Sequential metal mobilities

The diversity and complexity of the available approaches has identified the major difficulties associated with producing suitable guidelines that would allow comparisons between different laboratories and different countries in assessing metal mobility in the soil environment. This led to the development of single and sequential extraction procedures by the Standards, Materials and Testing (SM T - formerly the (European) Community Bureau of Reference (BCR) Programme of the European Union (1987)). Single extractants evaluated included 0.05 moll-1 EDTA, 0.43 moll-1 acetic acid, and 1 moll-1 ammonium acetate at pH 7. 

The behavior of elements (toxicity, bioavailability, and distribution) in the environment depends strongly on their chemical forms and type of binding and cannot be reliably predicted on the basis of the total concentration. In order to assess the mobility and reactivity of heavy metal (HM) species in solid samples (soils and sediments), batch sequential extraction procedures are used. HM are fractionated into operationally defined forms under the action of selective leaching reagents. 

Despite these shortcomings, common to any chemical extraction procedure, sequential dissolution techniques still furnish more useful information on metal binding, mobility and availability than can be obtained with only a single extractant. 

Zeien H., Brummer G.W. Determination of the mobility and binding of heavy metals in soil by sequential extraction. Mitteilungen der deutschen bodenkundlichen gesellschaft. 1991 66 397-400. 

Solid tailings were collected from test-pits and auger cuttings from boreholes in both the oxidized and reduced tailings. The tailings were analyzed for total metals. Sequential extraction of the tailings allowed the mobility of the metals to be assessed. Evaporites were also collected from the surface of the tailings. 

Davidson CM, Duncan AL, Littlejohn D, Garden LM. A critical evaluation of the three-stage BCR sequential extraction procedure to assess the potential mobility and toxicity of heavy metals in industrially-contaminated land. Anal. Chim. Acta 1998 363 45-55. 

Chester, R., Lin, F.J. and Murphy, K.J.T. (1989) A three stage sequential leaching scheme for the characterisation of the sources and environmental mobility of trace metals in the marine aerosol. Environ. Technol. Letts, 10, 887-900. 

Sequential extraction is now a well-established tool in soil and sediment analysis. Although the approach is unlikely to provide precise information on the mineral phases to which trace metals are bound, it does provide useful information on potential mobilities of heavy metal contaminants which is not available from (pseudo)total metal determination. The adoption of standard protocols, such as that recommended by BCR, means that reliable and comparable data can be obtained by different laboratories. 

Sequential extraction procedures include successive sample treatment with a series of extractants selected on the basis of their ability to dissolve analytes bound to different components of the matrix. The use of sequential extraction procedures simulating natural phenomena, (e.g., acid rain), can deliver detailed information about the origin, mode of occurrence, physiological availability, and mobilization of trace elements. It is also possible to estimate toxicological risk ensuing from the presence of different forms of metals and various phases containing those metals. A... 

In Chapter 3.4 a method has been presented for long-term prognosis of metal pollutant mobility, which combines column circulation leaching experiments at variable pH/E -conditions with sequential extraction procedures on the solid waste material before and after these experiments (Schoer FSrstner, 1987). Temporal release patterns are different for the individual elements (Figure 6-5) While at pH 5/400 mV release of cadmium seems to be completed within the experimental period mobilization of copper is still going on and the end point cannot be estimated from the data of the "kinetic" experiments. The same effect has been found for the examples of thallium and vanadium. For the other elements, the endpoint of release can be determined as approximately 10 mg cobalt, 0.6 mg cadmium, 600 mg zinc and 0.3 mg chromium, 2 mg barium and 20 mg lead (per 100 g of solid substrate treated with 140 L solu-... 

The objective of this study was to assess spatial and temporal changes of surface soil metals in Napoli urban area. Specifically, (i) the total content of Cu, Cr, Pb and Zn in surface and sub-surface soils of the city of Napoli was measured, (ii) the chemical and mineralogical forms of metals in soil were determined using a sequential fractionation procedure in order to assess mobility and availability to plants and (iii) current data were compared with those of a 1974 sampling, to define the accumulation or removal of metals. 

Simultaneous application of standard sequential leaching techniques can be used for geochemical characterization of anoxic, sulfide-bearing sediments in relation to the potential mobility of critical trace metals (Kersten and Fbrstner 1991). 

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