Sequential extraction procedures soil-sediment

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. 

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. 

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. 

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. 

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. 

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 reference materials currendy certified for metals extractable by a sequential extraction procedure are BCR CRM 601 and BCR CRM 701. The lack of reference materials in this area has prompted many workers to apply sequential extraction to other reference soils and sediments, certified for total metal contents, in an attempt to provide interim data useful in method validation. 

Rauret, G., Lopez-Sanchez, J.F., Sahuquillo, A., Davidson, C., Ure, A. and Quevauviller, Ph. (1999) Improvement of the BCR 3-step sequential extraction procedure prior to the certification of new sediment and soil reference materials./. Environ. Monit., 1, 57-61. 

Sequential extraction procedures have been applied for the purpose of isolating species of elements from particulate materials, soils and sediments (see Chapters 10 and 11). In sequential extraction procedures, samples are treated with a series of chemicals under rigorously controlled conditions of temperature, time and ratio of reactant to sample. The work of Tessier et al. (1979) resulted in a carefully designed procedure for the determination of species of elements in sediments. It has been used and modified by other investigators. Trace elements in the extracts are usually determined by means of AAS, ICPAES and ICP-MS. An example is the study of sequential extractions for the determination of 20 trace elements in ten certified geological reference materials (Hall et al., 1996). 

The other popular sequential extraction procedure is the protocol proposed by the Community Bureau of Reference, Commission of the European Community (known as the BCR protocol). The method was proposed on the basis of interlaboratory smdies undertaken in order to harmonize conditions for soil and sediment sample analysis. Based on the research data, in 1992 it was stated that application of EDTA or acetic acid solution is appropriate and sufficient for elimination of the bioaccessible fi action of metals from soil samples [62]. In the case of other samples, best results were achieved after application of a three-stage procedure with the following extractants ... 

Typical sequential extraction procedures employed for soils, sediment, and waste materials are based on the five-stage procedure of Tessier et al. [61]. For plant material (e.g., aquatic moss [3] and spinach [4]), sequential analysis with the use of water, EDTA, petroleum ether, ethyl acetate, butanol, methanol, and... 

Sutherland R. A. (2002) Comparison between non-residual A1 Co, Cu, Fe, Mn, Ni, Pb and Zn released by a three-step sequential extraction procedure and a dilute hydrochloric acid leach for soil and road deposited sediment. Appl. Geochem. 17, 353 —365. 

Quevauviller, Ph., Rauret, G., Muntau, H., Ure, A. M., Rubio, R., Ldpez-Sanchez, J. F., Fiedler, H. D., and Griepink, B. (1994). Evaluation of a sequential extraction procedure for the determination of extractable trace metal in sediments and soils. Fresenius J. Anal. Chem. 349, 808—814. 

The sequential extraction procedure can be carried out for the following trace metals Cd, Cr, Cu, Ni, Pb and Zn. A separate sub-sample of the sediment or soil should be dried in a layer of approximately 1 mm in depth in an oven at 105 2°C for 2-3 h and then weighed. From this, a correction to dry mass can be obtained and applied to all of the analytical results obtained (quantity per g dry sediment/soil). 

Specific chapters then focus on different projects on speciation analysis. Chapter 4 deals with interlaboratory studies on methylmercury in fish and sediment Chapter 5 describes the collaborative projects to certify organotins in sediment RMs and mussel tissues Chapter 6 gives an overview of the certification project on trimethyllead in simulated rainwater and urban dust Chapter 7 describes the certification project on arsenic species in fish tissues Chapter 8 focuses on the intercomparison and tentative certification of Se(IV) and Se(VI) in simulated freshwater Chapter 9 deals with a feasibility study to stabilize Cr species in solution followed by the certification of Cr(III) and Cr(VI) in lyophilized solutions and welding dust Chapter 10 gives a review of methods used for A1 speciation Chapter 11 develops the overall collaborative project to standardize single and sequential extraction procedures for soil and sediment analysis, followed by interlaboratory studies and certification of soil and sediment reference materials. 

Tessier A, Campbell PGC, Bisson M (1979) Sequential extraction procedure for the speciation of particulate trace metals. Anal Chem, 51 844-851 Ure AM, Quevauviller Ph, Muntau H, Griepink B (1993) Speciation of heavy metals in soils and sediments. An account of the improvement and harmonization of extraction techniques undertaken under the auspices of the BCR of the Commision of the European Communities. Int J Environ Anal Chem, 51 135-151 Wallmann K, Kersten M, Gruber J, Forstner U (1993) Artifacts in the determination of trace metal binding forms in anoxic sediments by sequential extraction. Int J Environ Anal Chem, 51 187-200... 

In the fields of environmental and exploration geochemistry, considerable use has been made of sequential extraction procedures in order to determine the bioavailability and geomobUity of trace metals (including V, Cr, As, Se, Cd and Pb). More recently, sequential extractions have been used to elucidate information relating to the speciation of metals in soils and sediments. The extraction protocol developed by Tessier " has traditionally been used (with modifications) and involves the sequential use of a variable cocktail of extractants including acetic acid, sodium acetate, magnesiumchloride, EDTA, acidified hydroxylamine hydrochloride, oxalic acid, sodium... 

Single and sequential extraction schemes have been designed in earlier years and widely used to assess the different retention/release of metals in soil and sediment samples (Tessier et al., 1979 Salomons and Forstner, 1980 Thomas et al., 1980 Meguellati et al., 1987). However, the lack of uniformity in the different procedures used did not allow the results to be compared worldwide or the procedures to be... 

For example, the speciation analysis of metal ions in sediments can be performed by sequential extraction, whereby each of the fractions obtained shows a different form of association of the metal in the soil matrix. As mentioned in Chapter 1, an accepted modem procedure is ... 

To remove PBO and its degradates from substrates such as sediment and soil, sequential extraction was carried out, generally at ambient temperatures, using solvents of differing polarity. Extraction procedures were designed to remove as much of the radioactive residue as possible without affecting the stability of PBO, Stability checks were employed where appropriate. Procedural recovery cheeks were also made to ensure that no losses of radioactivity occurred during work up procedures. 

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