Abstract
A comprehensive review of the single and sequential extraction schemes for metal fractionation in environmental samples such as soil and industrially contaminated soils, sewage sludge and sludge amended soils, road dust and run off, waste and miscellaneous materials along with other approaches of sequential extraction methods are being presented. A discussion on the application of chemometric methods in sequential extraction analysis is also being given. The study of single and sequential extraction methods for various reference materials are also being looked into. The review covers several aspects of the single and sequential extraction methodologies. The use of each reagents involved in these schemes are also discussed briefly. Finally the present upto date information by different workers in various fields of environmental geochemistry along with the possible future developments are also being outlined.
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1 Introduction
The biohazard in soils is attributable to several heavy metals and metalloids and often been assessed by determination of their total soil contents and national guidelines based on such total contents are currently in use in various countries. Total soil contents, however, reflect the geological origins of soils as well as the anthropogenic inputs such as pollutants from industrial processes and are poor indicators of mobility or bioavailability. A more relevant assessment of the contents of elemental contamination in the environmental context can be made by measuring the “ pseudototal” element contents of a soil by the analysis of strong acid or aqua regia digests of soils. Pseudototal soil contents give an assessment, therefore, of the maximum potentially soluble or mobile contents of metals and, in the case of environmental metal contaminants, usually not bound in silicates, a measure of the maximum potential hazard that could occur in long term or in extreme environmental regimes.
But selective extraction methods have been used to target element species in soil, or elements bound to, or associated with, particular soil phases or compounds. Examples include the use of extractants to release, metals on exchange sites, or metals bound or associated with soil organic matter etc. Thus, undoubtedly selective extraction methods can assess the amounts of mobile or potentially mobile species which in turn may correlate with plant-available contents under certain environmental or agricultural conditions.
For heavy metals and other potentially toxic elements, selective extraction methods are not only useful for the assessment of the mobile and potentially mobile species, for the plant availability etc., but the data also has been highly useful in the land use assessment.
A schematic diagram of the different approaches to the determination of heavy metals in soil is shown in Fig. 1.
Different approaches applied in general to the determination of heavy metals in soil (% values shown above are only tentative as they are metal and matrix dependent)
Single and sequential extraction methods are also used in more fundamental studies such as : to elucidate the soil chemistry, to assess the structure and composition of soil components and to improve understanding of the processes in the soil that control the mobilization and retention of nutrient and toxic elements as well as to illuminate their transport mechanisms.
Both single and sequential extraction methods are of major interest to the environmental scientist particularly in the study of the fate of environmental pollutants. Many of the extractants intended to target particular phase are also used in functional studies. The specificity of many of these reagents can be improved by combining a series of them in a sequential extraction scheme in which the residue from a first extraction is used as the material for a second extraction and so on through a number of stages. The soil phase attacked by each extracting reagent is thus restricted by the preceding extraction in the series and is thereby made more specific. Even if, all these procedures are operationally defined, they provide useful information.
Before going into the detailed discussion of the single and sequential extraction procedures a brief particulars on the methods of sample collection and the pretreatment are important so that proper sample has been collected for the analysis. Hence the details are given below.
2 Methods of Sample Collection and Pretreatment
2.1 Sample Collection
The procedure of sample collection mainly depends on the nature of the investigation. In an agricultural context where environmental contamination has occurred in a horizontal homogeneous way, e.g. by aerial deposition or fertilizer treatment, some 20–25 “sample units” for an area of 1–2 ha of arable soil are taken to plough depth (25 cm approx.) by auger at intervals along a W-shaped track or alternatively from 20–25 approximately equal squares covering the area.
For permanent pasture or grassland a sampling depth of 10 cm is appropriate. These sampling units are combined to provide for the laboratory a bulk sample, in a polyethylene bag, of some 1–2 kg field-moist soil.
On land contaminated by industrial activities first a systematic survey is made of samples from a grid of 50 m2 at a fixed depth of 50 cm. These samples are inspected and the results of this preliminary study act as a guide to further sampling and analysis. Moreover, sampling at a fixed depth will not be useful where the site has been subject to landfill (Fortunati et al. 1994) or as is often the case on industrial sites, has been built up by deposits of fill material itself contaminated. Rubio and Ure (1993) suggested an alternative strategy of judgment sampling which makes use of historical and anecdotal information to choose relevant sampling locations. These can then be sampled at the surface of the soil or profile samples taken from a soil pit dug to a depth of at least 1 m. As land from industrial sites often is essentially made-up ground, the samples are only nominally soils and the profile samples cannot be categorised into the usual horizons of natural soils. Examination of the soil profile can, however, identify distinct layers with distinguishable visual, olfactory or textual characteristics. Sampling of such identified layers down the soil profile can not only establish the vertical distribution of a contaminant but identify a particular profile layer material as a source of contamination (Davidson et al. 1998).
Rubio and Ure (1993) had also furnished excellent information on different aspects of sampling (a) Sample contamination using inappropriate materials, containers and tools as well as the possibility of losses of analyte during sample handling (b) Minimum sample weight criteria for representative sampling of dry soils in the field (c) Use of traps and continuous flow centrifugation methods for sediments and bottom sediments collection by grabs or cores and their comparison (d) Drying and storage temperatures to be controlled to minimize any significant changes in the metal species (e) Preservation of samples in an inert atmosphere or by irradiation and (f) Difficulties of establishing definitive protocols for sampling and sample pretreatment along with the need for selecting the appropriate technique in each particular case.
In a nutshell, it can be concluded that, there is a clear possibility of obtaining biased results when sampling only once. Distinction should also be made between sampling of (1) natural, agricultural, grassland, forest, or moorland soil where to some extent element distribution and speciation can be regarded as homogeneous and (2) industrially contaminated soils will usually have an element distribution and speciation that is heterogeneous not only over the surface area but also with depth. In the first case, representative samples of the area top soils may be required. In the second case, statistical sampling may be desirable but will often be uneconomic, and the so-called judgmental sampling using selected pit sampling of soil profiles may be required.
Filgueris et al. (2002) and Hlavay et al. (2004) have discussed in depth the various procedures to be followed for the sampling of different materials and the readers can refer these documents for the full particulars regarding sampling methodologies.
2.2 Sample Pretreatment
Ideally, sample pretreatment should not disturb the original metal distribution and keep it during storage prior to application of sequential extraction.
Drying seems to accelerate the crystallization of solids such as Fe–Mn oxides and promotes Fe, Mn and S oxidation, causing an increase in metals bound to Fe and Mn oxyhydroxides to the detriment of more labile phases made up of the exchangeable and carbonatic fractions (Bordas and Bourg 1998). However, the difficulties of storing field moist soils at temperatures low enough to minimise changes in soil nature with time, produced by microbiological action, or by chemical transformations due, for example, to oxidation or reduction and to prevent losses of volatile species, all make the use of moist soils only justifiable in limited special cases. Furthermore, not only are homogenisation and sub sampling of such wet soil difficult but, in consequence, representative sub samples of less than about 100 g are difficult to achieve. In addition this last limitation makes the preparation of certified reference materials with such large minimal sample weights difficult if not impracticable (Ure 1994). Hence in practice, in most soil laboratories, generally soil is dried before analysis. As further advantages, extractable metal contents are temporally stable in dried samples over periods of months, even years which is a necessary requirement for preparation of soil and sediment reference materials.
When drying the soil would entail a loss of essential information it is preferable to make use of the soil solution it self obtained, for example, by centrifugation or displacement rather than trying to use field moist soils (Ure 1996). Drying of the samples should be carried out at <30°C because soils stored over a 12-month period at 40°C showed small but significant increases in acetic acid extractable Cu, Cr but not for Cd, Ni, Pb and Zn ( Rauret et al. 2000a).
Bearing in mind that no method of sample pre-treatment preserves the speciation intact, metal extractability should at least be affected to a similar extent for the metals of interest. This is far from being the rule since metals such as Pb and Cd differed in their response to several pre-treatments (Davidson et al. 1999b). Quevauviller et al. (1993) concluded that sequential extraction should be applied to wet, sieved sediment immediately after sampling so that environmentally relevant information is obtained. It is clear that the last recommendation cannot always be followed. In a comparison of several pre-treatment methods such as freeze-drying, air-drying and oven-drying at 105°C, (Bordas and Bourg 1998) it was found that Cd, Cu and Pb were the metals most sensitive to pre-treatments, freeze-drying and air-drying being the least disturbing procedures. Davidson et al. (1999a) found in a study with industrially-contaminated soil for assessment of variability sources that repeatability was higher for air-dried samples than for field-moist soils, but larger amount of metals were extracted, suggesting alterations in the metal distribution during drying.
After drying, sieving is necessary to obtain the appropriate particle size distribution. This operation should be made with nylon sieves in order to avoid metal contamination. The smaller particle size used in sediments (<63 μm) in comparison with soils (<2 mm) means that the representative sub-sample used for extraction can be much smaller (i.e. 0.5–2 g for sediments vs. 5–20 g for soils).
Another aspect to be considered is the need for remixing before extraction. The influence of the manual remixing procedure to avoid segregation of particles in the bottle and the grinding of the solid sample has been studied for soils (Clevenger 1990). The stability of the soils was generally independent of the particle size distribution, but the metal extractability was affected by grinding, so rehomogenisation merely by manual shaking was recommended. Sahuquillo and Rauret (2003) covered several aspects of the sample pretreatment for sequential extraction procedures and the readers can refer the document for full particulars.
Once the extracts are separated from the solid material, acidification is recommended to guarantee a better conservation (Whalley and Grant 1994; Ianni et al. 2001).
Regarding sewage sludge, thermal treatment at 180°C was seen to benefit its use as soil fertilizer since on the one hand metal leaching is decreased and, on the other hand, losses of organic matter are negligible, thereby keeping its usefulness as organic amendment (Obrador et al. 2001).
Detailed guidelines on long and short term storage of soil samples had been brought by International Organization for Standardization (ISO 18512 2006) and the readers may refer the document for details. However as a conclusion, sample storage seems to be generally less critical to the analysis of extractable metal fractions than air-drying, but it is likely to enhance the effects of air-drying in the case of redox sensitive elements.
3 Extractants for Assessing the Easily Leachable or Potentially Leachable Trace Element Contents of Soils and Other Related Materials
The use of chemical extractants for the assessment or prediction of agricultural crop plant contents, plant growth or health and the estimation of the likelihood of plant or animal (consuming the fodder plant) deficiency or toxicity has been a major topic of study in agricultural laboratories for almost half a century, therefore exists a large body of information on soil extractants for these purposes and for which plant contents and soil extractable contents are correlated and the methods validated over several years for different crops and different soil types. The extractants developed are not universal reagents, but remain, to varying degrees, soil and crop specific.
Table 1 lists some of the extractants used for assessing plant-available element contents in an agricultural context. It is also important to note that, the validity of these extractants as measures of plant availability is much less well established in the case of industrial polluted soils where concentrations observed are well in excess of normal soil contents hence correlations are relatively unsubstantiated in those cases.
3.1 Single Extractants
In recent years the assessment of the mobile fractions of heavy metals in soils, as an indication of potential risk of toxic species entering the biosphere and the possible negative effects on ground water quality and their availability to plants and the need to evaluate the environmental impact has been a major topic of investigation all over the world as the number of contamination sites are increasing. One of the approaches most widely used to study metal mobility in soils is the use of single extraction procedures using unbuffered salt solutions that can be considered as good models for simulating raining and flooding events. Several studies have demonstrated the suitability of these extraction procedures for the prediction of plant uptake in soils (Menzies et al. 2007 and the references therein). While many of the extractants listed in Table 1 have been applied to various heavy metal assignment, the principal extraction reagents now favored are the weak salt solutions of calcium chloride, sodium nitrate and ammonium nitrate. They are also called as unbuffered mild extractants or neutral salt solutions and a more productive approximation to the soil solution in a better buffered state is obtained by extraction with weak neutral salt solutions and presently 0.01 M calcium chloride has been recommended in The Netherlands (Houba et al. 1996) and 0.1 M sodium nitrate in Switzerland (Hani and Gupta 1986) and 1 M ammonium nitrate in Germany (DINV 19730 1995) as a standard national protocols.
Single extractants can also be considered in their role of releasing elements from particular soil phases with which they are bound or associated. Extractants can be classified according to these soil phases or binding types and many of the extractants listed in Table 2 can be regarded in this light. The classification of extractants in this way is extremely useful in the study of soil chemistry and in elucidating the mechanisms of metal binding, transformation or release in soils. Unfortunately most available extractants are less specific than desired. More than one target site may be attacked or the release from the target site be less than complete. Table 2 lists only some typical single extractants with their ideal target species, but, in view of the limitations noted above, they are best regarded as extractants whose extracted phases are operationally defined i.e., by the procedure used to isolate them. Despite these limitations, which should always be borne in mind when interpreting results, the concept of a single extractant releasing element species or elements associated with a particular soil phase is still a useful one and constitutes one of the few tools available for assessing the binding and mobilisation of elements in soils.
3.2 Reagents and Targeted Phases
To understand more intricately the several reactions and mechanisms involved between the various reagents and the different phases, the fraction associated with each soil phase (or element binding type) have been considered individually and the reagents used to extract them are discussed in a more detailed way hereunder. Excellent and very exhaustive information had also been provided by Pickering (1986), Ure and Davidson (2001), Gleyzes et al. (2002a) which can be consulted for more details.
3.2.1 The Soil Solution
The water-soluble metals can be determined from the saturation paste extract of a soil or by extracting soil with deionised water at a certain soil–water ratio. (Svete et al. 2000).
This phase contains the water soluble species made up of free ions and ions complexed with soluble organic matter and other constituents. It constitutes the most mobile and potentially the most available metal and metalloid species. The concentration of trace element nutrients or pollutants in the soil solution is very low and only accessible to the most sensitive analytical techniques. This phase can be isolated by centrifugation (Linehan et al. 1985), displacement (Campbell et al. 1988; Sanders 1983), filtration (Brummer 1986), ultrafiltration (Wang and Benoit 1996) or dialysis etc.,(Lee and Zheng 1994; Ure and Davidson 2001).
3.2.2 Exchangeable/Non-specifically Sorbed Fraction
This fraction includes weakly adsorbed metals retained on the solid surface by relatively weak electrostatic interaction, metals that can be released by ion-exchange processes etc. Changes in the ionic composition, influencing adsorption–desorption reactions, or lowering of pH could cause remobilisation of metals from this fraction. (Krishnamurti et al. 1995; Arunachalam et al. 1996; Narwal et al. 1999; Ahnstrom and Parker 2001). Exchangeable metal ions are a measure of those trace metals which are released most readily into the environment. Metals corresponding to the exchangeable fraction usually represent a small portion of the total metal content in soil, sewage sludges, and sediments and can be replaced by neutral salts (Rauret 1998). Thus, this fraction generally accounted for less than 2% of the total metals in soil present, the exceptions to this were the macro-elements, K, Ca and Mn (Emmerson et al. 2000).
Readily exchangeable fraction, also described as non-specifically adsorbed fraction, can be released by the action of cations such as K, Ca, Mg or (NH4) displacing metals weakly bound electrostatistically on organic or inorganic sites. These cations have been widely employed for this purpose, generally at relatively high concentrations (Beckett 1989, pp.143–176). Examples are tabulated in Table 2. Neutral salts of strong acids and bases have the advantage that they do not affect the pH at the exchange sites, nor do they attack silicate or oxyhydroxide phases. Ammonium salts of strong acids, such as NH4Cl or NH4NO3, however, can lower the pH and encourage the hydrolysis of clays. Salts of weak acids, such as acetates, can, conversely, increase the pH with possible precipitation of metal hydroxides – an effect countered, however, by the complexation of metals by acetate. This complexation also inhibits the readsorption of released metals (Gomez-Ariza et al. 2000).
3.2.3 Specifically Sorbed Fraction
Less readily exchangeable fraction, bound by covalent forces, i.e. specifically sorbed species, are not easily displaced by major cations such as K or Ca, but require the hydrogen ion or a “soft” cation such as Pb (I) or Cu (I) to displace them from organic or inorganic sites. Magnesium salts have been reported to have a somewhat stronger displacing action than those of calcium in the case of sorbed Co and Zn (Tessier et al. 1979). Hydrogen ion in the form of 0.5 M acetic acid has been used in non-calcareous soils for copper (Berrow and Mitchell 1980) and for cobalt sorbed on iron oxyhydroxide sites (McLaren et al. 1986). Acetic acid lacks specificity in that it partly attacks carbonate and silicate phases (Rapin and Forstner 1983).
Copper acetate (0.125 M) has been used to displace metals sorbed on organic matter and on oxyhydroxides of iron (Soon and Bates 1982), while 0.05 M lead nitrate released specifically bound copper (Miller et al. 1986).
3.2.4 Organically Complexed Metal Fraction
The trace metals may be associated through complexation or bioaccumulation process with various forms of organic material such as living organisms, detritus or coatings on mineral particles . Organic substances exhibit a high degree of selectivity for divalent ions compared to monovalent ions and in aquatic systems, the probable order of binding strength for metal ions onto organic matter being Hg > Cu > Pb > Zn > Ni > Co (Filgueiras et al. 2002). Metallic pollutants associated with oxidizable phases are assumed to remain in the soil for longer periods but may be mobilised by decomposition processes (Kennedy et al. 1997). Degradation of organic matter under oxidising conditions can lead to a release of soluble trace metals bound to this component. Amounts of trace metals bound to sulfides might be extracted during this step (Marin et al. 1997). The organic fraction released in the oxidizable step is not considered very mobile or available since it is thought to be associated with stable high molecular weight humic substances that release small amounts of metals in a slow manner (Filgueiras et al. 2002).
The most widely used procedure involves the oxidation of organic material by hydrogen peroxide with a subsequent extraction with ammonium acetate to prevent readsorption or precipitation of released metals. A detailed procedural protocol for this method has been published by Ure et al. (1995). In some cases formation of oxalate can occur during oxidation with hydrogen peroxide (Harada and Inoko 1977) and this can attack iron and manganese oxyhydroxides and release metals sorbed on clays (Ure and Davidson 2001). H2O2 – ammonium citrate does not completely leach metals associated with sulfides. Other oxidising reagents, such as H2O2/ascorbic acid or HNO3 + HCl are used which can dissolve sulfides with enhanced selectivity, but, on the other hand, silicates are attacked to some extent (Klock et al. 1986).
Oxidation with alkaline sodium hypochlorite has also been recommended (Shuman 1983) although the fraction of organically bound metals released showed considerable variability in different soil horizons (Papp et al. 1991).
An alternative approach uses sodium or potassium pyrophosphate (0.1 M at pH 10) to disperse colloidal organic material by complexing the floculating Ca, Al or Fe cations. This reagent is more selective for the easily soluble organic fraction i.e., metals associated with humic and fulvic acids. Minimal degradation of Mn and Fe oxides and silicates have been indicated for hypochloride (Hall et al. 1996b; Hall and Pelchat 1999; Neel et al. 2007; Iwegbue et al. 2007).
Complexing extractants such as EDTA or DTPA can, by virtue of their strong complexing ability, displace metals from insoluble organic or organometallic complexes in addition to those sorbed on inorganic soil components (Berrow and Mitchell 1980; Ure and Davidson 2001).
Non-oxidising reagents have also been attempted for extraction of metals associated with organic matter such as sodium dodecyl sulfate in NaHCO3 mixed with an organic compound such as diaminemethane, N,N-dimethyl formamide, dimethylsulfoxide, etc. (Batley 1989).
3.2.5 The Carbonate Phase
Carbonate can be an important adsorbent for many metals when organic matter and Fe–Mn oxides are less abundant in the aquatic system (Stone and Droppo 1996). The carbonate form is a loosely bound phase and liable to change with environmental conditions. This phase is susceptible to changes in pH, being generally targeted by use of a mild acid. The time required for complete solubilisation of carbonates depend on several factors such as particle size of the solid, type and amount of carbonate in the sample,etc (Beck et al. 2001).
The most common reagent for the extraction of trace metals from carbonate phases in soil is 1 M sodium acetate acidified to pH 5 with acetic acid. Carbonate phases effectively attacked include dolomite, but the presence of acetic acid also promotes the release of metals specifically sorbed on inorganic and organic substrates (Tessier et al. 1979; Ahnstrom and Parker 1999).
3.2.6 The Fraction Associated with Hydrous Oxides of Iron and Manganese
Hydrous oxides of manganese and iron are extracted together and are the well known “sinks” in the surface environment for heavy metals. Scavenging by these secondary oxides, present as coatings on mineral surfaces or as fine discrete particles, can occur by any or a combination of the following mechanisms: coprecipitation; adsorption; surface complex formation; ion exchange; and penetration of the lattice (Hall et al. 1996a).
The amorphous oxyhydroxides of iron and manganese strongly sorb trace elements, initially in exchangeable forms, but increasingly with time are transformed to less mobile, specifically adsorbed forms. Acidified hydroxylamine hydrochloride 0.1 M releases metals mainly from amorphous manganese oxide phases with little attack on iron oxide phases (Shuman 1982). Increasing the hydroxylamine hydrochloride concentration to 0.5 M (Sahuquillo et al. 1999) and decreasing the pH from 2 to 1.5 (Chao 1972; Sahuquillo et al. 1999) provides effective attack on the iron oxide phases while still releasing metals from manganese oxide phases.
Sodium dithionate has been used in combination with sodium citrate and sodium bicarbonate in a range of concentrations (Beckett 1989, pp.163–164) and usually at pHs between 5.8 and 7.3 for the reduction of both crystalline and amorphous iron oxide phases and release of sorbed trace metals (McKeague and Day 1966). It is little used for heavy metal studies because of contamination of the dithionite with zinc and the possibility of precipitation of metal sulfides (Gibson and Farmer 1986). Additionally, this reagent easily attacks silicates (Rozenson and Heller 1978). The ascorbic acid/ammonium oxalate reagent offers several advantages over the previous reagent, since it can be achieved with a high purity degree, and does not attack silicates (Shuman 1982; Bibak et al. 1994).
Although acid (pH 3) ammonium oxalate has been widely used to dissolve iron and aluminium oxides and release bound trace metals, nevertheless, leaching of metals associated with organic matter is likely to occur as a result of the complexing capacity of oxalate (Slavek et al. 1982) and further, the extraction is sensitive to light and particularly to ultraviolet light (Endredy 1963). Schwertmann (1964) showed that in the dark the amorphous iron oxides were mainly attacked and under ultraviolet illumination the crystalline phases were dissolved as effectively as the dithionate reagent. Heavy metals are released, with the exception of lead and cadmium whose oxalates are poorly soluble and which coprecipitate with calcium oxalate (Chao and Zhou 1983). The use of oxalic acid at the lower pH of 2.5 improved the performance relative to acid ammonium oxalate in that Cd was almost unaffected although lead was partially lost by precipitation (Sahuquillo et al. 1999).
3.2.7 Strong Acid-extractable Fraction: Pseudototal Trace Element Contents
Digestion in strong acids such as nitric acid, hydrochloric acid or mixture such as aqua regia that do not dissolve the silicate matrix can give an estimate of the maximum amounts of elements that are potentially mobilisable with changing environmental conditions. It is therefore a useful tool in the assessment of the long-term potential risk of heavy or toxic metals entering the biosphere. Such reagents do not mobilise trace elements from geological, silicate parent materials but dissolve metal pollutants which largely enter the soil environment in non-silicate-bound forms. The terms pseudototal analysis and pesudototal contents are useful in expressing the environmental role of such strong acid digestion procedures. Aqua regia digestion is now a well-used procedure (ISO 11466 (1995)) with a legal status in some European countries and has been used as a reference procedure in the preparation of soil and sediment reference materials certified for extractable contents by the European Community of Bureau of Reference (BCR), now the Standards, Mesaurement and Testing Program (SM&T).
4 Other Selective Extraction Methods
Extraction procedures have also been developed for the determination of the anionic species in soils of elements such as sulfur which are important as binding sites for metals as well as for its own mobility and availability (Cordos et al. 1995). The important biosignificant element selenium has similarly received attention along with Cr, Pt, Tl for their estimation in the soil solution or soil extracts (Blaylock and James 1993; Seby et al. 1997; Zbiral et al. 2002). Review by Gleyzes et al. (2002a) provides an excellent information on the extraction of anionic species of As, Se, Cr etc. Recent review by Smichowski et al. (2005) also furnishes a detailed account of metal fractionation of atmospheric aerosols using various sequential chemical extraction procedures.
5 Sequential Extraction Methods
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 principal advantage claimed for sequential extraction over the use of single extractants is that the phase specificity is improved. This occurs because each reagent has a different chemical nature (e.g. a dilute acid, reducing or oxidising agent) and the steps are performed roughly in order of increasing “vigour.” Hence, in a typical procedure, the first species to be isolated are those already in the soil solution or sediment pore water, perhaps together with those loosely attached at cation-exchange sites in the matrix. This is generally followed by stepwise attack on the carbonate phase, iron and manganese oxyhydroxides and organic matter. Finally, more refractory soil components, sometimes including the primary silicates, may be dissolved. With the use of additional reagents, the minerological phases may be further subdivided: for example, many procedures involve separate attacks on the more labile, amorphous iron oxyhydroxides and the more refractory, crystalline forms etc.
Although there are several sequential extraction procedures in the literature the Tessier scheme and the BCR scheme are usually the most adopted methods by various workers. Hence the brief details of these two methods are given in Tables 3 and 4 respectively.
The BCR sequential extraction scheme which was originally developed for the analysis of heavy metals in sediments, has been standardized and reference materials are available and has been successfully applied to a variety of matrices, including calcareous soils (Alvarez et al. 2006), contaminated soils (Pueyo et al. 2003), road side soils (Yusuf 2006), industrially contaminated soils (Van Herreweghe et al. 2003), sewage sludge (Lihareva et al. 2006), Sludge amended soil (Rauret et al. 2000a), fly ash (Smeda and Zyrnicki 2002), mining waste (Margui et al. 2006) etc.
6 Quality Control of the Extraction Data
The usefulness of a certified reference material (CRM) for validation of analytical methodology depends critically on how well the certified values are established and special difficulties exist when the species to be determined are isolated via an operationally-defined procedure (Sahuquillo et al. 1999). The Community Bureau of Reference (BCR, now the Standards, Measurements and Testing Programme), being aware of this drawback, has undertaken in the past few years a series of inter-laboratory studies leading to certification of extractable contents of several metals according to a common three-stage sequential extraction scheme i.e., BCR SES (Ure et al. 1993; Quevauviller et al. 1997b; Rauret et al. 2000a) and single extractions with EDTA and acetic acid (Quevauviller et al. 1997a, c). So far, the results of these studies have conducted to the certification of two sediments (CRMs BCR 601 and 701) using the BCR SES (original and modified) and sewage sludge amended and calcareous soils (CRMs BCR 483, 484, 600 and 700) using single extraction with EDTA and acetic acid. Indicative values for extractable metal contents from CRM BCR 483 upon application of the modified BCR SES have also been published (Rauret et al. 2000b). Samples which are the candidates for reference materials to be certified for extractable metal contents are firstly characterized through homogeneity tests (i.e. comparison of between-bottle and within bottle precision) and stability studies (i.e. by studying the extractable metal contents for bottles kept at −20, +20 and +40°C during a period of 12 months). Marin et al. (1997) concluded their study saying that the sequential extraction scheme recommended by BCR is sufficiently repeatable and reproducible to be applied to metal distribution studies, but also advised that care must be taken when this procedure is applied to soil and sediments with extremely different chemical compositions. For example, Sulkowski and Hirner (2006) discussed in detail the problems associated with the sequential extraction of high carbonate content soils.
The sediments certified so far for extractable metal content (i.e. CRMs 601 and 701) were homogeneous at 1 g mass level, the mass used for sequential extraction. It should be stressed that certified materials are subjected to exhaustive mixing, homogenisation and sieving operations including in some cases further coning and quartering so that the sample size is conveniently diminished before bottling. Pre-treatment of real samples for determination of extractable metals is usually less stringent, and hence extraction results could be affected by non-homogeneity in a larger extent. In spite of the efforts made towards certification of soil and sediments for sequential extraction, unacceptable spread of results were observed in some cases and indicative values instead of certified ones have been provided (Quevauviller 1998a). Evidently, the operationally defined character of SES means that conditions established must be strictly followed if a good agreement has to be obtained between fractionation results from different labs. This includes consideration of the type of mechanical shaker and extraction vessel and the method of separating the extract from the soil residue used etc.
Thus, the effects of the shaker type and speed and room temperature have been thoroughly discussed (Quevauviller 1998b).The effect of the shaking speed has been found to be more significant as well as the type of shaker, and speed should be adjusted so that the mixture is maintained in suspension during extraction. The temperature of extraction should be kept constant at 20 ± 2°C. In conclusion, close control of the conditions of extraction are necessary so that reproducible and reliable extractable contents of soils or sediments can be obtained.
The ISO protocol (11466) for aqua regia leaching has been adapted for extraction of metals in the residue from the third stage of the BCR scheme as well as for the original sample as an internal check on the sequential extraction (Rauret et al. 2000a). Finally, in many inter-laboratory studies, most of the errors detected were due to the calibration of the method rather than to the application of the extraction scheme (Lopez-Sanchez et al. 1998). Rauret et al. (2000a) also gave a detailed account of specific recommendations for metal determination in extracts by commonly used techniques such as ICP-OES and ETAAS etc.
Given the importance of validation for accuracy assessment, additional reference materials comprising different types of soil, sediments, sewage sludge, particulate matter etc., with certified extractable metal contents are urgently needed for quality control. But due to the non availability of sufficient reference materials in this area has prompted many workers to apply sequential extraction procedures to other reference soils and sediments which have been certified for total metal contents, in an attempt to provide interim data, useful in method validation.
Some workers ( Pueyo et al. 2005) had given action limits for aqua regia and CaCl2 extraction procedures for a contaminated soil sample so that those samples can also be used as quality control materials (QCMs). Venelinov and Sahuquillo (2006) gave an excellent account of producing quality control materials to meet the shortage of certified reference materials. The QCMs can be used for the own internal use to optimize the cost of materials and as a complimentary materials and not intended for validation purposes.
Several studies of application of sequential extraction methods to different reference materials had been carried out by various workers and few selected examples are Zemberyova et al. 2006, 2007; Vasile et al. 2006; Larner et al. 2006; Matus et al. 2006; Kubova et al. 2004, 2005; Pueyo et al. 2005; Sutherland and Tack 2002, 2003; Rauret and Lopez-Sanchez 2001; Rauret et al. 1999; Ho and Evans 1997; Hall et al. 1996a, b; Li et al. 1995a, b etc.
7 Other Approaches of Extraction
A major limitation to the wide spread adoption of sequential extraction for trace element sequestration is the lengthy sample processing time (e.g. the Tessier and BCR sequential extraction schemes require an overall operation time of about 18 and 51 h, respectively). Hence, various authors have attempted to develop more rapid means of extraction, involving ultrasonic or microwave assistance and also continuous flow extraction techniques and rotating coiled columns etc. The goal of such studies is generally to obtain performance similar to that of a well-established methods.
In general, comparing the ultrasonic bath and microwave methods, it can be stated that, from an analytical point of view, the optimal ultrasonic method gives more accurate results than the microwave method and in particular ultrasonic versions of the BCR SES showed a better performance than the Tessier ones in order to attain similar extractability as compared with the conventional SES (Perez-Cid et al. 2001, Filgueiras et al. 2002).
A continuous-flow extraction technique has been proposed by Shiowatana et al. (2001) which has the potential to be an effective and accurate method for fractionating arsenic in soil samples. Benefits of the continuous system as compared with the batch system include the removal of errors associated with repeated centrifugation, filtration and washing. Continuous-flow extraction is faster than batch extraction and is likely to minimise readsorption problems also. An interesting feature to be investigated with continuous-flow extraction is the kinetics of the leaching process of each metal and the chemical associations present.
Rotating coiled columns (RCC) earlier used in countercurrent chromatography have been successfully applied to the leaching of heavy metals from soils and sediments by Fedotov et al. (2005). The use of multistage continuous extraction in rotating coiled columns allowed the reduction of the contact time needed for the separation of each fraction as well as heating being unnecessary for sample oxidation.
Lu et al. (2003) had applied the microwave extraction procedures for the rare earth element analysis in soils using single extractants like 0.05 M EDTA, 0.1 M acetic acid, 0.1 M HCl, 0.05 M Ca Cl2. The recommended technique shortened the operational time and improved the precision and the results were generally consistent with those obtained by using conventional methods.
Acceleration of sequential or single extractions with microwaves yielded, in general, a worse performance as compared with the use of ultrasound (Perez-Cid et al. 2001; Filgueiras et al. 2002) and best results being again obtained for metal partitioning in sewage sludge. It should be noted that heating caused by microwave treatment could cause significant changes on metal extraction, mainly in the labile phases. Also, working with soils, a set of optimised conditions were recommended for each stage according to the type of soil and the metal to be determined on applying a microwave sequential extraction procedure (Campos et al. 1998). So far, only a few studies have been carried out on acceleration of sequential extractions. Further investigations are clearly needed to assess metal extractability, redistribution and readsorption when replacing conventional treatments (i.e. magnetic stirring, conductive heating) by other involving ultrasonic or microwave energies.
Some of the useful references in this area are Buanuam et al. 2006; Nakazato et al. 2006; Tongtavee et al. 2005; Sun et al. 2004, 2005; Vaeisaenen and Kiljunen 2005; Chomchoei et al. 2004, 2005; Jimoh et al. 2005; Katasonova et al. 2005; Krasnodebska-Ostrega et al. 2003; Davidson and Delevoye 2001.
8 Application of Chemometric Techniques
Chemometric techniques are applied by different workers in the field of sequential extraction to improve experimental design and also to gain as much useful information as possible from experimental results.
Analysis of variance (ANOVA and MANOVA) has been used to investigate the influence of location on forms of metals in roadside soil (Nowak 1995). Multiple regression analysis has proved valuable in processing sequential extraction data to obtain information on plant availability of trace metals in soils (Qian et al. 1996; Zhang et al. 1998).
Chemometrics have also been used by some workers to overcome some of the intrinsic deficiencies of sequential extraction, such as non-specificity. Barona and Romero (1996) used principal component analysis (PCA) to establish relationships between the amounts of metals released at each stage of a sequential extraction procedure and bulk soil properties, and demonstrated that carbonates played a dominant role in governing metal partitioning in the soil studied. The same workers employed multiple regression analysis to study soil remediation. Zufiaurre et al. (1998) also used PCA to confirm their interpretation of phase association and hence potential bioavailability of heavy metals in sewage sludge.
An interesting, and somewhat radical, departure from traditional extraction methodology was proposed by Cave and Wragg (1997). They demonstrated that, with an appropriate chemometric mixture resolution procedure, a simple, non-specific extraction could provide information on metal binding in soil SRM 2710 similar to that obtained by a Tessier sequential extraction. The method used a central composite design, with extraction time, nitric acid concentration and sample : extractant ratio as variables, together with PCA.
Abollino et al. (2002) have used hierarchical cluster analysis (HCA) and principal component analysis (PCA) and discriminant analysis (DA) to obtain a visual representation of the data set and gain insight into the distribution of the pollutants by detecting similarities or differences which would be more difficult to identify only by looking at the tables.
Perez and Valiente (2005) in their studies on pollution trends in an abandoned mining site by utilizing sequential extraction procedure and chemometric treatment of the data employing pattern recognition techniques PCA, HCA observed interesting trends. Statistical evaluation of the results allowed in identification of groups of samples with similar characteristics and observation of correlations between variables determining the pollution trends and distribution of heavy minerals within the studied area.
Tokalioglu and Kartal (2006a) investigated the uptake of several elements by different plants growing in the three vegetable gardens by using statistical methods such as correlation analysis, PCA and cluster analysis. The chemometric treatment of the data allowed a considerable reduction in the number of variables and the detection of structure in the relationships between metals that would give information about the relation between soil and plant systems.
Stanimirova et al. (2006) suggested an advanced method of chemometric data treatment utilizing Tucker N-way method in order to evaluate the level of pollution in soil from a contaminated site because the classical two-way approaches such as PCA are not as good at revealing the complex relationships present in environmental data sets. They also suggest that in the future, applying N-way methods and particularly Tucker method will become increasingly popular when analysing data sets arranged in three-(or higher ways), which is the case for most environmental data sets.
Various other workers have used the different chemometric techniques and some of the interesting examples are of Sarbu et al. 2007; Topalovic et al. 2006; Song and Greenway 2006; Lucho-Constantino et al. 2005; Palumbo-Roe et al. 2005; Cave et al. 2004; Tokalioglu and Kartal 2003; Santamaria-Fernandez et al. 2002, 2003; Eichfeld et al. 2002; Maiz et al. 2000.
9 Applications of Sequential and Single Extraction Procedures
The sequential extraction procedures have been applied to various types of materials like soil, industrially contaminated soil, sewage sludge and sludge amended soil, road dust and run-off, waste and miscellaneous. The full details of the methodologies adopted by several workers for different variety of samples have been furnished in Table 5 (soil), Table 6 (industrially contaminated soil), Table 7 (sewage sludge and sludge amended soil), Table 8 (road dust and run-off), and Table 9 (waste materials and miscellaneous samples), respectively.
Radionuclides is another important area where single and sequential extraction methods are widely employed. Kennedy et al. (1997), Ure and Davidson, (2001) had reviewed the various methods employed by different workers and therefore, only important applications of sequential extraction to both natural and anthropogenic nuclides are discussed in Table 10.
Finally some of the selected examples of the application of single extractant procedures to soils and other related materials have also been furnished in Table 11.
10 Concluding Remarks
It is now well established that total metal concentrations show poor correlation with either bio-availability for plants or organisms or for predicting the impact of contaminated soils (or sediments) on ground or surface water quality.
Geochemical maps of surface soils and sediments have been prepared on the basis of total concentration. These maps allow to identify “hot spots” and can be used to classify polluted sites. However, by their nature they do not provide information on the reactivity, mobility or accessability of the metals. For instance it is well known that chromium occurs at high levels in ceratin rock types where it is largely immobile. Extension of this types of maps to include the “reactivity” of the metals as determined by a harmonized extraction schemes offer new possibilities. One can envisage maps which show the distribution of a certain metal fraction which is linked to plant availability or leachability. With this kind of regional maps it will be possible to identify problem areas for further investigations and, in combination with soil properties, predict the impact of changing land use, large scale civil engineering activities or acid deposition on metal behaviour etc.
A recent comprehensive study by Menzies et al. (2007) regarding the evaluation of single extractants for estimation of the phytoavailable trace metals in soils, of all the extractant types examined (104 studies and 4,500 individual data points collected from earlier works), they conclude that neutral salt solutions tended to provide the best relationship between soil-extractable trace metal and plant tissue accumulation. Of the six relationships examined for neutral salts (1 M NH4CH3COO for Cd, 1 M NH4NO3 for Cd, 0.1 M NaNO3 for Zn, and 0.01 M CaCl2 for Cd, Zn and Ni), all had R 2 values ≥0.50 other than the 1 M NH4NO3 for Cd (R 2 = 0.412). Similarly, trace metal concentrations determined by extraction using complexing agents (such as the widely used DTPA and EDTA extractants) or acid extractants (such as 0.1 M HCl) were generally poorly correlated to plant uptake.Unlike chelating extractants (such as DTPA), neutral salts remove the metal from the soil solid phase by swamping the soil with the desorbing cation (McLaughlin et al. 2000). Menzies et al. (2007) also indicate that further research is required to investigate the effectiveness of these neutral salt extractants for wide variety of soils.
Basta et al. (2005) in their excellent article concluded that caution is recommended when using only single soil extractants for the estimation of phytoavailability because they only measure metal availability (i.e., soil and residual factors) but plant physiology and rhizosphere biochemistry can alter the relationship between the extractant and plant tissue concentration. For example, rape (Brassica napus L.) had a concentration ratio of plant to soil of 1.7 for Cd, about 10 times greater than corn (Zea mays L.) (CF = 0.18) grown on the same soil treated with biosolids.
Other methods using diffusive gradients in thin films (DGT) [Nolan et al. (2005) and Nolan (2005); Song et al. (2004) ] and lux-marked bacteria (Palmer et al. 1998) rhizosphere-based study by Feng et al. (2005) reported to offer good correlations with the phytoavailable trace metals but little data is currently available to adequately assess these new approaches.
The use of different neutral salt solutions at our laboratory for the estimation of extractable elements in wide variety of soil samples indicated that the magnitude of extraction is similar for all the three commonly used reagents like 0.1 M NaNO3, 0.01 M CaCl2, 1 M NH4NO3 although in absolute terms the values obtained from 0.1 M NaNO3 are marginally lower than 0.05 M CaCl2 and 1 M NH4NO3 (Pueyo et al. 2004). This clearly indicate that any one of the above three reagents is suitable for the estimation of phytoavailable or easily leachable metals. So efforts should be made to harmonize the single extraction protocol similar to BCR sequential scheme so that interlaboratory comparisons can be made and finally reproducible and accurate procedure can be established.
Our studies dealing with the use of complexing agents like DTPA, EDTA and strong acids like HCl for the extraction of various metals in different soil and other related materials, gave very high recoveries as reported by earlier workers (Menzies et al. 2007), hence they may not be suitable for phytoavailable metal contents evaluation investigations.
Although the sequential extraction approach is unlikely to provide precise information on the mineral phases to which trace metals are bound, it does provide information on potential mobility of metal contaminants. Attempts to quantitatively predict phytoavailability and toxicity from sequential extraction data alone have not typically been successful (McLaughlin et al. 2000 and references therein). This is not only due to limitations of analytical fractionation or speciation techniques, but also to the complexity of the interactions between metals and biota, which needs to be taken into account when estimating metal phytoavailability. Sequential extraction methods should be used in conjunction with plant bioassays to determine residual effects on phytoavailability. Similar conclusions had also been drawn by Iwegbue et al. (2007) in their review article on the application of various extraction schemes applied to composts and compost amended soils. They also recommend that along with chemical extraction tests bioassay tests should also be carried out, together, so that complete field test situation will emerge which can significantly help in evaluating the level of risk of heavy metals.
Further, a recent interesting study by Neel et al. (2007) indicate that compared to direct in situ measurements, extractions gave meaningful estimations of the relative trace element mobility, which is of interest for phytoremediation purposes or for assessing the element phytoavailability with out costly direct plant analyses. However, they also mention that previous mineralogical analyses and the examination of the solid residues will provide crucial information for the interpretation of the extraction results. At the end they also conclude that extraction approach alone is not appropriate for long term matters such as sewage sludge risk assessment etc.
The primary importance of proper sampling protocols has also been emphasized, since the sampling error can cause erroneous results even using highly sophisticated analytical methods and instruments. It was recognised that vital information on the distribution of trace-metal fractions may be lost in some cases when (soil or sediment) samples are dried. However, drying is certainly the best compromise for achieving stability of samples and interlaboratory comparability because of the better homogeneity of dried samples. Further work is certainly recommended to determine the bestway of stabilising samples (including reference materials) so that original trace element pattern is maintained during storage, prior to application of methods such as single and sequential extraction.
Even though sequential extraction procedures provide useful information in environmental studies and an increasing number of publications have appeared over the last few years, they also have some limitations and draw backs (Kheboian and Bauer 1987; Nirel and Morel 1990; Scheckel et al. 2003).
Julian and Collado (2002) in their review of sequential chemical extraction (SCE) of heavy metals mention that the major problems in sequential chemical extraction are 1. reagent selectivity 2. operative definition of methods 3. elemental redistribution or readsorption 4. variable experimental conditions 5. scarcity of certified reference materials 6. major difficulty to validate the existing methods 7. and the evaluation of its precision. In spite of all the above methodological problems, they indicate that, at the present time sequential extraction protocols constitute the better approach to describe the geochemical association of trace elements with different fractions of solid materials.
Sutherland and Tack (2003) has emphasized that sequential extraction procedures should always be applied with full consideration of their limitations. Unless improved procedures can be developed that can be properly standardized and provide better, more conceptually defined fractions for a range of samples, it will be difficult to select one single scheme for future use. However, the optimized BCR procedure is currently the only scheme that is harmonized and standardized, and certified reference samples are available (most recently CRM 701). This is a significant advantage and provides for a degree of comparability between researchers generally not obtainable for the other procedures presented in the literature. Having said this, it is likely that the more commonly used sequential extraction schemes (i.e. the Tessier procedure) will continue to be employed by researchers. It is paramount that researchers rigorously follow the stated procedures for a selected scheme. In addition, caution should always be exercised when interpreting the results of sequential extractions, because artifacts of the scheme not only depend on the operational conditions of the method, but also vary from element to element and between different sample types.
D’Amore et al. (2005) in their review on methods for speciation of metals in soils also reveal interesting information on the shortcomings of chemical extraction methods such as (a) As the present sequential chemical extraction methods were meant for trace metals in sediment materials, their application to heavily contaminated soils may be suspect when concentrations of the “trace” metals are no longer trace but major constituents. In this situation, the metal chemistry is no longer dominated by the other major components of the system, but is itself controlling the chemistry of other elements (b) Comparisons of sequential extractions with thermodynamic models and direct instrumental analyses are lacking (c) Application of sequential extraction methods to lead contaminated, phosphorous amended samples results in the formation of pyromorphite [ Pb4(PO4)3Cl] during the extraction steps and hence the over – and underestimation of metal concentrations in particular steps of an extraction method could pose serious consequences in addressing risk assessments based solely on extraction results. Finally, they conclude that extraction schemes can be a useful tool in metal partioning but always be confirmed by other methods.
The increasing number of scientific papers making reference to the so-called “ BCR scheme “ (referring to the sequential extraction protocol) and the single extraction procedures (in particular EDTA) illustrates that these procedures are now internationally recognised as reference methods for soil and sediment studies (Quevauviller 2002).
Dahlin et al. (2002a, b) in their studies indicate that sequential extraction procedures which were developed to characterise pollutant species in normal soil and sediments may be unsuitable for industrial site materials which contain larger pollutant particles, encapsulated pollutants, and / or man-made materials e.g., slags, metals and plastics. However, they mention that if employed as part of a comprehensive, site – specific characterization study, sequential extractions could be a very useful tool. This study reveals that more standardized sequential chemical extraction procedures should be developed for various types of materials to get more reliable and useful data about the different phases of the toxic metals in wide variety of materials.
Song et al. (2004) in their review of sequential extraction technology pointout that this technology will develop in two new directions i.e., microwave heating-continuous flow-sequential extraction (MCSE) technology and microwave heating-ultrasonic vibrating-kinetic-parallel extraction (MUKPE) technology. SCE methods have also been utilized for the identification of the status of available nitrogen in the soil by Kodashima et al. (2005); and Bacon et al. (2006) combined SCE with isotope analysis as a tool to assess mobilization of lead into streams and several others used SCE methods for wide variety of applications.
Significant developments are being made in the application of chemometric procedures in SCE and Stanimirova et al. (2006) proposed a Tucker N – way method which can analyse data sets arranged in three – (or higher ways), which is the case for most environmental data.
Due to non availability of sufficient reference materials in this area, prompted many workers to apply SCE procedures to other reference soils and sediments which have been certified for total metal contents, in an attempt to provide interim data, useful in method validation. However, efforts should be made to develop more CRMs exclusively for single and sequential extraction procedures covering various types of materials.
From this review, it appears that all the reagents used in the various schemes have advantages and disadvantages and there is not an ideal reagent or an ideal protocol for general use. Therefore, the choice of procedure must be related to a definite objective, taking into account the nature of the sample: sediment; soil; sludge; or, industrially-polluted soil etc. Interpretation of results must not be based on the mineralogical fraction targeted but rather on the reagent used, implying a perfect knowledge of its action on the solid phase. A complementary approach characterising the solid residue can help in understanding the action of the reagent and the association of trace elements to the matrix. Finally, in the not-too-remote future, combination of the application of well-designed sequential extraction schemes and speciation studies of the solutions obtained will give a better view of the potential for transfer of trace elements in the environment and of the risks involved.
Finally, it may be emphasized that, some of the areas which needs more attention are (a) preparation of new reference materials with certified extractable contents (b) harmonisation of sequential extraction schemes in order to facilitate comparability of data (c) design of accelerated schemes based on the existing ones so that overall treatment times are diminished (d) development of extraction schemes specifically optimised for the characteristics of the target sample (e) implementation of on-line manifolds for performing sequential extractions (f) development of single extraction methodology simultaneously so that operation procedures are simplified (g) development of small-scale extractions to minimise sample and reagent consumption (h) application of sequential extraction for characterisation of metal mobility for different variety of environmental samples because till date, huge amount of data is available for soils and sediments but there are limited number of applications for coal fly ash, solid waste incineration bottom ash, airborne dust, etc. (i) use of chemometric approaches for robustness testing of sequential extraction protocols, for finding relationships between soil metal fractions and plant uptake, and for characterisation of pollution sources from partitioning results in soils and sediments (j) more investigations of changes in metal mobility in soils subjected to amendment etc.
At the outset, authors experiences in the field of single and sequential chemical extractions for the past few years ( Lopez-Sanchez et al. 1998, 2002; Pueyo et al. 2001a, b, 2003, 2005; Sahuquillo et al. 1999, 2002, 2003; Sahuquillo and Rauret 2003; Rauret et al. 1999, 2000a, b, 2001; Llaurado et al. 2001) reveal that there are lot of advantages of these methods and still enormous scope for further research and developments in these areas because pollution sites are increasing throughout the world and there is urgent need of methods for faster, reliable and cost-effective pollution assessment so that suitable remedial measures can be taken up on priority basis at an appropriate time.
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One of the authors (C.R.M.Rao) is thankful to the Spanish Ministry of Science and Education for a sabbatical fellowship for his stay in Spain (SAP 2005-0143) and also to Government of India for the sanction of necessary leave. The authors also thank DGICYT (Project CTQ 2006-02924) for financial support.
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Rao, C.R.M., Sahuquillo, A. & Lopez Sanchez, J.F. A Review of the Different Methods Applied in Environmental Geochemistry For Single and Sequential Extraction of Trace Elements in Soils and Related Materials. Water Air Soil Pollut 189, 291–333 (2008). https://doi.org/10.1007/s11270-007-9564-0
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DOI: https://doi.org/10.1007/s11270-007-9564-0