueous species in Na-K-H-S-Se-0 system for the conditions SS = 10 mol/kg H2O, ESe = 10 mol/kg H2O, ionic strength = 1, and temperature = 150°C. Dashed lines are the ratio ho-a i- /a i- in logarithmic units. Stability fields for native sulfur and native selenium and the boundaries between predominance regions of oxidized and reduced selenium species are omitted for clarity (Shikazono, 1978b). 

The defect population in the doped solid under moist conditions then consists of V%, OHo, h, and Y /x. The domains over which each species is dominant for conductivity can be represented diagrammatically when data concerning the conductivity of the solid has been measured (Fig. 8.20). In this representation, the conductivity fields are bounded by lines tracing the locus where the transport number for a pair of defects is equal to 0.5. (The diagram could equally well be drawn in terms of domains delineating the defect species that predominate.)... 

Figure 8.21C shows the Eh-pH diagram for phosphorus at a solute total molality of 10 ". Within the stability field of water, phosphorus occurs as orthophos-phoric acid H3PO4 and its ionization products. The predominance limits are dictated by the acidity of the solution and do not depend on redox conditions. 

Sulfur (figure 8.21D) is present in aqueous solutions in three oxidation states (2—, 0, and 6+). The field of native S, at a solute total molality of 10, is very limited and is comparable to that of carbon (for both extension and Eh-pH range). Sulfide complexes occupy the lower part of the diagram. The sulfide-sulfate transition involves a significant amount of energy and defines the limit of predominance above which sulfates occur. 

Figure 8.22A shows the Eh-pH diagram of iron in the Fe-O-H system at T = 25 °C and P = 1 bar. The diagram is relatively simple the limits of predominance are drawn for a solute total molality of 10 . Within the stability field of water, iron is present in the valence states 1+ and 3-I-. In figure 8.22A, it is assumed that the condensed forms are simply hematite Fe203 and magnetite Fe304. Actually, in the 3-1- valence state, metastable ferric hydroxide Fe(OH)3 and metastable goe-thite FeOOH may also form, and, in the 1+ valence state, ferrous hydroxide Fe(OH)2 may form. It is also assumed that the trivalent solute ion is simply Fe ", whereas, in fact, various aqueous ferric complexes may nucleate [i.e., Fe(OH), Fe(OH)2+, etc.]. 

The two-mica granite and the muscovite Ieucogranite phases of the NPSG fall in the field of specialized granites on a Rb-Sr-Ba ternary variation diagram for granitic intrusions (Fig. 3). Based on the elevated uranium contents in these two phases, it is likely that the uranium at Long Lake was derived predominantly from them. 

Fig. 10.15 Observed Na 3p — n = 25,26 spectra in varying strengths of microwave field overlaid on an energy level diagram of the Na m = 0 states. The baseline of each spectrum is located at the amplitude of the microwave field. Note that only odd sidebands of the p states occur and that the predominant effect of higher microwave fields is to add more...

The Eh-pH predominance diagram for Asv and As111 species is shown in Fig. 5.9. The upper and lower boundaries represent the stability field for water. 

However, in sulphides and related minerals, the effects of covalent bonding predominate and orbital overlap must be taken into account. Thus, concepts of molecular orbital theory are described in chapter 11 and applied to aspects of the sulfide mineralogy of transition elements. Examples of computed energy diagrams for molecular clusters are also presented in chapter 11. There, it is noted that the fundamental 3d orbital energy splitting parameter of crystal field theory, A, receives a similar interpretation in the molecular orbital theory. 

Although the predominant source of arsenic and metals to most soils and sediments in New England is sulfide-rich rock, the extensive application of arsenical pesticides and herbicides (lead arsenate, calcium arsenate, and sodium arsenate, and others) on apple, blueberry, and potato fields may have been a possible anthropogenic source of arsenic and lead. The main objective of this study was to determine the lead isotopic compositions of commonly used pesticides, such as lead arsenate, sodium metarsenite, and calcium arsenate, in order to assist in future isotopic comparisons and to better characterize this anthropogenic source of Pb. The pesticides plot along a linear trend in isotope diagrams, for example, in values of... 

Fig. 15 Left EH-pH diagram for the system Fe-02-H20 (at 25°C, the numbers 1-11 correspond to the reaction equations described in the text for the calculation of the stability fields, modified after Langmuir 1997) Right EH-pH diagram for the system Fe-02-H20-C02 (at 25°C, P(C02) = Iff2 atm), for fields where the total activity is < 10"6 (1) resp. < 10"4 (2) mol/ L the predominant, precipitating mineral phase is outlined (modified after Garrels and Christ 1965).

Hydrolysis reactions written in stepwise form are considered first. For iron these have been listed with their constants in Table A 12.2, Part I, from which it is clear that the boundaries between fields of predominance of the successive hydrolysis species are drawn at pH = pK for each stepwise reaction. Figure 12.1 and Table A 12.2, Part I indicate that FeOH and Fe(OH)2 complexes never predominate and so will not appear on the Eh-pH diagram. Drawing the vertical pH-dependent boundaries for Fe(lll) reactions 1 to 4 downward from the O2/H2O boundary and the boundary for reaction 8 upward from the H2O/H2 boundary, it becomes clear which redox reactions between Fe(lII) and Fe(II) species generate the boundaries that must next be considered. These reactions (9 to 14) and the Eh/pH equations of their boundaries are listed in Table A12.2, Part II. The resultant Eh-pH plot for Fe aqueous species only is shown in Fig. 12.8. 

Figure 12.16 Eh-pH diagram for thermodynamically stable substances in the system S-02-H20 at 25°C, showing the fields of predominance of the aqueous species and of elemental sulfur for S(aq) = I0 mol/kg at aqueous/S° boundaries.

Figure 13.27 Eh-pH diagram for the system Np-Oj-H O at 25°C and 1 bar total pressure for INp(aq) = 10" M, showing the stability fields of predominant aqueous species.

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