Three-stage Sequential Extraction Procedure Research Articles

Three-stage sequential extraction procedure for metal partitioning in polluted soils and sediments.

The sequential extraction procedure proposed by the Standard, Measurements and Testing program "SM&T" of the European Union has been applied to evaluate the amounts of Cr, Cu, Mn, Ni, Pb and Zn, extracted at each stage and indirectly their mobility and bioavailability in soil and sediment samples from a polluted area. Analysis of the extracts was carried out by flame atomic absorption spectrometry (FAAS). No significant matrix interferences were found except for Cr in the acetic acid and hydroxylammonium chloride extracts, which required determination by the standard additions method. Both of soils and sediments studied show similar partitioning of Mn, Ni, Cu and Cr. Mn and Ni can be considered to have significant component bound to the acido-soluble fraction, whereas, Cr and Cu occur largely in the organic and residual phases. The partitioning of Pb and Zn was different between soil and sediment. In terms of mobility and bioavailability, in soils, Ni and Mn can be regarded as moderately available followed by a lower availability of Pb while Cu, Zn and Cr have a very limited availability. In sediments, a higher availability (short-medium term) of Mn and Zn was presumed followed by a lower availability of Ni and Pb, whereas, Cu and Cr, occurring largely in the organic and residual phases, were of very limited availability.

Continuous-flow system for reduction of metal readsorption during sequential extraction of soil

A continuous-flow sequential extraction system developed in our laboratory for metal fractionation in solid materials has shown many advantages over the batch system such as speed of extraction, simple procedure, less risk of contamination and less dependency on extraction conditions. This report emphasizes the reduction of readsorption problem achieved from this system. Five metals (Pb, Cu, Zn, Ni and Cd) were studied because of their significance as environmental contaminants. A method of standard addition into extractants was used to evaluate readsorption phenomena in a three-stage sequential extraction procedure proposed by the Commission of the European Communities Bureau of Reference (BCR). The continuous-flow system showed less degree of readsorption compared with the batch system. The extent of readsorption was found to be dependent on elements, properties of soil especially organic matter content, extracting flow rate and metal concentration level. Metal readsorption from acid soluble fraction to oxidizable fraction was greater for soil of high organic matter than soil of low organic matter content. Lower extracting flow rate and lower concentration of metals in sample showed lower degree of readsorption. The behavior of metal readsorption during extraction was observed as two patterns. In pattern one, readsorption was observed at the earlier step and the adsorbed metal was leached in the following step. In pattern two, readsorption and desorption were observed in the same extraction step.

Evaluation of performance of time-saving extraction devices in the BCR three-step sequential extraction procedure.

Two alternative extraction methods--a routine ultrasonic bath and a microwave oven--were developed and optimized for their eventual exploitation in the three-stage sequential extraction procedure proposed by the European Standards, Measurements and Testing (S, M & T) Programme, formerly Bureau Communitaire de Reference (BCR), for the operationally defined speciation of Cd, Cr, Cu, Ni and Zn. The conventional BCR three-stage sequential extraction procedure was modified at each stage, applying ultrasonic or microwave devices in order to shorten the required 16 h of shaking in all three steps. The experimental tests and the optimization of the operating parameters were carried out on a highly homogenized estuarine sediment reference material (RM S7) prepared by the Joint Research Centre (JRC) of ISPRA. Extractable metal contents were measured by inductively coupled plasma-mass spectrometry (ICP-MS) and the results obtained in each fraction by all three methods were statistically compared for all the studied elements. The conventional and newly developed alternative extraction methods were finally compared by the analysis of BCR 601, which is certified for the three-step BCR sequential extraction procedure. According to the statistical evaluation of the results, the proposed accelerated sequential extraction methods are valid alternatives to conventional shaking, with much shorter extraction times.

Speciation of rare earth elements in soil and accumulation by wheat with rare earth fertilizer application

A greenhouse study was conducted to investigate the accumulation of rare earth elements (REEs), La, Ce, Pr and Nd, in winter wheat ( Triticum aestivum L.), and the speciation of these elements in soil following the application of REE-based fertilizers. Improved crop yield was confirmed by the experiment. The accumulation behavior of La, Ce, Pr and Nd in wheat varied depending on the concentration of REE fertilizer application, i.e. increased with increasing REE concentration at low fertilization application, constant over the medium REE range, and decreased with increasing REE concentration at high fertilizer application. Significant negative correlation was obtained between REE contents in roots and soil pH ( r=−0.5787 to −0.8442 for La). REEs in both the fertilized and unfertilized soils were fractionated by a three-stage sequential extraction procedure into three chemically distinct fractions: water soluble, exchangeable and carbonate bound (B1), Fe–Mn oxide bound (B2), and organic and sulfide bound (B3). REEs in fertilized soils were found mainly in the B2 and B3 fractions, with only a small amount in the B1 fraction. REEs in B1 and B2 fractions were negatively correlated with soil pH ( r=−0.6892 to −0.8927 and −0.7462 to −0.9482). Significant correlation was obtained between REEs in B1 fraction and REE contents in root. The correlation coefficients ranged from 0.6159 to 0.7410 when fertilizer application was lower than 20.0 mg/kg soil. No acceptable relationship was observed between REE contents in shoot and any of the extractable fractions in soils.

Effect of sample pretreatment upon the metal speciation in sediments by a sequential extraction procedure

Sample pretreatment, such as drying, is sometimes necessary for the speciation analysis of trace metals in sediments. However, this pretreatment may exert some effects upon the speciation distribution. In this study, the heavy metals (Cu, Pb, Cr, Cd, Zn and Ni) in reservoir surface sediments were fractionated to water soluble, exchangeable and carbonate bound (B1), Fe-Mn oxide bound (B2), organic matter and sulfide bound (B3) by a three-stage sequential extraction procedure. The effect of different drying methods (oven-drying at 85oC, air-drying at 20°C and freeze-drying) on metal speciation distribution was investigated. Compared to the fresh wet sample, none of the drying methods completely preserve the initial chemical speciation distribution of the elements. The B1 fraction was particularly sensitive to sample pretreatment methods. Among the elements, zinc was especially perturbed by air-drying, care must be exercised because air-drying was commonly used in sediment pretreatment.

The influence of rare earth element fertilizer application on the distribution and bioaccumulation of rare earth elements in plants under field conditions

This field study describes the distribution and bioaccumulation of rare earth elements (REEs) in wheat, rice and vegetables grown in Jiangxi, Anhui, Beijing and Heilongjiang provinces, located in southern and northern China after application of REE fertilizer at different levels. The results indicate that accumulation of REEs in different parts of plants follows the order: root>leaf>stem>grain. The concentrations of REEs in root, leaf and stem increase remarkably when the REE fertilizers were applied. However, no significant accumulation was observed in cereal grains except for those grown at the Jiangxi site. A significant accumulation of REEs in the edible parts of vegetables was observed, therefore, fertilizers containing REEs are not recommended. The exogenous REEs in the fertilized soil were extracted by a three-stage sequential extraction procedure into three fractions: (1) water soluble, exchangeable and carbonate bound (B1), (2) Fe–Mn oxide bound (B2) and (3) organic matter and sulfide bound (B3). The results demonstrated that all of the exogenous REEs were present in the fractions B1, B2 and B3 with ranges of 1.49–13.91%, 35.23–70.34% and 19.14–60.78% of total, respectively. No exogenous REEs entered the residual form within the field study period. However, the chemical forms of exogenous REEs in the fertilized soil and their bioavailability are dependent on the physicochemical properties of soil and plant species. It seems that REEs present in fraction B1 are more easily available to plants.

Some sources of variability in application of the three-stage sequential extraction procedure recommended by BCR to industrially-contaminated soil

Copper, lead, manganese and zinc have been determined in extracts of industrially-contaminated soil to investigate possible sources of variability in the three-stage sequential extraction procedure recommended by BCR. Analysis was performed by flame or electrothermal atomic absorption spectrometry, using reagent matched, multi-element standard solutions. Interferences were assessed by use of a single-point standard addition. Although the distribution of contaminants in the soil was expected to be heterogeneous, little difference in precision was found when replicate samples ranging in mass from 1–5 g were extracted, provided the extractant:soil ratio was kept constant. Significant differences in operational speciation were, however, obtained when the procedure was carried out independently by three different analysts. This may be due to the critical effect of pH in Step 2. Reproducibility was higher for air-dried than for field-moist soils, but larger amount of metals were extracted, suggesting alterations in speciation occurred during drying.

A preliminary investigation of the operational and isotopic speciation of uranium in sediments

Uranium was determined in extracts of inter-tidal sediment, obtained from a contaminated harbour, using the three-stage sequential extraction procedure recommended by BCR. The element was found mainly in association with reducible, or reducible and oxidisable, phases and the overall concentration was enhanced (up to 6.7 μg g–1 dry weight) with respect to typical, UK levels. The 238U/235U ratio has been measured in digests of stream sediments obtained from the vicinity of a uranium enrichment plant. Significant enhancement in 235U with respect to the natural 238U/235U ratio (137.88:1) was observed at some locations.

Use of a certified reference material for extractable trace metals to assess sources of uncertainty in the BCR three-stage sequential extraction procedure

Various potential sources of irreproducibility in the BCR three-stage sequential extraction procedure have been investigated using the lake sediment CRM 601. Of the variables considered, the pH of the hydroxylamine hydrochloride in Step 2 proved most important. Factors such as the type of acid used in pH adjustment, the temperature and duration of extraction, and working under nitrogen did not affect precision, although they did alter the amounts of metals extracted. Improved precision was obtained when the hydroxylamine hydrochloride concentration was increased from 0.1 to 0.5 mol l −1 and when the speed of the centrifugation was increased from 1500 to 3000 × g. The addition of a MgCl 2 wash between the steps of the extraction procedure gave rise to increased uncertainty. Although it did not adversely affect reproducibility, the use of filtration to separate solid and liquid phases was not recommended since it promoted dissolution of non-target phases. Neither ammonium hydrogen oxalate nor oxalic acid proved suitable alternatives in Step 2 owing to precipitation of insoluble lead salts, particularly in the presence of calcium.

Effect of sample preparation on the operational speciation of cadmium and lead in a freshwater sediment

The three-stage sequential extraction procedure recommended by BCR has been applied in the operational speciation of cadmium and lead in a relatively unpolluted freshwater sediment, following various types of sample pre-treatment. Changes in speciation occurred when the sediment was air-dried, oven-dried or frozen. Cadmium speciation was particularly perturbed by freezing, whilst lead was more affected by oven-drying. Metals were determined by electrothermal atomic absorption spectrometry, using direct calibration with reagent-matched standard solutions.

Evaluation of plant availability of rare earth elements in soils by chemical fractionation and multiple regression analysis

This case field study describes the distribution of rare earth elements (REEs) in different soil fractions obtained by a sequential extraction procedure and plant availability with single correlation and multiple regression analysis. Soil and plant samples were collected from a rural region of Beijing, China. Plant samples (corn, rice) were segmented into grain, stem, leaf and root. The results indicated that REE contents in different parts of plants followed the order: root>leaf>stem>grain. REEs in soils were extracted by a three-stage sequential extraction procedure into three fractions: water soluble, exchangeable and carbonate bound (B1), Fe–Mn oxide bound (B2), and organic matter and sulfide bound (B3). Chemical fractionation showed that the concentration of REEs in fraction B1 was close to or lower than 1% of the total REEs in soils, and in fractions B2 and B3 was in the range of 1.42–3.35 and 7.32–30.88% of total REEs in soil collected from corn fields, respectively. In soils from rice fields the concentration of REEs in fractions B1, B2 and B3 ranged from 0.58 to 1.05, 13.47 to 21.60 and 7.53 to 25.37% of total REEs, respectively. Correlation and multiple regression analysis indicated that total content of REEs in soils and the sum extracted with the sequential extraction procedure were poor indicators for REE uptake by plants. For La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb and Y, consistently significant positive correlations existed between contents in fraction B1 and concentrations in leaves of corn ( r=0.5662–0.7866) and rice ( r=0.5847–0.8262). As for other parts of rice and corn, significant correlations were observed sparsely. Multiple regression analysis was utilized to obtain the ‘best’ regression equations for predicting plant uptake of REEs. The study demonstrated that the sequential extraction procedure applied, though operationally defined, could provide valuable information on plant uptake of REEs in soils.

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.

Inhibition of Pb redistribution by two complexing agents (cryptand and NTA) during a sequential extraction of soil models

We previously reported a severe redistribution of Pb in both model soils and spiked real soil systems during two different sequential extraction schemes, and concluded that a manganese oxide and humic acid were the two main solid phases responsible for scavenging Pb liberated at the first extraction step. As an extension to that study, we have been examining a range of modifications to the first extraction step (0.11 M acetic acid, 16 h) of the three-stage sequential extraction procedure proposed by the Commission of the European Communities Bureau of Reference (BCR, now Standards, Measurements and Testing Programme) for a possible means of counteracting the Pb redistribution problem. In this study, the potential usefulness of two structurally-distinct soluble complexing ligands was investigated; cryptand 2.2.2 (a macrocyclic crown-like ligand) and nitrilotriacetic acid (NTA). The addition of each ligand to the first extraction solution of the BCR procedure generally improved the percentages of Pb recovered from our multi-phase model soils, with Pb originally spiked on calcite. However, the effectiveness of the cryptand and NTA at inhibiting redistribution (at concentrations of 9 × 10 −3 and 1 × 10 −4 M, respectively) was dependent on the percentages by weight of hausmannite and humic acid in each model system. In the presence of no more than 2% hausmannite, both cryptand and NTA, at the concentration used, could satisfactorily recover about 60% of the Pb, provided that the percentages of humic acid in the systems were not higher than 10% and 5%, respectively. The important preliminary criteria for selecting a potential useful soluble complexing agent suggested by the results of this study are that the ligand possess 1. (a) a high stability-constant for the ion involved, and 2. (b) a semi-encapsulated metal binding site (which appears to inhibit direct surface extraction from occurring). For Pb, we also found it desirable to decrease the extraction time of the first step in the BCR scheme as an aid to minimising opportunity for redistribution.

Assessment of the extent of lead redistribution during sequential extraction by two different methods

Synthetic model soils were used to evaluate redistribution of lead (Pb) in both the three-stage sequential extraction procedure proposed by the Commission of the European Communities Bureau of Reference (BCR, now Standards, Measurements and Testing Programme) and the commonly applied procedure of Tessier et al. (1979). Redistribution of Pb in both schemes was found to be substantial, sufficient to render meaningless any attempts to quantify the original phase associations of Pb in cases where these are not known. In the model systems, the manganese oxide and humic acid phases were found to be responsible for most of this redistribution in both schemes. Despite the fact that both schemes allow severe Pb redistribution to occur, differences are apparent in the manner in which this occurs. Whereas Tessier et al.'s procedure in all cases yielded the highest recovery of Pb at the reducing step, the three-stage BCR procedure released the most Pb at the oxidizing step, both irrespective of where it originated. These differences are attributable to a combination of high temperature (96 °C) and the presence of acetic acid (25%, v v ) in Tessier et al.'s reducing step. These conditions were found to reduce the sorptive capacity of humic acid by more than 50%, in addition to retaining more of the Pb in solution by acetate ions. Extensive redistribution in the three-stage BCR procedure was also observed in systems where natural soils were mixed with Pb-spiked synthetic phases. In keeping with the results related to synthetic soil models, the organic matter and Fe/Mn oxide fractions of the natural soils appear to dominate the redistribution of Pb during the extraction.

Evaluation of a sequential extraction procedure for the speciation of heavy metals in sediments

The three-stage sequential extraction procedure for speciation of heavy metals, proposed by the Commission of the European Communities Bureau of Reference (BCR), has been applied to a freshwater sediment collected from the River Clyde, Lanarkshire, UK. Initial studies were carried out using flame atomic absorption spectrometry (FAAS). Iron, manganese and zinc could be determined in the sediment extracts by FAAS, but the technique proved insufficiently sensitive for the determination of chromium, copper, nickel and lead. Detection methods based on electrothermal AAS were therefore developed and applied. Furnace conditions were optimised for the determination of the four analytes mentioned, plus molybdenum and vanadium, in acetic acid, hydroxylammonium chloride and ammonium acetate matrices. Interferences by components in the sediment extracts necessitated analysis by the method of standard additions in most cases. The sequential extraction procedure was found to be both repeatable and reproducible. The amounts of analytes released by the sequential extraction procedure plus aqua regia digestion of the residue remaining after extraction were similar to those released by pseudo-total digestion of the sediment (using aqua regia).

Three-stage sequential extraction procedure for the determination of metals in river sediments

A three-stage sequential extraction procedure, following a protocol proposed by a European working group coordinated and supported by the Community Bureau of Reference (BCR), has been applied to two river sediments from an industrial region of East Catalonia, Spain. The extractant solutions were: Step One, acetic acid (0.11 mol l −1); Step Two, hydroxylammonium chloride (0.1 mol l −1, pH 2); Step Three, hydrogen peroxide (8.8 mol l −1) oxidation followed by extraction with ammonium acetate (1 mol l −1, pH 2). The six elements determined in the extracts were Cd, Cr, Cu, Ni, Pb and Zn. Analysis of the extracts was carried out by flame atomic absorption spectrometry (FAAS). There were slight differences in analyte sensitivity between calibrants prepared in the extractant matrices and acidified aqueous calibrants. No significant interferences were found except for Cr in the acetic acid and hydroxylammonium chloride extracts, which required determination by the standard additions method. The other elements (plus all the elements in the ammonium acetate extracts) were determined by direct calibration against calibrant solutions prepared in the same matrices as the samples.