Speciation Of Metals In Soils Research Articles

Metal Bioaccumulation and Toxicity in Soils — Why Bother with Speciation?

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Metal Bioaccumulation and Toxicity in Soils—Why Bother with Speciation?

This review assesses metal speciation in soils, including analytical techniques used for measurement and the advantages and disadvantages of using chemical speciation information, in both the solid and aqueous phase, to predict adverse effects of metal contamination in soils and for use in soil protection policies. Other techniques used to assess metal lability and bioavailability in soil are also discussed.

Heavy metal uptake by barley growing in polluted soils: relationship with heavy metal speciation in soils

One soil polluted by lead and zinc and three soils contaminated with different copper (Cu), lead (Pb), and zinc (Zn) solutions in the laboratory were sown with barley (Hordeum distichum) to study the relationship between metal uptake and metal speciation in soil. The pH and organic matter content played a fundamental role in metal speciation and in particular in the amount of metal in exchangeable form. The results show that Zn is transported from roots towards leaves and a major uptake occurs. Cu and Pb are accumulated to a lesser extent than Zn and immobilized by root cells; for Cu and Pb a restriction of internal transport occurs. Tests performed by contaminating the soil during plant growth showed that soil having an acidic pH value and a lower organic matter content was not able to bind the metals, which were consequently sorbed by plants in high amounts.

A flow system for the determination of metal speciation in soil by sequential extraction

A flow extraction system with on-line and off-line flame atomic absorption spectrometric (FAAS) detection was developed to speed up, facilitate, and improve the accuracy of sequential extraction for metal speciation in solid materials. A flow extraction system with off-line detection was more advantageous than the on-line detection. In the proposed system, extraction was performed in a closed extraction chamber where extractants were flowing through sequentially. The extract from each extraction was collected in 4–10 fractions to obtain a total of approximately 180 ml for subsequent FAAS determination. The system is simple, easy to construct and operate. It has less risk of contamination and human error. A widely used three-step sequential extraction scheme was used to evaluate the novel system by analyzing Ca, Fe, Mn, Cu, and Zn in a certified reference material. The extraction time for three-step sequential extraction of soil sample was 4 h. The analytical results for Ca, Fe, Mn, Cu, and Zn of a soil certified reference material using the proposed system were compared with those of the conventional batch extraction.

The Importance of Nickel Phytoavailable Chemical Species Characterization in Soil for Phytoremediation Applicability

ABSTRACT In this work Ni speciation in natural and spiked soils (with similar total concentration) was studied. Spiked soils were prepared by addition of NiSO4.6H2O to obtain concentration similar to the one in natural soils. In soils mobile species were determined with a simplified sequential extraction as follows: H2O for water-soluble metal, KNO3 for exchangeable species, DTPA for complexed/adsorbed species. Results show that in spiked soils the exchangeable and adsorbed Ni concentrations are considerably higher than in natural soils. A study of plant uptake was carried out in order to evaluate the relation between mobile species and phyto-availability. Alfalfa (Medicago sativa L.), even though it is not a hyperaccumulator, was selected for its tolerance to high metal concentrations in soil. Preliminary results show a very high correlation between Ni mobile species and Ni uptake by alfalfa. Significant differences were found between spiked and natural soils. In the latter, high levels of total Ni did not correspond to relevant uptake as in the case of spiked soil. Results stress the importance of evaluating preliminarly heavy metal speciation in soil before planning phytoremediation procedures.

Metal Speciation in Soil: Extraction of Exchangeable Cations from a Calcareous Soil with a Magnesium Nitrate Solution

The exchangeable fraction of metals in a calcareous soil was studied with a magnesium nitrate solution and its extraction procedure was optimised. Parameters like extraction time, pH and concentration of the extractant solution, number of extractions or soil/solution ratio were examined. Experiments showed that after a 1-hour extraction, an equilibrium was established between extracted and re-sorbed metal. Moreover, carrying-out successive extractions re-sorbed quantities and maximum extracted ones could be estimated. Initial pH was fixed at 7 to avoid carbonates dissolution or hydroxides precipitation. A concentration of 0.5 mol/L and a soil/solution ratio of 1/20 were chosen. Leachate analyses showed a good selectivity of the extraction toward other soil fractions.

Sequential extraction for the speciation of some heavy metals in soils

The five step sequential extraction for speciation of copper and nickel originally designed for sediments has been applied to soil samples. The extractant solutions were: 1 mol/l ammonium acetate, 1 mol/l hydroxylammonium chloride in 25% acetic acid (1∶1), 0.1 mol/l hydrochlorid acid, 0.5 mol/l sodium hydroxide and 8 mol/l nitric acid. The residue was decomposed by HF and HNO3. Using this procedure the metal fraction bound to the organic matter can be distinguished. The concentrations of analytes were determined in the soil extracts by FAAS and ETAAS. Accuracy was assessed by comparing the sum of the contents of copper and nickel in soil extracts with the total certified values of CRMs of soils. The overall recovery values for nickel was 84–105% and for copper 105–114%.

Removal and redistribution of metals from contaminated soils by a sequential extraction method

The fate of heavy metals in a contaminated soil is dependent on both the total amount of metals and the chemical forms in which they exist. A widely applied sequential extraction method was used to study the metal speciation in soils. Sequential extraction for metal-contaminated individual soil components and synthetic soils was performed. The experimental results show that the specific reagents were effective for the extraction of metals from the corresponding individual soil components; but the impact of non-corresponding reagents on other soil components was also significant. The chemical forms in which metals exist in the synthetic soils were complicated and different. Metals in soils might be released and redistributed during the sequential extraction process, but knowledge about this behavior and the extent of metal redistribution is ambiguous.

Speciation of Heavy Metals in Soils, Sediments, and Sludges Using D.C. Plasma Atomic Emission Spectrometry Coupled with Ion Chromatography

Abstract An analytical method has been developed that can provide more reliable analytical information than the classical fractionation method which has been used by soil scientists for years to determine heavy metal contents of soil, sediment, and sludge samples. The new approach utilizes d.c. plasma atomic emission spectrometry in combination with ion chromatography (DCPAES-IC), whereby the DCPAES provides element selective measurements of the chromatographic effluents. In this way, the combined analytical system provides information on all the species of a metal present in the different steps of the fractionation approach. The DCPAES-IC approach also addresses questions pertaining to completeness of extraction, interference, and reagent concentration effects.

Validation of the methods for heavy metal speciation in soils and sediments

The knowledge of the chemical forms of metals is used to assess their availability and uptake by plants, and in sediments the forms of metals determine their transport and mobility in the aquatic media. This information may be obtained by determining chemical forms of metals (speciation) or different phases in which the metals are bound, obtained by applying extraction schemes. The analytical methods used include different steps and all of them must be validated. We report here the recommendations to minimize the errors in this kind of analysis. For validation the use of Certified Reference Materials (CRMs) and the participation in interlaboratory exercises are highly recommended.

Approaches to Sampling and Sample Pretreatments for Metal Speciation in Soils and Sediments

Abstract The main aspects of sampling and sample pretreatment in metal speciation studies of soils and sediments are discussed. The risks of sample contamination by the use of inappropriate materials, containers and tools as well as the possibilities of losses of analyte during sample handling are pointed out. Field sampling methods are described and minimum sample weight criteria for representative sampling of dry soils are presented. Sampling by traps and continuous flow centrifugation methods for suspended sediments and of bottom sediments by grabs or corers are compared. To avoid significant changes in some metal species drying and storage temperatures may need to be controlled and preservation in an inert atmosphere or by irradiation is discussed. The difficulties of establishing definitive protocols for sampling and sample pretreatment are emphasized as well as the need for selecting the appropriate technique in each particular case.

Speciation of metals in soils and mobilization by acid‐base treatment

To estimate the behaviour of metals in soils during pH change, polluted soil from an industrial area was subjected to sulphuric acid and calcium hydroxide additions. The behaviour of metals was studied by speciation techniques with separation into five fractions, namely: exchangeable, linked to carbonates, organic matter + sulphides or Fe‐Mn oxides and rest (associated to primary materials). As extractants, ammonium acetate, acetic acid, hydrogen peroxide and hydoxylamine were successively used. Determined metals include macro‐ (Na, K, Ca, Mg, Fe) and microconstituents (Cu, Pb, Mn, Zn) measured by AAS directly from extractant solutions and after HNO3—FH—HCIO4 digestion for total content. Analytical data are discussed as percentages of each metal in the different fractions at different soil pH values. The following results were obtained: (i) some macroconstituents (Mg, K, Na) are not influenced by acid‐base treatments of soil, (ii) calcium is probably retained as gypsum by soil treated with sulphuric acid, but an increase of its exchangeable amount was observed, (iii) most of Fe remains in the residual fraction, but the amount available for plants increases at low soil pH, and, (iv) the studied microconstituents become more soluble and presumably available for plants at lower soil pH values. As a whole, no differences were seem for fractions associated to Fe—Mn oxides or organic matter + sulphides, a decrease in the total amount of metals as carbonates and minima at intermediate pH values for exchangeable and metals linked to residual fraction were observed. This behaviour is further discussed in the sense that soil is a natural buffer, whose pH changes may be explained by conventional acid‐base equilibria. From the experimental data it was possible to estimate the acidity constant, which is probably related to the presence of clays in the soil studied.

Phosphatic fertilizers as a source of heavy metals in protected cultivation

Abstract In order to quantify the uptake by lettuce plants (Lactuca sativa L.) of heavy metals contained in the phosphate fertilizers and to evaluate their residual bioavailability in the growth substrate a greenhouse experiment was carried out. The superphosphate added to the soil contained varying amounts of heavy metals, particularly Cd, Cr and Zn which accumulated in the soil and increased the amounts available for plant uptake. The accumulation of metals observed in the plant tissues did not have physiological effects on lettuce and did not affect negatively the growth of the plants. The heavy metal speciation in soil indicated that metals extractable by EDTA increased by increasing the phosphorus added. These amounts of metals, adsorbed or complexed in the soil after harvest, represent in protected environments a potentially bioavailable species for the following cultivations.

Simulation of heavy metals extraction in soil samples compared with experimental results

Abstract The localization of heavy metals in soil is usually performed by means of chemical reagents and classical sequential procedures. The accuracy of selective extraction is, in fact, weak. In this study we compare the results of classical calculation with experimental results to show the limitations of the speciation of heavy metals in soils when using chemical reagents.

Speciation of Metals in Soils

Abstract Our studies have centred around the speciation of cadmium and manganese in three soils, one of which has been amended with sewage sludge. We report here some of our preliminary results, obtained by a coupled high performance liquid chromatography (HPLC)—graphite furnace atomic absorption spectroscopy (GFAAS) technique. The results indicate that cadmium may exist in different forms in soil, but only one form in soil-pore waters. Manganese is present in different soil-pore waters in different forms. The presence of sewage sludge in the soil can also affect the quantity and form of metals in water extracts.

Ionic activities of trace metals in sludge-amended soils

Trace metal bioavailability and mobility in soil systems is a function of both the metal speciation in the soil solution and the distribution of metals between solid soil components and the soil solution. A method has been developed to quantify metal speciation in sludge-amended soils which requires extraction of the soil with 0.01 M Ca(NO 3) 2, 0.01 M CaCl 2, and 0.01 M Ca(NO 3) 2 plus 5 × 10 −5 M Na 2EDTA. This method was applied to a silt loam surface soil amended with 408 T ha −1 (20%) of a sludge containing high concentrations of trace metals. Calculations showed that, of the Cd, Zn, Cu, Ni, and Fe in solution, 80, 54, 0.7, 0.03, and 10 −4.7% were present as uncomplexed ions, respectively. Logarithms of the relative stability constants with respect to Cd (K Cd M) calculated for the naturally occurring soluble complexing agents present in the Ca(NO 3) 2 extract were −0.55, −2.69, −3.85, and −7.80 for Zn, Cu, Ni, and Fe, respectively. Relative stability constants calculated with respect to Cd for the solid phase yielded similar values for the respective metals.

Speciation of heavy metals in soils based on charge separation

An experimental approach to the speciation of heavy metals in soil extracts has been developed. The proposed methodology differentiates between differently charged metal species in water and ammonium acetate soil extracts. Furthermore a distinction has been made between labile and stable metal species by using a Chelex 100 chelating ion exchange resin.