Sequential extractions

Sequential extraction involves treatment of a sample of soil or sediment with a series of reagents in order to partition the trace element content. The reagents used are generally similar to those employed as single extractants to liberate metals bound to particular components of the sample matrix, discussed above. The growth in popularity of the approach is demonstrated by the marked increase in publications in which it has featured over the past decade (Fig. 10.2). 

The principal advantage claimed for sequential extraction over the use of single extractants is that the phase specificity is improved. This occurs because each 

Chemical speciation in soils and related materials by selective chemical extraction 277 

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. 

The philosophy and limitations of sequential extraction are described in detail in Chapter 11, where the use of the approach in sediment research is also reviewed. Although originally developed for application to sediments, sequential extractions have since been applied successfully in many soil studies. Recent work 

In addition to appropriate laboratory ware, including carefully cleaned beakers, volumetric cylinders, flasks, etc. (see Section 12.1.4), the following reagent solutions are used  

Preferably, all chemicals and acids used for the preparation of the reagents should represent the highest analytical grade available with regard to TE impurities (see Section 12.1.5). Solutions A and B are stable indefinitely. Solution C should be prepared for the number of samples to be processed during one week. Solution D1 must be kept cool and dark and should be discarded when the volume in the bottle falls below 20 % of the total volume. 

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The procedure of simultaneous extracting-spectrophotometric determination of nitrophenols in wastewater is proposed on the example of the analysis of mixtures of mono-, di-, and trinitrophenols. The procedure consists of extraction concentrating in an acid medium, and sequential back-extractions under various pH. Such procedures give possibility for isolation o-, m-, p-nitrophenols, a-, P-, y-dinitrophenols and trinitrophenol in separate groups. Simultaneous determination is carried out by summary light-absorption of nitrophenol-ions. The error of determination concentrations on maximum contaminant level in natural waters doesn t exceed 10%. The peculiarities of application of the sequential extractions under fixed pH were studied on the example of mixture of simplest phenols (phenol, o-, m-, />-cresols). The procedure of their determination is based on the extraction to carbon tetrachloride, subsequent back-extraction and spectrophotometric measurement of interaction products with diazo-p-nitroaniline. 

Despite the recent efforts for settling operational conditions for metal and metalloid fractionation, conventional batch sequential extraction schemes lack automation and are rather time consuming and laborious. Two additional main problems are the phase overlapping and possible re-adsorption of released elements. 

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. 

In the present work it has been shown that on-line coupling of flowthrough fractionation in RCC with ICP-EAS detection enables not only the fast and efficient fractionation of trace elements (TE) in environmental solids to be achieved but allows real-time studies on the leaching process be made. A novel five-step sequential extraction scheme was tested in on-line mode. The optimal conditions for the fractionation were chosen. Investigating elution curves provides important information on the efficiency of the reagents used, the leaching time needed for the separation of each fraction, and the potential mobility of HM forms. 

Ruttenberg, K. C. (1992). Development of a sequential extraction method for different forms of phosphorus in marine sediments. Limnol. Oceanogr. 37, 1460-1482. 

Babraj, J., Cuthbertson, D.J., Rickhuss, P., Meier-Augenstein, W., Smith, K, Bohe, J., Wolfe, R.R., Gibson, J.N., Adams, C. Rennie, M.J. (2002). Sequential extracts of human bone show differing collagen synthetic rates. Biochemical Society Transactions, Vol. 30, pp. 61-65. 

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. 

Isolation and sequential extraction of cell wall polysaccharides from soy meal... 

Sequential extraction was performed as described by Redgwell and Selvendran [1], WUS was sequentially extracted with chelating agent (ChSS), dilute alkali (DASS), 1 M alkali (1 MASS) and 4 M alkali (4 MASS) and a cellulose-rich residue (RES) remained. 

WUS was sequentially extracted, which resulted in two pectin-rich extracts (ChSS and DASS), two hemicellulose-rich extracts (1 MASS and 4 MASS) and a residue. The yield of these extracts on sugar basis and the sugar composition is shown in Table 2. 

The isolation of polysaccharides from soy meal was successful, WUS contained only 2.1% of protein and 92% of the polysaccharides present in soy meal were recovered in WUS. The sugar composition of both the soy meal and WUS are similar and allow the conclusion that during the isolation procedure no sugar residues were specifically removed. The pectin-rich extracts (ChSS and DASS) obtained after sequential extraction of the WUS were the most abundant. 

Thorium is a highly insoluble element, mainly carried in the particulate form in river waters. This is well shown by Th data for the MacKenzie river (Vigier et al. 2001) and for the Kalix river (Andersson et al. 1995 Porcelli et al. 2001) in both cases, more than 95% of Th is carried by >0.45 pm particles. An important part of this Th is included within detrital material. This is illustrated by sequential extractions performed on sediments from the Witham river (Plater et al. 1992), which show the very low amount of Th in ion-exchangeable and organic-bound fractions compared to Th in Fe-Mn oxides... 

Denmark 1.5 days after the explosion. Air samples collected at Roskilde, Denmark on April 27-28, contained a mean air concentration of 241Am of 5.2 pBq/m3 (0.14 fCi/m3). In May 1986, the mean concentration was 11 pBq/m3 (0.30 fCi/m3) (Aarkrog 1988). Whereas debris from nuclear weapons testing is injected into the stratosphere, debris from Chernobyl was injected into the troposphere. As the mean residence time in the troposphere is 20-40 days, it would appear that the fallout would have decreased to very low levels by the end of 1986. However, from the levels of other radioactive elements, this was not the case. Sequential extraction studies were performed on aerosols collected in Lithuania after dust storms in September 1992 carried radioactive aerosols to the region from contaminated areas of the Ukraine and Belarus. The fraction distribution of241 Am in the aerosol samples was approximately (fraction, percent) organically-bound, 18% oxide-bound, 10% acid-soluble, 36% and residual, 32% (Lujaniene et al. 1999). Very little americium was found in the more readily extractable exchangeable and water soluble and specifically adsorbed fractions. 

This procedure was compared with sequential extractive techniques employing alkaline hydrolysis of dried plant tissue followed by extraction of the acidified mixture with ethyl acetate. Fractions were individually evaluated for phytotoxic properties. Selected fractions from those showing a positive response were analyzed by gas-liquid chromatography. Structural identification and characterization of the individual components in these selected fractions were accomplished by gas chromatography-mass spectrometry. 

The crude organic solvent preparations were produced by sequential extractions of the plant material with petroleum ether (PE), followed by dichloromethane (DCM), and finally methanol (MeOH). 

Advantages Low cost No grinding Broad applicability High b.p. solvent contamination of analyte Low investment Simple equipment Simultaneous extractions in series Low investment Simple equipment Rapid Economic solvent use Good reproducibility Low investment Simple equipment Economical Simple equipment Not traumatic Almost solvent free Concentrated analyte Rapid Low temperatures Rapid Automated Simultaneous extraction Low solvent use Rapid User friendly Automated Sequential extractions Not analyst labour intensive... 

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. 

Biester H., Scholz C. Determination of mercury binding forms in contaminated soils Mercury pyrolysis versus sequential extractions. Environ Sci Technol 1997 31 233-239. 

Chang A.C., Page A.L., Wameke J.E., Grgurevic E. Sequential extraction of soil heavy metals following a sludge applications. J Environ Qual 1984 13 33-38. 

Clevenger T.E. Use of sequential extraction to evaluate the heavy metals in mining wastes. Water Air Soil Pollut 1990 50 241-254. 

Hall G.E.M., Gauthier G., Pelchat J.C., Pelchate P., Vaive J.E. Application of a sequential extraction scheme to ten geological certified reference materials for the determination of 20 elements. J Anal At Spectrom 1996 11 787-796. 

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. 

Kim, N.D. and Fergusson, J.E., Effectiveness of a commonly used sequential extraction techniques in determining the speciation of cadmium in soils. Sci Total Environ 1991 105 191-209. 

Mahan K.I., Foderaro T.A., Garza T.L., Martinez R.M., Maroney G.A., Trivisonne M.R., Willging E.M. Microwave digestion techniques in the sequential extraction of cadmium, iron, chromium, manganese, lead and zinc in sediments. Anal Chem 1987 59 938-945. 

Martin J.M., Nirel P., Thomas A.J., Sequential extraction techniques Promises and Problems, Mar Chem 1987 22 313-341. 

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. 

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