Mercury enrichment factors

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. 

Large scale enrichment of lithium for thermonuclear uses took place at the Oak Ridge National Laboratory in the 1950 s. The enrichment primarily employed ion exchange between aqueous/organic solutions and amalgam, commonly mercury-based (Palko et al. 1976). Electrochemical separation has also been employed for such operations (Umeda et al. 2001). These practices have not been taken up in academic laboratories in the intervening years, partly as they tend to be most effective only with relatively pure starting materials, partly because of the difference in scales involved. Enrichment factors of Li of 1-7% are typical for these techniques (Symons 1985). 

Most of the discrimination between inorganic and methylmercury thus occurs during trophic transfer, while the major enrichment factor is between water and the phytoplankton. This also has been reported for the diatom Thalassiosura weissflogii in a marine food chain (Mason et al. 1996). Methylmercury was accumulated in the cell cytoplasm, and its assimilation by copepods was 4 times more efficient than the assimilation of inorganic mercury. Bioaccumulation has been demonstrated for predator fish in both freshwater and marine systems and in marine mammals (see Section 5.4.4). Bioaccumulation of methylmercury in aquatic food chains is of interest, because it is generally the most important source of nonoccupational human exposure to this compound (EPA 1984b WHO 1990, 1991). 

Isotopic enrichment has also been found by monoisotopic photosensitization for mixtures of natural mercury and alkyl chlorides and vinyl chloride by similar processes. Isotopic enrichment is dependent on such factors as lamp temperatures, flow rates, and substrate pressures. Enrichment increases with decreasing lamp temperature and increasing flow rate, since process (VIII-1) is more ellicient at low temperatures and Cl atoms react with natural mercury containing higher fractions of 202Hg in (VIII-3) at higher flow rates of HC1 or under intermittent illumination. The intermittent illumination results in higher enrichment than the steady illumination. 

The contents of some trace elements in the continental crust, shales, soils, bituminous coals and plankton are given in Table 1.1 to provide some perspective when considering other aspects of these elements. In each of these situations, organic matter is associated with the elements to a greater or a lesser degree. This is not usually very marked with crustal rocks except shales, but may be a major factor for some elements in surface soils and coals. The data in Table 1.1 show that, for some elements, e.g. beryllium, cadmium, cobalt and molybdenum, the contents of the various reservoirs are similar, while for others, there may be enrichments relative to the crust, e.g. boron and sulfur in many shales, soils and coals, mercury, nickel and selenium in many shales, and germanium in some coals. 

Mercury Hg species are generally toxic. Inorganic Hg salts, however, are less dangerous than methylated forms. The latter are extremely more toxic (by a factor of 100) and can be enriched up to 10000-fold in fish. 

Information on the land biota is also tentative. Because most land plants accumulate woody parts for an extended period of their life, the overall amount of mercury in the total land biomass is proportionally greater than that in water biota. The huge amount of litter produced in forests (10 tons/year) (25) carries a large amount of the immobilized mercury to the forest floor. From there, it is incorporated into humus and finally into the soil. For this reason the uppermost soil (Ao and Ai horizons) is enriched in mercury by a factor of 2 to 4 with respect to that in underlying layers. To a smaller extent the same situation applies to agricultural land and pastures. The removal of mercury from soil by cropping represents barely 5% in 1000 years, which means that agricultural soil is really not in equilibrium with respect to natural soils. 

Trauser and co-workers investigated the application of molecular or short-path distiUation in the enrichment of the lithium isotopes. They developed single and multi-stage apparatus and found separation factors between 1.052 and 1.064 for one stage in the temperature range from 535 to 627 °C. In a similar way the mercury isotopes were separated. 

A very good relationship is obtained between the porosity, from mercury poro-simetry measurements, and permeability of hardened pastes. However, this method caimot be recommended in the case of concretes, as it has been mentioned in Chap. 5. A representative concrete sample cannot be satisfactorily reduced to the small specimen for mercury porosimetiy measurements. Therefore different methods of concrete enrichment in paste component are used the last one being the concrete porosity controlling factor. The concretes with lightweight aggregate are an exception in this case. There is usually too much paste in these small samples of concrete and the methods of concrete enrichment in paste worsen additionally the situation. 

The high value of the preexponential factor may be explained by a considerable change in the properties of the surface layer with temperature. An alloy containing 2.1 at.% Hg is nearly saturated at room temperature, while at a temperature of about 80°C, the solubility of mercury increases about 1.5 times[403]. Consequently, at increased temperatures, the alloy is far from saturation. We can therefore expect a lower surface concentration for mercury and higher for gallium. The enrichment of the surface by a metal with a lower overpotential increases (in absolute magnitude) the temperature coefficient of the overpotential, i.e. increases the activation energy and the preexponential factor. 

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