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
Umeň University
S-901 87 Umeň
Sweden
FAX: +46-90 13 63 10 Tel: +46-90 16 54 81