Partitioning Of Metals In Sediments Research Articles

Metal partitioning in sediments and risk assessment of a ramsar site-Cochin estuary, South West India

Geochemical partitioning of sediment bound metals in Cochin estuary were determined by sequential extraction to unravel the processes controlling their distribution, mobility and contamination status. Most of the metals recorded higher content at residual and exchangeable fractions. Abundance of Cd, Zn and Mn in bioavailable form (promoted by surface adsorption on fine grained sediments) was sourced to industrial operations, municipal sewage and agriculture effluents. Pyrite formation effectively controls the fraction distribution of redox sensitive elements- Mn and Fe. Scavenging by Fe-Mn oxide was dominant process regulating the accretion of metals -Zn, Ni, Cr, Co, Cd, Mn whereas organic matter complexation facilitate the accumulation of the metals-Mn, Co, Cu and Zn. Contamination due to Zn and Cd was substantiated by enrichment factor (range for Zn: 4.42–57.22; range for Cd: 0.02–0.37), contamination factor (range for Zn: 0.51–22.19; range for Cd: 3.30–60.67 and geoaccumulation index (range for Zn: 0.10–4.45; range for Cd: 0.66–12.18). Elevated PLI (Range: 0.19–1.58) indicated metal inputs from anthropogenic activities. Potential ecological risk values >40 implied that Cd, Zn and Pb can induce risk to the ecosystem.

Heavy metal partitioning in sediments and bioaccumulation in commercial fish species of three major reservoirs of river Cauvery delta region, India

Chemical partitioning of heavy metals (Fe, Mn, Cu, Cr, Pb, Zn, Ni) were determined in surface sediments of three reservoirs at the Delta region of Cauvery River, India. The abundance of metals in sediments varied in the following descending order: Fe, Mn, Cr, Zn, Cu, Ni, and Pb. Higher concentrations of Zn, Pb, Fe, Mn and Cu in exchangeable and carbonate fraction indicated toxicity risk to the biota. Therefore, to understand the extent of bioaccumulation, six commercial fish species were collected from the same sites and analyzed for heavy metals distribution in different organs. Among the metals found in fish samples, iron was observed in the highest concentration, followed by Zn, Pb, Cr, Mn, Cu, and Ni. Concentrations of Pb, Cr and Zn in many fish samples exceeded the permissible limits of Food and Agriculture Organization. The concentrations of Pb (17.7–31.7%), Cr (6.2–15.1%), Cu (15.2-30.5%) and Zn (30–40%) associated with exchangeable and carbonate fractions had significant positive correlation with the respective metal concentrations in fish. Among the fish species, Catla catla and Etroplus suratensis showed the highest accumulation of metals suggesting risk for human consumption.

Heavy Metal Partitioning in Sediments from Rivers Flowing Through Coal Fields in Mpumalanga, South Africa

AbstractThe association of elements Cd, Co, Cr, Fe, Mn, Pb, Ti, and V with various geochemical phases in the sediments from the Olifants, Klein Olifants, Wilge rivers and a tributary of the Olifants River was studied using a four‐step sequential extraction scheme. By employing enrichment factors, these elements were found to be contaminating the sediments. Sequential extraction enabled partitioning of the metals into exchangeable, reducible, oxidizable, and residual fractions. Most of the elements were found to exist in the residual fraction, characterized by stable compounds. Application of risk assessment code to the exchangeable fraction revealed that most of the elements posed a medium risk to aquatic life, with the exception of Co, Pb, and Mn, which were classified into the high‐risk category. Non‐residual/more bioavailable fractions were examined using statistics. Correlation analysis was employed to understand the interaction between the more bioavailable fractions of metals with the reducible phase consisting of oxides of Fe and Mn. These oxides contribute to the adsorption of trace metals onto sediments. Co, V, Pb, Cr, and Cd in the reducible fraction were found to be associated with Fe‐oxides, while V, Cr, and Ti were associated with Mn‐oxides, as indicated by significantly high correlation coefficients. Through cluster and factor analysis three anthropogenic activities associated with mining and use of coal and iron and steel manufacturing were found to be contributing metals to the sediments.

Comparative spatial metal concentrations and partitioning in bottom sediments of two tropical freshwater lake basins, Kenya

AbstractMetal concentrations in the water, sediments and biota in alkaline, near neutral water tropical freshwater lake basins were determined and a comparative analysis made on the spatial and accumulated trace metal concentrations, and geochemical partitioning of iron (Fe), manganese (Mn), copper (Cu), chromium (Cr), zinc (Zn), nickel (Ni), cadmium (Cd) and lead (Pb) in bottom lake sediments. Variable accumulated levels were found in the analysed sediments and aquatic biological tissues. Metal partitioning data in the present study from the two areas exhibits similar trends in magnitude. The residual fractions retained relatively higher amounts of metals. Relatively higher contents of Pb, Cr and Cu were found in Lake Victoria sediments. Cd, Ni, Mn and Pb are of concern because a higher percentage of these metals were retained in the more bioavailable fractions in Lake Naivasha. Total levels of sediment mercury were comparable for the two lake basins. Metals were found in significant levels in the tissues of aquatic organisms, highlighting the potential risks to other levels within the aquatic food chain. Although the exposure risk was not evaluated, it is important to continue monitoring these elements in fish, as sediments were found to contain considerable quantities of the metals. Numerous sediment extraction techniques data applied for evaluating metal partitioning in sediments cannot be compared because of the use of different methods. Nevertheless, the information is still useful in differentiating and understanding pollution loads from natural versus anthropogenic sources and their associated risks.

Metal partitioning in sediments and mineralogical controls on the acid mine drainage in Ribeira da Água Forte (Aljustrel, Iberian Pyrite Belt, Southern Portugal)

► Organic-rich waste water drives the release of metals from acid drainage sediments. ► Precipitation/dissolution of Fe-hydroxides and sulfates controls water chemistry. ► Schwertmannite formation and transformation to Fe-hydroxides maintains low pH. ► Metals keep high concentration in solution with no effective attenuation mechanism. This work focuses on the geochemical processes taking place in the acid drainage in the Ribeira da Água Forte, located in the Aljustrel mining area in the Iberian Pyrite Belt. The approach involved water and stream sediment geochemical analyses, as well as other techniques such as sequential extraction, Mössbauer spectroscopy, and X-ray diffraction. Ribeira da Água Forte is a stream that drains the area of the old mine dumps of the Aljustrel mine, which have for decades been a source of acid waters. This stream flows to the north for a little over than 10 km, but mixes with a reduced, organic-rich, high pH waste water from the municipal waste water pools of the village. This water input produces two different results in the chemistry of the stream depending upon the season: (i) in the winter season, effective water mixing takes place, and the flux of acid water from the mine dumps is continuous, resulting in the immediate precipitation of the Fe from the acid waters; (ii) during the summer season, acid drainage is interrupted and only the waste water feeds the stream, resulting in the reductive dissolution of Fe hydroxides and hydroxysulfates in the stream sediments, releasing significant quantities of metals into solution. Throughout the year, water pH stays invariably within 4.0–4.5 for several meters downstream of this mixing zone even when the source waters come from the waste water pools, which have a pH around 8.4. The coupled interplay of dissolution and precipitation of the secondary minerals (hydroxides and sulfates), keeps the system pH between 3.9 and 4.5 all along the stream. In particular, evidence suggests that schwertmannite may be precipitating and later decomposing into Fe hydroxides to sustain the stream water pH at those levels. While Fe content decreases by 50% from solution, the most important trace metals are only slightly attenuated before the solution mixes with the Ribeira do Rôxo stream waters. Concentrations of As are the only ones effectively reduced along the flow path. Partitioning of Cu, Zn and Pb in the contaminated sediments also showed different behavior. Specific/non-specific adsorption is relevant for Cu and Zn in the upstream branch of Ribeira da Água Forte with acid drainage conditions, whereas the mixture with the waste water causes that the association of these metals with oxyhydroxide to be more important. Metals bound to oxyhydroxides are on the order of 60–70% for Pb, 50% for Cu and 30–60% for Zn. Organic matter is only marginally important around the waste water input area showing 2–8% Cu bound to this phase. These results also show that, although the mixing process of both acid and organic-rich waters can suppress and briefly mitigate some adverse effects of acid drainage, the continuing discharge of these waste waters into a dry stream promotes the remobilization of metals fixed in the secondary solid phases in the stream bed back into solution, a situation that can hardly be amended back to its original state.

Comparison of sequential extraction and principal component analysis for determination of heavy metal partitioning in sediments: the case of protected Lagoon El Kelbia (Tunisia)

A sequential extraction procedure applied to surface sediments from El Kelbia Lagoon determined the partitioning of Ba, Co, Cr, Cu, Ni, Sr, V and Zn among evaporites, carbonates, Fe–Mn oxides, organic matter and silicates. To validate the procedure, the sequential extraction results (SER) were compared to principal components analysis (PCA) using major and trace element concentrations and mineralogical quantitative data of surface sediments. SER showed that a part of Sr was highly mobile; Cu, Sr, and Zn and a part of Co and Ni were mobile depending on pH conditions; Cr and V were strongly bound to silicate phases; Co and Ni were partitioned between carbonates, oxides and silicates, and a great part of Ba and Sr were bound to organic matter or sulfides. An agreement was found with PCA in terms of partitioning among minerals for most trace elements. Moreover, the absence of correlation between Ca concentrations and the abundances of calcite, gypsum or dolomite could be explained by an important fraction of Ca bound to organic matter. Also, unexpected negative correlations between abundances of smectite and illite could be explained by a transformation of illite into smectite. Thus, SER and PCA were mostly convergent, which enabled: (1) validation of the extraction procedure used, and (2) refinement of interpretations of the origin and relations between minerals.

Partitioning of metals in sediments of the Haraz River (Southern Caspian Sea basin)

The Haraz River flows northwards through the Iranian Alborz mountains in the central region of Mazandaran province and empties into the Caspian Sea. This area has been a rich source of minerals from times immemorial. About 45 mines (coal, limestone, sand and gravel, etc.) have been operational for the last eight decades. Towards the estuary, the river receives a discharge of industrial, agricultural and urban wastes. Eight sediment samples from the Haraz River and its main tributaries were collected and analyzed for base metals as well as Sr and Fe. The chemical partitioning of metals (Cu, Zn, As, Cd, Pb, Fe, Ni, Cr, Co and Sr) in each sample was determined in four fractions (acid-soluble, reducible, oxidizable and residual). The total content of each metal was also determined. The results showed relatively higher concentrations of Cd, As, Sr and Pb in comparison to that of shale. However, based on the chemical partitioning of metals, it is found that Pb, Co, Cd and Sr are the most mobile metals. In spite of the high As concentrations in sediments, it is not likely that this element is a major hazard for the aquatic environment, since it is found mainly in the residual fraction. Cadmium is the metal that showed the highest percentages in the acid-soluble fraction (the most labile) and the lowest in the residual fraction. However, Fe, Cr and Ni are present in the greatest percentages in the residual fraction, which implies that these metals are strongly linked to the sediments.

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.

Factors Controlling the Geochemical Partitioning of Trace Metals in Estuarine Sediments

The geochemical partitioning of trace metals in sediments is of great importance in risk assessment and remedial investigation. Selected factors that may control the partitioning behavior of Cu, Pb and Zn in non-sulfidic, estuarine sediments were examined with the use of combined sorption curve—sequential extraction analysis. This approach, which has not been previously used to examine estuarine sediments, allowed determination of sorption parameters for Cu, Pb and Zn partitioning to individual geochemical fractions. Partitioning behavior in sulfidic sediments was also determined by sequentially extracting Cu, Pb, and Zn from synthetic sulfide minerals and from natural sediment and pure quartz sand after spiking with acid-volatile sulfide (AVS). Trace metal sorption to the “carbonate” fraction (pH 5, NaOAc extraction) increased with metal loading due to saturation of sorption sites associated with the “Fe-oxide” (NH2OH·HCl extraction) and “organic” (H2O2 extraction) fractions in non-sulfidic sediments. Freundlich parameters describing sorption to the “Fe-oxide” and “organic” fractions were controlled by the sediment Fe-oxide and organic carbon content, respectively. Sequential extraction of Cu from pure CuS, AVS-spiked sediment and AVS-spiked quartz sand showed that AVS-bound Cu was quantitatively recovered in association with the “organic” fraction. However, some AVS-bound Pb and Zn were recovered by the NH2OH·HCl step (which has been previously interpreted as “Fe-oxide” bound metals) in the sequential extraction procedure used in this study. This indicates that the sequential extraction of Pb and Zn in sulfidic sediments may lead to AVS-bound metals being mistaken as Fe-oxide bound species. Caution should therefore be exercised when interpreting sequential extraction results for Pb and Zn in anoxic sediments.

The impact of a salinity barrier on the partitioning of heavy metals in sediments of a tropical backwater system

Concentrations of heavy metals (Cd, Co, Cr, Cu and Fe) in surface sediments and their partitioning behaviour between exchangeable, reducible (Fe-Mn oxide bound) and organic/residual phases of the sediments in a typical backwater system of Kerala, viz. the southern upstream part of Cochin Estuarine System (South India), have been studied. Spatial and temporal variations of trace metals are discussed with special reference to pH, dissolved oxygen, salinity, organic carbon and texture of sediment. Metal concentrations in the tide gated part of the estuary were found to be significantly higher when compared to metal concentrations reported from the unrestricted part of the Cochin estuarine system and also those from many of the unpolluted estuaries worldwide. The higher levels of heavy metals in the study area and their characteristic distribution and partitioning behaviour in the surficial sediments were attributed to the presence of a salinity barrier across the backwater system and also by the massive use of pesticides and chemical fertilizers in the vast area of agricultural land near the backwater system.

Determination and partitioning of metals in sediments along the Suez Canal by sequential extraction

The application of sequential extraction technique was used to determine the chemical association of heavy metals in five different chemical phases (exchangeable F1, bound to carbonate F2, bound to Fe–Mn oxides F3, bound to organic matter F4 and residual F5) for sediment samples collected from the Suez Canal. From the obtained data, it can be seen that the surplus of metal contaminants introduced into the sediment from sources usually exists in relatively unstable chemical forms. A high proportion of the studied metals remained in the residual fraction. Most of remaining portion of metals was bound to ferromanganese oxides fraction. The low concentrations of metals in the exchangeable fraction indicated that the sediments of Suez Canal were relatively unpolluted.

Short-term accumulation of Cd and Cu from water, sediment and algae by the amphipod Melita plumulosa and the bivalve Tellina deltoidalis

The benthic organisms Melita plumulosa (amphipod) and Tellina deltoidalis (bivalve) are currently being used as test species for assessing contamination of estuarine-marine sediments in whole-sediment toxicity tests in Australia. Radiotracer techniques were used to determine the rates of accumulation (at 21 ± 3°C) of cadmium and copper by these organisms from water, algae and sediment sources. A biokinetic model was then developed to separately quantify influx and efflux rates from the metal sources. For exposures to seawater, metal uptake (1 g -1 d -1 ) and efflux rates (% d -1 ) were 0.028 and 6.0 (Cd) and 0.12 and 16 (Cu) for M. plumulosa and 0.012 and 8.5 (Cd) and 0.19 and 11 (Cu) for T. deltoidalis. Net accumulation of metals from the dissolved phase was greater for Cu than Cd for both organisms. For sediment exposures, metal assimilation efficiencies (AEs, %) and efflux rates (% d -1 ) were 22 ± 10 and 19 ± 3 (Cd) and 7.8 ± 3 and 31 ± 4 (Cu) for M. plumulosa, and 25 ± 3 and 17 ± 3 (Cd) and 30 ± 2 and 20 ± 4 (Cu) for T. deltoidalis. AEs and efflux rates for algae were 56 ± 4 and 9.8 ± 1 (Cd) and 33 ± 5 and 16 ± 4 (Cu) for M. plumulosa, and 73 ± 3 and 5.0 ± 2 (Cd) and 49 ± 4 and 4.3 ± 1 (Cu) for T. deltoidalis. For M. plumulosa, the presence of algae increased the rate of accumulation of both Cu and Cd from sediments, and indicated that M. plumulosa fed more rapidly, but not necessarily selectively, in the presence of algae. Ingestion rates of sediments were estimated as 0.20 and 0.08 g g -1 d -1 for M. plumulosa and T. deltoidalis, respectively. The biokinetic models indicated that metal accumulation by the organisms would increase greatly as partitioning to the pore waters increased or if organisms increased their ingestion of metal-contaminated algae in preference to sediments. If it is assumed that the toxic effect of accumulated metals is independent of accumulation pathway, then changes in sediment properties or in an organism's feeding behaviour would be expected to greatly affect the toxicity of metals to the organism. The study highlights the need for careful scrutiny of metal partitioning in sediments when data are to be used to estimate toxicity thresholds or effects concentrations, before such values are adopted in sediment quality guidelines.

The use of flotation techniques in the remediation of heavy metal contaminated sediments and soils: an overview of controlling factors

The potential use of flotation as a remediation technique for heavy metal contaminated sediments and soils is discussed. In general, the flotation process is much less selective for heavy metals in sediments and soils compared to the results obtained in common mineral ore flotation practice. The following factors could be isolated as responsible for this different behaviour: the metal distribution over the different geochemical phases (metal partitioning), the metal distribution over the different size fractions (metal fractionation) and the presence of organic matter (humic substances). These controlling factors were illustrated by performing Denver flotation experiments on a natural canal sediment and column flotation tests on synthetic sediments. It is emphasised that sequential extraction techniques can be used as a tool for determining the original metal partitioning in sediments and soils and as a result helps to select the most appropriate procedure of flotation (e.g. a phase-specific collector). A chemical pre-treatment step (e.g. sulphidisation, hydroxylation) prior to the flotation of sediments and soils can be beneficial depending on the in-situ metal partitioning and serves to transfer heavy metals, which are associated with different speciation sources (oxides, sulphides, clay minerals, organic matter, etc.), into one unique-good floatable-chemical speciation form (e.g. oxides, sulphides). The selective flotation of metal-bearing particles from fine-textured sediments and soils is very difficult and therefore a major challenge for future research is to tackle the problems related with the flotation of these fines. Organic matter, which is rather selectively flotated, has an adverse effect on the metal (sulphide) flotation selectivity due to its adsorption onto mineral surfaces.

Partitioning of metals in sediments from the Odiel River (Spain)

The Odiel River runs through an area known as the Iberian pyrite belt in the province of Huelva in the southwestern corner of the Iberian Peninsula. Its waters are acidic and contain large amounts of heavy metals from erosion and mining activity. In addition, the Odiel River estuary is one of the most industrialized areas in southern Spain and, consequently, receives the discharge of industrial and urban waste. Seventeen sediment samples from the Odiel and its main tributaries were analyzed for this study. The chemical partitioning of metals (Cu, Zn, Cd, Pb, Fe, Ni, Cr and Co) in each sample was determined in four fractions (acid-soluble, reducible, oxidizable and residual). The total content of each of the metals was also determined. The results showed high concentrations of Fe, Cu, Zn, Pb and Cd, undoubtedly as a result of contamination from the mining and industrial activity mentioned above. However, the concentrations of the other metals analyzed were low since there are no sources of pollution by them in the Odiel River. Based on the chemical distribution of metals, we found that Cd, Zn and Cu are the most mobile metals (i.e., metals that can pass easily into the water under changing environmental conditions). Cd is the metal that showed the highest percentages in the acid-soluble fraction (the most labile) and the lowest in the residual fraction. However, Pb, Fe, Cr and Ni are present in the greatest percentages in the residual fraction, which implies that these metals are strongly linked to the sediments.

Comparison of the standard SM&T sequential extraction method with small-scale ultrasound-assisted single extractions for metal partitioning in sediments.

A single extraction method based on the reagents employed in the SM&T (standards, measurement and testing programme) sequential extraction scheme for partitioning of Cd, Cr, Cu, Ni, and Pb in sediments has been developed. Small-scale extractions adapted to electrothermal atomic absorption spectrometric determinations (i.e. 25 mg mass in 1 mL extractant) have been used for the first time in conjunction with selective reagents using conventional operating conditions (i.e. conductive heating, magnetic stirring) or ultrasound-assisted extraction. Small-scale slurry preparations were implemented for determination of the residual/total metal content. The methodology developed enabled drastic reduction of extraction times, yielding extractable contents in accordance with those obtained by application of the conventional SM&T method, which can be regarded as a standard method. An important saving in the amount of sample and reagents is also achieved with small-scale extractions. Homogeneity of sediments at the mg level proved acceptable - precision was similar for between-batch and within-batch measurements. Method development was performed with CRM BCR 601 lake sediment for which extractable amounts of several metals are known. An application to a "real-world" sediment is also presented.

Insights into sequential chemical extraction procedures from quantitative XRD: a study of trace metal partitioning in sediments related to frog malformities

Sequential chemical extraction (SCE) and quantitative X-ray diffraction (QXRD) were used in combination to assess trace metal speciation and availability in sediment from two separate marshes in western Vermont, USA, one with high (45%) and one with low (<5%) northern leopard frog ( Rana pipiens) malformity rates. The types of malformities observed in the field are very similar to malformities produced in laboratory studies where frogs are exposed to waters and sediments with elevated levels of trace metals such as Co, Cu, Ni, and Zn. Total recoverable metals, as determined by US EPA method 200.2 (HCl–HNO 3), are statistically higher in the malformity-affected marsh than in the marsh with low malformity rates, suggesting possible causality. Analysis of trace metal speciation by SCE and QXRD, however, implies that the most common reservoir of Co, Cr, Cu, Ni, and Zn in the sediment is trioctahedral clay minerals, including chlorite, high charge smectite and vermiculite. The only trace metal that is not associated with trioctahedral clay is Pb, which appears to be contained in poorly crystalline or amorphous oxyhydroxides. The proportions of trace metals in the exchangeable fraction are variable, with values of ≤3% of total Cr and Pb, 2–17% of total Cu and Ni, and up to 24% of total Co and Zn. Total metals concentrations may be slightly elevated relative to background values, and the partitioning of Co, Cu, Ni, and Zn into the exchangeable fraction implies a mechanism for interaction with frogs. Furthermore, partitioning of Pb into poorly crystalline or amorphous oxyhydroxides suggests that it might be mobilized when water levels rise and cause reduction and dissolution of oxyhydroxides in submerged sediment. The difference in speciation between Pb on the one hand, and Co, Cr, Cu, Ni, and Zn on the other, implies that only Pb has an anthropogenic source. Mn speciation differs from all other metals in that it is subequally partitioned into exchange sites, hydroxides and trioctahedral clay. Fe is predominantly partitioned into trioctahedral clay, with lesser amounts in hydroxide form. QXRD indicates that expandable trioctahedral clay dissolves progressively throughout the SCE procedure, including during extractions intended to (1) oxidize sulfides and organic matter, and (2) reduce oxides and hydroxides, indicating that SCE procedures are not entirely phase selective. It also underscores the need for quantitative mineralogical analysis in conjunction with SCE, and has important implications for the interpretation of metal speciation by SCE alone and without corroborating QXRD analyses.

Evaluation and comparison of two selective extraction procedures for heavy metal speciation in sediments.

Two sequential extraction procedures (the former proposed by the Standard, Measurements and Testing-SM&T-program of European Union and the latter derived from those of Tessier and Forstner with various modifications) have been compared, by analyzing a reference material (CRM 601) certified for the content of heavy metals in the three solutions resulting from the SM&T procedure. Scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX) was used to evaluate the composition of the residues after various extractions. Reliable results, both in terms of precision and accuracy, were obtained for the first two steps of the SM&T scheme. As regards the third step, because it was modified during this work, a direct comparison of our results with those certified is not possible. SEM analysis confirmed that the scheme previously used in the laboratory was more aggressive compared to the SM&T procedure, which has also the advantage that the only CRM available for metal partitioning in sediments is certified using this method.

Heavy metal partitioning in sediments from the lagoon of Venice (Italy)

In this study heavy metal pollution in surface sediments of the Lagoon of Venice (Italy) was investigated. Attention was focused on the evaluation of the amounts of heavy metals that can potentially be released into the overlying water column, under particular environmental conditions, thus becoming bioavailable. To this end, the partitioning of heavy metals among different geochemical phases was determined by a procedure of sequential selective extractions. This approach can provide useful information for assessing the risk posed to the Lagoon ecosystem by metal contaminated sediments and for estimating the anthropogenic heavy metal content. In general, higher levels of contamination were found in sediments sampled in areas situated in the proximity of sources of pollutants and in degraded sites, which presented limited water circulation and prevailing anoxic conditions. The results of the sequential extraction procedure showed that partitioning is dependent on the metal considered: in particular, Pb, Cu, Zn and Cd were bound mainly to the most labile phases of sediments, whereas Cr and Ni were found prevalently in the residual fraction.

Comparison of the feasibility of three extraction procedures for trace metal partitioning in sediments from south-west Spain

The feasibility of three sequential extraction schemes (a modification of the Tessier procedure, the scheme proposed by Meguellati and the protocol designed by BCR (now called the Standards, Measurements and Testing Programme, M&T) have been compared to study the distribution of As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb and Zn in sediment samples. The comparison has been performed by analyzing Certified Reference Material (CRM-601), a test material (S-12) and seven sediments from the Odiel Marshes Natural Park (located at the Atlantic coast of southern Spain). Samples were classified as sandy (with low iron oxide and organic matter contents) and clay–silty (with high iron oxide and organic matter contents) sediments. A higher metal mobility, especially under reducing conditions, was more properly assessed using the modified Tessier scheme compared to both the BCR and Meguellati procedures, these two later presenting comparative results for the reducible and residual phases. Significant Hg losses were found using the BCR procedure but the quantification of the acid phase for Cd, Cr and Ni was more reliable than that obtained with the modified Tessier and Meguellati schemes.

Sequential extraction of trace metals from sediments

With the aim of harmonizing the methodology associated with the trace metal partitioning in sediments, a group of European researchers proposed a three-step sequential extraction scheme using acetic acid, hydroxylamine hydrochloride and ammonium acetate as extracting agents. The metal content in each extract is mainly determined by AAS or ICP.