Sediments speciation methods

In analytical work on speciation, methods of wet sample preparation are very important parts of the overall scheme of analysis. Constraints on preparation methods include low concentrations of analytes, often less than 0.1 mgg-1, stabilities of the analytes, and the need for suitable solutions for instrumental techniques of elemental determinations. Volume of sample and type of matrix must be considered. Procedures for the quantitative recoveries of organometallic compounds from sediments and organic matrices can be time-consuming. Their efficiencies and reliabilities must be thoroughly tested for each type of sample for analysis. 

In order to validate speciation methods more effectively, a number of certified reference materials (CRMs) have been produced to allow laboratories to measure the accuracy of their techniques. A range of sediment and biota materials are available such as the sediment materials PACS-2 from the National Research Council of Canada, NIES-12 from the National Institute for Environmental Studies, Japan and the CRM-462 from Community Bureau of Reference (BCR), EU. These materials have been rigorously homogeneity and stability tested and the levels of organotins have been certified by a range of techniques utilizing either definitive methods or multiple independent methods. 

As for soils, similar chemical extraction methods have been developed to determine the speciation of Fe in sediments (Heron et al. 1994 Kostka and Luther III, 1994). For Fe oxides, some modifications of the scheme used in soils are needed. In particular, oxalate may not be applied if large amounts of Fe" are present because in oxalate solution, Fe " will catalyze the dissolution of better crystalline Fe oxides such as goethite and, therefore, the method will not be be specific for ferrihydrite. Fe bound in sulphides and carbonate is extracted by HCl prior to dithionite which, as in soils, extracts most of the Fe" oxides. The latter can also be extracted by a Ti" -EDTA solu-... 

In the environment, thorium and its compounds do not degrade or mineralize like many organic compounds, but instead speciate into different chemical compounds and form radioactive decay products. Analytical methods for the quantification of radioactive decay products, such as radium, radon, polonium and lead are available. However, the decay products of thorium are rarely analyzed in environmental samples. Since radon-220 (thoron, a decay product of thorium-232) is a gas, determination of thoron decay products in some environmental samples may be simpler, and their concentrations may be used as an indirect measure of the parent compound in the environment if a secular equilibrium is reached between thorium-232 and all its decay products. There are few analytical methods that will allow quantification of the speciation products formed as a result of environmental interactions of thorium (e.g., formation of complex). A knowledge of the environmental transformation processes of thorium and the compounds formed as a result is important in the understanding of their transport in environmental media. For example, in aquatic media, formation of soluble complexes will increase thorium mobility, whereas formation of insoluble species will enhance its incorporation into the sediment and limit its mobility. 

For the determination of organotin compounds (tributyltin, triphenyltin, triethyltin, and tetra-ethyltin) a MAE is proposed before the normal phase (NP) HPLC/UV analysis [35], In organotin and arsenic speciation studies, hydride generation is the most popular derivatization method, combined with atomic absorption and fluorescence spectroscopy or ICP techniques [25,36], Both atmospheric pressure chemical ionization (APCI)-MS and electrospray ionization ESI-MS are employed in the determination of butyltin, phenyltin, triphenyltin, and tributyltin in waters and sediments [37], A micro LC/ESI-ion trap MS method has been recently chosen as the official EPA (Environmental Protection Agency) method (8323) [38] it permits the determination of mono-, di-, and tri- butyltin, and mono-, di-, and tri-phenyltin at concentration levels of a subnanogram per liter and has been successfully applied in the analysis of freshwaters and fish [39], Tributyltin in waters has been also quantified through an automated sensitive SPME LC/ESI-MS method [40],... 

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. 

Calle, M.B. de la, Scerbo, R., Chiavarini, S., Quevauviller, Ph. and Morabito, R. (1997) Comparison of derivatization methods for the determination of butyl- and phenyl-tin compounds in mussels by gas chromatography. Appl. Organometal. Chem., 11, 693. Cimara, C., Cobo, M.G., Palacios, M.A., Munoz, R. and Donard, O.FX. (1996) Selenium speciation analyses in water and sediment matrices. In Quality Assurance for Environmental Analysis (eds Quevauviller, Ph., Maier, E.A. and Griepink, B.), Vol. 10. Elsevier, Amsterdam, 237. 

Development of new in situ analytical methods for species determination - There is a disparity between the amount of speciation information obtained from laboratory studies on well-defined systems, and that which is applicable to the real environmental situation (Buffle, 1990). Development of in situ methods will facilitate more relevant measurements which minimise perturbation of the system. Systems particularly prone to measurement-induced artefacts include anoxic waters, the sediment-water interface and colloidal materials. Methods for accurate measurement of free metal ion concentration over a much broader concentration range than that currently available are particularly required. 

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. 

Kersten, M. and Forstner, U. (1989) Speciation of trace elements in sediments. In Trace Elements Speciation Analytical Methods and Problems (ed. Batley, G.). CRC Press, Boca Raton, FL, pp. 245-317. 

Methods of Speciation and Fractionation. It is apparent that in order to understand the mobility of arsenic and its availability for reactions, methods of speciation and fractionation must be applied to sediment samples in field and laboratory studies. In this paper speciation refers to the separation and quantitative determination of inorganic arsenic, methanearsonic acid, and cacodylic acid. Compartmentalization involves identifying the major compartments for arsenic in a heterogeneous system (e.g. aqueous, adsorbed, occluded,...) and determining the amounts of arsenic in each compartment. Fractionation involves the extraction of arsenic from operationally defined fractions of the solid phase of an aquatic system (e.g. sediment). 

Several analytical methods for speciating arsenic have been reported. They include chromatographic techniques such as electrophoresis and ion-exchange (17), paper chromatography (18) and HPLC (19) selective volatilization of arsenic compounds to analogous arsines followed by GC-MES (20) boiling point separation/spectral emission (21) and atomic absorption (22). The above techniques have been applied to samples such as commercial pesticides (20),coal and fly ash (23),rocks, sediments, soils and minerals (24, 22),plant tissue (18), bovine liver (23),and water samples T25). 

Rates of changes in arsenic speciation were studied by spiking anaerobic sediments with arsenic standards, incubating the sediments, and monitoring arsenic speciation over a period of two months. The incubation experiments were modeled using the methods described in the previous section. 

Solid-phase speciation has been measured both by wet chemical extraction and, for arsenic, by instrumental methods principally X-ray absorption near edge structure spectroscopy (XANES) (Brown et al., 1999). La Force et al. (2000) used XANES and selective extractions to determine the likely speciation of arsenic in a wetland affected by mine wastes. They identified seasonal effects with As(El) and As(V) thought to be associated with carbonates in the summer, iron oxides in the autumn and winter, and silicates in the spring. Extended X-ray absorption fine stmcture spectroscopy (EXAES) has been used to determine the oxidation state of arsenic in arsenic-rich Californian mine wastes (Eoster et al., 1998b). Typical concentrations of arsenic in sods and sediments (arsenic <20 mg kg ) are often too low for EXAFS measurements, but as more powerful photon beams become available, the use of such techniques should increase. 

The measurement of standard reference materials (SRMs) provides the best method for ensuring that an analytical procedure is producing accurate results in realistic matrices. Many SRMs are available (Govindaraju, 1994 Rasmussen and Andersen, 2002). The most widely used are those supplied by National Institute of Standards and Technology (NIST). Arsenic and selenium concentrations have been certified in a range of natural waters, sediments, and soils (Tables 4 and 5). The SLRS range of certified standards from the National Research Council of Canada also includes several river waters with much lower arsenic concentrations than the NIST standards ( 0.2-l pgL ). Certified standards for As(III)/As(V) and Se(lV)/Se(VI) speciation are available commercially (e.g., SPEX Certiprep speciation standards). 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

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