Reid M.K.* and Spencer K.L. Department of Geography, Queen Mary, University of London, Mile End Road, London, E1 4NS. En

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 RSD%s 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 RSD%s 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 RSD%s are generally below 10% for all methods (see error bars) and all elements. Overall, the lowest RSD%s 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. RSD%s for the BCR conventional method are lower than all others (Tables 5 and 6), with highest RSD%s for the BCR microwave method. RSD%s for the Tessier microwave method appear lower than the Tessier conventional method. These RSD%s 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). RSD%s are shown on each. Table 5: RSD%s 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@qmul.ac.uk +44 (0) 20 7882 3363 Table 6: RSD%s for the BCR sequential extractions (analysed in triplicate). Table 2: Conventional and microwave BCR methods