Reid M.K.* and Spencer K.L.

1
A comparison of conventional and microwave
assisted sequential extraction methods for Cu, Pb
and Zinc in estuarine sediments
Introduction Total metal analysis alone is
unable to accurately assess contamination by, and
mobility of, metals in the environment (Marin et
al., 1997 Usero et al., 1998). Sequential
extractions (despite limitations discussed by
e.g. Martin et al., 1987) are frequently used to
gain more insight into metal behaviour and
availability in the environment (Ryssen et al.,
1999 Tuzen 2003 Bryan and Langston, 1992).
They have also been used to identify diagenetic
processes in sediment (Spencer et al., 2003) and
to provide greater discrimination between
contaminants with similar sources (Li et al.,
1995 Rieuwerts et al., 2000). One of the major
criticisms is the time required for extraction
(4-5 days) and microwave assisted techniques have
been investigated as a means of reducing this
extraction time (e.g. Mahan et al., 1987 Ipolyi
et al., 2002 Perez-Cid et al., 1999). However
studies are limited or results are inconclusive.
Therefore we have investigated the use of two
microwave procedures using estuarine sediment
collected from the Medway Estuary, Kent, UK, and
BCR CRM 701. Methods Successively
aggressive reagents are used to sequentially
extract metals associated with operationally
defined (e.g. reagent concentration, time,
temperature, pH) sediment fractions. A microwave
assisted BCR protocol (Ipolyi et al., 2002) and a
microwave Tessier protocol (Mahan et al., 1987
Perez-Cid et al., 1999) were tested along with
conventional methods (Tessier - Tessier et al.,
1979 BCR - Rauret et al., 1999). Zinc, Cu and Pb
were then analysed by AAS following digestion,
evaporation and rehydration in a 2 HNO3 (Ouddane
et al., 1997 Li et al., 1995). The CEM MARS X
microwave uses a ramp to temperature rather than
W setting (therefore maximum temperatures were
set 25oC lower than the reagent boiling point,
for the BCR method temperatures were kept below
75oC). Following all steps (Tables 1 and 2),
samples were centrifuged and the supernatant was
decanted and stored, samples were then rinsed in
distilled water and centrifuged again before the
next step. A microwave assisted Aqua Regia
extraction was used for the pseudo total and the
final step of all sequential extraction
procedures. Samples were all analysed in
triplicate.
Results for CRM sediment
Conclusions 1. Microwave methods drastically
reduce the time required for extraction from
4.5 days to 1.5 days (including the final step)
and provide a viable alternative to the
conventional methods. 2. Microwave and
conventional techniques show good reproducibility
for the CRM sediment and RSDs are comparable to
those in other work (Bruder-Hubscher et al.,
2002). 3. There is good agreement between the
microwave and conventional methods. Recovery
percentages for the CRM sediments compared to
Aqua Regia totals are comparable to those found
by Mossop and Davidson (2003). 4. Reproducibility
is not as high for estuarine sediments although
this may be the result of inhomogeneity of the
wet sediments. All RSDs for the BCR conventional
method are below 20 therefore in line with other
work (e.g. Mahan et al., 1987, Spencer et al.,
2003). Although the Tessier microwave method has
some high RSD values, other are relatively low,
especially for Cu. 5. Results show that either
of the methods would be appropriate for use,
selection depends on the requirements of
analysis. The BCR method has less fractions and
therefore is likely to generate lower errors
(Mester et al., 1998), however the Tessier method
will provide more information about the earlier
(potentially more bioavailable) fractions. 6.
There are still problems to be rectified,
especially the Tessier microwave method for the
organic fraction as some samples had to be re-run
because sample loss due to excessive foaming.
Comparisons will need to be drawn between the
Tessier methods on the same estuarine sediment,
and more elements should be tested to provide
more comprehensive results. 7. CRM results show
that microwave methods can be reliably compared
to conventional results, and the speed of the
microwave methods makes them more accessible. It
is recommended that comparisons be tested on the
sediment in question, prior to analysis.
Cu (Figure 1) Each of the microwave methods
extracts similar concentrations to the respective
conventional method (although the microwave
extracts Cu in the exchangeable fraction, and
lower concentrations in the carbonate fraction
than the conventional) for each fraction and the
sum totals of all methods (Table 3) appear in
good agreement. However, the Tessier method
extracts much higher concentrations in the
organic fraction as opposed to the BCR which
extracts greater concentrations from the Fe-Mn
oxide fraction. Zn (Figure 2) Again for Zn, the
microwave and conventional extractions for each
of the methods compare well. However, there are
differences between the organic and sulphide
fraction and Fe-Mn oxide fraction for the Tessier
methods. None of the methods extract values
equal to the Aqua Regia total, but the Tessier
conventional and BCR conventional extract the
closest values (Table 3). Pb (Figure 3) The CRM
values given for exchangeable and carbonate, and
organic and sulphide fractions were below
analytical detection. However a significant
proportion of Pb is extracted in the organic
fraction by the Tessier microwave method which
also extracts lower concentrations of Pb in the
Fe-Mn oxide fractions than all other methods.
Perez-Cid et al (1999) found increased extraction
of Pb in the fourth step, but the first three
steps of their work compared well to the
conventional. Reproducibility and accuracy RSDs
are generally below 10 for all methods (see
error bars) and all elements. Overall, the lowest
RSDs are found for the microwave BCR for Cu,
Tessier conventional for Zn, and Tessier
microwave for Pb. In terms of recovery, the BCR
conventional method is the most accurate,
followed by the Tessier conventional. Recoveries
for all are within 17 of the Aqua Regia total
(except microwave methods for Zn).
Table 3 Average sum of extractions compared to
CRM values (i.e. steps 1-3 BCR, 1-4 Tessier) and
Aqua Regia totals (i.e. steps 1-4 BCR, 1-5
Tessier).
Acknowledgements Reid M.K. would like to thank
Queen Mary, University of London for the
provision of studentship, University of London,
Central Research Fund for fieldwork expenses, and
the BGRG Postgraduate Conference Fund for
conference support
Results for Estuarine Sediments Sequential
extractions were carried out on wet sediment.
Values for the estuarine sediments for Tessier
methods are not comparable with each other as
different fresh sediments had to be
used. RSDs for the BCR conventional
method are lower than all others (Tables 5 and
6), with highest RSDs for the BCR microwave
method. RSDs for the Tessier microwave method
appear lower than the Tessier conventional
method. These RSDs are comparable to those found
by other workers (e.g. Mahan et al., 1987).
There is less agreement for natural
estuarine sediments between conventional and
microwave methods than for reference materials.
Drying sediment is thought to alter partitioning
so wet sediments were used, however it is harder
to homogenise increasing sample variability
(Baeyens et al., 2003 Ure, 1996) which may
lessen the reproducibility. For Cu the microwave
BCR method extracts concentrations in the organic
fraction, whereas the conventional does not, the
microwave method also extracts higher
concentrations in the residual. Lead is only
found in the Fe-Mn fraction by the BCR
conventional method, but the microwave method
extracts concentrations in the residual fraction
as well.
References Baeyens W., Montney F., Leermakers M.,
and Bouillon S. (2003) Anal. Bioanal. Chem. 376
890-901. Bruder-Hubscher V., Lagarde F., Leroy
M.J.F., Coughanowr C., Enguehard F. (2002) Anal.
Chim. Acta 451 285-295. Bryan G.W. and Langston
W.J. (1992) Environ. Pollut. 76 89-131. Ipolyi
I., Brunori C., Cremisini C., Fodor P., Macaluso
L., and Marabito R. (2002) J Environ. Monit. 4
541-548. Li X., Coles B.J., Ramsey M.H., Thornton
I. (1995) Chem. Geol. 124 109-123 Mahan K.I.,
Foderaro T.A., Garze T.L., Martinez R.M., Maroney
G.A., Trivisonno M.R., and Willging E.M. (1987)
Anal. Chem. 59 938-945. Marin B., Valladon M.,
Polve M., and Monaco A. (1997) Anal. Chim. Acta
342 91-112. Martin J.M., Nirel P., and Thomas
A.J. (1987) Mar. Chem. 22 313-341. Mester Z.,
Cremisini C., Ghiara E., and Morabito R. (1998)
Anal. Chim. Acta 359 133-142. Mossop K.F. and
Davidson C.M. (2003) Anal. Chim. Acta 478
111-118. Ouddane B., Martin E., Boughreit A.,
Fischer J.C., and Wartel M. (1997) Mar. Chem. 58
189-201. Perez-Cid B., Lavilla I., and Bendicho
C. (1999) Anal. Chim. Acta 378 201-210. Rauret
G., Lopez-Sanchez J.F., Sahuquillo A., Davidson
C., Ure A., and Quevauviller Ph. (1999) J.
Environ. Monitor. 1 57-61. Rieuwerts J.S., Farago
M.E., Cikrt M., and Bencko V. (2000) Water, Air,
Soil Pollut. 122 203-229. Ryssen R. v.,
Leermakers M., Baeyens W. (1999) Environ. Sci.
Policy 2 75-86. Spencer, K.L., Cundy, A.B., and
Croudace, I.W. (2003) Estuar, Coast. Shelf S.
57(1-2) 43-54. Tessier A., Campbell P.G.C., and
Bisson M. (1979) Anal. Chem. 51 (7)
844-851. Tuzen M. (2003) Microchem. J. 74
105-110. Ure A.M. (1996) Sci. Total Environ. 178
3-10. Usero J., Gamero M., Morillo J., and Gracia
I. (1998) Environ. Int. 24 (4) 487-496.
Table 1 Conventional and microwave Tessier
methods
Figure 4 Comparison of BCR conventional (C) and
microwave (M) methods on 3 triplicate estuarine
samples (1-3). RSDs are shown on each.
Table 5 RSDs for the Tessier sequential
extractions methods (triplicates for the
conventional and duplicates for microwave) (nd
non-detect).
Reid M.K. and Spencer K.L. Department of
Geography, Queen Mary, University of London, Mile
End Road, London, E1 4NS. England Corresponding
author m.warner_at_qmul.ac.uk 44 (0) 20 7882 3363
Table 6 RSDs for the BCR sequential
extractions (analysed in triplicate).
Table 2 Conventional and microwave BCR methods