Sequential extraction procedure

The behavior of elements (toxicity, bioavailability, and distribution) in the environment depends strongly on their chemical forms and type of binding and cannot be reliably predicted on the basis of the total concentration. In order to assess the mobility and reactivity of heavy metal (HM) species in solid samples (soils and sediments), batch sequential extraction procedures are used. HM are fractionated into operationally defined forms under the action of selective leaching reagents. 

Thomas, R. R, Ure, A. M., Davidson, C. M., Littlejohn, D., Rauret, G., Rubio, R., and Lopez-Sanchez, J. F., Three-Stage Sequential Extraction Procedure for the Determination of Metal in River Sediments, Analytica Chimica Acta 286, 1994, 423 29. 

Ahnstrom and Parker (2001) studied Cd reactivity in metal-contaminated soils using a coupled stable isotope dilution-sequential extraction procedure. They found that in uncontaminated arid soil and in... 

Ahnstrom Z.A.S., Parker D.R. Cadmium Reactivity in Metal-Contaminated Soils Using a Coupled Stable Isotope Dilution-Sequential Extraction Procedure. Environ Sci... 

Badri M.A., Aston S.R. A comparative study of sequential extraction procedures in the geochemical fractionation of heavy metals in estuarine sediments. International Conference Amsterdam, pp 705-708. 1981. 

Ho M.D., Evans G.J. Operational speciation of cadmium, copper, lead and zinc in the NIST standard reference materials 2710 and 2711 (Montana soil) by the BCR sequential extraction procedure and flame atomic absorption spectrometry. Anal Commun 1997 34 353-364. 

Rapin F., Tessier A., Campbell P.G.C., Carignan R. Potential artifacts in the determination of metal partitioning in sediments by a sequential extraction procedure. Environ Sci Technol 1986 20 836-840. 

Shannon R.D., White J.R. The selectivity of a sequential extraction procedure for the determination of iron oxyhydroxides and iron sulfides in lake sediments. Biogeochem 1991 14 193-208. 

Tessier, A., Campell, P.G.C., and Bisson, M., Sequential extraction procedure for the speciation of particulate trace metals. Anal Chem 1979 51 844... 

Wenzel W.W., Kirchbaumer N., Prohaska T., Stingeder G., Lombi E., Adriano D.C. Arsenic fractionation in soils using an improved sequential extraction procedure. Anal Chimica Acta 2001 436 309-323. 

In this study, 30 soil samples from rice farmland in Zhejiang province, in eastern China. We sampled the cultivation layer soils during rice harvest season. The soils are acidified and the average value of pH is 5.82. A sequential extraction procedure... 

In order to validate the hypothesis mentioned above, the Ni retention capacity of the Lac Tio waste rock was estimated using a batch sorption test performed on a fresh (C1) and weathered (C4) sample, followed by a 3-step Sequential Extraction Procedure (or SEP). The batch sorption test was done using a 10 mg/L Ni solution with an initial pH of 6, an ionic force adjusted to 0.05 M with NaN03 and with a liquid/solid ratio of 25. Some of the batch sorption results are presented in Figure 3. 

Whalley C, Grant A. Assessment of the phase selectivity of the European Community Bureau of Reference (BCR) sequential extraction procedure for metals in sediment. Anal. Chim. Acta 1994 291 287-295. 

Fernandez E, Jimenez R, Lallena AM, Aguilar J. Evaluation of the BCR sequential extraction procedure applied for two unpolluted Spanish soils. Environ. Pollut. 2004 131 355-364. 

Davidson CM, Duncan AL, Littlejohn D, Garden LM. A critical evaluation of the three-stage BCR sequential extraction procedure to assess the potential mobility and toxicity of heavy metals in industrially-contaminated land. Anal. Chim. Acta 1998 363 45-55. 

Mossop KF, Davidson CM. Comparison of original and modified BCR sequential extraction procedures for the fractionation of copper, iron, lead, manganese and zinc in soils and sediments. Anal. Chim. Acta 2003 478 111-118. 

Gleyzes C, Tellier S, Astruc M. Fractionation studies of trace elements in contaminated soils and sediments a review of sequential extraction procedures. TrAC Trend Anal. Chem. 2002 21 451—467. 

Scheckel, K. G., Impellittteri, C. A., Ryan, J. A. McEvoy, T. 2003. Assessment of a sequential extraction procedure for perturbed lead-contaminated samples with and without phosphorus amendments. Environmental Science Technology, 37, 1892-1898. 

Yanase, N., Nightingale, T., Payne, T. Duerden, P. 1991. Uranium distribution in mineral phases of rock by sequential extraction procedure. Radiochimica Acta, 52/53, 387-393. 

Much work has been reported on the evaluation of sequential extraction procedures. The three-stage sequential extraction procedure for speciation of heavy metals proposed by the Commission of the European Communities Bureau of References (BCR) was found to be acceptable and reproducible with some modifications [29]. In another study, when applied to real soils and sediments, this (unmodified) BCR method was queried [30]. Lopez-Sanchez et al. [31 ] found that significant results can be obtained when different sequential extraction procedures are used. 

Some work on sediments is reported here in the belief that it may also be useful in the analysis of soil samples. Thus Asikainen and Nikolaides [33] have carried out a sequential extraction study of chromium from contaminated aquifer sediments and found that 65% of the chromium was extractable. Of this amount 25% was exchangeable, 11% was bound to organic matter and 30% was bound to iron and manganese oxide surfaces. Thomas et al. [34] also investigated the use of BCR sequential extraction procedures for river sediments, and found the method to work well. Real et al. [35] improved sequential extraction by optimising microwave heating. 

Real et al. [341] showed that optimising the microwave heating procedure would optimise the results obtained in sequential extraction procedures. 

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

This chapter considers methods of trace element speciation, and their application to soils, that involve selective chemical extraction techniques. It will be concerned firstly with extraction by single selective reagents and secondly with the development and application of sequential extraction procedures for soils and related materials. Sequential extraction procedures for sediments are discussed in depth in Chapter 11. Speciation in the soil solution and modelling aspects of its interaction with soil solid phases are comprehensively covered in Chapter 9 and will not be considered here. 

Various workers have questioned the ability of sequential extraction to provide accurate information on the mineralogical phases with which trace elements are associated in soil or sediments (e.g. Nirel and Morel, 1990). Problems, including non-selectivity of reagents and readsorption of analytes following release, are frequently reported. Hence, nowadays, most environmental analytical chemists accept that sequential extraction should be considered an operational form of speciation, in which the fractions isolated are defined purely by the sequence of reagents used, and not as a means to determine information on the specific mineralogical phases to which trace elements are bound. Modern sequential extraction procedures label the fractions obtained in terms of the type of chemical reaction used to isolate them, in order to emphasise this, e.g. reducible or oxidisable species. Unfortunately, this distinction is not always made clear in the wider environmental literature. 

Recently, a group of experts working under the auspices of the Commission of the European Communities, Community Bureau of Reference (BCR) sought to improve comparability between sequential extraction results obtained by different laboratories, and proposed a simple, three-stage sequential extraction procedure for sediment (Table 10.3) (Ure et d., 1993a, b). The protocol was then refined through two sets of interlaboratory trials (Quevauviller et d., 1994) and studies were conducted to assess whether adequate homogeneity and stability could be achieved to allow certification of a reference material for metals extractable by the procedure (Fiedler et d., 1994). 

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