Background
Analysis of Lead in Soil
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What is it? What is its history? |
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Lead is metal that has found use since ancient times, famously in plumbing. Indeed, Plumbum is the Latin word for lead. Roman cooking and storage utensils were made of lead and they used lead sugar (lead(II) acetate) as a sweetening agent until they had discovered (by the time of Ceasar Augustus) that the last two uses were toxic! (from Britannica, primary source Vitruvius?) Lead compounds have also long been used in paints: lead white (PbCO3)2 · Pb(OH)2 and red lead Pb3O4 (aka minium). Minium and litharge PbO were quite important in Lavoisier's ♣ experiments leading to modern chemical theory.
Until recently (1989), tetraethyl lead Pb(C2H5)4 was used in large quantities as a gasoline additive in the US. Lead is no longer used that way because it is considered a significant contributor to air pollution. Currently, large amounts of lead are used in batteries, lining industrial pipes and tanks, in X-ray apparatus and in radiation shielding. (You wear it when receiving an x-ray at the dentist!) Lead is used in solder and a large number of low melting alloys.
The sources of lead have varied over time. In the US , mines in Missouri , Kansas , and Oklahoma all provided lead in the 1700's. As the American west opened up, mines in these locations came into operation. At one time, American mines contributed one third of the world production of lead, but since the mid 1900's the major US galena (primary lead ore) deposits have been exhausted. Other large lead deposits are found in Australia , Canada , Mexico , Peru , Serbia , and Russia (from Encarta).
Fortunately, metallic lead and insoluble lead compounds are not absorbed through the skin. However, ingestion of lead and of lead compounds does result in the binding of lead to blood cells and accumulation of lead in the bone. Sugar of lead (lead (II) acetate) mentioned above as a sweetener used by the Romans (until they learned better) likely was formed in vats of vinegar that had been sealed using lead. Acids will convert lead metal into Pb(II) salts. The problem with lead cooking utensils was likely an effect of this same chemistry caused by the acids in foods being prepared.
The major concern regarding lead toxicity in the US is in children because their rapid bone growth results in accumulation of more lead than in adults. The major source of this exposure is lead paint which was commonly used in this country before 1978, when it was banned from most uses. Webmd.com is a good source of additional information regarding lead toxicity as are other sites that do not require registration such as one maintained by the State of Wisconsin.
Where did it come from?
To answer this question, we rely on cosmic and nuclear chemistry. Most elements, or more accurately their nuclei, are synthesized in stars by fusion of lighter nuclei, a process known as Stellar Nucleogenesis. In the first stage, four hydrogen nuclei fuse to form one helium-4 nucleus. In the second stage, helium nuclei fuse to form carbon-12, oxygen-16, neon-20, and magnesium-24. In the third stage, elements as heavy as iron-56 are formed (the isotope with the highest nuclear binding energy). This is the end of the line for fusion reactions that yield energy. In the fourth stage, in very heavy stars much more massive than our own sun, the core of the star collapses. The protons in iron and other elements combine with electrons to form neutrons and a very dense neutron star. These stars may then violently collapse due to the huge gravitational force exerted by their mass. The resulting stellar novas produce great streams of neutrons.
Stable isotopes of elements heavier than iron are formed in Neutron Capture Reactions: the stepwise capture of single neutrons followed by a beta decay forming a new isotope one atomic unit heavier (slow or s-process). After sufficient time has elapsed, all of the elements up to bismuth-209 are synthesized.
Isotopes with a mass greater than bismuth-209 cannot form in this way since the intervening nuclei’s half lives are too short. Instead the heaviest elements are formed by multiple neutron capture reactions in quick succession when the density of neutrons is high during the supernova process (rapid or r-process). The heaviest isotope observed to be synthesized in this manner is californium-254.
After billions of years, the debris from earlier novas recombines to form new solar systems, of which ours is one. Because all of the nuclei with atomic numbers greater than 83 (bismuth) are radioactive, only a few of these isotopes have long enough half lives that they can be incorporated into the minerals on planets. These isotopes are thorium-232, uranium-235, and uranium-238.
Lead (and bismuth) are unique in that they can be formed both by s-process of neutron capture AND by radioactive decay of heavier elements. All nuclei can be placed into one of 4 decay series by dividing its mass number (sum of neutrons and protons) by 4 and using the remainder (7 divided by 4 equals 1 remainder 3) which identifies the series - 4n+0, 4n+1, 4n+2, 4n+3. The stable end product of each of these series is either a lead or bismuth isotope (Table 1). The members of all of these series except the 4n+1 can be found on earth in small amounts in radioactive equilibrium in uranium and thorium ores. No members of the neptunium series except the end product bismuth-209 are found on earth since the half life of the longest lived isotope in the series is short compared to the age of the earth.
The combination of geochemical processes and the unique chemistry of each element (including that of lead, thorium, and uranium) result in the segregation different elements into different rocks/minerals. These minerals end up with different isotopes of lead following the radioactive decay of the segregated thorium-232, uranium-235 and uranium-238 deposits. For example a rock which initially had thorium-232 present would (after billions of years) become enriched in lead-208. Because of the diverse origins of the lead isotopes (relative to any other element) and the likelihood of geochemical differentiation, no other element found on earth exhibits as wide a range in its natural abundance variability (Table 1).
How much is there? and where?
As result of this nuclear chemistry, the best estimates of the natural abundance of lead (all isotopes) in the earth’s crust is 10 to 14 ppm. This number has been derived from the chemical analysis of lead in crustal rocks and estimates of the abundance of those rock types in the crust, although other estimates range from 5 to 25 ppm. Lead content varies in igneous rock types from 49 ppm in granite (G1 rock) to 8 ppm in basalt (W1 rock) and in sedimentary rock, sandstone for example, the concentration is 7 ppm. The concentration of lead in seawater is only 3 x 10-5 ppm (Brittanica). For comparison, silicon is a major component of the earth’s crust and its abundance is estimated to be 270000 ppm by mass (webelements).
Because the Th 232 and U 238 are so long lived, significant amounts of these isotopes have existed on earth throughout all of geological time. As the earth cooled the interior structure of the planet differentiated into core, mantle, and crust. The upper mantle is assumed to be a relatively homogeneous silicate/perovskite material (i.e. peridotite). The sizes of uranium and thorium atoms made them a poor fit into the crystal structures of these minerals deep in the earth. As a result, these atoms stayed in the liquid state and were forced up into the crust of the earth as molten magma. Consequently, most of the uranium and thorium present in the earth (and therefore most of the lead) has been concentrated in the crust of the earth.
Of course, lead is not recovered at random from crustal rocks. The magma that carried lead and lead precursor isotopes to the earth's surface cooled into deposits of minerals with high lead concentrations (lead ores). Lead metal is refined from these ores, the most economically useful of which, is galena. This ore contains lead largely as the lead (II) sulfide, PbS, and is recovered by roasting the ore in air.
One of the primary hypotheses of this research depends on the fact that lead ores from different locations do NOT contain the same ratios of lead isotopes because of variations in the ratios of the lead, thorium, and uranium present in the ore-forming magmas and subsequent geochemical transformations.
If lead is present at 13 ppm, what is mass of lead on average in 1 g of earth's crust?
Next, express your mass result as the average percent of lead in the crust.
Why do we care about lead in Omaha?
Omaha has a well documented environmental problem with lead and a good deal is already known about it thanks in particular to the Environmental Protection Agency (EPA). According to the EPA notice,
"The Asarco facility conducted lead refining operations from the early 1870s until 1996. The Asarco facility is located on approximately 23 acres on the west bank of the Missouri River in downtown Omaha. During the operational period, lead and other heavy metals were emitted into the atmosphere through smoke stacks. The pollutants were transported downwind in various directions and deposited on the ground surface due to the combined process of turbulent diffusion and gravitational settling. In addition, Gould, Inc. operated as a lead battery recycling plant and was considered a secondary lead smelter in the area. The Gould, Inc., plant closed in 1982. Several other businesses in the Omaha area utilized lead in their manufacturing process. Subsequently in 1998, the Omaha City Council solicited assistance from the U.S. Environmental Protection Agency (EPA) in addressing problems with lead contamination in the Omaha area. The EPA initiated the process to investigate the lead contamination in the area under the authority of the Comprehensive Environmental Response, Compensation, and Liability Act."
A great deal of information on the lead pollution in Omaha is available from additional EPA web pages: the current status, a remedial investigation residential yard soil report (pdf 604 p 24.5 MB), a feasibility study residential yard soil report (pdf 78 p 0.5 MB) and a proposed plan residential yard soils (pdf 20 p 50 kB). One of the maps (page 427) contained in the EPA remedial investigation residential yard soil report is shown below. If you read the article, it explains that the data displayed on the map "would be difficult to backtransform into estimates of lead concentration, but the pattern of high and low concentrations is clearly shown. The highest values occur near the refinery source, and decrease with distance from the source."
Print Resources
The Encyclopedia Britannica
Silberberg, M. S. Chemical Connections “Origin of the Elements in Stars” pages 1074-1075, in “Chemistry, the molecular nature of matter and change” 2003 McGraw Hill.
Emsley, J. “The Elements” 2nd ed. 1991 Clarendon Press, Oxford.
Anders, E and Ebihara, M. 1982 “Solar-system abundances of the elements”. Geochim. Cosmochim Acta, 46: 2363-2380.
Krauskopf, K. B. “Introduction to Geochemistry” 1967 McGraw Hill
Additional Web Resources
Stellar evolution
http://fti.neep.wisc.edu/neep533/SPRING1999/lecture7.pdf
http://cosmos.phy.tufts.edu/~zirbel/laboratories/Elements.pdf
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/nucsyn.html
Decay Series
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radser.html
http://www.ead.anl.gov/pub/doc/natural-decay-series.pdf
http://www.chem.ox.ac.uk/icl/heyes/LanthAct/A2.html
Animation of the nuclear decay process
http://www.eserc.stonybrook.edu/ProjectJava/Radiation/
(Be sure to play around with the time interval setting a bit. Depending on the decay series try anything from 10 years to 100,000,000 years.)
Isotopic Abundance
http://www.iupac.org/publications/pac/1998/pdf/7001x0217.pdf
http://www.iupac.org/publications/pac/1984/pdf/5606x0675.pdf
Elemental Abundances
http://www.treatiseongeochemistry.com/contents/sample1.pdf
http://solarsystem.wustl.edu/metsoc03.pdf
http://en.wikipedia.org/wiki/Cosmochemical_Periodic_Table_of_the_Elements_in_the_Solar_System
Crustal Abundances
http://www.webelements.com/webelements/scholar/elements/periodic-table/geological.html
http://www.webelements.com/webelements/scholar/properties/definitions/abund-crust.html
Half lives