A Critical Evaluation of Single Extractions from the SMT Program to Determine Trace Element Mobility in Sediments

Hindawi Publishing Corporation Applied and Environmental Soil Science Volume 2012, Article ID 672914, 15 pages doi:10.1155/2012/672914 Research Article A Critical Evaluation of Single Extractions from the SMT Program to Determine Trace Element Mobility in Sediments Valérie Cappuyns1, 2 1 Center for Economics and Corporate Sustainability (CEDON), University College Brussels (HUB), Warmoesberg 26, 1000 Brussels, Belgium 2 Department of Earth and Environmental Sciences, KULeuven, Celestijnenlaan 200E, 3001 Heverlee, Belgium Correspondence should be addressed to Valérie Cappuyns, Received 30 November 2011; Revised 27 March 2012; Accepted 4 April 2012 Academic Editor: Larissa Macedo dos Santos Copyright © 2012 Valérie Cappuyns. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Two commonly applied single extractions procedures, namely extractions with ammonium-EDTA and acetic acid, were evaluated based on the analysis of 72 samples from alluvial sediments. For most trace elements (Cu, Zn, Cd, Ni, As, and Pb), a significant linear relationship could be established between their ammonium-EDTA or acetic acid extractable concentrations and their total concentrations, the organic carbon content, pH, and Fe , Al, and/or Ca content in the sediments. The scientific understanding of trace element partitioning in the complex soil-water system with these simple models is rather limited, but they offer the opportunity to use data from single extractions in a more comprehensive way. Despite the fact that these extractions cannot directly be related to the bioavailability of elements, they can provide input data for use in risk assessment models. Additionally, they also offer possibilities to perform a fast screening of the mobilizable pool of elements in soils and/or sediments. 1. Introduction The contamination of soils and sediments is widespread and is a potential threat for the environment in the short and long term. The impact of trace elements in soils and sediments on the environment depends on their speciation, mobility, and bioavailability. Over the past decades, the term “heavy metals” has increasingly been used, without any consistency to denote trace element contamination of environmental media. An overview of the use of the term “heavy metals” in scientific dictionaries and relevant literature can be found in Duffus [1]. Since “heavy metals” is a poor scientific term and many alternatives exit [2], we will use the term “trace elements” in the present study to refer to As, Cd, Cu, Cr, Ni, Pb, and Zn. Talking about trace metals would be incorrect because arsenic is actually a metalloid. Before discussing the different methods for determination of “trace element” availability in soils and/or sediments and before addressing the pros and cons of single and sequential extraction procedures, the difference between soils and sediments will be clarified, as well as the terminology used throughout this paper. 1.1. Soils versus Sediments. Soils and sediments are different matrixes from many viewpoints, especially under the environmental context. “Soil” can be defined as a “threedimensional body with properties that reflect the impact of climate, vegetation, fauna, and topography on soils parent material over a variable time span. Soils are still in a process of change. As a result of “soil formation” or “pedogenesis,” soil profiles show signs of differentiation or alteration of the soil material [3].” “Sediment” can be described as “material that is transported by water and settles down from the water column [4]. In freshly deposited alluvial sediments, signs of differentiation or alteration of the material are sometimes not yet observable.” Nevertheless, in soil classification, specific designations are foreseen for this kind of “material”: alluvial soils can often be classified as Fluvisols, which “exhibit a stratified profile that reflects their depositional history or an irregular layering of humus and mineral sediments in which the content of organic carbon decreases with depth [5].” The qualifiers fluvic and spolic are used to indicate, respectively, the regular deposition of fresh sediments or the deposition of dredged sediments on a soil. 2 Throughout this paper, the term “sediment” will be used to indicate both the river sediments and the alluvial soils that consist of dredged-sediment derived soils and overbank sediments, regardless of their specific origin (e.g., overbank flooding, dredged-sediment derived soils, etc.), the degree of alteration, or the catchment width. 1.2. Trace Element Mobility in Soils and Sediments: Experimental Approach. “Trace element mobility” is an operationally defined term, which is determined by an approach used to determine the mobile, labile, or available metal species in soils and sediments. Although spectroscopic tools such as X-ray adsorption fine structure (XAFS) spectroscopy can give information on the coordination chemistry of metals (e.g., [6]), the quantification of the most mobile species is still difficult. The composition of soil pore water is important from an environmental point of view because it gives an indication of the “actual mobility” of trace elements and because the uptake of trace elements by plants occurs via the pore water. Moreover, pore water is also the carrier for elements to the groundwater. Leaching is the process by which inorganic or organic contaminants in the pore water are moved to deeper soil layers or to the groundwater by infiltrating water. However, the pore water composition only gives a momentary picture of trace element mobility since pore water composition can change over time. To assess trace element mobility in the long term (referred to as “potential mobility,” including physicochemically and biologically available metal pools) and under changing environmental conditions a variety of leaching and extractions tests are used. According to Peijnenburg et al. [7], three approaches can be distinguished to quantify physicochemically and biologically available metal pools in the soil: (1) direct measurement or modelling of metal activities, (2) assessment of operationally defined element fractions by means of single and sequential extractions, and (3) application of semipermeable devices, such as ion exchange resins/membranes and toxicity tests with membrane devices. For example, the in situ technique of diffusive gradients in thin films (DGT) is used for measuring effective soil solution concentrations and the additional element concentration supplied from the solid phase. The direct measurement of metal activities in pore water is rather complex, and there is not always an agreement between measured and model concentrations of free metal ion activities [8]. Several investigations have also been performed to compare the results of diffuse gradi (...truncated)