Earth enrichment factor

Uranium is not a very rare element. It is widely disseminated in nature with estimates of its average abundance in the Earth s crust varying from 2 to 4 ppm, close to that of molybdenum, tungsten, arsenic, and beryllium, but richer than such metals as bismuth, cadmium, mercury, and silver its crustal abundance is 2.7 ppm. The economically usable tenor of uranium ore deposits is about 0.2%, and hence the concentration factor needed to form economic ore deposits is about 750. In contrast, the enrichment factors needed to form usable ore deposits of common metals such as lead and chromium are as high as 3125 and 1750, respectively. 

The special position of the Earth among the terrestrial planets is also shown by the availability of free water. On Venus and Mars, it has not until now been possible to detect any free water there is, however, geological and atmospheric evidence that both planets were either partially or completely covered with water during their formation phase. This can be deduced from certain characteristics of their surfaces and from the composition of their atmospheres. The ratio of deuterium to hydrogen (D/H) is particularly important here both Mars and Venus have a higher D/H ratio than that of the Earth. For Mars, the enrichment factor is around 5, and in the case of Venus, 100 (deBergh, 1993). 

One useful method for systematizing data on sources and ambient aerosols Is the use of enrichment factors with respect to the Earth s crust, EFpT-ngt defined by (37, 38) ... 

Table V. Enrichment Factors for Fine Particles with Respect to Earth s Crust ...

Table 9.11 shows the aerodynamic mass median diameter (MMD) for some typical inorganics that are common components of tropospheric particles. Also shown are the calculated crystal enrichment factors, EFcrusl. These are a measure of the enrichment of the element in the airborne particles compared to that expected for the earth s crust, using aluminum as the reference element. Thus EF,.rust for a particular element X is defined as... 

For example, the most common elements in the earth s crust (Table 9.12 and Fig. 9.34) are O, Si, Al, Fe, Mg, Ca, Na, K, and Ti. These elements have MMDs > 3 fxva and enrichment factors that are generally less than three (Table 9.11). That the enrichment... 

Particulate emissions data for 21 studies of coal-fired power plants were compiled for use in receptor models. Enrichment factors were calculated (relative to Al) with respect to the earth s crust (EFcrust) and to the input coal (EFcoai). Enrichment factors for input coals relative to crustal material were also calculated. Enrichment factors for some elements that are most useful as tracers of coal emissions (e.g., As, Se) vary by more than ten-fold. The variability can be reduced by considering only the types of plants used in a given area, e.g., plants with electrostatic precipitators (ESPs) burning bituminous coal. For many elements (e.g., S, Se, As, V), EFcrust values are higher for plants with scrubbers than for plants with ESPs. For most lithophiles, EFcrust values are similar for the coarse (>2.5 ym) and fine (<2.5 ym) particle fractions. 

We also calculated "enrichment factors" to simplify comparisons between different plants. About half of the particulate matter in the atmosphere is suspended soil, so we have used enrichment factors with respect to the earth s crust to help identify potential coal tracer elements ... 

Before discussing the emitted particles, let us consider the compositions of the coals themselves. Figures 1 and 2 show enrichment factors for eastern and western coals relative to the earth s crust, EFcrust(coal), respectively. Soluble elements such as the alkali metals and Mn are depleted in coal, and apparently are leached from it during its formation. Sodium, Ca, and Mn are more depleted in eastern than in western coals, whereas K, Rb, and... 

Figure 1. Enrichment factors with respect to the earth s crust (14) for coals from the eastern United States. Based on coal compositions determined by the Illinois State Geological Survey (15). Range shown for each element is from Xg/ag to xg-ag, where xg and ag are geometric means and standard deviations.

Enrichment factors for stack emissions relative to the earth s crust are shown in Figure 3 for 56 elements. Averages are based on all available data from the 21 studies. An EFcrust of about 1 indicates that an element is at about the same relative concentration in coal-fired plant emissions as in soil. An EFcrust of less than 1 indicates that an element is depleted relative to soil, while a value greater than one indicates an enrichment of the element in coal-fired plant emissions. Elements such as Na, K, Si, Ca, Rb and Mn are depleted relative to the crust because of depletion in the input coals. Most lithophiles, e.g., Sc, Ti, Fe, and rare earths, have EFs of about 1. 

Table II. Enrichment Factors with Respect to the Earth s Crust and to Input Coal for All Plants and for Eastern Plants with ESPs. Geometric Ranges are given for 22 Selected Elements on Total Particles.

Soil is the product formed when the rocks of the earth s crust are exposed at the surface and are subjected to various physical, chemical, and, eventually, biological weathering processes. The minerals in these rocks are predominantly silicates, which dominate the characteristics of most soils. Table 1 shows those elements that are found in the crust above an average concentration of 1% and their corresponding soil content. The importance of aluminosilicates in soil is clear from the enrichment factors of approximately 1 for O, Si, and Al. Some loss occurs of K, Fe, Ca, Na, and Mg as a result of soil processes. But two elements, C and N, show considerable enrichment in soil because of the crucial role played by organic matter. 

Table 1 Average elemental concentrations in the earth s crust and in soil, and the enrichment factor in soil...

Figure 1 Ionic radius (in angstrom) versus ionic charge for lithophile major and trace elements in mantle sihcates. Also shown are ranges of enrichment factors in average continental crust, using the estimate of (Rudnick and Fountain, 1995), relative to the concentrations in the primitive mantle (or hulk silicate Earth ) (source McDonough and Sun, 1995).

There is —90 ppm of phosphorus in the silicate Earth (McDonough et al., 1985), and the bulk Earth is estimated to have — 0.1 wt.% phosphorus. Using the relationships in Figure 6 the core is thus estimated to have —0.20 wt.% phosphorus (Table 4). Thus, 90% of the planet s inventory of phosphorus is in the core (Table 6) and the core s metal/silicate phosphorus enrichment factor is —22. Similarly, the core hosts —90% of the planet s carbon budget, and has a metal/silicate enrichment factor only slightly lower at —17. 

Figure 1 Enrichment factors of elements ( ) average shale as compared with the Earth s upper continental...

We turn next to consider the nonvolatile alkali and alkaline earth elements and the insoluble components of mineral origin. Their major natural sources are the Earth s crust and the ocean, respectively. We expect the chemical composition of the aerosol to reflect the relative contributions of elements from both reservoirs, provided other contributions from anthropogenic or volcanic sources are negligible. In Section 7.4.4 it has been noted, however, that his premise does not hold for all constituents of the aerosol. Some trace components are considerably enriched compared with their crustal abundances. It is appropriate, therefore, to inquire whether the observations confirm our expectations at least for the major elements listed in Table 7-13, or whether deviations occur also in these cases. As Rahn (1975a,b) has shown, the problem may be approached in two ways, either by calculating enrichment factors defined by... 

Atmospheric deposition is also a major source of metal input into many aquatic ecosystems (Salomons 1986). Helmers and Schrems (1995) reported for the tropical North Atlantic Ocean that wet trace element deposition dominates over dry input. From the increased enrichment factors relative to the Earth s crust, the determined trace metal concentrations were assumed to originate from anthropogenic sources. For atmospheric wet depositional fluxes of selected trace elements at two mid-Atlantic sites, Kim et al. (2000) reported that at least half of the Cr and Mn and more than 90% of the Cd, Zn, Pb, and Ni are from non-crustal (presumably anthropogenic) sources. 

Fig. 18.31 The concentrations of chemical elements in the crystalline micrometeorites (MMs) at Cap Prudhomme, Antarctica, in the size range 100-400 pm deviate significantly from the chemical composition of the rocks in the crust of the Earth. The enrichment and depletion of the MMs is expressed as the logarithm to the base 10 of the ratios of the concentrations of the MMs divided by the concentrations in crustal rocks as reported by Taylor and McLennan (1985). The elements that are enriched in the MMs and their respective enrichment factors include primarily Ir (4700), Au (333), Ni (43), Se (36), Cr (34), Fe (28), Co (8.8), As (4.8), and Sb (2.2). The MMs were analyzed by C. Koeberl by instrumental neutron activation analysis (INAA)...

All ore mineral deposits lie in or on solid rocks of which the Earth s crust is predominantly composed. The geological processes which are responsible for the formation of rocks also form the ore bodies associated with them. For the formation of an ore body, the metal or metals concerned must be enriched to a considerably higher level than their normal crustal abundance. The degree of such enrichment below which the extraction cost makes the processing of the ore uneconomical is termed the concentration factor. Typical values of the concentration factor for some of the common metals are given in Table 1.5. 

Elderfield and Greaves [629] have described a method for the mass spectromet-ric isotope dilution analysis of rare earth elements in seawater. In this method, the rare earth elements are concentrated from seawater by coprecipitation with ferric hydroxide and separated from other elements and into groups for analysis by anion exchange [630-635] using mixed solvents. Results for synthetic mixtures and standards show that the method is accurate and precise to 1% and blanks are low (e.g., 1() 12 moles La and 10 14 moles Eu). The method has been applied to the determination of nine rare earth elements in a variety of oceanographic samples. Results for North Atlantic Ocean water below the mixed layer are (in 10 12 mol/kg) 13.0 La, 16.8 Ce, 12.8 Nd, 2.67 Sm, 0.644 Eu, 3.41 Gd, 4.78 Dy, 407 Er, and 3.55 Yb, with enrichment of rare earth elements in deep ocean water by a factor of 2 for the light rare earth elements, and a factor of 1.3 for the heavy rare earth elements. 

Trace metals, such as copper, nickel, cobalt, zinc, and various rare earth elements, tend to coprecipitate with or adsorb onto Fe-Mn oxides. As shown in Table 18.1, this causes these elements to be highly enriched in the hydrogenous deposits as compared to their concentrations in seawater. The degree of enrichment is dependent on various environmental factors, such as the redox history of the underlying sediments and hydrothermal activity. This makes the composition of the oxides geographically variable. 

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