Speciation of trace metals in sediments

The term species generally refers to the molecular forms of an element or a cluster of atoms of different elements in a given (in this case solid) matrix (Bernhard et al., 1986a). The term form is also used to indicate uncertainty or lack of knowledge about the exact nature of the species one expects to find in an environmental sample. Conditional speciation assessment techniques like sequential extractions or leaching tests are not able to yield information at the true molecular level in solid phases thus the more general term form is used rather than species when referring to the results of such techniques commonly applied to date to soils and sediments. 

The term speciation , according to Bernhard et al. (1986a), encompasses three aspects (1) the actual distribution among molecular level entities in a given matrix, (2) the processes responsible for an observable distribution (species distribution), and (3) the analytical methods used (species analysis). Aspects (1) and (3) are compatible with the definition given by Ure (1990) and presented in Chapter 1. The second aspect, (2) above, is now seldom used in chemistry and 

This chapter will focus on the potential limitations of analytical methods used for speciation of trace elements in solid, but not necessarily natural samples, both from the molecular level point of view (e.g. XANES) and from the environmental assessment point of view (sequential extractions). 

Qualitative Elemental Analysis and Morphology of Individual Particles 

Identification of Major Compounds (Including Lead Species) 

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Kersten M. and Forstner U. (1995) Speciation of trace metals in sediments and combustion waste. In Chemica Speciation in the Environment (eds. A. M. Ure and C. M. Davidson). Blackie Academic and Professional, London, pp. 234-275. 

Tessier. A. and Campbell. P.G.C.. Partitioning of trace metals in sediments In metal speciation. In Theory, analysis and application, J.R. Kramer and H. E. Allen (Eds). Lewis Publisher. 

As for the overall metal removal efficiency, the best results were obtained for Pb and Zn as well as Cd (if precipitation at the cathode is considered), with more than 60% of each metal being mobilized as a result of the electrokinetic process. This result is likely related to multiple mechanisms, including speciation of trace metals in the original sediment, thermodynamic stability, and formation kinetics of metal-EDTA complexes, as well as ionic mobility of individual trace metal ions. However, due to the presumably interconnected effects of the mentioned mechanisms, a precise quantification of their individual contribution to trace metal removal from sediment is hard to be attained. 

In 1993-1994, two saltwater inflow events caused a change from anoxic to oxic conditions in the deep and bottom water of the Gotland Basin. A further stagnation period established from 1995 until 2003, and permanent anoxic conditions below 150 m were recorded since 1999. Under this context, the change in the environmental conditions coupled with a lifting of the redoxcline was a major process that influenced the speciation of trace metals, the resulting change in their solubility in the water column, and their export to anoxic sediments. 

Tessier A, Campbell PGC, Bisson M (1979) Sequential extraction procedure for the speciation of particulate trace metals. Anal Chem, 51 844-851 Ure AM, Quevauviller Ph, Muntau H, Griepink B (1993) 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 Commision of the European Communities. Int J Environ Anal Chem, 51 135-151 Wallmann K, Kersten M, Gruber J, Forstner U (1993) Artifacts in the determination of trace metal binding forms in anoxic sediments by sequential extraction. Int J Environ Anal Chem, 51 187-200... 

Marin, B., Valladon, M., Polve, M. and Monaco, A. (1997) Reproducibility testing of a sequential extraction scheme for the determination of trace metal speciation in a marine reference sediment by ICP-MS. Anal. Chim. Acta, 342, 91-112. 

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