Lead enrichments

American toad, Bufo americanus tadpoles held in lead-enriched waters (0, 500, 750, or 1000 pg Pb/L) for 144 h then given choice of lead-containing solutions (0, 500, 750, or 1000 pg/L) in fluvorium No deaths. Toads did not prefer or avoid lead-containing solutions 48... 

Steele, C.W., S. Strickler-Shaw, and D.H. Taylor. 1991. Failure of Bufo americanus tadpoles to avoid lead-enriched water. Jour. Herpetol. 25 241-243. 

Fig. 31. CD of the supernatant after ethanol precipitation of the ruthenium complex bound S-DNA. Binding to B-DNA is stereoselective and leads enrichment of the supernatant in the unbound A-isomer 1761...

As seen in Figure 2.26, lead ions are fairly evenly distributed across the surface, proving cation exchange. However, there are spots where the concentration of lead is higher than average. In other words, lead enrichments can be observed. 

The results of the quantitative analysis of montmorillonite samples obtained with different Ca-montmorillonite/lead ion ratio and pH are shown in Table 2.14 for the surfaces with even lead distribution. The chemical composition of lead enrichments is shown in Table 2.15. 

As discussed previously in this chapter, there are spots where the lead content (and the elementary composition) is higher and so-called lead enrichments are formed. Table 2.15 shows that the composition of lead enrichments can be rather different in different enrichments. Besides the increase in lead concentration, other changes in elementary composition may also occur. There are examples of lead enrichment (Figure 2.27 Sample 3 in Table 4) where the average composition remains unchanged even though the lead concentration is elevated. However, there are lead enrichments where the increase in lead concentration is accompanied by a simultaneous increase in iron (Sample 4 in Table 2.15), or a simultaneous increase of calcium and lead (Sample 2). Lead enrichment in these cases is likely due to lead coprecipitation reactions with other minerals (iron oxide and other calcium silicate phases) that were present in the sample. 

As mentioned in Section 2.10.3.1, the resolution of SEM (Figure 2.26) is about 1 pm. Thus, the size of enrichments greater than 1 pm can be measured by SEM. When smaller spots of lead enrichments are found, their size should be determined by another method below 1 pm resolution. For example, atomic force microscopy (AFM) can be a suitable method since it has nanometer resolution. This technique,... 

The SEM picture shows two types of particles (1) lead enrichments without montmorillonite, and (2) montmorillonite particles with lead enrichment on it. The montmorillonite particles are much larger, their size can reach 10 pm, and their elemental SEM maps show even distributions of Al, Si, Mg, Fe, Ca, and O only the distribution of Pb shows spots with higher concentration (white spots in Figure 2.29). 

Lead enrichments can be detected only when the system contains both lead ions and montmorillonite. However, they cannot be observed on mica flakes immersed... 

Of course, only the weakly adsorbed lead enrichments can be transferred onto the surface of mica. The presence of other, more strongly adsorbed, lead enrichments on montmorillonite cannot be excluded. 

The precipitation and colloid formation of different metal oxide hydroxides is known in soils when the concentration of the ions reaches the value of stability products. In this case, the precipitation can be explained by the thermodynamic properties of the bulk solution. In the lead ion/calcium-montmorillonite system, however, the production of lead enrichments cannot be explained by the... 

As shown in Chapter 2, Section 2.10.3, the system of lead ion/calcium-montmoril-lonite lead enrichments may form when, based on thermodynamic conditions, their formation is not expected. The process was considered as surface precipitation or coprecipitation, depending on the chemical composition of the lead enrichments. Since lead enrichments are fairly bulky (a few hundred nanometers to micrometers), their migration rate is practically zero. Therefore, if they form under natural conditions, they should be observed on the surface of natural clay. In fact, lead enrichments were found on clay mineral surfaces from lake sediments (Figure 3.6 Nagy et al. 2003a). 

Under natural conditions, the thermodynamic conditions are obviously unknown, so the lead enrichment formation mechanism is also unknown. However, the formation of nano- and microparticles containing lead ions under environmental conditions is especially intriguing. At least a portion of lead ions in nature will form similar enrichments. The process, all types of precipitation, can decrease the migration rate of lead ions. If the precipitation of any substance occurs, its migration rate can decrease. 

Brenan J.-M., Shaw H. F., and Ryerson F. J. (1995) Experimental evidence for the origin of lead enrichment in convergent-margin magma. Nature 378, 54-56. 

Brannvall M. L., Kurkkio H., Bindler R., Emteryd O., and Renberg I. (2001) The role of pollution versus natural geological sources for lead enrichment in recent lake sediments and surface forest soils. Environ. Geol. 40, 1057-1065. 

A. Kabata-Pendias and K.P. Henry (1991) Ecological consequences of arsenic, cadmium, mercury and lead enrichment in European Soils . In Lead, Mercury, Cadmium and Arsenic in the Environment, SCOPE 32, T.C. Hutchinson, K. Meema and C. Gordon, eds., J. Wiley, pp. 36-77. 

New Pd-Pb catalysts comprising of the intermetallic compound PdsPb has been reported with improved properties over commercially available Lindlar s catalysts for the selective reduction of 2-butyne and phenylacety-lene, in which overreduction and isomerisation are diminished. While commercially sourced Lindlar catalyst (Pd-Pb-CaCOs) was found to be comprised of palladium with highly dispersed lead covering its ciystallites, the intermetallic compound PdsPb catalyst appears to contain much more homogeneously dispersed alloy crystallites, although with lead enriched in the surface layer. ... 

Soils and waters affected by leaded gas, smelter emissions, mining wastes and by-products. Dusts, soils, and debris containing lead-bearing paint. Foods grown in lead-rich soils. Soils and dusts derived from naturally lead-enriched rocks. Waters that have leached lead from supply pipes. 

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