Speciation of Lead
Background
Environmental pollution from lead is mainly a problem arising from the use
of tetraalkyllead compounds as anti-knock additives in petrol. Although
this use is diminishing, the more stable forms, tri- and dialkyllead are
fairly persistent in the environment. It has been shown that, even in remote
areas with no direct sources of emission from trafic, the concentrations of
alkyllead species found are related to, and correlated with, the introduction
and proliferation of motor vehicles. The lead species typically present in
environmental samples are shown in the figure below. Of these, tetra- and
triethyllead are the most toxic towards humans, and 10 - 100 times more
poisonous than inorganic lead. In addition, the nature of the organometallic
forms is such that they are more lipophilic and can more easily penetrate
biological membranes. Consequently, alkyllead species may be bioaccumulated
in food chains.
CH3
C2H5
C2H5
C2H5
C2H5
CH3 Pb CH3
CH3 Pb CH3
CH3 Pb CH3
CH3 Pb C2H5
C2H5 Pb C2H5
CH3
CH3
C2H5
C2H5
C2H5
tetramethyllead trimethylethyllead
dimethyldiethyllead methyltriethyllead tetraethyllead
CH3
C2H5
C2H5
C2H5
CH3 Pb
+
CH3 Pb +
CH3 Pb +
CH3 Pb +
CH3
CH3
C2H5
C2H5
trimethyllead dimethylethyllead
methyldiethyllead
triethyllead
CH3 Pb 2+
CH3 Pb 2+
C2H5 Pb 2+
CH3
C2H5
C2H5
dimethyllead
methylethyllead
diethyllead
Pb 2+
inorganic lead
|
Lead species present in the environment
The question as to whether lead can be biomethylated is not yet fully
understood and remains a subject of heated debate, but there may be natural
processes producing methyllead species as well as anthropogenic sources.
The cycling of lead in the environment is illustrated below. The major sources
are evaporation losses incurred during transport and handling of leaded
petrol, and emissions in motor vehicle exhaust. It has been suggested that up
to about 3 % of the tetraalkyllead in petrol is emitted, either without
degradation or as ionic alkyllead. The bulk of the tetraalkyllead species are,
however, completely combusted and primarily emitted as particulate, inorganic
lead.
Environmental cycling of lead species
Analytical methods
A variety of analytical methods have been used for the determination of
lead species in biological and environmental samples. The most commonly
employed, and that used by our group, involves an
initial isolation of the species from the sample matrix, followed by extraction
into an organic solution. For the ionic alkyllead compounds, this is achieved
using a complexing agent such as diethyldithiocarbamate or dithizone. These
complexes are then derivatized by Grignard reaction into their tetraalkyllead
analogues, yielding compounds suitable for separation by gas chromatography:-
R3PbX +
R'MgX => R3
PbR' +
MgX2
R2PbX2 +
2 R'MgX => R2
PbR'2 +
2 MgX2
R = methyl, ethyl
R' = propyl, butyl
|
The separated analytes are then detected using atomic absorption spectrometry
equipped with a quartz tube atomizer, microwave-induced plasma atomic emission
spectrometry or mass spectrometry.
A list of references to our work on the speciation of lead can be found
here.
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This document was last updated on August 9, 1996 by
mag@alchemy.chem.umu.se:
Magnus Johansson
Department of Analytical Chemistry
Umeå University
S-901 87 Umeå
Sweden
FAX: +46-90 13 63 10 Tel: +46-90 16 54 81