The utilization of modified BCR three-step sequential extraction procedure for the fractionation of Cd, Cr, Cu, Ni, Pb and Zn in soil reference materials of different origins

A modified three-step sequential extraction procedure for the fractionation of heavy metals, proposed by the Commission of the European Communities Bureau of Reference (BCR) has been applied to the Slovak reference materials of soils (soil orthic luvisols, soil rendzina and soil eutric cambisol), which represent pedologically different types of soils in Slovakia. Analyses were carried out by flame or electrothermal atomic absorption spectrometry (FAAS or ETAAS). The fractions extracted were: exchangeable (extraction step 1), reducible-iron/manganese oxides (extraction step 2), oxidizable-organic matter and sulfides (extraction step 3). The sum of the element contents in the three fractions plus aqua-regia extractable content of the residue was compared to the aqua-regia extractable content of the elements in the origin soils. The accuracy obtained by comparing the determined contents of the elements with certified values, using BCR CRM 701, certified for the extractable contents (mass fractions) of Cd, Cr, Cu, Ni, Pb and Zn in sediment following a modified BCR-three step sequential extraction procedure, was found to be satisfactory.     

Ultrasound and microwave-assisted extraction of metals from sediment: a comparison with the BCR procedure

In this paper we investigate alternatives to mechanical stirring for the extraction of the mobile fraction of metals from sediment, and analyze whether these techniques can reduce extraction time and improve reproducibility. We compare the quantities of metal extracted from BCR601 and BCR701 certified sediments using ultrasound bath, microwave-assisted extraction and the first step in the certified BCR sequential extraction procedure. Some environmentally important not-certified metals such as As, Mn, Co, Fe and Al have been included in this study. In the case of microwave-assisted extraction, we compare tests in which samples are exposed to constant, low power irradiation with tests using pulsed high power.

In the tests using the ultrasound bath, less metal was extracted than with the other extractive techniques and standard deviations were comparable to those obtained with the BCR procedure; in assays using microwaves at constant power, extraction efficiencies were different for different metals and for different reference materials and, in some cases, standard deviations were higher than those for the reference method. In contrast, tests with microwaves and constant temperature produced encouraging results: R.S.D.s lay in the 2–4% range, both for certified and not-certified metals; these values are very low compared to those for the reference method. Extraction efficiencies for certified metals were close to 100% for Cd, Zn, Cu and Ni and around 80% for Pb and Cr.     

Fractionation of heavy metals in sediment by using microwave assisted sequential extraction procedure and determination by inductively coupled plasma mass spectrometry

In this work, a fast microwave assisted extraction procedure was developed and optimized for the heavy metals (Cu, Ni, Cr, Pb and Cd) partitioning in the three-stage sequential extraction procedure proposed by the European Standards, Measurements and Testing (SM&;T) Program, formerly the Community Bureau of Reference (BCR). The microwave oven procedure was optimized to obtain extraction efficiencies similar to the conventional BCR procedure, in less time, while using smaller volumes of reagents. In the optimization process, three variables (extraction time, ramping time and microwave power) were considered as factors and as a response the concentration of different metal ions in each individual BCR fraction. Interactions between analytical factors and their optimized levels were investigated using a central composite design. Extractable metals obtained by both comparable methodologies were measured by inductively coupled plasma mass spectrometry. With the use of optimal microwave conditions, steps 1–3 of the sequential extraction (including the hydrogen peroxide digestion in step 3) could be completed between 21 and 22 min. Detection limits were between 1 and 18 ng l^{− 1}. The accuracy of the proposed method was checked with a certified reference material (CRM) of Lake Sediment BCR 701. Values obtained were in accordance with those reported for the certified material with only a few exceptions. Different origin sediments (river and marine) were analyzed by both BCR and MW procedures, and the results obtained were comparable according to the t-paired-test for a 95% confidence level.     

Comparison of original and modified BCR sequential extraction procedures for the fractionation of copper, iron, lead, manganese and zinc in soils and sediments

This article describes a detailed comparison between the original BCR sequential extraction procedure, step 2 of which involves treatment with 0.1 mol l⁻¹ hydroxylammonium chloride at pH 2, and the revised BCR procedure (step 2: 0.5 mol l⁻¹ hydroxylammonium chloride at pH 1.5). An intermediate protocol was also evaluated in which 0.5 mol l⁻¹ hydroxylammonium chloride at pH 2 was used. The procedures were applied to five soil and sediment substrates: a sewage sludge-amended soil, two different industrially contaminated soils, a river sediment and an inter-tidal sediment. Extractable iron and manganese concentrations were measured to assess the effects of the procedural modifications on dissolution of the reducible matrix components. Trace elements copper, lead and zinc were also determined. Statistical analysis (two-tailed t-tests at 95% confidence interval) indicated that recovery of iron in step 2 was not markedly enhanced when the intermediate protocol was used. However, significantly greater amounts were isolated with the revised BCR scheme than with the original procedure. Copper behaved similarly to iron. Lead recoveries were increased by use of both modified protocols, with the greatest effect occurring for the revised BCR extraction. In contrast, manganese and zinc extraction did not vary markedly between procedures. The work indicates that the revised BCR sequential extraction provides better attack on the iron-based components of the reducible matrix for a wide range of soils and sediments.     

Metal fractionation in olive oil and urban sewage sludges using the three-stage BCR sequential extraction method and microwave single extractions

The conventional three-stage BCR sequential extraction method was employed for the fractionation of heavy metals in sewage sludge samples from an urban wastewater treatment plant and from an olive oil factory. The results obtained for Cu, Cr, Ni, Pb and Zn in these samples were compared with those attained by a simplified extraction procedure based on microwave single extractions and using the same reagents as employed in each individual BCR fraction. The microwave operating conditions in the single extractions (heating time and power) were optimized for all the metals studied in order to achieve an extraction efficiency similar to that of the conventional BCR procedure. The measurement of metals in the extracts was carried out by flame atomic absorption spectrometry. The results obtained in the first and third fractions by the proposed procedure were, for all metals, in good agreement with those obtained using the BCR sequential method. Although in the reducible fraction the extraction efficiency of the accelerated procedure was inferior to that of the conventional method, the overall metals leached by both microwave single and sequential extractions were basically the same (recoveries between 90.09 and 103.7%), except for Zn in urban sewage sludges where an extraction efficiency of 87% was achieved. Chemometric analysis showed a good correlation between the results given by the two extraction methodologies compared. The application of the proposed approach to a certified reference material (CRM-601) also provided satisfactory results in the first and third fractions, as it was observed for the sludge samples analysed.     

Speeding up of a three-stage sequential extraction method for metal speciation using focused ultrasound

The three-stage sequential extraction procedure, proposed by the European Community Bureau of Reference (BCR), has been applied for speciation of copper, chromium, nickel, lead and zinc in a sludge sample collected from an urban wastewater treatment plant. The conventional BCR sequential extraction method has been modified, in each stage, applying ultrasonic energy by means of a probe (handling at an adequate sonication power and time) in order to shorten the required operation time. Extractable metal contents obtained by both the conventional and the accelerated ultrasonic extraction method, were measured by Flame-Atomic Absorption Spectrometry. Results obtained in each fraction by both methods were statistically compared (P=0.95) for all the studied elements and no significant differences were found except for chromium and zinc in the third fraction (oxidisable). For all metals the extraction percentage was>95%. The proposed accelerated sequential extraction method could be a valid alternative to the conventional shaking with a much shorter operating time.     

Should acid ammonium oxalate replace hydroxylammonium chloride in step 2 of the revised BCR sequential extraction protocol for soil and sediment?

The revised, four-step BCR sequential extraction for soil or sediment has been compared with an alternative procedure in which 0.2 mol l⁻¹ ammonium oxalate (pH 3) replaced 0.5 mol l⁻¹ hydroxylammonium chloride (pH 1.5) in step 2, the reducible step. A variety of substrates were studied: BCR CRM601, a sewage sludge amended soil, two industrial soils, and a steel manufacturing by-product (basic oxygen furnace filter cake). Greater amounts of iron were recovered in step 2 when acid ammonium oxalate was used, for all substrates. Similar trends were observed for copper. Manganese and zinc were not strongly affected by the procedural modification, except for zinc in the two industrial soils, where oxalate extraction proved more efficient than use of hydroxylammonium chloride. A large proportion of the calcium and lead isolated in step 2 of the BCR procedure was not released until step 3 when the alternative procedure with oxalate in step 2 was used. This is probably due to rapid precipitation of analyte oxalates from solution. Thus, whilst oxalate offers superior dissolution of iron-containing matrix components, it should not be used if calcium or lead concentrations are to be measured. Selection of the most appropriated sequential extraction protocol for use in a particular study must always be carried out on the basis of “fitness for purpose” criteria. However, the revised BCR protocol, involving use of 0.5 mol l⁻¹ NH₂OH·HCl in the reducible step, appears to be more generally applicable than procedures involving acid ammonium oxalate.     

Voltammetric behaviour at gold electrodes immersed in the BCR sequential extraction scheme media: Application of underpotential deposition-stripping voltammetry to determination of copper in soil extracts

The development of mercury-free electroanalytical systems for in-field analysis of pollutants requires a foundation on the electrochemical behaviour of the chosen electrode material in the target sample matrices. In this work, the behaviour of gold working electrodes in the media employed in the BCR sequential extraction protocol, for the fractionation of metals in solid environmental matrices, is reported. All three of the BCR sequential extraction media are redox active, on the basis of acidity and oxygen content as well as the inherent reducing or oxidising nature of some of the reagents employed: 0.11 M acetic acid, 0.1 M hydroxylammonium chloride (adjusted to pH 2) and 1 M ammonium acetate (adjusted to pH 2) with added trace hydrogen peroxide. The available potential ranges together with the demonstrated detection of target metals in these media are presented. Stripping voltammetry of copper or lead in the BCR extract media solutions reveal a multi-peak behaviour due to the stripping of both bulk metal and underpotential metal deposits. A procedure based on underpotential deposition-stripping voltammetry (UPD-SV) was evaluated for application to determination of copper in 0.11 M acetic acid soil extracts. A preliminary screening step in which different deposition times are applied to the sample enables a deposition time commensurate with UPD-SV to be selected so that no bulk deposition or stripping occurs thus simplifying the shape and features of the resulting voltammograms. Choice of the suitable deposition time is then followed by standards addition calibration. The method was validated by the analysis of a number of BCR 0.11 M acetic acid soil extracts. Good agreement was obtained been the UPD-SV method and atomic spectroscopic results.     

Comparison of single and sequential extraction procedures for the study of rare earth elements remobilisation in different types of soils

With the continual increase in the utilisation of rare earth elements (REE) for industrial and agricultural purposes, research into the environmental and biogeochemical behaviour of REE had attracted much interest in recent times. This study principally describes the distribution of REE in four different types of soils like lateritic soil (S-1), in situ natural soil (S-2), soil contaminated by mining activity (S-3) and accidentally polluted soil (S-4) utilizing the optimised BCR sequential extraction procedure and partial extractions with various types of single extractants such as unbuffered salt solutions 0.1 M NaNO₃, 0.01 M CaCl₂, 1 M NH₄NO₃; complexing agents 0.005 M DTPA and 0.05 M EDTA; acid solutions 0.43 M CH₃COOH and 1 M HCl. Comparison of the sum of the four BCR fractions, which included an aqua regia attack on the residue, with the pseudo-total aqua regia digest values to assess the accuracy of the BCR partioning approach has been undertaken. Partial extraction results with several single extractants have also been reported for all the REE elements including yttrium which have been analysed by the optimised BCR procedure. Results obtained after 24 h extraction with each of the single extractant have also been discussed. The extraction with 1 M HCl during 24 h yielded similar quantities of REE as those released under the combined steps of 1, 2 and 3 of the BCR sequential extraction for all the four different type of soil samples indicating that this reagent can be used successfully to estimate the total extractable contents of REE in various types of soil samples.     

A comparison of an optimised sequential extraction procedure and dilute acid leaching of elements in anoxic sediments, including the effects of oxidation on sediment metal partitioning

The effect of oxidation of anoxic sediment upon the extraction of 13 elements (Cd, Sn, Sb, Pb, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, As) using the optimised Community Bureau of Reference of the European Commission (BCR) sequential extraction procedure and a dilute acid partial extraction procedure (4 h, 1 mol L⁻¹ HCl) was investigated. Elements commonly associated with the sulfidic phase, Cd, Cu, Pb, Zn and Fe exhibited the most significant changes under the BCR sequential extraction procedure. Cd, Cu, Zn, and to a lesser extent Pb, were redistributed into the weak acid extractable fraction upon oxidation of the anoxic sediment and Fe was redistributed into the reducible fraction as expected, but an increase was also observed in the residual Fe. For the HCl partial extraction, sediments with moderate acid volatile sulfide (AVS) levels (1-100 μmol g⁻¹) showed no significant difference in element partitioning following oxidation, whilst sediments containing high AVS levels (>100 μmol g⁻¹) were significantly different with elevated concentrations of Cu and Sn noted in the partial extract following oxidation of the sediment. Comparison of the labile metals released using the BCR sequential extraction procedure (ΣSteps 1-3) to labile metals extracted using the dilute HCl partial extraction showed that no method was consistently more aggressive than the other, with the HCl partial extraction extracting more Sn and Sb from the anoxic sediment than the BCR procedure, whilst the BCR procedure extracted more Cr, Co, Cu and As than the HCl extraction.     

Microanalytical flow-through method for assessment of the bioavailability of toxic metals in environmental samples

The application of a recently proposed microanalytical flow-through system for on-line sequential extraction of heavy metals from solid samples of environmental interest is described. Using various extraction schemes (a nitric acid scheme, a two-stage extraction scheme using two reagents applied in the BCR procedure) and comparison with the common batch sequential BCR procedure, the suitability of the system for fast screening of solid environmental samples is demonstrated. By pumping leaching agents sequentially through the sample held in a micro cartridge, the different metal fractions present can be assessed in less than an hour. Method evaluation was performed using SRM 1648 urban particulate matter and BCR 701 lake sediment reference material certified for extractable metals. The need for and design of laboratory internal reference material suitable for simulating the natural (dynamic) processes of metal release into the environment is also discussed. For the first time correlation is sought between fractionation techniques and physiologically based methods for assessment of the bioaccessibility of metals in biomatrices.     

Kinetic speciation of BCR reference materials

The accurate study of heavy metal speciation is important in environmental monitoring. There has been much work developing various operationally defined speciation methods for soil and sediment, but there is a need to compare the different approaches by evaluating them for the same sample. In this article, a kinetic method was applied for the heavy metal speciation of the two BCR reference materials, CRM601 and BCR701, which have been specifically developed as materials to evaluate the validated BCR three-step sequential extraction method. When EDTA was used as an extractant, 81.0% of Cd, 68.0% of Cu, 21.5% of Ni, 80.3% of Pb and 71.9% of Zn was extracted from CRM601. For BCR701, the removal ratios were 92.0, 52.3, 18.7, 50.6 and 67.5% with EDTA and 95.7, 25.2, 20.0, 52.4 and 68.5% with hydroxylamine hydrochloride as an extractant, for Cd, Cu, Ni, Pb and Zn respectively. A two-component kinetic model was applied to the extraction curve and the extractable metals were readily classified into two categories, namely, labile fraction and non-labile fractions. The rate constants obtained from the regression model were found to be useful in quantifying the lability of an element. The rate constants obtained from the labile fractions in BCR701 were higher than that of obtained from CRM601, which indicated the high lability of metals in BCR701. When compared with the sequential extraction data, it seemed that the lability of an element was positively correlated to the first step extraction fraction.     

Mercury determination in solid phases from application of the modified BCR-sequential extraction procedure: a valuable tool for assessing its mobility in sediments

The present paper presents a feasibility study for the evaluation of mercury mobility in sediments by application of the modified BCR three-step sequential extraction procedure (BCR-SEP). The aim of the study was to evaluate the effect of acidification, reduction and oxidation processes on mercury mobility from sediments, once the SEP was validated with other trace metals.As extractable mercury amounts were mostly found below detection limits of the advanced mercury analyser (AMA-254) used for measuring the extracts (L.D.<0.5 ng), the use of a solid sampling atomic absorption spectrometer with a specially designed furnace for Hg atomisation was found to be an optimal technique to quantify Hg in the solid residues coming from the SEP.With this approach, mercury was found to be hardly mobile and only for one of the moderately polluted sediments (BCR CRM 320), extractable mercury (exchangeable fraction, BCR procedure step 1) has been found. An oxidation step with H(2)O(2) was required to obtain extractability up to 64% of the total content in a highly polluted lagoon sediment.     

Comparison of three sequential extraction procedures for fractionation of arsenic from highly polluted mining sediments

Three sequential extraction procedures were evaluated for the study of fractionation of arsenic in environmental solid samples. The procedures considered were as follows: i) the standardized and widely recognised BCR procedure, conceived for the study of the partitioning of heavy metals; ii) the procedure developed by Manful, who adapted a phosphorus scheme for arsenic fractionation; and iii) a novel sequential extraction scheme especially devised for arsenic. The efficiency and suitability of these methods and the corresponding extraction steps for partitioning arsenic obtained from the most important solid forms were tested by application of the methods to real sediment samples heavily polluted by mining activity. Results showed the BCR scheme was inappropriate for arsenic fractionation. The procedure could, nevertheless, be a first approach for the assessment of arsenic partitioning, because its first extraction step can be regarded as adequate for the estimation of the most easily mobilizable arsenic. Although the Manful scheme results in a more differentiated arsenic pattern, some drawbacks arise from the lack of selectivity of some of the reagents used, for example overlapping of specific target phases, overestimation of adsorbed arsenate because of inadequate coprecipitation processes, and the inability to discriminate among amorphous and crystalline oxyhydroxides which are mainly responsible for arsenic retention. The novel procedure achieves the most suitable arsenic fractionation, because the main phases retaining arsenic are selectively targeted according to mobilization potential. In addition, the simplicity of its extraction steps would enable automation in a continuous flow scheme.     

Evaluation of a sequential extraction procedure for the determination of extractable trace metal contents in sediments

The determination of extractable trace metals in sediments using sequential extraction procedures has been performed in many laboratories within the last ten years in order to study environmental pathways (e.g. mobility of metals, bounding states). However, the results obtained by different laboratories could hardly be compared due to lack of harmonized schemes. Owing to the need for standardization and subsequent validation of extraction schemes for sediment analysis, the Measurements and Testing Programme (formerly BCR Programme) of the European Commission has organized a project to adopt a sequential extraction procedure that could be used as a mean of comparison of data of extractable trace metals in sediments. A scheme was designed after a series of investigations on existing schemes and tested in interlaboratory studies. This paper presents the results of two round-robin exercises on extractable trace metals using this sequential extraction protocol and describes the final version of the extraction procedure amended according to the most recent improvements.     

A rapid fractionation method for heavy metals in soil by continuous-flow sequential extraction assisted by focused microwaves

A microwave-assisted continuous-flow sequential extraction system was developed for rapid fractionation analysis of heavy metals in soil. Insertion of pressure-adjusted air between the extractants provided stable flows of the extractants without mutual mixing and back-pressure influence of a column packed with soil, thereby facilitating reliable continuous-flow extractions. In addition, use of pure water as a pumping solvent removed metal contamination because of direct contact between corrosive extractants and the pump containing metallic materials. Focused microwave irradiation to the soil accelerated the selective extractions of the acid-soluble and reducible fractions of heavy metals in soil in the first and second steps of the sequential extraction conditions, as defined by the Commission of the European Bureau of Reference (BCR). The microwave-assisted continuous-flow extraction provided high correlations in amounts of six heavy metals except Zn in the first step and Cu in the second step extracted from a reference sludge soil, BCR CRM 483, with a conventional batchwise extraction proposed by BCR; continuous-flow extraction assisted by conductive heating provided lower correlations for all the six metals. The proposed method drastically reduced the time required for the sequence extraction to ca. 65 min without losing accuracy and precision of the fractionation analysis of heavy metals in soil, whereas the BCR batchwise method requires ca. 33 h. An erratum to this article can be found at     

Utilization of optimized BCR three-step sequential and dilute HCl single extraction procedures for soil-plant metal transfer predictions in contaminated lands

The prediction of soil metal phytoavailability using the chemical extractions is a conventional approach routinely used in soil testing. The adequacy of such soil tests for this purpose is commonly assessed through a comparison of extraction results with metal contents in relevant plants. In this work, the fractions of selected risk metals (Al, As, Cd, Cu, Fe, Mn, Ni, Pb, Zn) that can be taken up by various plants were obtained by optimized BCR (Community Bureau of Reference) three-step sequential extraction procedure (SEP) and by single 0.5 mol L(-1) HCl extraction. These procedures were validated using five soil and sediment reference materials (SRM 2710, SRM 2711, CRM 483, CRM 701, SRM RTH 912) and applied to significantly different acidified soils for the fractionation of studied metals. The new indicative values of Al, Cd, Cu, Fe, Mn, P, Pb and Zn fractional concentrations for these reference materials were obtained by the dilute HCl single extraction. The influence of various soil genesis, content of essential elements (Ca, Mg, K, P) and different anthropogenic sources of acidification on extraction yields of individual risk metal fractions was investigated. The concentrations of studied elements were determined by atomic spectrometry methods (flame, graphite furnace and hydride generation atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry). It can be concluded that the data of extraction yields from first BCR SEP acid extractable step and soil-plant transfer coefficients can be applied to the prediction of qualitative mobility of selected risk metals in different soil systems.     

Application of sequential extraction with supercritical CO(2), subcritical H(2)O,and an H(2)O/CO(2) mixture for estimation of environmentally mobile heavy metal fractions in sediments

Results from use of a new isolation procedure based on sequential extraction with supercritical CO(2), subcritical H(2)O, and an H(2)O/CO(2) mixture in the same supercritical fluid extractor have been compared with results from the BCR-recommended three-step sequential extraction procedure. The new procedure gives more detailed information about environmentally mobile fractions (water-soluble, bicarbonate-forming), and in less time (5-6 h), than the BCR procedure.     

Assessment of heavy metals leachability from metallo-organic sorbent-iron humate-with the aid of sequential extraction test

The sequential extraction test, known as a BCR procedure, was used to assess a leachability of heavy metals (Zn, Cd, Pb, Cu) from the metallo-organic sorbent—iron humate—loaded with these metals. The sequential test allowed to discriminate between various fractions of heavy metals, namely the acid-extractable fraction, the fraction bound to Fe oxides, and the fraction bound to organic matter. It was proven that the heavy metals are bound mainly to Fe oxides and organic matter, and thus they may be relatively hardly liberated into the environment. The BCR sequential extraction test exhibited a very good repeatability, when it was applied to the loaded sorbent—relative standard deviations were typically lower than 10%.     

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 Commission of the European Communities

Abstract

An account is presented of a series of investigations and collaborative studies, initiated by BCR, on current methods of metal speciation by extraction of soils and sediments with chemical reagents. It was established by extensive consultation with European experts that the diverse procedures used could be harmonized into agreed methods. These methods, including both single extractant and sequential extraction procedures were subjected to collaborative, interlaboratory trials and the results, presented briefly here, showed that it was both possible and desirable that reference soils and sediments, characterised by certified values for extractable contents, be prepared. As a consequence of these studies two soils have been prepared and will shortly be the subject of interlaboratory analysis with a view to certification of their EDTA and acetic acid extractable contents of some heavy metals. Following this workshop a feasibility study of the agreed sequential extraction procedure will, it is believed, shortly lead to certification of sediments for contents extractable by a defined sequential extraction procedure.