Lead 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. 

Metal ions such Cu, Cd, and Pb can be preconcentrated from water samples using liquid membranes containing 40% w/w of di-2-ethylhexylphosphoric acid in kerosene diluent in a PTFE support. The liquid membrane can be coupled on-line to an atomic absorption spectrometer and has been shown to be stable for at least 200 h with extraction efficiencies over 80%, and enrichment factors of 15 can be obtained. A liquid membrane has also been used for sample cleanup and enrichment of lead in urine samples prior to determination by atomic absorption spectrometry [100]. The experimental setup for metal enrichment is shown in Fig. 13.4. Lead was enriched 200 times from urine [80] and several metals were enriched 200 times from natural waters [88]. Using hollow fiber... 

On the other hand, in the literature reviewed for Table 9.11, particulate lead has an MMD of only 0.55 fim. This is consistent with the major source of lead to the atmosphere in the 1970s being combustion of leaded gasoline, the remainder being due mainly to smelting processes. As a result, Pb is primarily found in the accumulation mode and has a large enrichment factor... 

A negative value of adsorption energy leads to a high value of the enrichment factor Yb/Xb and to a sharp decrease of v with the first additions of B in A. Equation (4.8) shows that factors favoring adsorption of B are (i) a low value of ofv compared to and (ii) weak A-B interactions i.e., a tendency of A-B alloy to form a miscibility gap (A 0). 

Although the model was developed for a A-B alloy over the whole composition range (Li et al. 1989), application to the case of a solution that is infinitely dilute in B leads to simple expressions for the slope of enrichment factor at the interface YB/XB where YB is the molar fraction of B in the interfacial monolayer ... 

Kolthoff and Noponen mixed 25 ml of 0.11 Af barium nitrate, 10 ml of 0.1 M lead nitrate, and 25 ml of 0.1 M sodium sulfate. After precipitation and digestion for 1 h, 59% of the lead remained in the precipitate. Calculate the recovery and separation (enrichment) factors for barium. 

Another evaluation of metal transfer rates was made by Sposito (1986) on the basis of the data compiled by Buat-Menard (1985). Despite many uncertainties, the conclusion emerges from Sposito s review (1986) that the trace metals Cu, Zn, Ag, Sb, Sn, Hg, and Pb are the most potentially hazardous on a global or regional scale. Lead is of acute concern on the global scale, because of its prominent showing in all the enrichment factors and transfer rates considered. These conclusions are in accordance with those of Andreae et al. (1986a) in a Dahlem report. 

Gastropods, jellyfish, and sea anemones accumulate lead from sea water by an enrichment factor of 2600, scallops by a factor of 5300 the kidneys of scallops were shown to contain 137 ppm of lead (material dried at 110 °C)59). 

Evaporation is a method for preconcentration used in the analysis of water samples. The enrichment factor is between 10 and 10 This approach however may lead to losses of certain elements due to adsorption on the walls of the vessels. That is why evaporation nowadays is practically used in the preconcentrations of radionuclides from waters since the carriers prevent adsorption (Das et al., 1983 Mizuike, 1983 Toelgyessy and Kyrs, 1989). 

More sophisticated approaches to compound selection aim to use knowledge of ligand and/or receptor to improve the odds of finding hits, thereby leading to a good enrichment factor, the increase in the proportion of hits found in any given sample of compounds compared with the proportion expected for a random sample. For example, if screening a full library of... 

The crucial metric for assessing the performance of docking codes is the extent to which a dataset of compounds can be enriched such that only a much smaller subset needs to be prepared and assayed to identify hits or leads. Following Pearhnan and Charifson [36], the enrichment factor can be calculated according to Equation 111. 

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